Gradle User Manual

Version 4.7

Gradle build tool source code is open and licensed under the Apache License 2.0. Gradle user manual and DSL references are licensed under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Table of Contents

About Gradle
Introduction
Overview
Working with existing builds
Installing Gradle
Command-Line Interface
The Gradle Wrapper
The Gradle Daemon
Dependency Management for Java Projects
Executing Multi-Project Builds
Composite builds
Build Environment
Troubleshooting
Embedding Gradle using the Tooling API
Build Cache
Writing Gradle build scripts
Build Script Basics
Build Init Plugin
Writing Build Scripts
Authoring Tasks
Working With Files
Using Ant from Gradle
Build Lifecycle
Logging
Authoring Multi-Project Builds
Using Gradle Plugins
Standard Gradle plugins
The Project Report Plugin
The Build Dashboard Plugin
Comparing Builds
Publishing artifacts
The Maven Plugin
The Signing Plugin
Ivy Publishing (new)
Maven Publishing (new)
The Distribution Plugin
The Announce Plugin
The Build Announcements Plugin
Dependency management
Introduction to Dependency Management
Declaring Dependencies
Managing Dependency Configurations
Declaring Repositories
Inspecting Dependencies
Managing Transitive Dependencies
Working with Dependencies
Customizing Dependency Resolution Behavior
Troubleshooting Dependency Resolution
Extending the build
Writing Custom Task Classes
Writing Custom Plugins
Gradle Plugin Development Plugin
Organizing Build Logic
Lazy Configuration
Initialization Scripts
Testing Build Logic with TestKit
Building JVM projects
Java Quickstart
Building Java & JVM projects
Testing in Java & JVM projects
The Base Plugin
The Java Plugin
The Java Library Plugin
Web Application Quickstart
The War Plugin
The Ear Plugin
The Jetty Plugin
The Application Plugin
The Java Library Distribution Plugin
Groovy Quickstart
The Groovy Plugin
The Scala Plugin
The ANTLR Plugin
The Checkstyle Plugin
The CodeNarc Plugin
The FindBugs Plugin
The JDepend Plugin
The PMD Plugin
The JaCoCo Plugin
The OSGi Plugin
The Eclipse Plugins
The IDEA Plugin
Best practices
Authoring Maintainable Build Scripts
Organizing Build Logic
The Software model
Rule based model configuration
Software model concepts
Implementing model rules in a plugin
Building Play applications
Building native software
Extending the software model
Glossary
Dependency Management Terminology
Dependency Types
Repository Types
The Dependency Cache
Appendix
A. Gradle Samples
B. Potential Traps
C. The Feature Lifecycle
D. Documentation licenses

List of Examples

1. Excluding tasks
2. Abbreviated camel case task name
3. Obtaining detailed help for tasks
4. Information about properties
5. Running the Wrapper task
6. The generated distribution URL
7. Providing options to Wrapper task
8. The generated distribution URL
9. Executing the build with the Wrapper batch file
10. Upgrading the Wrapper version
11. Checking the Wrapper version after upgrading
12. Customizing the Wrapper task
13. The generated distribution URL
14. Specifying the HTTP Basic Authentication credentials using system properties
15. Specifying the HTTP Basic Authentication credentials in distributionUrl
16. Configuring SHA-256 checksum verification
17. Dependency declarations for a Java-based project
18. Definition of a module dependency
19. Usage of Maven central repository
20. Usage of a local Ivy directory
21. Publishing to a Maven repository
22. Listing the projects in a build
23. Dependencies of my-app
24. Declaring a command-line composite
25. Declaring a separate composite
26. Depending on task from included build
27. Build that does not declare group attribute
28. Declaring the substitutions for an included build
29. Depending on a single task from an included build
30. Depending on a tasks with path in all included builds
31. Setting properties with a gradle.properties file
32. Specifying system properties in gradle.properties
33. Setting a project property via gradle.properties
34. Setting a project property via environment variable
35. Changing JVM settings for Gradle client JVM
36. Changing JVM settings for forked Gradle JVMs
37. Set Java compile options for JavaCompile tasks
38. Prevent releasing outside of CI
39. Configuring an HTTP proxy using gradle.properties
40. Configuring an HTTPS proxy using gradle.properties
41. Using the tooling API
42. Configure the local cache
43. Pull from HttpBuildCache
44. Configure remote HTTP cache
45. Allow untrusted SSL certificate for HttpBuildCache
46. Recommended setup for CI push use case
47. Consistent setup for buildSrc and main build
48. Init script to configure the build cache
49. Your first build script
50. Execution of a build script
51. A task definition shortcut
52. Using Groovy in Gradle's tasks
53. Using Groovy in Gradle's tasks
54. Declaration of task that depends on other task
55. Lazy dependsOn - the other task does not exist (yet)
56. Dynamic creation of a task
57. Accessing a task via API - adding a dependency
58. Accessing a task via API - adding behaviour
59. Accessing task as a property of the build script
60. Adding extra properties to a task
61. Using AntBuilder to execute ant.loadfile target
62. Using methods to organize your build logic
63. Defining a default task
64. Different outcomes of build depending on chosen tasks
65. Accessing property of the Project object
66. Using local variables
67. Using extra properties
68. Configuring arbitrary objects
69. Configuring arbitrary objects using a script
70. Groovy JDK methods
71. Property accessors
72. Method call without parentheses
73. List and map literals
74. Closure as method parameter
75. Closure delegates
76. Defining tasks
77. Defining tasks - using strings for task names
78. Defining tasks with alternative syntax
79. Accessing tasks as properties
80. Accessing tasks via tasks collection
81. Accessing tasks by path
82. Creating a copy task
83. Configuring a task - various ways
84. Configuring a task - with closure
85. Defining a task with closure
86. Adding dependency on task from another project
87. Adding dependency using task object
88. Adding dependency using closure
89. Adding a 'must run after' task ordering
90. Adding a 'should run after' task ordering
91. Task ordering does not imply task execution
92. A 'should run after' task ordering is ignored if it introduces an ordering cycle
93. Adding a description to a task
94. Overwriting a task
95. Skipping a task using a predicate
96. Skipping tasks with StopExecutionException
97. Enabling and disabling tasks
98. Custom task class
99. Ad-hoc task
100. Ad-hoc task declaring a destroyable
101. Using runtime API with custom task type
102. Using skipWhenEmpty() via the runtime API
103. Inferred task dependency via task outputs
104. Inferred task dependency via a task argument
105. Declaring a method to add task inputs
106. Declaring a method to add a task as an input
107. Failed attempt at setting up an inferred task dependency
108. Setting up an inferred task dependency between output dir and input files
109. Setting up an inferred task dependency with files()
110. Setting up an inferred task dependency with builtBy()
111. Ignoring up-to-date checks
112. Runtime classpath normalization
113. Task rule
114. Dependency on rule based tasks
115. Adding a task finalizer
116. Task finalizer for a failing task
117. How to copy a single file
118. Using implicit string paths
119. Prefer task/project properties over hard-coded paths
120. Using multiple arguments with from()
121. Using a flat filter
122. Using a deep filter
123. Copying an entire directory
124. Copying an entire directory, including itself
125. Archiving a directory as a ZIP
126. Using the Base Plugin for its archive name convention
127. Unpacking a ZIP file
128. Creating a Java uber or fat JAR
129. Manually creating a directory
130. Moving a directory using the Ant task
131. Renaming files as they are copied
132. Truncating filenames as they are copied
133. Deleting a directory
134. Deleting files matching a specific pattern
135. How to minimize the number of hard-coded paths in your build
136. Locating files
137. Creating a path relative to a parent project
138. Creating a file collection
139. Implementing a file collection
140. Using a file collection
141. Filtering a file collection
142. Creating a file tree
143. Changing Ant default exclusions for a copy task
144. Using a file tree
145. Using an archive as a file tree
146. Specifying a set of files
147. Appending a set of files
148. Specifying copy task source files and destination directory
149. Selecting the files to copy
150. Renaming files as they are copied
151. Filtering files as they are copied
152. Sharing copy specifications
153. Sharing copy patterns only
154. Nested copy specs
155. Copying files using the copy() method without up-to-date check
156. Copying files using the copy() method with up-to-date check
157. Using the Sync task to copy dependencies
158. Archiving a directory as a ZIP
159. Creation of ZIP archive
160. Configuration of archive task - custom archive name
161. Configuration of archive task - appendix & classifier
162. Activating reproducible archives
163. Using an Ant task
164. Passing nested text to an Ant task
165. Passing nested elements to an Ant task
166. Using an Ant type
167. Using a custom Ant task
168. Declaring the classpath for a custom Ant task
169. Using a custom Ant task and dependency management together
170. Importing an Ant build
171. Task that depends on Ant target
172. Adding behaviour to an Ant target
173. Ant target that depends on Gradle task
174. Renaming imported Ant targets
175. Setting an Ant property
176. Getting an Ant property
177. Setting an Ant reference
178. Getting an Ant reference
179. Fine tuning Ant logging
180. Single project build
181. Hierarchical layout
182. Flat layout
183. Lookup of elements of the project tree
184. Modification of elements of the project tree
185. Adding of test task to each project which has certain property set
186. Notifications
187. Setting of certain property to all tasks
188. Logging of start and end of each task execution
189. Using stdout to write log messages
190. Writing your own log messages
191. Writing a log message with placeholder
192. Using SLF4J to write log messages
193. Configuring standard output capture
194. Configuring standard output capture for a task
195. Customizing what Gradle logs
196. Multi-project tree - water & bluewhale projects
197. Build script of water (parent) project
198. Multi-project tree - water, bluewhale & krill projects
199. Water project build script
200. Defining common behavior of all projects and subprojects
201. Defining specific behaviour for particular project
202. Defining specific behaviour for project krill
203. Adding custom behaviour to some projects (filtered by project name)
204. Adding custom behaviour to some projects (filtered by project properties)
205. Running build from subproject
206. Evaluation and execution of projects
207. Evaluation and execution of projects
208. Running tasks by their absolute path
209. Dependencies and execution order
210. Dependencies and execution order
211. Dependencies and execution order
212. Declaring dependencies
213. Declaring dependencies
214. Cross project task dependencies
215. Configuration time dependencies
216. Configuration time dependencies - evaluationDependsOn
217. Configuration time dependencies
218. Dependencies - real life example - crossproject configuration
219. Project lib dependencies
220. Project lib dependencies
221. Fine grained control over dependencies
222. Build and Test Single Project
223. Partial Build and Test Single Project
224. Build and Test Depended On Projects
225. Build and Test Dependent Projects
226. Applying a script plugin
227. Applying a core plugin
228. Applying a community plugin
229. Applying plugins only on certain subprojects.
230. Using plugins from custom plugin repositories.
231. Plugin resolution strategy.
232. Complete Plugin Publishing Sample
233. Applying a binary plugin
234. Applying a binary plugin by type
235. Applying a plugin with the buildscript block
236. Using the Build Dashboard plugin
237. Defining an artifact using an archive task
238. Defining an artifact using a file
239. Customizing an artifact
240. Map syntax for defining an artifact using a file
241. Configuration of the upload task
242. Using the Maven plugin
243. Creating a standalone pom.
244. Upload of file to remote Maven repository
245. Upload of file via SSH
246. Customization of pom
247. Builder style customization of pom
248. Modifying auto-generated content
249. Customization of Maven installer
250. Generation of multiple poms
251. Accessing a mapping configuration
252. Using the Signing plugin
253. Sign with GnuPG
254. Configure the GnupgSignatory
255. Signing a configuration
256. Signing a configuration output
257. Signing a task
258. Signing a task output
259. Conditional signing
260. Signing a POM for deployment
261. Applying the “ivy-publish” plugin
262. Publishing a Java module to Ivy
263. Publishing additional artifact to Ivy
264. customizing the publication identity
265. Customizing the module descriptor file
266. Publishing multiple modules from a single project
267. Declaring repositories to publish to
268. Choosing a particular publication to publish
269. Publishing all publications via the “publish” lifecycle task
270. Generating the Ivy module descriptor file
271. Publishing a Java module
272. Example generated ivy.xml
273. Applying the 'maven-publish' plugin
274. Adding a MavenPublication for a Java component
275. Adding additional artifact to a MavenPublication
276. customizing the publication identity
277. Modifying the POM file
278. Publishing multiple modules from a single project
279. Declaring repositories to publish to
280. Publishing a project to a Maven repository
281. Publish a project to the Maven local repository
282. Generate a POM file without publishing
283. Using the distribution plugin
284. Adding extra distributions
285. Configuring the main distribution
286. publish main distribution
287. Applying the announce plugin
288. Configure the announce plugin
289. Using the announce plugin
290. Using the build announcements plugin
291. Using the build announcements plugin from an init script
292. Declaring a dependency with a concrete version
293. Declaring a dependency without version
294. Declaring a dependency with a dynamic version
295. Declaring a dependencies with a changing version
296. Declaring multiple file dependencies
297. Declaring project dependencies
298. Resolving a JavaScript artifact for a declared dependency
299. Resolving a JavaScript artifact with classifier for a declared dependency
300. Declaring and using a custom configuration
301. Extending a configuration from another configuration
302. Declaring JCenter repository as source for resolving dependencies
303. Declaring a custom repository by URL
304. Declaring multiple repositories
305. Declaring the JGit dependency with a custom configuration
306. Rendering the dependency report for a custom configuration
307. Declaring the JGit dependency and a conflicting dependency
308. Using the dependency insight report for a given dependency
309. Giving a reason for choosing a certain module version in a dependency declaration
310. Using the dependency insight report with custom reasons
311. Define dependency constraints
312. Unresolved artifacts for transitive dependencies
313. Excluding transitive dependency for a particular dependency declaration
314. Excluding transitive dependency for a particular configuration
315. Enforcing a dependency version
316. Enforcing a dependency version on the configuration-level
317. Disabling transitive dependency resolution for a declared dependency
318. Disabling transitive dependency resolution on the configuration-level
319. Depending on a BOM to import its dependency constraints
320. Iterating over the dependencies assigned to a configuration
321. Iterating over the artifacts resolved for a module
322. Walking the resolved and unresolved dependencies of a configuration
323. Accessing a Maven module's metadata artifact
324. Forcing a consistent version for a group of libraries
325. Using a custom versioning scheme
326. Blacklisting a version with a replacement
327. Changing dependency group and/or name during resolution
328. Substituting a module with a project
329. Substituting a project with a module
330. Conditionally substituting a dependency
331. 'Latest' version selector
332. Custom status scheme
333. Custom status scheme by module
334. Ivy component metadata rule
335. Rule source component metadata rule
336. Component selection rule
337. Component selection rule with module target
338. Component selection rule with metadata
339. Component selection rule using a rule source object
340. Declaring a module replacement
341. Specifying default dependencies on a configuration
342. Enabling dynamic resolve mode
343. Dynamic version cache control
344. Changing module cache control
345. Defining a custom task
346. A hello world task
347. A customizable hello world task
348. A build for a custom task
349. A custom task
350. Using a custom task in another project
351. Testing a custom task
352. Defining an incremental task action
353. Running the incremental task for the first time
354. Running the incremental task with unchanged inputs
355. Running the incremental task with updated input files
356. Running the incremental task with an input file removed
357. Running the incremental task with an output file removed
358. Running the incremental task with an input property changed
359. Declaring a command line option
360. Using a command line option
361. Declaring available values for an option
362. Listing available values for option
363. Creating a unit of work implementation
364. Submitting a unit of work for execution
365. Waiting for asynchronous work to complete
366. Submitting an item of work to run in a worker daemon
367. A custom plugin
368. A custom plugin extension
369. A custom plugin with configuration closure
370. Evaluating file properties lazily
371. Mapping extension properties to task properties
372. A build for a custom plugin
373. Wiring for a custom plugin
374. Using a custom plugin in another project
375. Applying a community plugin with the plugins DSL
376. Testing a custom plugin
377. Using the Java Gradle Plugin Development plugin
378. Nested DSL elements
379. Managing a collection of objects
380. Using the Java Gradle Plugin Development plugin
381. Using the gradlePlugin {} block.
382. Using inherited properties and methods
383. Using injected properties and methods
384. Configuring the project using an external build script
385. Custom buildSrc build script
386. Adding subprojects to the root buildSrc project
387. Running another build from a build
388. Declaring external dependencies for the build script
389. A build script with external dependencies
390. Ant optional dependencies
391. Using a read-only and configurable property
392. Using file and directory property
393. Implicit task dependency
394. List property
395. Using init script to perform extra configuration before projects are evaluated
396. Declaring external dependencies for an init script
397. An init script with external dependencies
398. Using plugins in init scripts
399. Declaring the TestKit dependency
400. Declaring the JUnit dependency
401. Using GradleRunner with JUnit
402. Using GradleRunner with Spock
403. Making the code under test classpath available to the tests
404. Injecting the code under test classes into test builds
405. Injecting the code under test classes into test builds for Gradle versions prior to 2.8
406. Using the Java Gradle Development plugin for generating the plugin metadata
407. Automatically injecting the code under test classes into test builds
408. Reconfiguring the classpath generation conventions of the Java Gradle Development plugin
409. Specifying a Gradle version for test execution
410. Testing cacheable tasks
411. Clean build cache between tests
412. Using the Java plugin
413. Building a Java project
414. Adding Maven repository
415. Adding dependencies
416. Customization of MANIFEST.MF
417. Adding a test system property
418. Publishing the JAR file
419. Eclipse plugin
420. Java example - complete build file
421. Multi-project build - hierarchical layout
422. Multi-project build - settings.gradle file
423. Multi-project build - common configuration
424. Multi-project build - dependencies between projects
425. Multi-project build - distribution file
426. Applying the Java Plugin
427. Declaring dependencies
428. Declaring custom source directories
429. Declaring custom source directories additively
430. Setting Java compiler options
431. Configure Java 6 build
432. Defining a custom task to create a 'sources' JAR
433. Creating a Java uber or fat JAR
434. Customization of MANIFEST.MF
435. Creating a manifest object.
436. Separate MANIFEST.MF for a particular archive
437. Saving a MANIFEST.MF to disk
438. Using a custom doclet with Javadoc
439. Defining a custom Javadoc task
440. Filtering tests in the build script
441. Changing the default test report and results directories
442. Creating a unit test report for subprojects
443. JUnit Categories
444. JUnit Platform Tags
445. Grouping TestNG tests
446. Enabling JUnit Platform to run your tests
447. JUnit Jupiter dependencies
448. JUnit Vintage dependencies
449. Filter specific engines
450. Preserving order of TestNG tests
451. Grouping TestNG tests by instances
452. Setting up working integration tests
453. Defining a working integration test task
454. Applying the Base Plugin
455. Using the Java plugin
456. Custom Java source layout
457. Assembling a JAR for a source set
458. Generating the Javadoc for a source set
459. Running tests in a source set
460. Registering incremental annotation processors
461. An isolated annotation processor
462. An aggregating annotation processor
463. Declaring annotation processors
464. Customization of MANIFEST.MF
465. Creating a manifest object.
466. Separate MANIFEST.MF for a particular archive
467. Saving a MANIFEST.MF to disk
468. Using the Java Library plugin
469. Declaring API and implementation dependencies
470. Making the difference between API and implementation
471. Declaring API and implementation dependencies
472. Configuring the Groovy plugin to work with Java Library
473. War plugin
474. Running web application with Gretty plugin
475. Using the War plugin
476. Customization of war plugin
477. Using the Ear plugin
478. Customization of ear plugin
479. Using the application plugin
480. Configure the application main class
481. Configure default JVM settings
482. Configure custom directory for start scripts
483. Include output from other tasks in the application distribution
484. Automatically creating files for distribution
485. Using the Java library distribution plugin
486. Configure the distribution name
487. Include files in the distribution
488. Groovy plugin
489. Dependency on Groovy
490. Groovy example - complete build file
491. Using the Groovy plugin
492. Custom Groovy source layout
493. Configuration of Groovy dependency
494. Configuration of Groovy test dependency
495. Configuration of bundled Groovy dependency
496. Configuration of Groovy file dependency
497. Configure Java 6 build for Groovy
498. Using the Scala plugin
499. Custom Scala source layout
500. Declaring a Scala dependency for production code
501. Declaring a Scala dependency for test code
502. Declaring a version of the Zinc compiler to use
503. Forcing a scala-library dependency for all configurations
504. Forcing a scala-library dependency for the zinc configuration
505. Adjusting memory settings
506. Forcing all code to be compiled
507. Configure Java 6 build for Scala
508. Explicitly specify a target IntelliJ IDEA version
509. Using the ANTLR plugin
510. Declare ANTLR version
511. setting custom max heap size and extra arguments for ANTLR
512. Using the Checkstyle plugin
513. Using the config_loc property
514. Customizing the HTML report
515. Using the CodeNarc plugin
516. Using the FindBugs plugin
517. Customizing the HTML report
518. Using the JDepend plugin
519. Using the PMD plugin
520. Applying the JaCoCo plugin
521. Configuring JaCoCo plugin settings
522. Configuring test task
523. Configuring violation rules
524. Configuring test task
525. Using application plugin to generate code coverage data
526. Coverage reports generated by applicationCodeCoverageReport
527. Using the OSGi plugin
528. Configuration of OSGi MANIFEST.MF file
529. Using the Eclipse plugin
530. Using the Eclipse WTP plugin
531. Partial Overwrite for Classpath
532. Partial Overwrite for Project
533. Export Classpath Entries
534. Customizing the XML
535. Using the IDEA plugin
536. Partial Rewrite for Module
537. Partial Rewrite for Project
538. Export Dependencies
539. Customizing the XML
540. A task declaring the group and description
541. Executing logic during configuration should be avoided
542. Executing logic during execution phase is preferred
543. A build script using conditional logic to create a task
544. A binary plugin implementing imperative logic
545. A build script applying a plugin that encapsulates imperative logic
546. Using inherited properties and methods
547. Using injected properties and methods
548. Configuring the project using an external build script
549. Custom buildSrc build script
550. Adding subprojects to the root buildSrc project
551. Running another build from a build
552. Declaring external dependencies for the build script
553. A build script with external dependencies
554. Ant optional dependencies
555. applying a rule source plugin
556. a model creation rule
557. a model mutation rule
558. creating a task
559. a managed type
560. a String property
561. a File property
562. a Long property
563. a boolean property
564. an int property
565. a managed property
566. an enumeration type property
567. a managed set
568. a scalar collection
569. strongly modelling sources sets
570. a DSL example applying a rule to every element in a scope
571. DSL configuration rule
572. Configuration run when required
573. Configuration not run when not required
574. DSL creation rule
575. DSL creation rule without initialization
576. Initialization before configuration
577. Nested DSL creation rule
578. Nested DSL configuration rule
579. DSL configuration rule for each element in a map
580. Nested DSL property configuration
581. a DSL example showing type conversions
582. a DSL rule using inputs
583. model task output
584. Using the Play plugin
585. The components report
586. Selecting a version of the Play Framework
587. Adding dependencies to a Play application
588. A Play 2.6 project
589. Adding Guice dependency in Play 2.6 project
590. Configuring extra source sets to a Play application
591. Adding extra source sets to a Play application
592. Configuring Scala compiler options
593. Configuring routes style
594. Configuring a custom asset pipeline
595. Configuring dependencies on Play subprojects
596. Add extra files to a Play application distribution
597. Applying both the Play and IDEA plugins
598. Defining a library component
599. Defining executable components
600. Sample build
601. Dependent components report
602. Dependent components report
603. Report of components that depends on the operators component
604. Report of components that depends on the operators component, including test suites
605. Assemble components that depends on the passing/static binary of the operators component
606. Build components that depends on the passing/static binary of the operators component
607. Adding a custom check task
608. Running checks for a given binary
609. The components report
610. The 'cpp' plugin
611. C++ source set
612. The 'c' plugin
613. C source set
614. The 'assembler' plugin
615. The 'objective-c' plugin
616. The 'objective-cpp' plugin
617. Settings that apply to all binaries
618. Settings that apply to all shared libraries
619. Settings that apply to all binaries produced for the 'main' executable component
620. Settings that apply only to shared libraries produced for the 'main' library component
621. The 'windows-resources' plugin
622. Configuring the location of Windows resource sources
623. Building a resource-only dll
624. Providing a library dependency to the source set
625. Providing a library dependency to the binary
626. Declaring project dependencies
627. Creating a precompiled header file
628. Including a precompiled header file in a source file
629. Configuring a precompiled header
630. Defining build types
631. Configuring debug binaries
632. Defining platforms
633. Defining flavors
634. Targeting a component at particular platforms
635. Building all possible variants
636. Defining tool chains
637. Reconfigure tool arguments
638. Defining target platforms
639. Registering CUnit tests
640. Configuring CUnit tests
641. Running CUnit tests
642. Registering GoogleTest tests
643. an example of using a custom software model
644. Declare a custom component
645. Register a custom component
646. Declare a custom binary
647. Register a custom binary
648. Declare a custom source set
649. Register a custom source set
650. Generates documentation binaries
651. Generates tasks for text source sets
652. Register a custom source set
653. an example of using a custom software model
654. components report
655. public type and internal view declaration
656. type registration
657. public and internal data mutation
658. example build script and model report output
659. Module dependencies
660. File dependencies
661. Generated file dependencies
662. Project dependencies
663. Gradle API dependencies
664. Gradle TestKit dependencies
665. Gradle's Groovy dependencies
666. Flat repository resolver
667. Adding central Maven repository
668. Adding Bintray's JCenter Maven repository
669. Adding Google Maven repository
670. Adding the local Maven cache as a repository
671. Adding custom Maven repository
672. Adding additional Maven repositories for JAR files
673. Accessing password-protected Maven repository
674. Ivy repository
675. Ivy repository with named layout
676. Ivy repository with pattern layout
677. Ivy repository with multiple custom patterns
678. Ivy repository with Maven compatible layout
679. Ivy repository with authentication
680. Maven repository that supports artifacts without metadata
681. Declaring a Maven and Ivy repository
682. Using the SFTP protocol for a repository
683. Declaring a S3 backed Maven and Ivy repository
684. Declaring a S3 backed Maven and Ivy repository using IAM
685. Declaring a Google Cloud Storage backed Maven and Ivy repository using default application credentials
686. Configure repository to use only digest authentication
687. Configure repository to use preemptive authentication
B.1. Variables scope: local and script wide
B.2. Distinct configuration and execution phase

About Gradle

Introduction

We would like to introduce Gradle to you, a build system that we think is a quantum leap for build technology in the Java (JVM) world. Gradle provides:

  • A very flexible general purpose build tool like Ant.

  • Switchable, build-by-convention frameworks a la Maven. But we never lock you in!

  • Very powerful support for multi-project builds.

  • Very powerful dependency management (based on Apache Ivy).

  • Full support for your existing Maven or Ivy repository infrastructure.

  • Support for transitive dependency management without the need for remote repositories or pom.xml and ivy.xml files.

  • Ant tasks and builds as first class citizens.

  • Groovy build scripts.

  • A rich domain model for describing your build.

In Overview you will find a detailed overview of Gradle. Otherwise, the guides are waiting, have fun :)

About this user guide

This user guide, like Gradle itself, is under very active development. Some parts of Gradle aren’t documented as completely as they need to be. Some of the content presented won’t be entirely clear or will assume that you know more about Gradle than you do. We need your help to improve this user guide. You can find out more about contributing to the documentation at the Gradle web site.

Throughout the user guide, you will find some diagrams that represent dependency relationships between Gradle tasks. These use something analogous to the UML dependency notation, which renders an arrow from one task to the task that the first task depends on.

Overview

Features

Here is a list of some of Gradle’s features.

Declarative builds and build-by-convention

At the heart of Gradle lies a rich extensible Domain Specific Language (DSL) based on Groovy. Gradle pushes declarative builds to the next level by providing declarative language elements that you can assemble as you like. Those elements also provide build-by-convention support for Java, Groovy, OSGi, Web and Scala projects. Even more, this declarative language is extensible. Add your own new language elements or enhance the existing ones, thus providing concise, maintainable and comprehensible builds.

Language for dependency based programming

The declarative language lies on top of a general purpose task graph, which you can fully leverage in your builds. It provides utmost flexibility to adapt Gradle to your unique needs.

Structure your build

The suppleness and richness of Gradle finally allows you to apply common design principles to your build. For example, it is very easy to compose your build from reusable pieces of build logic. Inline stuff where unnecessary indirections would be inappropriate. Don’t be forced to tear apart what belongs together (e.g. in your project hierarchy). Avoid smells like shotgun changes or divergent change that turn your build into a maintenance nightmare. At last you can create a well structured, easily maintained, comprehensible build.

Deep API

From being a pleasure to be used embedded to its many hooks over the whole lifecycle of build execution, Gradle allows you to monitor and customize its configuration and execution behavior to its very core.

Gradle scales

Gradle scales very well. It significantly increases your productivity, from simple single project builds up to huge enterprise multi-project builds. This is true for structuring the build. With the state-of-art incremental build function, this is also true for tackling the performance pain many large enterprise builds suffer from.

Multi-project builds

Gradle’s support for multi-project build is outstanding. Project dependencies are first class citizens. We allow you to model the project relationships in a multi-project build as they really are for your problem domain. Gradle follows your layout not vice versa.

Gradle provides partial builds. If you build a single subproject Gradle takes care of building all the subprojects that subproject depends on. You can also choose to rebuild the subprojects that depend on a particular subproject. Together with incremental builds this is a big time saver for larger builds.

Many ways to manage your dependencies

Different teams prefer different ways to manage their external dependencies. Gradle provides convenient support for any strategy. From transitive dependency management with remote Maven and Ivy repositories to jars or directories on the local file system.

Gradle is the first build integration tool

Ant tasks are first class citizens. Even more interesting, Ant projects are first class citizens as well. Gradle provides a deep import for any Ant project, turning Ant targets into native Gradle tasks at runtime. You can depend on them from Gradle, you can enhance them from Gradle, you can even declare dependencies on Gradle tasks in your build.xml. The same integration is provided for properties, paths, etc …​

Gradle fully supports your existing Maven or Ivy repository infrastructure for publishing and retrieving dependencies. Gradle also provides a converter for turning a Maven pom.xml into a Gradle script. Runtime imports of Maven projects will come soon.

Ease of migration

Gradle can adapt to any structure you have. Therefore you can always develop your Gradle build in the same branch where your production build lives and both can evolve in parallel. We usually recommend to write tests that make sure that the produced artifacts are similar. That way migration is as less disruptive and as reliable as possible. This is following the best-practices for refactoring by applying baby steps.

Groovy

Gradle’s build scripts are written in Groovy, not XML. But unlike other approaches this is not for simply exposing the raw scripting power of a dynamic language. That would just lead to a very difficult to maintain build. The whole design of Gradle is oriented towards being used as a language, not as a rigid framework. And Groovy is our glue that allows you to tell your individual story with the abstractions Gradle (or you) provide. Gradle provides some standard stories but they are not privileged in any form. This is for us a major distinguishing feature compared to other declarative build systems. Our Groovy support is not just sugar coating. The whole Gradle API is fully Groovy-ized. Adding Groovy results in an enjoyable and productive experience.

The Gradle wrapper

The Gradle Wrapper allows you to execute Gradle builds on machines where Gradle is not installed. This is useful for example for some continuous integration servers. It is also useful for an open source project to keep the barrier low for building it. The wrapper is also very interesting for the enterprise. It is a zero administration approach for the client machines. It also enforces the usage of a particular Gradle version thus minimizing support issues.

Free and open source

Gradle is an open source project, and is licensed under the ASL.

Why Groovy?

We think the advantages of an internal DSL (based on a dynamic language) over XML are tremendous when used in build scripts. There are a couple of dynamic languages out there. Why Groovy? The answer lies in the context Gradle is operating in. Although Gradle is a general purpose build tool at its core, its main focus are Java projects. In such projects the team members will be very familiar with Java. We think a build should be as transparent as possible to all team members.

In that case, you might argue why we don’t just use Java as the language for build scripts. We think this is a valid question. It would have the highest transparency for your team and the lowest learning curve, but because of the limitations of Java, such a build language would not be as nice, expressive and powerful as it could be.[1] Languages like Python, Groovy or Ruby do a much better job here. We have chosen Groovy as it offers by far the greatest transparency for Java people. Its base syntax is the same as Java’s as well as its type system, its package structure and other things. Groovy provides much more on top of that, but with the common foundation of Java.

For Java developers with Python or Ruby knowledge or the desire to learn them, the above arguments don’t apply. The Gradle design is well-suited for creating another build script engine in JRuby or Jython. It just doesn’t have the highest priority for us at the moment. We happily support any community effort to create additional build script engines.



[1] At http://www.defmacro.org/ramblings/lisp.html you find an interesting article comparing Ant, XML, Java and Lisp. It’s funny that the 'if Java had that syntax' syntax in this article is actually the Groovy syntax.

Working with existing builds

Installing Gradle

You can install the Gradle build tool on Linux, macOS, or Windows. This document covers installing using a package manager like SDKMAN!, Homebrew, or Scoop, as well as manual installation.

To upgrade Gradle, use of the Gradle Wrapper is the recommended.

You can find all releases and their checksums on the releases page.

Prerequisites

Gradle runs on all major operating systems and requires only a Java JDK version 7 or higher to run. To check, run java -version. You should see something like this:

❯ java -version
java version "1.8.0_151"
Java(TM) SE Runtime Environment (build 1.8.0_151-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.151-b12, mixed mode)

Gradle ships with its own Groovy library, therefore Groovy does not need to be installed. Any existing Groovy installation is ignored by Gradle.

Gradle uses whatever JDK it finds in your path. Alternatively, you can set the JAVA_HOME environment variable to point to the installation directory of the desired JDK.

Installing with a package manager

SDKMAN! is a tool for managing parallel versions of multiple Software Development Kits on most Unix-based systems.

❯ sdk install gradle 4.6

Homebrew is "the missing package manager for macOS".

❯ brew install gradle

Scoop is a command-line installer for Windows inspired by Homebrew.

❯ scoop install gradle

Chocolatey is "the package manager for Windows".

❯ choco install gradle

MacPorts is a system for managing tools on macOS:

❯ sudo port install gradle

↓ Proceed to next steps

Installing manually

Step 1. Download the latest Gradle distribution

The distribution ZIP file comes in two flavors:

  • Binary-only (bin)

  • Complete (all) with docs and sources

Need to work with an older version? See the releases page.

Step 2. Unpack the distribution

Linux & MacOS users

Unzip the distribution zip file in the directory of your choosing, e.g.:

❯ mkdir /opt/gradle
❯ unzip -d /opt/gradle gradle-4.6-bin.zip
❯ ls /opt/gradle/gradle-4.6
LICENSE  NOTICE  bin  getting-started.html  init.d  lib  media

Microsoft Windows users

Create a new directory C:\Gradle with File Explorer.

Open a second File Explorer window and go to the directory where the Gradle distribution was downloaded. Double-click the ZIP archive to expose the content. Drag the content folder gradle-4.6 to your newly created C:\Gradle folder.

Alternatively you can unpack the Gradle distribution ZIP into C:\Gradle using an archiver tool of your choice.

Step 3. Configure your system environment

For running Gradle, firstly add the environment variable GRADLE_HOME. This should point to the unpacked files from the Gradle website. Next add GRADLE_HOME/bin to your PATH environment variable. Usually, this is sufficient to run Gradle.

Linux & MacOS users

Configure your PATH environment variable to include the bin directory of the unzipped distribution, e.g.:

❯ export PATH=$PATH:/opt/gradle/gradle-4.6/bin

Microsoft Windows users

In File Explorer right-click on the This PC (or Computer) icon, then click PropertiesAdvanced System SettingsEnvironmental Variables.

Under System Variables select Path, then click Edit. Add an entry for C:\Gradle\gradle-4.6\bin. Click OK to save.

↓ Proceed to next steps

Verifying installation

Open a console (or a Windows command prompt) and run gradle -v to run gradle and display the version, e.g.:

❯ gradle -v

------------------------------------------------------------
Gradle 4.6
------------------------------------------------------------

Build time:   2018-02-21 15:28:42 UTC
Revision:     819e0059da49f469d3e9b2896dc4e72537c4847d

Groovy:       2.4.12
Ant:          Apache Ant(TM) version 1.9.9 compiled on February 2 2017
JVM:          1.8.0_151 (Oracle Corporation 25.151-b12)
OS:           Mac OS X 10.13.3 x86_64

If you run into any trouble, see the section on troubleshooting installation.

You can verify the integrity of the Gradle distribution by downloading the SHA-256 file (available from the releases page) and following these verification instructions.

Next steps

Now that you have Gradle installed, use these resources for getting started:

Command-Line Interface

The command-line interface is one of the primary methods of interacting with Gradle. The following serves as a reference of executing and customizing Gradle use of a command-line or when writing scripts or configuring continuous integration.

Use of the Gradle Wrapper is highly encouraged. You should substitute ./gradlew or gradlew.bat for gradle in all following examples when using the Wrapper.

Executing Gradle on the command-line conforms to the following structure. Options are allowed before and after task names.

gradle [taskName...] [--option-name...]

If multiple tasks are specified, they should be separated with a space.

Options that accept values can be specified with or without = between the option and argument; however, use of = is recommended.

--console=plain

Options that enable behavior have long-form options with inverses specified with --no-. The following are opposites.

--build-cache
--no-build-cache

Many long-form options, have short option equivalents. The following are equivalent:

--help
-h

Many command-line flags can be specified in gradle.properties to avoid needing to be typed. See the configuring build environment guide for details.

The following sections describe use of the Gradle command-line interface, grouped roughly by user goal. Some plugins also add their own command line options, for example --tests for Java test filtering. For more information on exposing command line options for your own tasks, see the section called “Declaring and Using Command Line Options”.

Executing tasks

You can run a task and all of its dependencies.

❯ gradle myTask

You can learn about what projects and tasks are available in the project reporting section.

Executing tasks in multi-project builds

In a multi-project build, subproject tasks can be executed with ":" separating subproject name and task name. The following are equivalent when run from the root project.

❯ gradle :mySubproject:taskName
❯ gradle mySubproject:taskName

You can also run a task for all subprojects using the task name only. For example, this will run the "test" task for all subprojects when invoked from the root project directory.

❯ gradle test

When invoking Gradle from within a subproject, the project name should be omitted:

❯ cd mySubproject
❯ gradle taskName

When executing the Gradle Wrapper from subprojects, one must reference gradlew relatively. For example: ../gradlew taskName. The community gdub project aims to make this more convenient.

Executing multiple tasks

You can also specify multiple tasks. For example, the following will execute the test and deploy tasks in the order that they are listed on the command-line and will also execute the dependencies for each task.

❯ gradle test deploy

Excluding tasks from execution

You can exclude a task from being executed using the -x or --exclude-task command-line option and providing the name of the task to exclude.

Figure 1. Example Task Graph

Example Task Graph

Example 1. Excluding tasks

Output of gradle dist --exclude-task test

> gradle dist --exclude-task test

> Task :compile
compiling source

> Task :dist
building the distribution

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

You can see that the test task is not executed, even though it is a dependency of the dist task. The test task’s dependencies such as compileTest are not executed either. Those dependencies of test that are required by another task, such as compile, are still executed.

Forcing tasks to execute

You can force Gradle to execute all tasks ignoring up-to-date checks using the --rerun-tasks option:

❯ gradle test --rerun-tasks

This will force test and all task dependencies of test to execute. It’s a little like running gradle clean test, but without the build’s generated output being deleted.

Continuing the build when a failure occurs

By default, Gradle will abort execution and fail the build as soon as any task fails. This allows the build to complete sooner, but hides other failures that would have occurred. In order to discover as many failures as possible in a single build execution, you can use the --continue option.

❯ gradle test --continue

When executed with --continue, Gradle will execute every task to be executed where all of the dependencies for that task completed without failure, instead of stopping as soon as the first failure is encountered. Each of the encountered failures will be reported at the end of the build.

If a task fails, any subsequent tasks that were depending on it will not be executed. For example, tests will not run if there is a compilation failure in the code under test; because the test task will depend on the compilation task (either directly or indirectly).

Task name abbreviation

When you specify tasks on the command-line, you don’t have to provide the full name of the task. You only need to provide enough of the task name to uniquely identify the task. For example, it’s likely gradle che is enough for Gradle to identify the check task.

You can also abbreviate each word in a camel case task name. For example, you can execute task compileTest by running gradle compTest or even gradle cT.

Example 2. Abbreviated camel case task name

Output of gradle cT

> gradle cT

> Task :compile
compiling source

> Task :compileTest
compiling unit tests

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

You can also use these abbreviations with the -x command-line option.

Common tasks

The following are task conventions applied by built-in and most major Gradle plugins.

Computing all outputs

It is common in Gradle builds for the build task to designate assembling all outputs and running all checks.

❯ gradle build

Running applications

It is common for applications to be run with the run task, which assembles the application and executes some script or binary.

❯ gradle run

Running all checks

It is common for all verification tasks, including tests and linting, to be executed using the check task.

❯ gradle check

Cleaning outputs

You can delete the contents of the build directory using the clean task, though doing so will cause pre-computed outputs to be lost, causing significant additional build time for the subsequent task execution.

❯ gradle clean

Project reporting

Gradle provides several built-in tasks which show particular details of your build. This can be useful for understanding the structure and dependencies of your build, and for debugging problems.

You can get basic help about available reporting options using gradle help.

Listing projects

Running gradle projects gives you a list of the sub-projects of the selected project, displayed in a hierarchy.

❯ gradle projects

You also get a project report within build scans. Learn more about creating build scans.

Listing tasks

Running gradle tasks gives you a list of the main tasks of the selected project. This report shows the default tasks for the project, if any, and a description for each task.

❯ gradle tasks

By default, this report shows only those tasks which have been assigned to a task group. You can obtain more information in the task listing using the --all option.

❯ gradle tasks --all

Show task usage details

Running gradle help --task someTask gives you detailed information about a specific task.

Example 3. Obtaining detailed help for tasks

Output of gradle -q help --task libs

> gradle -q help --task libs
Detailed task information for libs

Paths
     :api:libs
     :webapp:libs

Type
     Task (org.gradle.api.Task)

Description
     Builds the JAR

Group
     build

This information includes the full task path, the task type, possible command line options and the description of the given task.

Reporting dependencies

Build scans give a full, visual report of what dependencies exist on which configurations, transitive dependencies, and dependency version selection.

❯ gradle myTask --scan

This will give you a link to a web-based report, where you can find dependency information like this.

Build Scan dependencies report

Learn more in Inspecting Dependencies.

Listing project dependencies

Running gradle dependencies gives you a list of the dependencies of the selected project, broken down by configuration. For each configuration, the direct and transitive dependencies of that configuration are shown in a tree. Below is an example of this report:

❯ gradle dependencies

Concrete examples of build scripts and output available in the Inspecting Dependencies.

Running gradle buildEnvironment visualises the buildscript dependencies of the selected project, similarly to how gradle dependencies visualizes the dependencies of the software being built.

❯ gradle buildEnvironment

Running gradle dependencyInsight gives you an insight into a particular dependency (or dependencies) that match specified input.

❯ gradle dependencyInsight

Since a dependency report can get large, it can be useful to restrict the report to a particular configuration. This is achieved with the optional --configuration parameter:

Listing project properties

Running gradle properties gives you a list of the properties of the selected project.

Example 4. Information about properties

Output of gradle -q api:properties

> gradle -q api:properties

------------------------------------------------------------
Project :api - The shared API for the application
------------------------------------------------------------

allprojects: [project ':api']
ant: org.gradle.api.internal.project.DefaultAntBuilder@12345
antBuilderFactory: org.gradle.api.internal.project.DefaultAntBuilderFactory@12345
artifacts: org.gradle.api.internal.artifacts.dsl.DefaultArtifactHandler_Decorated@12345
asDynamicObject: DynamicObject for project ':api'
baseClassLoaderScope: org.gradle.api.internal.initialization.DefaultClassLoaderScope@12345
buildDir: /home/user/gradle/samples/userguide/tutorial/projectReports/api/build
buildFile: /home/user/gradle/samples/userguide/tutorial/projectReports/api/build.gradle

Software Model reports

You can get a hierarchical view of elements for software model projects using the model task:

❯ gradle model

Learn more about the model report in the software model documentation.

Command-line completion

Gradle provides bash and zsh tab completion support for tasks, options, and Gradle properties through gradle-completion, installed separately.

Figure 2. Gradle Completion

Gradle Completion

Debugging options

-?, -h, --help

Shows a help message with all available CLI options.

-v, --version

Prints Gradle, Groovy, Ant, JVM, and operating system version information.

-S, --full-stacktrace

Print out the full (very verbose) stacktrace for any exceptions. See also logging options.

-s, --stacktrace

Print out the stacktrace also for user exceptions (e.g. compile error). See also logging options.

--scan

Create a build scan with fine-grained information about all aspects of your Gradle build.

-Dorg.gradle.debug=true

Debug Gradle client (non-Daemon) process. Gradle will wait for you to attach a debugger at localhost:5005 by default.

-Dorg.gradle.daemon.debug=true

Debug Gradle Daemon process.

Performance options

Try these options when optimizing build performance. Learn more about improving performance of Gradle builds here.

Many of these options can be specified in gradle.properties so command-line flags are not necessary. See the configuring build environment guide.

--build-cache, --no-build-cache

Toggles the Gradle build cache. Gradle will try to reuse outputs from previous builds. Default is off.

--configure-on-demand, --no-configure-on-demand

Toggles Configure-on-demand. Only relevant projects are configured in this build run. Default is off.

--max-workers

Sets maximum number of workers that Gradle may use. Default is number of processors.

--parallel, --no-parallel

Build projects in parallel. For limitations of this option please see the section called “Parallel project execution”. Default is off.

--profile

Generates a high-level performance report in the $buildDir/reports/profile directory. --scan is preferred.

--scan

Generate a build scan with detailed performance diagnostics.

Build Scan performance report

Gradle daemon options

You can manage the Gradle Daemon through the following command line options.

--daemon, --no-daemon

Use the Gradle Daemon to run the build. Starts the daemon if not running or existing daemon busy. Default is on.

--foreground

Starts the Gradle Daemon in a foreground process.

--status (Standalone command)

Run gradle --status to list running and recently stopped Gradle daemons. Only displays daemons of the same Gradle version.

--stop (Standalone command)

Run gradle --stop to stop all Gradle Daemons of the same version.

-Dorg.gradle.daemon.idletimeout=(number of milliseconds)

Gradle Daemon will stop itself after this number of milliseconds of idle time. Default is 10800000 (3 hours).

Logging options

Setting log level

You can customize the verbosity of Gradle logging with the following options, ordered from least verbose to most verbose. Learn more in the logging documentation.

-Dorg.gradle.logging.level=(quiet,warn,lifecycle,info,debug)

Set logging level via Gradle properties.

-q, --quiet

Log errors only.

-w, --warn

Set log level to warn.

-i, --info

Set log level to info.

-d, --debug

Log in debug mode (includes normal stacktrace).

Lifecycle is the default log level.

Customizing log format

You can control the use of rich output (colors and font variants) by specifying the "console" mode in the following ways:

-Dorg.gradle.console=(auto,plain,rich,verbose)

Specify console mode via Gradle properties. Different modes described immediately below.

--console=(auto,plain,rich,verbose)

Specifies which type of console output to generate.

Set to plain to generate plain text only. This option disables all color and other rich output in the console output. This is the default when Gradle is not attached to a terminal.

Set to auto (the default) to enable color and other rich output in the console output when the build process is attached to a console, or to generate plain text only when not attached to a console. This is the default when Gradle is attached to a terminal.

Set to rich to enable color and other rich output in the console output, regardless of whether the build process is not attached to a console. When not attached to a console, the build output will use ANSI control characters to generate the rich output.

Set to verbose to enable color and other rich output like the rich, but output task names and outcomes at the lifecycle log level, as is done by default in Gradle 3.5 and earlier.

Showing or hiding warnings

By default, Gradle won’t display all warnings (e.g. deprecation warnings). Instead, Gradle will collect them and render a summary at the end of the build like:

Deprecated Gradle features were used in this build, making it incompatible with Gradle 5.0.

You can control the verbosity of warnings on the console with the following options:

-Dorg.gradle.warning.mode=(all,none,summary)

Specify warning mode via Gradle properties. Different modes described immediately below.

--warning-mode=(all,none,summary)

Specifies how to log warnings. Default is summary.

Set to all to log all warnings.

Set to summary to suppress all warnings and log a summary at the end of the build.

Set to none to suppress all warnings, including the summary at the end of the build.

Rich Console

Gradle’s rich console displays extra information while builds are running.

Gradle Rich Console

Features:

  • Progress bar and timer visually describe overall status

  • Parallel work-in-progress lines below describe what is happening now

  • Colors and fonts are used to highlight important output and errors

Execution options

The following options affect how builds are executed, by changing what is built or how dependencies are resolved.

--include-build

Run the build as a composite, including the specified build. See Composite Builds.

--offline

Specifies that the build should operate without accessing network resources. Learn more about options to override dependency caching.

--refresh-dependencies

Refresh the state of dependencies. Learn more about how to use this in the dependency management docs.

--dry-run

Run Gradle with all task actions disabled. Use this to show which task would have executed.

Environment options

You can customize many aspects about where build scripts, settings, caches, and so on through the options below. Learn more about customizing your build environment.

-b, --build-file

Specifies the build file. For example: gradle --build-file=foo.gradle. The default is build.gradle, then build.gradle.kts, then myProjectName.gradle.

-c, --settings-file

Specifies the settings file. For example: gradle --settings-file=somewhere/else/settings.gradle

-g, --gradle-user-home

Specifies the Gradle user home directory. The default is the .gradle directory in the user’s home directory.

-p, --project-dir

Specifies the start directory for Gradle. Defaults to current directory.

--project-cache-dir

Specifies the project-specific cache directory. Default value is .gradle in the root project directory.

-u, --no-search-upward (deprecated)

Don’t search in parent directories for a settings.gradle file.

-D, --system-prop

Sets a system property of the JVM, for example -Dmyprop=myvalue. See the section called “System properties”.

-I, --init-script

Specifies an initialization script. See Initialization Scripts.

-P, --project-prop

Sets a project property of the root project, for example -Pmyprop=myvalue. See the section called “Project properties”.

-Dorg.gradle.jvmargs

Set JVM arguments.

-Dorg.gradle.java.home

Set JDK home dir.

Bootstrapping new projects

Creating new Gradle builds

Use the built-in gradle init task to create a new Gradle builds, with new or existing projects.

❯ gradle init

Most of the time you’ll want to specify a project type. Available types include basic (default), java-library, java-application, and more. See init plugin documentation for details.

❯ gradle init --type java-library

Standardize and provision Gradle

The built-in gradle wrapper task generates a script, gradlew, that invokes a declared version of Gradle, downloading it beforehand if necessary.

❯ gradle wrapper --gradle-version=4.4

You can also specify --distribution-type=(bin|all), --gradle-distribution-url, --gradle-distribution-sha256-sum in addition to --gradle-version. Full details on how to use these options are documented in the Gradle wrapper section.

Continuous Build

Continuous Build allows you to automatically re-execute the requested tasks when task inputs change.

For example, you can continuously run the test task and all dependent tasks by running:

❯ gradle test --continuous

Gradle will behave as if you ran gradle test after a change to sources or tests that contribute to the requested tasks. This means that unrelated changes (such as changes to build scripts) will not trigger a rebuild. In order to incorporate build logic changes, the continuous build must be restarted manually.

Terminating Continuous Build

If Gradle is attached to an interactive input source, such as a terminal, the continuous build can be exited by pressing CTRL-D (On Microsoft Windows, it is required to also press ENTER or RETURN after CTRL-D). If Gradle is not attached to an interactive input source (e.g. is running as part of a script), the build process must be terminated (e.g. using the kill command or similar). If the build is being executed via the Tooling API, the build can be cancelled using the Tooling API’s cancellation mechanism.

Limitations and quirks

Continuous build is an incubating feature.

There are several issues to be aware with the current implementation of continuous build. These are likely to be addressed in future Gradle releases.

Build cycles

Gradle starts watching for changes just before a task executes. If a task modifies its own inputs while executing, Gradle will detect the change and trigger a new build. If every time the task executes, the inputs are modified again, the build will be triggered again. This isn’t unique to continuous build. A task that modifies its own inputs will never be considered up-to-date when run "normally" without continuous build.

If your build enters a build cycle like this, you can track down the task by looking at the list of files reported changed by Gradle. After identifying the file(s) that are changed during each build, you should look for a task that has that file as an input. In some cases, it may be obvious (e.g., a Java file is compiled with compileJava). In other cases, you can use --info logging to find the task that is out-of-date due to the identified files.

Restrictions with Java 9

Due to class access restrictions related to Java 9, Gradle cannot set some operating system specific options, which means that:

  • On macOS, Gradle will poll for file changes every 10 seconds instead of every 2 seconds.

  • On Windows, Gradle must use individual file watches (like on Linux/Mac OS), which may cause continuous build to no longer work on very large projects.

Performance and stability

The JDK file watching facility relies on inefficient file system polling on macOS (see: JDK-7133447). This can significantly delay notification of changes on large projects with many source files.

Additionally, the watching mechanism may deadlock under heavy load on macOS (see: JDK-8079620). This will manifest as Gradle appearing not to notice file changes. If you suspect this is occurring, exit continuous build and start again.

On Linux, OpenJDK’s implementation of the file watch service can sometimes miss file system events (see: JDK-8145981).

Changes to symbolic links

  • Creating or removing symbolic link to files will initiate a build.

  • Modifying the target of a symbolic link will not cause a rebuild.

  • Creating or removing symbolic links to directories will not cause rebuilds.

  • Creating new files in the target directory of a symbolic link will not cause a rebuild.

  • Deleting the target directory will not cause a rebuild.

Changes to build logic are not considered

The current implementation does not recalculate the build model on subsequent builds. This means that changes to task configuration, or any other change to the build model, are effectively ignored.

The Gradle Wrapper

The recommended way to execute any Gradle build is with the help of the Gradle Wrapper (in short just “Wrapper”). The Wrapper is a script that invokes a declared version of Gradle, downloading it beforehand if necessary. As a result, developers can get up and running with a Gradle project quickly without having to follow manual installation processes saving your company time and money.

Figure 3. The Wrapper workflow

The Wrapper workflow

In a nutshell you gain the following benefits:

  • Standardizes a project on a given Gradle version, leading to more reliable and robust builds.

  • Provisioning a new Gradle version to different users and execution environment (e.g. IDEs or Continuous Integration servers) is as simple as changing the Wrapper definition.

So how does it work? For a user there are typically three different workflows:

The following sections explain each of these use cases in more detail.

Adding the Gradle Wrapper

Generating the Wrapper files requires an installed version of the Gradle runtime on your machine as described in Installing Gradle. Thankfully, generating the initial Wrapper files is a one-time process.

Every vanilla Gradle build comes with a built-in task called wrapper. You’ll be able to find the task listed under the group "Build Setup tasks" when listing the tasks. Executing the wrapper task generates the necessary Wrapper files in the project directory.

Example 5. Running the Wrapper task

Output of gradle wrapper

> gradle wrapper
> Task :wrapper

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

To make the Wrapper files available to other developers and execution environments you’ll need to check them into version control. All Wrapper files including the JAR file are very small in size. Adding the JAR file to version control is expected. Some organizations do not allow projects to submit binary files to version control. At the moment there are no alternative options to the approach.

The generated Wrapper properties file, gradle/wrapper/gradle-wrapper.properties, stores the information about the Gradle distribution.

  • The server hosting the Gradle distribution.

  • The type of Gradle distribution. By default that’s the -bin distribution containing only the runtime but no sample code and documentation.

  • The Gradle version used for executing the build. By default the wrapper task picks the exact same Gradle version that was used to generate the Wrapper files.

Example 6. The generated distribution URL

gradle/wrapper/gradle-wrapper.properties

distributionUrl=https\://services.gradle.org/distributions/gradle-4.3.1-bin.zip


All of those aspects are configurable at the time of generating the Wrapper files with the help of the following command line options.

--gradle-version

The Gradle version used for downloading and executing the Wrapper.

--distribution-type

The Gradle distribution type used for the Wrapper. Available options are bin and all. The default value is bin.

--gradle-distribution-url

The full URL pointing to Gradle distribution ZIP file. Using this option makes --gradle-version and --distribution-type obsolete as the URL already contains this information. This option is extremely valuable if you want to host the Gradle distribution inside your company’s network.

--gradle-distribution-sha256-sum

The SHA256 hash sum used for verifying the downloaded Gradle distribution.

Let’s assume the following use case to illustrate the use of the command line options. You would like to generate the Wrapper with version 4.0 and use the -all distribution to enable your IDE to enable code-completion and being able to navigate to the Gradle source code. Those requirements are captured by the following command line execution:

Example 7. Providing options to Wrapper task

Output of gradle wrapper --gradle-version 4.0 --distribution-type all

> gradle wrapper --gradle-version 4.0 --distribution-type all
> Task :wrapper

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

As a result you can find the desired information in the Wrapper properties file.

Example 8. The generated distribution URL

gradle/wrapper/gradle-wrapper.properties

distributionUrl=https\://services.gradle.org/distributions/gradle-4.0-all.zip


Let’s have a look at the following project layout to illustrate the expected Wrapper files:

.
├── build.gradle
├── settings.gradle
├── gradle
│   └── wrapper
│       ├── gradle-wrapper.jar
│       └── gradle-wrapper.properties
├── gradlew
└── gradlew.bat

A Gradle project typically provides a build.gradle and a settings.gradle file. The Wrapper files live alongside in the gradle directory and the root directory of the project. The following list explains their purpose.

gradle-wrapper.jar

The Wrapper JAR file containing code for downloading the Gradle distribution.

gradle-wrapper.properties

A properties file responsible for configuring the Wrapper runtime behavior e.g. the Gradle version compatible with this version.

gradlew, gradlew.bat

A shell script and a Windows batch script for executing the build with the Wrapper.

You can go ahead and execute the build with the Wrapper without having to install the Gradle runtime. If the project you are working on does not contain those Wrapper files then you’ll need to generate them.

Using the Gradle Wrapper

It is recommended to always execute a build with the Wrapper to ensure a reliable, controlled and standardized execution of the build. Using the Wrapper looks almost exactly like running the build with a Gradle installation. Depending on the operating system you either run gradlew or gradlew.bat instead of the gradle command. The following console output demonstrate the use of the Wrapper on a Windows machine for a Java-based project.

Example 9. Executing the build with the Wrapper batch file

Output of gradlew.bat build

> gradlew.bat build
Downloading https://services.gradle.org/distributions/gradle-4.0-all.zip
.....................................................................................
Unzipping C:\Documents and Settings\Claudia\.gradle\wrapper\dists\gradle-4.0-all\ac27o8rbd0ic8ih41or9l32mv\gradle-4.0-all.zip to C:\Documents and Settings\Claudia\.gradle\wrapper\dists\gradle-4.0-al\ac27o8rbd0ic8ih41or9l32mv
Set executable permissions for: C:\Documents and Settings\Claudia\.gradle\wrapper\dists\gradle-4.0-all\ac27o8rbd0ic8ih41or9l32mv\gradle-4.0\bin\gradle

BUILD SUCCESSFUL in 12s
1 actionable task: 1 executed

In case the Gradle distribution is not available on the machine, the Wrapper will download it and store in the local file system. Any subsequent build invocation is going to reuse the existing local distribution as long as the distribution URL in the Gradle properties doesn’t change.

The Wrapper shell script and batch file reside in the root directory of a single or multi-project Gradle build. You will need to reference the correct path to those files in case you want to execute the build from a subproject directory e.g. ../../gradlew tasks.

Upgrading the Gradle Wrapper

Projects will typically want to keep up with the times and upgrade their Gradle version to benefit from new features and improvements. One way to upgrade the Gradle version is manually change the distributionUrl property in the Wrapper property file. The better and recommended option is to run the wrapper task and provide the target Gradle version as described in the section called “Adding the Gradle Wrapper”. Using the wrapper task ensures that any optimizations made to the Wrapper shell script or batch file with that specific Gradle version are applied to the project. As usual you’d commit the changes to the Wrapper files to version control.

Use the Gradle wrapper task to generate the wrapper, specifying a version. The default is the current version, which you can check by executing ./gradlew --version.

Example 10. Upgrading the Wrapper version

Output of ./gradlew wrapper --gradle-version 4.2.1

> ./gradlew wrapper --gradle-version 4.2.1

BUILD SUCCESSFUL in 4s
1 actionable task: 1 executed

Example 11. Checking the Wrapper version after upgrading

Output of ./gradlew -v

> ./gradlew -v
Downloading https://services.gradle.org/distributions/gradle-4.2.1-bin.zip
...................................................................
Unzipping /Users/claudia/.gradle/wrapper/dists/gradle-4.2.1-bin/dajvke9o8kmaxbu0kc5gcgeju/gradle-4.2.1-bin.zip to /Users/claudia/.gradle/wrapper/dists/gradle-4.2.1-bin/dajvke9o8kmaxbu0kc5gcgeju
Set executable permissions for: /Users/claudia/.gradle/wrapper/dists/gradle-4.2.1-bin/dajvke9o8kmaxbu0kc5gcgeju/gradle-4.2.1/bin/gradle

------------------------------------------------------------
Gradle 4.2.1
------------------------------------------------------------

Build time:   2017-10-02 15:36:21 UTC
Revision:     a88ebd6be7840c2e59ae4782eb0f27fbe3405ddf

Groovy:       2.4.12
Ant:          Apache Ant(TM) version 1.9.6 compiled on June 29 2015
JVM:          1.8.0_60 (Oracle Corporation 25.60-b23)
OS:           Mac OS X 10.13.1 x86_64

Customizing the Gradle Wrapper

Most users of Gradle are happy with the default runtime behavior of the Wrapper. However, organizational policies, security constraints or personal preferences might require you to dive deeper into customizing the Wrapper. Thankfully, the built-in wrapper task exposes numerous options to bend the runtime behavior to your needs. Most configuration options are exposed by the underlying task type Wrapper.

Let’s assume you grew tired of defining the -all distribution type on the command line every time you upgrade the Wrapper. You can save yourself some keyboard strokes by re-configuring the wrapper task.

Example 12. Customizing the Wrapper task

build.gradle

wrapper {
    distributionType = Wrapper.DistributionType.ALL
}

With the configuration in place running ./gradlew wrapper --gradle-version 4.1 is enough to produce a distributionUrl value in the Wrapper properties file that will request the -all distribution.

Example 13. The generated distribution URL

gradle/wrapper/gradle-wrapper.properties

distributionUrl=https\://services.gradle.org/distributions/gradle-4.1-all.zip


Check out the API documentation for more detail descriptions of the available configuration options. You can also find various samples for configuring the Wrapper in the Gradle distribution.

Authenticated Gradle distribution download

The Gradle Wrapper can download Gradle distributions from servers using HTTP Basic Authentication. This enables you to host the Gradle distribution on a private protected server. You can specify a username and password in two different ways depending on your use case: as system properties or directly embedded in the distributionUrl. Credentials in system properties take precedence over the ones embedded in distributionUrl.

Security Warning

HTTP Basic Authentication should only be used with HTTPS URLs and not plain HTTP ones. With Basic Authentication, the user credentials are sent in clear text.

Using system properties can be done in the .gradle/gradle.properties file in the user’s home directory, or by other means, see the section called “Gradle properties”.

Example 14. Specifying the HTTP Basic Authentication credentials using system properties

gradle.properties

systemProp.gradle.wrapperUser=username
systemProp.gradle.wrapperPassword=password


Embedding credentials in the distributionUrl in the gradle/wrapper/gradle-wrapper.properties file also works. Please note that this file is to be committed into your source control system. Shared credentials embedded in distributionUrl should only be used in a controlled environment.

Example 15. Specifying the HTTP Basic Authentication credentials in distributionUrl

gradle/wrapper/gradle-wrapper.properties

distributionUrl=https://username:password@somehost/path/to/gradle-distribution.zip


This can be used in conjunction with a proxy, authenticated or not. See the section called “Accessing the web through a HTTP proxy” for more information on how to configure the Wrapper to use a proxy.

Verification of downloaded Gradle distributions

The Gradle Wrapper allows for verification of the downloaded Gradle distribution via SHA-256 hash sum comparison. This increases security against targeted attacks by preventing a man-in-the-middle attacker from tampering with the downloaded Gradle distribution.

To enable this feature, download the .sha256 file associated with the Gradle distribution you want to verify.

Downloading the SHA-256 file

You can download the .sha256 file from the stable releases or release candidate and nightly releases. The format of the file is a single line of text that is the SHA-256 hash of the corresponding zip file.

Configuring checksum verification

Add the downloaded hash sum to gradle-wrapper.properties using the distributionSha256Sum property or use --gradle-distribution-sha256-sum on the command-line.

Example 16. Configuring SHA-256 checksum verification

gradle/wrapper/gradle-wrapper.properties

distributionSha256Sum=371cb9fbebbe9880d147f59bab36d61eee122854ef8c9ee1ecf12b82368bcf10


Gradle will report a build failure in case the configured checksum does not match the checksum found on the server for hosting the distribution. Checksum Verification is only performed if the configured Wrapper distribution hasn’t been downloaded yet.

The Gradle Daemon

From Wikipedia…

A daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user.

Gradle runs on the Java Virtual Machine (JVM) and uses several supporting libraries that require a non-trivial initialization time. As a result, it can sometimes seem a little slow to start. The solution to this problem is the Gradle Daemon: a long-lived background process that executes your builds much more quickly than would otherwise be the case. We accomplish this by avoiding the expensive bootstrapping process as well as leveraging caching, by keeping data about your project in memory. Running Gradle builds with the Daemon is no different than without. Simply configure whether you want to use it or not - everything else is handled transparently by Gradle.

Why the Gradle Daemon is important for performance

The Daemon is a long-lived process, so not only are we able to avoid the cost of JVM startup for every build, but we are able to cache information about project structure, files, tasks, and more in memory.

The reasoning is simple: improve build speed by reusing computations from previous builds. However, the benefits are dramatic: we typically measure build times reduced by 15-75% on subsequent builds. We recommend profiling your build by using --profile to get a sense of how much impact the Gradle Daemon can have for you.

The Gradle Daemon is enabled by default starting with Gradle 3.0, so you don’t have to do anything to benefit from it.

If you run CI builds in ephemeral environments (such as containers) that do not reuse any processes, use of the Daemon will slightly decrease performance (due to caching additional information) for no benefit, and may be disabled.

Running Daemon Status

To get a list of running Gradle Daemons and their statuses use the --status command.

Sample output:

  PID VERSION                 STATUS
28411 3.0                     IDLE
34247 3.0                     BUSY

Currently, a given Gradle version can only connect to daemons of the same version. This means the status output will only show Daemons for the version of Gradle being invoked and not for any other versions. Future versions of Gradle will lift this constraint and will show the running Daemons for all versions of Gradle.

Disabling the Daemon

The Gradle Daemon is enabled by default, and we recommend always enabling it. There are several ways to disable the Daemon, but the most common one is to add the line

org.gradle.daemon=false

to the file «USER_HOME»/.gradle/gradle.properties, where «USER_HOME» is your home directory. That’s typically one of the following, depending on your platform:

  • C:\Users\<username> (Windows Vista & 7+)

  • /Users/<username> (macOS)

  • /home/<username> (Linux)

If that file doesn’t exist, just create it using a text editor. You can find details of other ways to disable (and enable) the Daemon in the section called “FAQ” further down. That section also contains more detailed information on how the Daemon works.

Note that having the Daemon enabled, all your builds will take advantage of the speed boost, regardless of the version of Gradle a particular build uses.

Continuous integration

Since Gradle 3.0, we enable Daemon by default and recommend using it for both developers' machines and Continuous Integration servers. However, if you suspect that Daemon makes your CI builds unstable, you can disable it to use a fresh runtime for each build since the runtime is completely isolated from any previous builds.

Stopping an existing Daemon

As mentioned, the Daemon is a background process. You needn’t worry about a build up of Gradle processes on your machine, though. Every Daemon monitors its memory usage compared to total system memory and will stop itself if idle when available system memory is low. If you want to explicitly stop running Daemon processes for any reason, just use the command gradle --stop.

This will terminate all Daemon processes that were started with the same version of Gradle used to execute the command. If you have the Java Development Kit (JDK) installed, you can easily verify that a Daemon has stopped by running the jps command. You’ll see any running Daemons listed with the name GradleDaemon.

FAQ

How do I disable the Gradle Daemon?

There are two recommended ways to disable the Daemon persistently for an environment:

  • Via environment variables: add the flag -Dorg.gradle.daemon=false to the GRADLE_OPTS environment variable

  • Via properties file: add org.gradle.daemon=false to the «GRADLE_USER_HOME»/gradle.properties file

Note, «GRADLE_USER_HOME» defaults to «USER_HOME»/.gradle, where «USER_HOME» is the home directory of the current user. This location can be configured via the -g and --gradle-user-home command line switches, as well as by the GRADLE_USER_HOME environment variable and org.gradle.user.home JVM system property.

Both approaches have the same effect. Which one to use is up to personal preference. Most Gradle users choose the second option and add the entry to the user gradle.properties file.

On Windows, this command will disable the Daemon for the current user:

(if not exist "%USERPROFILE%/.gradle" mkdir "%USERPROFILE%/.gradle") && (echo. >> "%USERPROFILE%/.gradle/gradle.properties" && echo org.gradle.daemon=false >> "%USERPROFILE%/.gradle/gradle.properties")

On UNIX-like operating systems, the following Bash shell command will disable the Daemon for the current user:

mkdir -p ~/.gradle && echo "org.gradle.daemon=false" >> ~/.gradle/gradle.properties

Once the Daemon is disabled for a build environment in this way, a Gradle Daemon will not be started unless explicitly requested using the --daemon option.

The --daemon and --no-daemon command line options enable and disable usage of the Daemon for individual build invocations when using the Gradle command line interface. These command line options have the highest precedence when considering the build environment. Typically, it is more convenient to enable the Daemon for an environment (e.g. a user account) so that all builds use the Daemon without requiring to remember to supply the --daemon option.

Why is there more than one Daemon process on my machine?

There are several reasons why Gradle will create a new Daemon, instead of using one that is already running. The basic rule is that Gradle will start a new Daemon if there are no existing idle or compatible Daemons available. Gradle will kill any Daemon that has been idle for 3 hours or more, so you don’t have to worry about cleaning them up manually.

idle

An idle Daemon is one that is not currently executing a build or doing other useful work.

compatible

A compatible Daemon is one that can (or can be made to) meet the requirements of the requested build environment. The Java runtime used to execute the build is an example aspect of the build environment. Another example is the set of JVM system properties required by the build runtime.

Some aspects of the requested build environment may not be met by an Daemon. If the Daemon is running with a Java 7 runtime, but the requested environment calls for Java 8, then the Daemon is not compatible and another must be started. Moreover, certain properties of a Java runtime cannot be changed once the JVM has started. For example, it is not possible to change the memory allocation (e.g. -Xmx1024m), default text encoding, default locale, etc of a running JVM.

The “requested build environment” is typically constructed implicitly from aspects of the build client’s (e.g. Gradle command line client, IDE etc.) environment and explicitly via command line switches and settings. See Build Environment for details on how to specify and control the build environment.

The following JVM system properties are effectively immutable. If the requested build environment requires any of these properties, with a different value than a Daemon’s JVM has for this property, the Daemon is not compatible.

  • file.encoding

  • user.language

  • user.country

  • user.variant

  • java.io.tmpdir

  • javax.net.ssl.keyStore

  • javax.net.ssl.keyStorePassword

  • javax.net.ssl.keyStoreType

  • javax.net.ssl.trustStore

  • javax.net.ssl.trustStorePassword

  • javax.net.ssl.trustStoreType

  • com.sun.management.jmxremote

The following JVM attributes, controlled by startup arguments, are also effectively immutable. The corresponding attributes of the requested build environment and the Daemon’s environment must match exactly in order for a Daemon to be compatible.

  • The maximum heap size (i.e. the -Xmx JVM argument)

  • The minimum heap size (i.e. the -Xms JVM argument)

  • The boot classpath (i.e. the -Xbootclasspath argument)

  • The “assertion” status (i.e. the -ea argument)

The required Gradle version is another aspect of the requested build environment. Daemon processes are coupled to a specific Gradle runtime. Working on multiple Gradle projects during a session that use different Gradle versions is a common reason for having more than one running Daemon process.

How much memory does the Daemon use and can I give it more?

If the requested build environment does not specify a maximum heap size, the Daemon will use up to 1GB of heap. It will use the JVM’s default minimum heap size. 1GB is more than enough for most builds. Larger builds with hundreds of subprojects, lots of configuration, and source code may require, or perform better, with more memory.

To increase the amount of memory the Daemon can use, specify the appropriate flags as part of the requested build environment. Please see Build Environment for details.

How can I stop a Daemon?

Daemon processes will automatically terminate themselves after 3 hours of inactivity or less. If you wish to stop a Daemon process before this, you can either kill the process via your operating system or run the gradle --stop command. The --stop switch causes Gradle to request that all running Daemon processes, of the same Gradle version used to run the command, terminate themselves.

What can go wrong with Daemon?

Considerable engineering effort has gone into making the Daemon robust, transparent and unobtrusive during day to day development. However, Daemon processes can occasionally be corrupted or exhausted. A Gradle build executes arbitrary code from multiple sources. While Gradle itself is designed for and heavily tested with the Daemon, user build scripts and third party plugins can destabilize the Daemon process through defects such as memory leaks or global state corruption.

It is also possible to destabilize the Daemon (and build environment in general) by running builds that do not release resources correctly. This is a particularly poignant problem when using Microsoft Windows as it is less forgiving of programs that fail to close files after reading or writing.

Gradle actively monitors heap usage and attempts to detect when a leak is starting to exhaust the available heap space in the daemon. When it detects a problem, the Gradle daemon will finish the currently running build and proactively restart the daemon on the next build. This monitoring is enabled by default, but can be disabled by setting the org.gradle.daemon.performance.enable-monitoring system property to false.

If it is suspected that the Daemon process has become unstable, it can simply be killed. Recall that the --no-daemon switch can be specified for a build to prevent use of the Daemon. This can be useful to diagnose whether or not the Daemon is actually the culprit of a problem.

Tools & IDEs

The Gradle Tooling API (see Embedding Gradle using the Tooling API), that is used by IDEs and other tools to integrate with Gradle, always use the Gradle Daemon to execute builds. If you are executing Gradle builds from within you’re IDE you are using the Gradle Daemon and do not need to enable it for your environment.

How does the Gradle Daemon make builds faster?

The Gradle Daemon is a long lived build process. In between builds it waits idly for the next build. This has the obvious benefit of only requiring Gradle to be loaded into memory once for multiple builds, as opposed to once for each build. This in itself is a significant performance optimization, but that’s not where it stops.

A significant part of the story for modern JVM performance is runtime code optimization. For example, HotSpot (the JVM implementation provided by Oracle and used as the basis of OpenJDK) applies optimization to code while it is running. The optimization is progressive and not instantaneous. That is, the code is progressively optimized during execution which means that subsequent builds can be faster purely due to this optimization process. Experiments with HotSpot have shown that it takes somewhere between 5 and 10 builds for optimization to stabilize. The difference in perceived build time between the first build and the 10th for a Daemon can be quite dramatic.

The Daemon also allows more effective in memory caching across builds. For example, the classes needed by the build (e.g. plugins, build scripts) can be held in memory between builds. Similarly, Gradle can maintain in-memory caches of build data such as the hashes of task inputs and outputs, used for incremental building.

Dependency Management for Java Projects

This chapter explains how to apply basic dependency management concepts to Java-based projects. For a detailed introduction to dependency management, see Introduction to Dependency Management.

Dissecting a typical build script

Let’s have a look at a very simple build script for a Java-based project. It applies the Java Library plugin which automatically introduces a standard project layout, provides tasks for performing typical work and adequate support for dependency management.

Example 17. Dependency declarations for a Java-based project

build.gradle

apply plugin: 'java-library'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.hibernate:hibernate-core:3.6.7.Final'
    api 'com.google.guava:guava:23.0'
    testImplementation 'junit:junit:4.+'
}

The Project.dependencies{} code block declares that Hibernate core 3.6.7.Final is required to compile the project’s production source code. It also states that junit >= 4.0 is required to compile the project’s tests. All dependencies are supposed to be looked up in the Maven Central repository as defined by Project.repositories{}. The following sections explain each aspect in more detail.

Declaring module dependencies

There are various types of dependencies that you can declare. One such type is a module dependency. A module dependency represents a dependency on a module with a specific version built outside the current build. Modules are usually stored in a repository, such as Maven Central, a corporate Maven or Ivy repository, or a directory in the local file system.

To define an module dependency, you add it to a dependency configuration:

Example 18. Definition of a module dependency

build.gradle

dependencies {
    implementation 'org.hibernate:hibernate-core:3.6.7.Final'
}

To find out more about defining dependencies, have a look at Declaring Dependencies.

Using dependency configurations

A Configuration is a named set of dependencies and artifacts. There are three main purposes for a configuration:

Declaring dependencies

A plugin uses configurations to make it easy for build authors to declare what other subprojects or external artifacts are needed for various purposes during the execution of tasks defined by the plugin. For example a plugin may need the Spring web framework dependency to compile the source code.

Resolving dependencies

A plugin uses configurations to find (and possibly download) inputs to the tasks it defines. For example Gradle needs to download Spring web framework JAR files from Maven Central.

Exposing artifacts for consumption

A plugin uses configurations to define what artifacts it generates for other projects to consume. For example the project would like to publish its compiled source code packaged in the JAR file to an in-house Artifactory repository.

With those three purposes in mind, let’s take a look at a few of the standard configurations defined by the Java Library Plugin.

implementation

The dependencies required to compile the production source of the project which are not part of the API exposed by the project. For example the project uses Hibernate for its internal persistence layer implementation.

api

The dependencies required to compile the production source of the project which are part of the API exposed by the project. For example the project uses Guava and exposes public interfaces with Guava classes in their method signatures.

testImplementation

The dependencies required to compile and run the test source of the project. For example the project decided to write test code with the test framework JUnit.

Various plugins add further standard configurations. You can also define your own custom configurations in your build via Project.configurations{}. See Managing Dependency Configurations for the details of defining and customizing dependency configurations.

Declaring common Java repositories

How does Gradle know where to find the files for external dependencies? Gradle looks for them in a repository. A repository is a collection of modules, organized by group, name and version. Gradle understands different repository types, such as Maven and Ivy, and supports various ways of accessing the repository via HTTP or other protocols.

By default, Gradle does not define any repositories. You need to define at least one with the help of Project.repositories{} before you can use module dependencies. One option is use the Maven Central repository:

Example 19. Usage of Maven central repository

build.gradle

repositories {
    mavenCentral()
}

You can also have repositories on the local file system. This works for both Maven and Ivy repositories.

Example 20. Usage of a local Ivy directory

build.gradle

repositories {
    ivy {
        // URL can refer to a local directory
        url "../local-repo"
    }
}

A project can have multiple repositories. Gradle will look for a dependency in each repository in the order they are specified, stopping at the first repository that contains the requested module.

To find out more about defining repositories, have a look at Declaring Repositories.

Publishing artifacts

Dependency configurations are also used to publish files. Gradle calls these files publication artifacts, or usually just artifacts. As a user you will need to tell Gradle where to publish the artifacts. You do this by declaring repositories for the uploadArchives task. Here’s an example of publishing to a Maven repository:

Example 21. Publishing to a Maven repository

build.gradle

apply plugin: 'maven'

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://localhost/tmp/myRepo/")
        }
    }
}

Now, when you run gradle uploadArchives, Gradle will build the JAR file, generate a .pom file and upload the artifacts.

To learn more about publishing artifacts, have a look at Publishing artifacts.

Executing Multi-Project Builds

Only the smallest of projects has a single build file and source tree, unless it happens to be a massive, monolithic application. It’s often much easier to digest and understand a project that has been split into smaller, inter-dependent modules. The word “inter-dependent” is important, though, and is why you typically want to link the modules together through a single build.

Gradle supports this scenario through multi-project builds.

Structure of a multi-project build

Such builds come in all shapes and sizes, but they do have some common characteristics:

  • A settings.gradle file in the root or master directory of the project

  • A build.gradle file in the root or master directory

  • Child directories that have their own *.gradle build files (some multi-project builds may omit child project build scripts)

The settings.gradle file tells Gradle how the project and subprojects are structured. Fortunately, you don’t have to read this file simply to learn what the project structure is as you can run the command gradle projects. Here’s the output from using that command on the Java multiproject build in the Gradle samples:

Example 22. Listing the projects in a build

Output of gradle -q projects

> gradle -q projects

------------------------------------------------------------
Root project
------------------------------------------------------------

Root project 'multiproject'
+--- Project ':api'
+--- Project ':services'
|    +--- Project ':services:shared'
|    \--- Project ':services:webservice'
\--- Project ':shared'

To see a list of the tasks of a project, run gradle <project-path>:tasks
For example, try running gradle :api:tasks

This tells you that multiproject has three immediate child projects: api, services and shared. The services project then has its own children, shared and webservice. These map to the directory structure, so it’s easy to find them. For example, you can find webservice in <root>/services/webservice.

By default, Gradle uses the name of the directory it finds the settings.gradle as the name of the root project. This usually doesn’t cause problems since all developers check out the same directory name when working on a project. On Continuous Integration servers, like Jenkins, the directory name may be auto-generated and not match the name in your VCS. For that reason, it’s recommended that you always set the root project name to something predictable, even in single project builds. You can configure the root project name by setting rootProject.name.

Each project will usually have its own build file, but that’s not necessarily the case. In the above example, the services project is just a container or grouping of other subprojects. There is no build file in the corresponding directory. However, multiproject does have one for the root project.

The root build.gradle is often used to share common configuration between the child projects, for example by applying the same sets of plugins and dependencies to all the child projects. It can also be used to configure individual subprojects when it is preferable to have all the configuration in one place. This means you should always check the root build file when discovering how a particular subproject is being configured.

Another thing to bear in mind is that the build files might not be called build.gradle. Many projects will name the build files after the subproject names, such as api.gradle and services.gradle from the previous example. Such an approach helps a lot in IDEs because it’s tough to work out which build.gradle file out of twenty possibilities is the one you want to open. This little piece of magic is handled by the settings.gradle file, but as a build user you don’t need to know the details of how it’s done. Just have a look through the child project directories to find the files with the .gradle suffix.

Once you know what subprojects are available, the key question for a build user is how to execute the tasks within the project.

Executing a multi-project build

From a user’s perspective, multi-project builds are still collections of tasks you can run. The difference is that you may want to control which project’s tasks get executed. You have two options here:

  • Change to the directory corresponding to the subproject you’re interested in and just execute gradle <task> as normal.

  • Use a qualified task name from any directory, although this is usually done from the root. For example: gradle :services:webservice:build will build the webservice subproject and any subprojects it depends on.

The first approach is similar to the single-project use case, but Gradle works slightly differently in the case of a multi-project build. The command gradle test will execute the test task in any subprojects, relative to the current working directory, that have that task. So if you run the command from the root project directory, you’ll run test in api, shared, services:shared and services:webservice. If you run the command from the services project directory, you’ll only execute the task in services:shared and services:webservice.

For more control over what gets executed, use qualified names (the second approach mentioned). These are paths just like directory paths, but use ‘:’ instead of ‘/’ or ‘\’. If the path begins with a ‘:’, then the path is resolved relative to the root project. In other words, the leading ‘:’ represents the root project itself. All other colons are path separators.

This approach works for any task, so if you want to know what tasks are in a particular subproject, just use the tasks task, e.g. gradle :services:webservice:tasks .

Regardless of which technique you use to execute tasks, Gradle will take care of building any subprojects that the target depends on. You don’t have to worry about the inter-project dependencies yourself. If you’re interested in how this is configured, you can read about writing multi-project builds later in the user guide.

There’s one last thing to note. When you’re using the Gradle wrapper, the first approach doesn’t work well because you have to specify the path to the wrapper script if you’re not in the project root. For example, if you’re in the webservice subproject directory, you would have to run ../../gradlew build.

That’s all you really need to know about multi-project builds as a build user. You can now identify whether a build is a multi-project one and you can discover its structure. And finally, you can execute tasks within specific subprojects.

Composite builds

Composite build is an incubating feature. While useful for many use cases, there are bugs to be discovered, rough edges to smooth, and enhancements we plan to make. Thanks for trying it out!

What is a composite build?

A composite build is simply a build that includes other builds. In many ways a composite build is similar to a Gradle multi-project build, except that instead of including single projects, complete builds are included.

Composite builds allow you to:

  • combine builds that are usually developed independently, for instance when trying out a bug fix in a library that your application uses

  • decompose a large multi-project build into smaller, more isolated chunks that can be worked in independently or together as needed

A build that is included in a composite build is referred to, naturally enough, as an "included build". Included builds do not share any configuration with the composite build, or the other included builds. Each included build is configured and executed in isolation.

Included builds interact with other builds via dependency substitution. If any build in the composite has a dependency that can be satisfied by the included build, then that dependency will be replaced by a project dependency on the included build.

By default, Gradle will attempt to determine the dependencies that can be substituted by an included build. However for more flexibility, it is possible to explicitly declare these substitutions if the default ones determined by Gradle are not correct for the composite. See the section called “Declaring the dependencies substituted by an included build”.

As well as consuming outputs via project dependencies, a composite build can directly declare task dependencies on included builds. Included builds are isolated, and are not able to declare task dependencies on the composite build or on other included builds. See the section called “Depending on tasks in an included build”.

Defining a composite build

The following examples demonstrate the various ways that 2 Gradle builds that are normally developed separately can be combined into a composite build. For these examples, the my-utils multi-project build produces 2 different java libraries (number-utils and string-utils), and the my-app build produces an executable using functions from those libraries.

The my-app build does not have direct dependencies on my-utils. Instead, it declares binary dependencies on the libraries produced by my-utils.

Example 23. Dependencies of my-app

my-app/build.gradle

apply plugin: 'java'
apply plugin: 'application'
apply plugin: 'idea'

group "org.sample"
version "1.0"

mainClassName = "org.sample.myapp.Main"

dependencies {
    compile "org.sample:number-utils:1.0"
    compile "org.sample:string-utils:1.0"
}

repositories {
    jcenter()
}

Note: The code for this example can be found at samples/compositeBuilds/basic in the ‘-all’ distribution of Gradle.


Defining a composite build via --include-build

The --include-build command-line argument turns the executed build into a composite, substituting dependencies from the included build into the executed build.

Example 24. Declaring a command-line composite

Output of gradle --include-build ../my-utils run

> gradle --include-build ../my-utils run
> Task :processResources NO-SOURCE
> Task :my-utils:string-utils:compileJava
> Task :my-utils:string-utils:processResources NO-SOURCE
> Task :my-utils:string-utils:classes
> Task :my-utils:string-utils:jar
> Task :my-utils:number-utils:compileJava
> Task :my-utils:number-utils:processResources NO-SOURCE
> Task :my-utils:number-utils:classes
> Task :my-utils:number-utils:jar
> Task :compileJava
> Task :classes

> Task :run
The answer is 42


BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

Defining a composite build via settings.gradle

It’s possible to make the above arrangement persistent, by using Settings.includeBuild(java.lang.Object) to declare the included build in the settings.gradle file. The settings.gradle file can be used to add subprojects and included builds at the same time. Included builds are added by location. See the examples below for more details.

Defining a separate composite build

One downside of the above approach is that it requires you to modify an existing build, rendering it less useful as a standalone build. One way to avoid this is to define a separate composite build, whose only purpose is to combine otherwise separate builds.

Example 25. Declaring a separate composite

settings.gradle

rootProject.name='adhoc'

includeBuild '../my-app'
includeBuild '../my-utils'

In this scenario, the 'main' build that is executed is the composite, and it doesn’t define any useful tasks to execute itself. In order to execute the 'run' task in the 'my-app' build, the composite build must define a delegating task.

Example 26. Depending on task from included build

build.gradle

task run {
    dependsOn gradle.includedBuild('my-app').task(':run')
}

More details tasks that depend on included build tasks below.

Restrictions on included builds

Most builds can be included into a composite, however there are some limitations.

Every included build:

  • must have a settings.gradle file.

  • must not itself be a composite build.

  • must not have a rootProject.name the same as another included build.

  • must not have a rootProject.name the same as a top-level project of the composite build.

  • must not have a rootProject.name the same as the composite build rootProject.name.

Interacting with a composite build

In general, interacting with a composite build is much the same as a regular multi-project build. Tasks can be executed, tests can be run, and builds can be imported into the IDE.

Executing tasks

Tasks from the composite build can be executed from the command line, or from you IDE. Executing a task will result in direct task dependencies being executed, as well as those tasks required to build dependency artifacts from included builds.

There is not (yet) any means to directly execute a task from an included build via the command line. Included build tasks are automatically executed in order to generate required dependency artifacts, or the including build can declare a dependency on a task from an included build.

Importing into the IDE

One of the most useful features of composite builds is IDE integration. By applying the idea or eclipse plugin to your build, it is possible to generate a single IDEA or Eclipse project that permits all builds in the composite to be developed together.

In addition to these Gradle plugins, recent versions of IntelliJ IDEA and Eclipse Buildship support direct import of a composite build.

Importing a composite build permits sources from separate Gradle builds to be easily developed together. For every included build, each sub-project is included as an IDEA Module or Eclipse Project. Source dependencies are configured, providing cross-build navigation and refactoring.

Declaring the dependencies substituted by an included build

By default, Gradle will configure each included build in order to determine the dependencies it can provide. The algorithm for doing this is very simple: Gradle will inspect the group and name for the projects in the included build, and substitute project dependencies for any external dependency matching ${project.group}:${project.name}.

There are cases when the default substitutions determined by Gradle are not sufficient, or they are not correct for a particular composite. For these cases it is possible to explicitly declare the substitutions for an included build. Take for example a single-project build 'unpublished', that produces a java utility library but does not declare a value for the group attribute:

Example 27. Build that does not declare group attribute

build.gradle

apply plugin: 'java'

When this build is included in a composite, it will attempt to substitute for the dependency module "undefined:unpublished" ("undefined" being the default value for project.group, and 'unpublished' being the root project name). Clearly this isn’t going to be very useful in a composite build. To use the unpublished library unmodified in a composite build, the composing build can explicitly declare the substitutions that it provides:

Example 28. Declaring the substitutions for an included build

settings.gradle

rootProject.name = 'app'

includeBuild('../anonymous-library') {
    dependencySubstitution {
        substitute module('org.sample:number-utils') with project(':')
    }
}

With this configuration, the "my-app" composite build will substitute any dependency on org.sample:number-utils with a dependency on the root project of "unpublished".

Cases where included build substitutions must be declared

Many builds that use the uploadArchives task to publish artifacts will function automatically as an included build, without declared substitutions. Here are some common cases where declared substitutions are required:

  • When the archivesBaseName property is used to set the name of the published artifact.

  • When a configuration other than default is published: this usually means a task other than uploadArchives is used.

  • When the MavenPom.addFilter() is used to publish artifacts that don’t match the project name.

  • When the maven-publish or ivy-publish plugins are used for publishing, and the publication coordinates don’t match ${project.group}:${project.name}.

Cases where composite build substitutions won’t work

Some builds won’t function correctly when included in a composite, even when dependency substitutions are explicitly declared. This limitation is due to the fact that a project dependency that is substituted will always point to the default configuration of the target project. Any time that the artifacts and dependencies specified for the default configuration of a project don’t match what is actually published to a repository, then the composite build may exhibit different behaviour.

Here are some cases where the publish module metadata may be different from the project default configuration:

  • When a configuration other than default is published.

  • When the maven-publish or ivy-publish plugins are used.

  • When the POM or ivy.xml file is tweaked as part of publication.

Builds using these features function incorrectly when included in a composite build. We plan to improve this in the future.

Depending on tasks in an included build

While included builds are isolated from one another and cannot declare direct dependencies, a composite build is able to declare task dependencies on its included builds. The included builds are accessed using Gradle.getIncludedBuilds() or Gradle.includedBuild(java.lang.String), and a task reference is obtained via the IncludedBuild.task(java.lang.String) method.

Using these APIs, it is possible to declare a dependency on a task in a particular included build, or tasks with a certain path in all or some of the included builds.

Example 29. Depending on a single task from an included build

build.gradle

task run {
    dependsOn gradle.includedBuild('my-app').task(':run')
}

Example 30. Depending on a tasks with path in all included builds

build.gradle

task publishDeps {
    dependsOn gradle.includedBuilds*.task(':uploadArchives')
}

Current limitations and future plans for composite builds

We think composite builds are pretty useful already. However, there are some things that don’t yet work the way we’d like, and other improvements that we think will make things work even better.

Limitations of the current implementation include:

Improvements we have planned for upcoming releases include:

  • Better detection of dependency substitution, for build that publish with custom coordinates, builds that produce multiple components, etc. This will reduce the cases where dependency substitution needs to be explicitly declared for an included build.

  • The ability to target a task or tasks in an included build directly from the command line. We are currently exploring syntax options for allowing this functionality, which will remove many cases where a delegating task is required in the composite.

  • Making the implicit buildSrc project an included build.

  • Supporting composite-of-composite builds.

Build Environment

Gradle provides multiple mechanisms for configuring behavior of Gradle itself and specific projects. The following is a reference for using these mechanisms.

When configuring Gradle behavior you can use these methods, listed in order of highest to lowest precedence (first one wins):

  • Command-line flags such as --build-cache. These have precedence over properties and environment variables.

  • System properties such as systemProp.http.proxyHost=somehost.org stored in a gradle.properties file.

  • Gradle properties such as org.gradle.caching=true that are typically stored in a gradle.properties file in a project root directory or GRADLE_USER_HOME environment variable.

  • Environment variables such as GRADLE_OPTS sourced by the environment that executes Gradle.

Aside from configuring the build environment, you can configure a given project build using Project properties such as -PreleaseType=final.

Gradle properties

Gradle provides several options that make it easy to configure the Java process that will be used to execute your build. While it’s possible to configure these in your local environment via GRADLE_OPTS or JAVA_OPTS, it is useful to store certain settings like JVM memory configuration and Java home location in version control so that an entire team can work with a consistent environment.

Setting up a consistent environment for your build is as simple as placing these settings into a gradle.properties file. The configuration is applied in following order (if an option is configured in multiple locations the last one wins):

  • gradle.properties in project root directory.

  • gradle.properties in GRADLE_USER_HOME directory.

  • system properties, e.g. when -Dgradle.user.home is set on the command line.

The following properties can be used to configure the Gradle build environment:

org.gradle.caching=(true,false)

When set to true, Gradle will reuse task outputs from any previous build, when possible, resulting is much faster builds. Learn more about using the build cache.

org.gradle.caching.debug=(true,false)

When set to true, individual input property hashes and the build cache key for each task are logged on the console. Learn more about task output caching.

org.gradle.configureondemand=(true,false)

Enables incubating configuration on demand, where Gradle will attempt to configure only necessary projects.

org.gradle.console=(auto,plain,rich,verbose)

Customize console output coloring or verbosity. Default depends on how Gradle is invoked. See command-line logging for additional details.

org.gradle.daemon=(true,false)

When set to true the Gradle Daemon is used to run the build. Default is true.

org.gradle.daemon.idletimeout=(# of idle millis)

Gradle Daemon will terminate itself after specified number of idle milliseconds. Default is 10800000 (3 hours).

org.gradle.debug=(true,false)

When set to true, Gradle will run the build with remote debugging enabled, listening on port 5005. Note that this is the equivalent of adding -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005 to the JVM command line and will suspend the virtual machine until a debugger is attached. Default is false.

org.gradle.java.home=(path to JDK home)

Specifies the Java home for the Gradle build process. The value can be set to either a jdk or jre location, however, depending on what your build does, using a JDK is safer. A reasonable default is used if the setting is unspecified.

org.gradle.jvmargs=(JVM arguments)

Specifies the JVM arguments used for the Gradle Daemon. The setting is particularly useful for configuring JVM memory settings for build performance.

org.gradle.logging.level=(quiet,warn,lifecycle,info,debug)

When set to quiet, warn, lifecycle, info, or debug, Gradle will use this log level. The values are not case sensitive. The lifecycle level is the default. See the section called “Choosing a log level”.

org.gradle.parallel=(true,false)

When configured, Gradle will fork up to org.gradle.workers.max JVMs to execute projects in parallel. To learn more about parallel task execution, see the Gradle performance guide.

org.gradle.warning.mode=(all,none,summary)

When set to all, summary or none, Gradle will use different warning type display. See the section called “Logging options” for details.

org.gradle.workers.max=(max # of worker processes)

When configured, Gradle will use a maximum of the given number of workers. Default is number of CPU processors. See also performance command-line options.

The following example demonstrates usage of various properties.

Example 31. Setting properties with a gradle.properties file

gradle.properties

gradlePropertiesProp=gradlePropertiesValue
sysProp=shouldBeOverWrittenBySysProp
envProjectProp=shouldBeOverWrittenByEnvProp
systemProp.system=systemValue

build.gradle

task printProps {
    doLast {
        println commandLineProjectProp
        println gradlePropertiesProp
        println systemProjectProp
        println envProjectProp
        println System.properties['system']
    }
}

Output of gradle -q -PcommandLineProjectProp=commandLineProjectPropValue -Dorg.gradle.project.systemProjectProp=systemPropertyValue printProps

> gradle -q -PcommandLineProjectProp=commandLineProjectPropValue -Dorg.gradle.project.systemProjectProp=systemPropertyValue printProps
commandLineProjectPropValue
gradlePropertiesValue
systemPropertyValue
envPropertyValue
systemValue

System properties

Using the -D command-line option, you can pass a system property to the JVM which runs Gradle. The -D option of the gradle command has the same effect as the -D option of the java command.

You can also set system properties in gradle.properties files with the prefix systemProp.

Example 32. Specifying system properties in gradle.properties

systemProp.gradle.wrapperUser=myuser
systemProp.gradle.wrapperPassword=mypassword

The following system properties are available. Note that command-line options take precedence over system properties.

gradle.wrapperUser=(myuser)

Specify user name to download Gradle distributions from servers using HTTP Basic Authentication. Learn more in the section called “Authenticated Gradle distribution download”.

gradle.wrapperPassword=(mypassword)

Specify password for downloading a Gradle distribution using the Gradle wrapper.

gradle.user.home=(path to directory)

Specify the Gradle user home directory.

In a multi project build, “systemProp.” properties set in any project except the root will be ignored. That is, only the root project’s gradle.properties file will be checked for properties that begin with the “systemProp.” prefix.

Environment variables

The following environment variables are available for the gradle command. Note that command-line options and system properties take precedence over environment variables.

GRADLE_OPTS

Specifies command-line arguments to use when starting the Gradle client. This can be useful for setting the properties to use when running Gradle.

GRADLE_USER_HOME

Specifies the Gradle user home directory (which defaults to $USER_HOME/.gradle if not set).

JAVA_HOME

Specifies the JDK installation directory to use.

Project properties

You can add properties directly to your Project object via the -P command line option.

Gradle can also set project properties when it sees specially-named system properties or environment variables. If the environment variable name looks like ORG_GRADLE_PROJECT_prop=somevalue, then Gradle will set a prop property on your project object, with the value of somevalue. Gradle also supports this for system properties, but with a different naming pattern, which looks like org.gradle.project.prop. Both of the following will set the foo property on your Project object to "bar".

Example 33. Setting a project property via gradle.properties

org.gradle.project.foo=bar

Example 34. Setting a project property via environment variable

ORG_GRADLE_PROJECT_foo=bar

The properties file in the user’s home directory has precedence over property files in the project directories.

This feature is very useful when you don’t have admin rights to a continuous integration server and you need to set property values that should not be easily visible. Since you cannot use the -P option in that scenario, nor change the system-level configuration files, the correct strategy is to change the configuration of your continuous integration build job, adding an environment variable setting that matches an expected pattern. This won’t be visible to normal users on the system.

You can access a project property in your build script simply by using its name as you would use a variable.

If a project property is referenced but does not exist, an exception will be thrown and the build will fail.

You should check for existence of optional project properties before you access them using the Project.hasProperty(java.lang.String) method.

Configuring JVM memory

Gradle defaults to 1024 megabytes maximum heap per JVM process (-Xmx1024m), however, that may be too much or too little depending on the size of your project. There are many JVM options (this blog post on Java performance tuning and this reference may be helpful).

You can adjust JVM options for Gradle in the following ways:

The JAVA_OPTS environment variable is used for the Gradle client, but not forked JVMs.

Example 35. Changing JVM settings for Gradle client JVM

JAVA_OPTS="-Xmx2g -XX:MaxPermSize=256m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8"

You need to use the org.gradle.jvmargs Gradle property to configure JVM settings for the Gradle Daemon.

Example 36. Changing JVM settings for forked Gradle JVMs

org.gradle.jvmargs=-Xmx2g -XX:MaxPermSize=256m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8

Many settings (like the Java version and maximum heap size) can only be specified when launching a new JVM for the build process. This means that Gradle must launch a separate JVM process to execute the build after parsing the various gradle.properties files.

When running with the Gradle Daemon, a JVM with the correct parameters is started once and reused for each daemon build execution. When Gradle is executed without the daemon, then a new JVM must be launched for every build execution, unless the JVM launched by the Gradle start script happens to have the same parameters.

Certain tasks in Gradle also fork additional JVM processes, like the test task when using Test.setMaxParallelForks(int) for JUnit or TestNG tests. You must configure these through the tasks themselves.

Example 37. Set Java compile options for JavaCompile tasks

apply plugin: "java"

tasks.withType(JavaCompile) {
    options.compilerArgs += ["-Xdoclint:none", "-Xlint:none", "-nowarn"]
}

See other examples in the Test API documentation and test execution in the Java plugin reference.

Build scans will tell you information about the JVM that executed the build when you use the --scan option.

Build Environment in build scan

Configuring a task using project properties

It’s possible to change the behavior of a task based on project properties specified at invocation time.

Suppose you’d like to ensure release builds are only triggered by CI. A simple way to handle this is through an isCI project property.

Example 38. Prevent releasing outside of CI

build.gradle

task performRelease {
    doLast {
        if (project.hasProperty("isCI")) {
            println("Performing release actions")
        } else {
            throw new InvalidUserDataException("Cannot perform release outside of CI")
        }
    }
}

Output of gradle performRelease -PisCI=true --quiet

> gradle performRelease -PisCI=true --quiet
Performing release actions

Accessing the web through a HTTP proxy

Configuring an HTTP or HTTPS proxy (for downloading dependencies, for example) is done via standard JVM system properties. These properties can be set directly in the build script; for example, setting the HTTP proxy host would be done with System.setProperty('http.proxyHost', 'www.somehost.org'). Alternatively, the properties can be specified in gradle.properties.

Example 39. Configuring an HTTP proxy using gradle.properties

systemProp.http.proxyHost=www.somehost.org
systemProp.http.proxyPort=8080
systemProp.http.proxyUser=userid
systemProp.http.proxyPassword=password
systemProp.http.nonProxyHosts=*.nonproxyrepos.com|localhost

There are separate settings for HTTPS.

Example 40. Configuring an HTTPS proxy using gradle.properties

systemProp.https.proxyHost=www.somehost.org
systemProp.https.proxyPort=8080
systemProp.https.proxyUser=userid
systemProp.https.proxyPassword=password
systemProp.https.nonProxyHosts=*.nonproxyrepos.com|localhost

You may need to set other properties to access other networks. Here are 2 references that may be helpful:

NTLM Authentication

If your proxy requires NTLM authentication, you may need to provide the authentication domain as well as the username and password. There are 2 ways that you can provide the domain for authenticating to a NTLM proxy:

  • Set the http.proxyUser system property to a value like domain/username.

  • Provide the authentication domain via the http.auth.ntlm.domain system property.

Troubleshooting

The following is a collection of common issues and suggestions for addressing them. You can get other tips and search the Gradle forums and StackOverflow #gradle answers, as well as Gradle documentation from help.gradle.org.

Troubleshooting Gradle installation

If you followed the installation instructions, and aren’t able to execute your Gradle build, here are some tips that may help.

If you installed Gradle outside of just invoking the Gradle Wrapper, you can check your Gradle installation by running gradle --version in a terminal.

You should see something like this:

❯ gradle --version

-----------------------------------------------------------
Gradle 4.6
------------------------------------------------------------

Build time:   2018-02-21 15:28:42 UTC
Revision:     819e0059da49f469d3e9b2896dc4e72537c4847d

Groovy:       2.4.12
Ant:          Apache Ant(TM) version 1.9.9 compiled on February 2 2017
JVM:          1.8.0_151 (Oracle Corporation 25.151-b12)
OS:           Mac OS X 10.13.3 x86_64

If not, here are some things you might see instead.

Command not found: gradle

If you get "command not found: gradle", you need to ensure that Gradle is properly added to your PATH.

JAVA_HOME is set to an invalid directory

If you get something like:

ERROR: JAVA_HOME is set to an invalid directory

Please set the JAVA_HOME variable in your environment to match the location of your Java installation.

You’ll need to ensure that a Java Development Kit version 7 or higher is properly installed, the JAVA_HOME environment variable is set, and Java is added to your PATH.

Permission denied

If you get "permission denied", that means that Gradle likely exists in the correct place, but it is not executable. You can fix this using chmod +x path/to/executable on *nix-based systems.

Other installation failures

If gradle --version works, but all of your builds fail with the same error, it is possible there is a problem with one of your Gradle build configuration scripts.

You can verify the problem is with Gradle scripts by running gradle help which executes configuration scripts, but no Gradle tasks. If the error persists, build configuration is problematic. If not, then the problem exists within the execution of one or more of the requested tasks (Gradle executes configuration scripts first, and then executes build steps).

Debugging dependency resolution

Common dependency resolution issues such as resolving version conflicts are covered in Troubleshooting Dependency Resolution.

You can see a dependency tree and see which resolved dependency versions differed from what was requested by clicking the Dependencies view and using the search functionality, specifying the resolution reason.

Figure 4. Debugging dependency conflicts with build scans

Debugging dependency conflicts with build scans

The actual build scan with filtering criteria is available for exploration.

Troubleshooting slow Gradle builds

For build performance issues (including “slow sync time”), see the guide to Improving the Performance of Gradle Builds.

Android developers should watch a presentation by the Android SDK Tools team about Speeding Up Your Android Gradle Builds. Many tips are also covered in the Android Studio user guide on optimizing build speed.

Debugging build logic

Attaching a debugger to your build

You can set breakpoints and debug buildSrc and standalone plugins in your Gradle build itself by setting the org.gradle.debug property to “true” and then attaching a remote debugger to port 5005.

❯ gradle help -Dorg.gradle.debug=true --no-daemon

In addition, if you’ve adopted the Kotlin DSL, you can also debug build scripts themselves.

The following video demonstrates how to debug an example build using IntelliJ IDEA.

Figure 5. Interactive debugging of a build script

Interactive debugging of a build script

You must either stop running Gradle Daemons or run with --no-daemon when using debug mode.

Adding and changing logging

In addition to controlling logging verbosity, you can also control display of task outcomes (e.g. “UP-TO-DATE”) in lifecycle logging using the --console=verbose flag.

You can also replace much of Gradle’s logging with your own by registering various event listeners. One example of a custom event logger is explained in the logging documentation. You can also control logging from external tools, making them more verbose in order to debug their execution.

Additional logs from the Gradle Daemon can be found under GRADLE_USER_HOME/daemon/<gradle-version>/.

Task executed when it should have been UP-TO-DATE

--info logs explain why a task was executed, though build scans do this is searchable, visual way by going to the Timeline view and clicking on the task you want to inspect.

Figure 6. Debugging incremental build with a build scan

Debugging incremental build with a build scan

You can learn what the task outcomes mean from this listing.

Debugging IDE integration

Many infrequent errors within IDEs can be solved by "refreshing" Gradle. See also more documentation on working with Gradle in IntelliJ IDEA and in Eclipse.

Refreshing IntelliJ IDEA

NOTE: This only works for Gradle projects linked to IntelliJ.

From the main menu, go to View > Tool Windows > Gradle. Then click on the Refresh icon.

Figure 7. Refreshing a Gradle project in IntelliJ IDEA

Refreshing a Gradle project in IntelliJ IDEA

Refreshing Eclipse (using Buildship)

If you’re using Buildship for the Eclipse IDE, you can re-synchronize your Gradle build by opening the "Gradle Tasks" view and clicking the "Refresh" icon, or by executing the Gradle > Refresh Gradle Project command from the context menu while editing a Gradle script.

Figure 8. Refreshing a Gradle project in Eclipse Buildship

Refreshing a Gradle project in Eclipse Buildship

Getting additional help

If you didn’t find a fix for your issue here, please reach out to the Gradle community on the help forum or search relevant developer resources using help.gradle.org.

If you believe you’ve found a bug in Gradle, please file an issue on GitHub.

Embedding Gradle using the Tooling API

Introduction to the Tooling API

Gradle provides a programmatic API called the Tooling API, which you can use for embedding Gradle into your own software. This API allows you to execute and monitor builds and to query Gradle about the details of a build. The main audience for this API is IDE, CI server, other UI authors; however, the API is open for anyone who needs to embed Gradle in their application.

  • Gradle TestKit uses the Tooling API for functional testing of your Gradle plugins.

  • Eclipse Buildship uses the Tooling API for importing your Gradle project and running tasks.

  • IntelliJ IDEA uses the Tooling API for importing your Gradle project and running tasks.

Tooling API Features

A fundamental characteristic of the Tooling API is that it operates in a version independent way. This means that you can use the same API to work with builds that use different versions of Gradle, including versions that are newer or older than the version of the Tooling API that you are using. The Tooling API is Gradle wrapper aware and, by default, uses the same Gradle version as that used by the wrapper-powered build.

Some features that the Tooling API provides:

  • Query the details of a build, including the project hierarchy and the project dependencies, external dependencies (including source and Javadoc jars), source directories and tasks of each project.

  • Execute a build and listen to stdout and stderr logging and progress messages (e.g. the messages shown in the 'status bar' when you run on the command line).

  • Execute a specific test class or test method.

  • Receive interesting events as a build executes, such as project configuration, task execution or test execution.

  • Cancel a build that is running.

  • Combine multiple separate Gradle builds into a single composite build.

  • The Tooling API can download and install the appropriate Gradle version, similar to the wrapper.

  • The implementation is lightweight, with only a small number of dependencies. It is also a well-behaved library, and makes no assumptions about your classloader structure or logging configuration. This makes the API easy to embed in your application.

Tooling API and the Gradle Build Daemon

The Tooling API always uses the Gradle daemon. This means that subsequent calls to the Tooling API, be it model building requests or task executing requests will be executed in the same long-living process. The Gradle Daemon contains more details about the daemon, specifically information on situations when new daemons are forked.

Quickstart

As the Tooling API is an interface for developers, the Javadoc is the main documentation for it. We provide several samples that live in samples/toolingApi in your Gradle distribution. These samples specify all of the required dependencies for the Tooling API with examples for querying information from Gradle builds and executing tasks from the Tooling API.

To use the Tooling API, add the following repository and dependency declarations to your build script:

Example 41. Using the tooling API

build.gradle

repositories {
    maven { url 'https://repo.gradle.org/gradle/libs-releases' }
}

dependencies {
    compile "org.gradle:gradle-tooling-api:${toolingApiVersion}"
    // The tooling API need an SLF4J implementation available at runtime, replace this with any other implementation
    runtime 'org.slf4j:slf4j-simple:1.7.10'
}

The main entry point to the Tooling API is the GradleConnector. You can navigate from there to find code samples and explore the available Tooling API models. You can use GradleConnector.connect() to create a ProjectConnection. A ProjectConnection connects to a single Gradle project. Using the connection you can execute tasks, tests and retrieve models relative to this project.

Gradle version and Java version compatibility

Provider side

The current version of Tooling API supports running builds using Gradle versions 1.2 and later. However, support for running builds with Gradle versions older than 2.6 is deprecated and will be removed in Tooling API version 5.0.

Consumer side

The current version of Gradle supports running builds via Tooling API versions 2.0 and later. However, support for running builds via Tooling API versions older than 3.0 is deprecated and will be removed in Gradle 5.0.

You should note that not all features of the Tooling API are available for all versions of Gradle. For example, build cancellation is only available when a build uses Gradle 2.1 and later. Refer to the documentation for each class and method for more details.

Java version

The Tooling API requires Java 8 or later. Java 7 is currently still supported but will be removed in Gradle 5.0. The Gradle version used by builds may have additional Java version requirements.

Build Cache

The build cache feature described here is different from the Android plugin build cache.

Overview

The Gradle build cache is a cache mechanism that aims to save time by reusing outputs produced by other builds. The build cache works by storing (locally or remotely) build outputs and allowing builds to fetch these outputs from the cache when it is determined that inputs have not changed, avoiding the expensive work of regenerating them.

A first feature using the build cache is task output caching. Essentially, task output caching leverages the same intelligence as up-to-date checks that Gradle uses to avoid work when a previous local build has already produced a set of task outputs. But instead of being limited to the previous build in the same workspace, task output caching allows Gradle to reuse task outputs from any earlier build in any location on the local machine. When using a shared build cache for task output caching this even works across developer machines and build agents.

Apart from task output caching, we expect other features to use the build cache in the future.

A complete guide is available about using the build cache. It covers the different scenarios caching can improve, and detailed discussions of the different caveats you need to be aware of when enabling caching for a build.

Enable the Build Cache

By default, the build cache is not enabled. You can enable the build cache in a couple of ways:

Run with --build-cache on the command-line

Gradle will use the build cache for this build only.

Put org.gradle.caching=true in your gradle.properties

Gradle will try to reuse outputs from previous builds for all builds, unless explicitly disabled with --no-build-cache.

When the build cache is enabled, it will store build outputs in the Gradle user home. For configuring this directory or different kinds of build caches see the section called “Configure the Build Cache”.

Task Output Caching

Beyond incremental builds described in the section called “Up-to-date checks (AKA Incremental Build)”, Gradle can save time by reusing outputs from previous executions of a task by matching inputs to the task. Task outputs can be reused between builds on one computer or even between builds running on different computers via a build cache.

We have focused on the use case where users have an organization-wide remote build cache that is populated regularly by continuous integration builds. Developers and other continuous integration agents should pull cache entries from the remote build cache. We expect that developers will not be allowed to populate the remote build cache, and all continuous integration builds populate the build cache after running the clean task.

For your build to play well with task output caching it must work well with the incremental build feature. For example, when running your build twice in a row all tasks with outputs should be UP-TO-DATE. You cannot expect faster builds or correct builds when enabling task output caching when this prerequisite is not met.

Task output caching is automatically enabled when you enable the build cache, see the section called “Enable the Build Cache”.

What does it look like

Let us start with a project using the Java plugin which has a few Java source files. We run the build the first time.

$> gradle --build-cache compileJava
:compileJava
:processResources
:classes
:jar
:assemble

BUILD SUCCESSFUL

We see the directory used by the local build cache in the output. Apart from that the build was the same as without the build cache. Let’s clean and run the build again.

$> gradle clean
:clean

BUILD SUCCESSFUL
$> gradle --build-cache assemble
:compileJava FROM-CACHE
:processResources
:classes
:jar
:assemble

BUILD SUCCESSFUL

Now we see that, instead of executing the :compileJava task, the outputs of the task have been loaded from the build cache. The other tasks have not been loaded from the build cache since they are not cacheable. This is due to :classes and :assemble being lifecycle tasks and :processResources and :jar being Copy-like tasks which are not cacheable since it is generally faster to execute them.

Cacheable tasks

Since a task describes all of its inputs and outputs, Gradle can compute a build cache key that uniquely defines the task’s outputs based on its inputs. That build cache key is used to request previous outputs from a build cache or push new outputs to the build cache. If the previous build is already populated by someone else, e.g. your continuous integration server or other developers, you can avoid executing most tasks locally.

The following inputs contribute to the build cache key for a task in the same way that they do for up-to-date checks:

  • The task type and its classpath

  • The names of the output properties

  • The names and values of properties annotated as described in the section called “Custom task types”

  • The names and values of properties added by the DSL via TaskInputs

  • The classpath of the Gradle distribution, buildSrc and plugins

  • The content of the build script when it affects execution of the task

Task types need to opt-in to task output caching using the @CacheableTask annotation. Note that @CacheableTask is not inherited by subclasses. Custom task types are not cacheable by default.

Built-in cacheable tasks

Currently, the following built-in Gradle tasks are cacheable:

All other built-in tasks are currently not cacheable.

Some tasks, like Copy or Jar, usually do not make sense to make cacheable because Gradle is only copying files from one location to another. It also doesn’t make sense to make tasks cacheable that do not produce outputs or have no task actions.

Third party plugins

There are third party plugins that work well with the build cache. The most prominent examples are the Android plugin 3.1+ and the Kotlin plugin 1.2.21+. For other third party plugins, check their documentation to find out whether they support the build cache.

Declaring task inputs and outputs

It is very important that a cacheable task has a complete picture of its inputs and outputs, so that the results from one build can be safely re-used somewhere else.

Missing task inputs can cause incorrect cache hits, where different results are treated as identical because the same cache key is used by both executions. Missing task outputs can cause build failures if Gradle does not completely capture all outputs for a given task. Wrongly declared task inputs can lead to cache misses especially when containing volatile data or absolute paths. (See the section called “Task inputs and outputs” on what should be declared as inputs and outputs.)

The task path is not an input to the build cache key. This means that tasks with different task paths can re-use each other’s outputs as long as Gradle determines that executing them yields the same result.

In order to ensure that the inputs and outputs are properly declared use integration tests (for example using TestKit) to check that a task produces the same outputs for identical inputs and captures all output files for the task. We suggest adding tests to ensure that the task inputs are relocatable, i.e. that the task can be loaded from the cache into a different build directory (see @PathSensitive).

In order to handle volatile inputs for your tasks consider configuring input normalization.

Configure the Build Cache

You can configure the build cache by using the Settings.buildCache(org.gradle.api.Action) block in settings.gradle.

Gradle supports a local and a remote build cache that can be configured separately. When both build caches are enabled, Gradle tries to load build outputs from the local build cache first, and then tries the remote build cache if no build outputs are found. If outputs are found in the remote cache, they are also stored in the local cache, so next time they will be found locally. Gradle pushes build outputs to any build cache that is enabled and has BuildCache.isPush() set to true.

By default, the local build cache has push enabled, and the remote build cache has push disabled.

The local build cache is pre-configured to be a DirectoryBuildCache and enabled by default. The remote build cache can be configured by specifying the type of build cache to connect to (BuildCacheConfiguration.remote(java.lang.Class)).

Built-in local build cache

The built-in local build cache, DirectoryBuildCache, uses a directory to store build cache artifacts. By default, this directory resides in the Gradle user home directory, but its location is configurable.

Gradle will periodically clean-up the local cache directory by removing entries that have not been used recently to conserve disk space.

For more details on the configuration options refer to the DSL documentation of DirectoryBuildCache. Here is an example of the configuration.

Example 42. Configure the local cache

settings.gradle

buildCache {
    local(DirectoryBuildCache) {
        directory = new File(rootDir, 'build-cache')
        removeUnusedEntriesAfterDays = 30
    }
}

Remote HTTP build cache

Gradle has built-in support for connecting to a remote build cache backend via HTTP. For more details on what the protocol looks like see HttpBuildCache. Note that by using the following configuration the local build cache will be used for storing build outputs while the local and the remote build cache will be used for retrieving build outputs.

Example 43. Pull from HttpBuildCache

settings.gradle

buildCache {
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
    }
}

You can configure the credentials the HttpBuildCache uses to access the build cache server as shown in the following example.

Example 44. Configure remote HTTP cache

settings.gradle

buildCache {
    remote(HttpBuildCache) {
        url = 'http://example.com:8123/cache/'
        credentials {
            username = 'build-cache-user'
            password = 'some-complicated-password'
        }
    }
}

You may encounter problems with an untrusted SSL certificate when you try to use a build cache backend with an HTTPS URL. The ideal solution is for someone to add a valid SSL certificate to the build cache backend, but we recognize that you may not be able to do that. In that case, set HttpBuildCache.isAllowUntrustedServer() to true:

Example 45. Allow untrusted SSL certificate for HttpBuildCache

settings.gradle

buildCache {
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
        allowUntrustedServer = true
    }
}

This is a convenient workaround, but you shouldn’t use it as a long-term solution.

Configuration use cases

The recommended use case for the build cache is that your continuous integration server populates the remote build cache with clean builds while developers pull from the remote build cache and push to a local build cache. The configuration would then look as follows.

Example 46. Recommended setup for CI push use case

settings.gradle

ext.isCiServer = System.getenv().containsKey("CI")

buildCache {
    local {
        enabled = !isCiServer
    }
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
        push = isCiServer
    }
}

If you use a buildSrc directory, you should make sure that it uses the same build cache configuration as the main build. This can be achieved by applying the same script to buildSrc/settings.gradle and settings.gradle as shown in the following example.

Example 47. Consistent setup for buildSrc and main build

settings.gradle

apply from: new File(settingsDir, 'gradle/buildCacheSettings.gradle')

buildSrc/settings.gradle

apply from: new File(settingsDir, '../gradle/buildCacheSettings.gradle')

gradle/buildCacheSettings.gradle

ext.isCiServer = System.getenv().containsKey("CI")

buildCache {
    local {
        enabled = !isCiServer
    }
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
        push = isCiServer
    }
}

It is also possible to configure the build cache from an init script, which can be used from the command line, added to your Gradle user home or be a part of your custom Gradle distribution.

Example 48. Init script to configure the build cache

init.gradle

gradle.settingsEvaluated { settings ->
    settings.buildCache {
        // vvv Your custom configuration goes here
        remote(HttpBuildCache) {
            url = 'https://example.com:8123/cache/'
        }
        // ^^^ Your custom configuration goes here
    }
}

Build cache and composite builds

Gradle’s composite build feature allows including other complete Gradle builds into another. Such included builds will inherit the build cache configuration from the top level build, regardless of whether the included builds define build cache configuration themselves or not.

The build cache configuration present for any included build is effectively ignored, in favour of the top level build’s configuration. This also applies to any buildSrc projects of any included builds.

How to set up an HTTP build cache backend

Gradle provides a Docker image for a build cache node, which can connect with Gradle Enterprise for centralized management. The cache node can also be used without a Gradle Enterprise installation with restricted functionality.

Implement your own Build Cache

Using a different build cache backend to store build outputs (which is not covered by the built-in support for connecting to an HTTP backend) requires implementing your own logic for connecting to your custom build cache backend. To this end, custom build cache types can be registered via BuildCacheConfiguration.registerBuildCacheService(java.lang.Class, java.lang.Class). For an example of what this could look like see the Gradle Hazelcast plugin.

Gradle Enterprise includes a high-performance, easy to install and operate, shared build cache backend.

Writing Gradle build scripts

Build Script Basics

Projects and tasks

Everything in Gradle sits on top of two basic concepts: projects and tasks.

Every Gradle build is made up of one or more projects. What a project represents depends on what it is that you are doing with Gradle. For example, a project might represent a library JAR or a web application. It might represent a distribution ZIP assembled from the JARs produced by other projects. A project does not necessarily represent a thing to be built. It might represent a thing to be done, such as deploying your application to staging or production environments. Don’t worry if this seems a little vague for now. Gradle’s build-by-convention support adds a more concrete definition for what a project is.

Each project is made up of one or more tasks. A task represents some atomic piece of work which a build performs. This might be compiling some classes, creating a JAR, generating Javadoc, or publishing some archives to a repository.

For now, we will look at defining some simple tasks in a build with one project. Later chapters will look at working with multiple projects and more about working with projects and tasks.

Hello world

You run a Gradle build using the gradle command. The gradle command looks for a file called build.gradle in the current directory.[2] We call this build.gradle file a build script, although strictly speaking it is a build configuration script, as we will see later. The build script defines a project and its tasks.

To try this out, create the following build script named build.gradle.

Example 49. Your first build script

build.gradle

task hello {
    doLast {
        println 'Hello world!'
    }
}

In a command-line shell, move to the containing directory and execute the build script with gradle -q hello:

What does -q do?

Most of the examples in this user guide are run with the -q command-line option. This suppresses Gradle’s log messages, so that only the output of the tasks is shown. This keeps the example output in this user guide a little clearer. You don’t need to use this option if you don’t want to. See Logging for more details about the command-line options which affect Gradle’s output.

Example 50. Execution of a build script

Output of gradle -q hello

> gradle -q hello
Hello world!

What’s going on here? This build script defines a single task, called hello, and adds an action to it. When you run gradle hello, Gradle executes the hello task, which in turn executes the action you’ve provided. The action is simply a closure containing some Groovy code to execute.

If you think this looks similar to Ant’s targets, you would be right. Gradle tasks are the equivalent to Ant targets, but as you will see, they are much more powerful. We have used a different terminology than Ant as we think the word task is more expressive than the word target. Unfortunately this introduces a terminology clash with Ant, as Ant calls its commands, such as javac or copy, tasks. So when we talk about tasks, we always mean Gradle tasks, which are the equivalent to Ant’s targets. If we talk about Ant tasks (Ant commands), we explicitly say Ant task.

A shortcut task definition

This functionality is deprecated and will be removed in Gradle 5.0 without replacement. Use the methods Task.doFirst(org.gradle.api.Action) and Task.doLast(org.gradle.api.Action) to define an action instead, as demonstrated by the rest of the examples in this chapter.

There is a shorthand way to define a task like our hello task above, which is more concise.

Example 51. A task definition shortcut

build.gradle

task hello << {
    println 'Hello world!'
}

Again, this defines a task called hello with a single closure to execute. The << operator is simply an alias for doLast.

Build scripts are code

Gradle’s build scripts give you the full power of Groovy. As an appetizer, have a look at this:

Example 52. Using Groovy in Gradle's tasks

build.gradle

task upper {
    doLast {
        String someString = 'mY_nAmE'
        println "Original: " + someString
        println "Upper case: " + someString.toUpperCase()
    }
}

Output of gradle -q upper

> gradle -q upper
Original: mY_nAmE
Upper case: MY_NAME

or

Example 53. Using Groovy in Gradle's tasks

build.gradle

task count {
    doLast {
        4.times { print "$it " }
    }
}

Output of gradle -q count

> gradle -q count
0 1 2 3 

Task dependencies

As you probably have guessed, you can declare tasks that depend on other tasks.

Example 54. Declaration of task that depends on other task

build.gradle

task hello {
    doLast {
        println 'Hello world!'
    }
}
task intro(dependsOn: hello) {
    doLast {
        println "I'm Gradle"
    }
}

Output of gradle -q intro

> gradle -q intro
Hello world!
I'm Gradle

To add a dependency, the corresponding task does not need to exist.

Example 55. Lazy dependsOn - the other task does not exist (yet)

build.gradle

task taskX(dependsOn: 'taskY') {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

Output of gradle -q taskX

> gradle -q taskX
taskY
taskX

The dependency of taskX to taskY is declared before taskY is defined. This is very important for multi-project builds. Task dependencies are discussed in more detail in the section called “Adding dependencies to a task”.

Please notice that you can’t use shortcut notation (see the section called “Shortcut notations”) when referring to a task that is not yet defined.

Dynamic tasks

The power of Groovy can be used for more than defining what a task does. For example, you can also use it to dynamically create tasks.

Example 56. Dynamic creation of a task

build.gradle

4.times { counter ->
    task "task$counter" {
        doLast {
            println "I'm task number $counter"
        }
    }
}

Output of gradle -q task1

> gradle -q task1
I'm task number 1

Manipulating existing tasks

Once tasks are created they can be accessed via an API. For instance, you could use this to dynamically add dependencies to a task, at runtime. Ant doesn’t allow anything like this.

Example 57. Accessing a task via API - adding a dependency

build.gradle

4.times { counter ->
    task "task$counter" {
        doLast {
            println "I'm task number $counter"
        }
    }
}
task0.dependsOn task2, task3

Output of gradle -q task0

> gradle -q task0
I'm task number 2
I'm task number 3
I'm task number 0

Or you can add behavior to an existing task.

Example 58. Accessing a task via API - adding behaviour

build.gradle

task hello {
    doLast {
        println 'Hello Earth'
    }
}
hello.doFirst {
    println 'Hello Venus'
}
hello.doLast {
    println 'Hello Mars'
}
hello {
    doLast {
        println 'Hello Jupiter'
    }
}

Output of gradle -q hello

> gradle -q hello
Hello Venus
Hello Earth
Hello Mars
Hello Jupiter

The calls doFirst and doLast can be executed multiple times. They add an action to the beginning or the end of the task’s actions list. When the task executes, the actions in the action list are executed in order.

Shortcut notations

There is a convenient notation for accessing an existing task. Each task is available as a property of the build script:

Example 59. Accessing task as a property of the build script

build.gradle

task hello {
    doLast {
        println 'Hello world!'
    }
}
hello.doLast {
    println "Greetings from the $hello.name task."
}

Output of gradle -q hello

> gradle -q hello
Hello world!
Greetings from the hello task.

This enables very readable code, especially when using the tasks provided by the plugins, like the compile task.

Extra task properties

You can add your own properties to a task. To add a property named myProperty, set ext.myProperty to an initial value. From that point on, the property can be read and set like a predefined task property.

Example 60. Adding extra properties to a task

build.gradle

task myTask {
    ext.myProperty = "myValue"
}

task printTaskProperties {
    doLast {
        println myTask.myProperty
    }
}

Output of gradle -q printTaskProperties

> gradle -q printTaskProperties
myValue

Extra properties aren’t limited to tasks. You can read more about them in the section called “Extra properties”.

Using Ant Tasks

Ant tasks are first-class citizens in Gradle. Gradle provides excellent integration for Ant tasks by simply relying on Groovy. Groovy is shipped with the fantastic AntBuilder. Using Ant tasks from Gradle is as convenient and more powerful than using Ant tasks from a build.xml file. From the example below, you can learn how to execute Ant tasks and how to access Ant properties:

Example 61. Using AntBuilder to execute ant.loadfile target

build.gradle

task loadfile {
    doLast {
        def files = file('../antLoadfileResources').listFiles().sort()
        files.each { File file ->
            if (file.isFile()) {
                ant.loadfile(srcFile: file, property: file.name)
                println " *** $file.name ***"
                println "${ant.properties[file.name]}"
            }
        }
    }
}

Output of gradle -q loadfile

> gradle -q loadfile
 *** agile.manifesto.txt ***
Individuals and interactions over processes and tools
Working software over comprehensive documentation
Customer collaboration  over contract negotiation
Responding to change over following a plan
 *** gradle.manifesto.txt ***
Make the impossible possible, make the possible easy and make the easy elegant.
(inspired by Moshe Feldenkrais)

There is lots more you can do with Ant in your build scripts. You can find out more in Using Ant from Gradle.

Using methods

Gradle scales in how you can organize your build logic. The first level of organizing your build logic for the example above, is extracting a method.

Example 62. Using methods to organize your build logic

build.gradle

task checksum {
    doLast {
        fileList('../antLoadfileResources').each { File file ->
            ant.checksum(file: file, property: "cs_$file.name")
            println "$file.name Checksum: ${ant.properties["cs_$file.name"]}"
        }
    }
}

task loadfile {
    doLast {
        fileList('../antLoadfileResources').each { File file ->
            ant.loadfile(srcFile: file, property: file.name)
            println "I'm fond of $file.name"
        }
    }
}

File[] fileList(String dir) {
    file(dir).listFiles({file -> file.isFile() } as FileFilter).sort()
}

Output of gradle -q loadfile

> gradle -q loadfile
I'm fond of agile.manifesto.txt
I'm fond of gradle.manifesto.txt

Later you will see that such methods can be shared among subprojects in multi-project builds. If your build logic becomes more complex, Gradle offers you other very convenient ways to organize it. We have devoted a whole chapter to this. See Organizing Build Logic.

Default tasks

Gradle allows you to define one or more default tasks that are executed if no other tasks are specified.

Example 63. Defining a default task

build.gradle

defaultTasks 'clean', 'run'

task clean {
    doLast {
        println 'Default Cleaning!'
    }
}

task run {
    doLast {
        println 'Default Running!'
    }
}

task other {
    doLast {
        println "I'm not a default task!"
    }
}

Output of gradle -q

> gradle -q
Default Cleaning!
Default Running!

This is equivalent to running gradle clean run. In a multi-project build every subproject can have its own specific default tasks. If a subproject does not specify default tasks, the default tasks of the parent project are used (if defined).

Configure by DAG

As we later describe in full detail (see Build Lifecycle), Gradle has a configuration phase and an execution phase. After the configuration phase, Gradle knows all tasks that should be executed. Gradle offers you a hook to make use of this information. A use-case for this would be to check if the release task is among the tasks to be executed. Depending on this, you can assign different values to some variables.

In the following example, execution of the distribution and release tasks results in different value of the version variable.

Example 64. Different outcomes of build depending on chosen tasks

build.gradle

task distribution {
    doLast {
        println "We build the zip with version=$version"
    }
}

task release(dependsOn: 'distribution') {
    doLast {
        println 'We release now'
    }
}

gradle.taskGraph.whenReady {taskGraph ->
    if (taskGraph.hasTask(release)) {
        version = '1.0'
    } else {
        version = '1.0-SNAPSHOT'
    }
}

Output of gradle -q distribution

> gradle -q distribution
We build the zip with version=1.0-SNAPSHOT

Output of gradle -q release

> gradle -q release
We build the zip with version=1.0
We release now

The important thing is that whenReady affects the release task before the release task is executed. This works even when the release task is not the primary task (i.e., the task passed to the gradle command).

Where to next?

In this chapter, we have had a first look at tasks. But this is not the end of the story for tasks. If you want to jump into more of the details, have a look at Authoring Tasks.

Otherwise, continue on to the tutorials in Java Quickstart and Dependency Management for Java Projects.



[2] There are command line switches to change this behavior. See Command-Line Interface)

Build Init Plugin

The Build Init plugin is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

The Gradle Build Init plugin can be used to bootstrap the process of creating a new Gradle build. It supports creating brand new projects of different types as well as converting existing builds (e.g. An Apache Maven build) to be Gradle builds.

Gradle plugins typically need to be applied to a project before they can be used (see the section called “Using plugins”). The Build Init plugin is an automatically applied plugin, which means you do not need to apply it explicitly. To use the plugin, simply execute the task named init where you would like to create the Gradle build. There is no need to create a “stub” build.gradle file in order to apply the plugin.

It also leverages the wrapper task to generate the Gradle Wrapper files for the project.

Tasks

The plugin adds the following tasks to the project:

Table 1. Build Init plugin - tasks

Task name Depends on Type Description

init

wrapper

InitBuild

Generates a Gradle project.

wrapper

-

Wrapper

Generates Gradle wrapper files.

What to set up

The init supports different build setup types. The type is specified by supplying a --type argument value. For example, to create a Java library project simply execute: gradle init --type java-library.

If a --type parameter is not supplied, Gradle will attempt to infer the type from the environment. For example, it will infer a type value of “pom” if it finds a pom.xml to convert to a Gradle build.

If the type could not be inferred, the type “basic” will be used.

The init plugin also supports generating build scripts using either the Gradle Groovy DSL or the Gradle Kotlin DSL. The build script DSL to use defaults to the Groovy DSL and is specified by supplying a --dsl argument value. For example, to create a Java library project with Kotlin DSL build scripts simply execute: gradle init --type java-library --dsl kotlin.

All build setup types include the setup of the Gradle Wrapper.

Note that the migration from Maven builds only supports the Groovy DSL for generated build scripts.

Build init types

As this plugin is currently incubating, only a few build init types are currently supported. More types will be added in future Gradle releases.

pom” (Maven conversion)

The “pom” type can be used to convert an Apache Maven build to a Gradle build. This works by converting the POM to one or more Gradle files. It is only able to be used if there is a valid “pom.xml” file in the directory that the init task is invoked in or, if invoked via the “-p” command line option, in the specified project directory. This “pom” type will be automatically inferred if such a file exists.

The Maven conversion implementation was inspired by the maven2gradle tool that was originally developed by Gradle community members.

The conversion process has the following features:

  • Uses effective POM and effective settings (support for POM inheritance, dependency management, properties)

  • Supports both single module and multimodule projects

  • Supports custom module names (that differ from directory names)

  • Generates general metadata - id, description and version

  • Applies maven, java and war plugins (as needed)

  • Supports packaging war projects as jars if needed

  • Generates dependencies (both external and inter-module)

  • Generates download repositories (inc. local Maven repository)

  • Adjusts Java compiler settings

  • Supports packaging of sources and tests

  • Supports TestNG runner

  • Generates global exclusions from Maven enforcer plugin settings

java-application

The “java-application” build init type is not inferable. It must be explicitly specified.

It has the following features:

  • Uses the “application” plugin to produce a command-line application implemented using Java

  • Uses the “jcenter” dependency repository

  • Uses JUnit for testing

  • Has directories in the conventional locations for source code

  • Contains a sample class and unit test, if there are no existing source or test files

Alternative test framework can be specified by supplying a --test-framework argument value. To use a different test framework, execute one of the following commands:

  • gradle init --type java-application --test-framework spock: Uses Spock for testing instead of JUnit

  • gradle init --type java-application --test-framework testng: Uses TestNG for testing instead of JUnit

java-library

The “java-library” build init type is not inferable. It must be explicitly specified.

It has the following features:

  • Uses the “java” plugin to produce a library Jar

  • Uses the “jcenter” dependency repository

  • Uses JUnit for testing

  • Has directories in the conventional locations for source code

  • Contains a sample class and unit test, if there are no existing source or test files

Alternative test framework can be specified by supplying a --test-framework argument value. To use a different test framework, execute one of the following commands:

  • gradle init --type java-library --test-framework spock: Uses Spock for testing instead of JUnit

  • gradle init --type java-library --test-framework testng: Uses TestNG for testing instead of JUnit

scala-library

The “scala-library” build init type is not inferable. It must be explicitly specified.

It has the following features:

  • Uses the “scala” plugin to produce a library Jar

  • Uses the “jcenter” dependency repository

  • Uses Scala 2.10

  • Uses ScalaTest for testing

  • Has directories in the conventional locations for source code

  • Contains a sample scala class and an associated ScalaTest test suite, if there are no existing source or test files

  • Uses the Zinc Scala compiler by default

groovy-library

The “groovy-library” build init type is not inferable. It must be explicitly specified.

It has the following features:

  • Uses the “groovy” plugin to produce a library Jar

  • Uses the “jcenter” dependency repository

  • Uses Groovy 2.x

  • Uses Spock testing framework for testing

  • Has directories in the conventional locations for source code

  • Contains a sample Groovy class and an associated Spock specification, if there are no existing source or test files

groovy-application

The “groovy-application” build init type is not inferable. It must be explicitly specified.

It has the following features:

  • Uses the “groovy” plugin

  • Uses the “application” plugin to produce a command-line application implemented using Groovy

  • Uses the “jcenter” dependency repository

  • Uses Groovy 2.x

  • Uses Spock testing framework for testing

  • Has directories in the conventional locations for source code

  • Contains a sample Groovy class and an associated Spock specification, if there are no existing source or test files

“basic”

The “basic” build init type is useful for creating a fresh new Gradle project. It creates a sample build.gradle file, with comments and links to help get started.

This type is used when no type was explicitly specified, and no type could be inferred.

Writing Build Scripts

This chapter looks at some of the details of writing a build script.

The Gradle build language

Gradle provides a domain specific language, or DSL, for describing builds. This build language is based on Groovy, with some additions to make it easier to describe a build.

A build script can contain any Groovy language element.[3] Gradle assumes that each build script is encoded using UTF-8.

The Project API

In the tutorial in Java Quickstart we used, for example, the apply() method. Where does this method come from? We said earlier that the build script defines a project in Gradle. For each project in the build, Gradle creates an object of type Project and associates this Project object with the build script. As the build script executes, it configures this Project object:

Getting help writing build scripts

Don’t forget that your build script is simply Groovy code that drives the Gradle API. And the Project interface is your starting point for accessing everything in the Gradle API. So, if you’re wondering what 'tags' are available in your build script, you can start with the documentation for the Project interface.

  • Any method you call in your build script which is not defined in the build script, is delegated to the Project object.

  • Any property you access in your build script, which is not defined in the build script, is delegated to the Project object.

Let’s try this out and try to access the name property of the Project object.

Example 65. Accessing property of the Project object

build.gradle

println name
println project.name

Output of gradle -q check

> gradle -q check
projectApi
projectApi

Both println statements print out the same property. The first uses auto-delegation to the Project object, for properties not defined in the build script. The other statement uses the project property available to any build script, which returns the associated Project object. Only if you define a property or a method which has the same name as a member of the Project object, would you need to use the project property.

Standard project properties

The Project object provides some standard properties, which are available in your build script. The following table lists a few of the commonly used ones.

Table 2. Project Properties

Name Type Default Value

project

Project

The Project instance

name

String

The name of the project directory.

path

String

The absolute path of the project.

description

String

A description for the project.

projectDir

File

The directory containing the build script.

buildDir

File

projectDir/build

group

Object

unspecified

version

Object

unspecified

ant

AntBuilder

An AntBuilder instance

The Script API

When Gradle executes a script, it compiles the script into a class which implements Script. This means that all of the properties and methods declared by the Script interface are available in your script.

Declaring variables

There are two kinds of variables that can be declared in a build script: local variables and extra properties.

Local variables

Local variables are declared with the def keyword. They are only visible in the scope where they have been declared. Local variables are a feature of the underlying Groovy language.

Example 66. Using local variables

build.gradle

def dest = "dest"

task copy(type: Copy) {
    from "source"
    into dest
}

Extra properties

All enhanced objects in Gradle’s domain model can hold extra user-defined properties. This includes, but is not limited to, projects, tasks, and source sets. Extra properties can be added, read and set via the owning object’s ext property. Alternatively, an ext block can be used to add multiple properties at once.

Example 67. Using extra properties

build.gradle

apply plugin: "java"

ext {
    springVersion = "3.1.0.RELEASE"
    emailNotification = "build@master.org"
}

sourceSets.all { ext.purpose = null }

sourceSets {
    main {
        purpose = "production"
    }
    test {
        purpose = "test"
    }
    plugin {
        purpose = "production"
    }
}

task printProperties {
    doLast {
        println springVersion
        println emailNotification
        sourceSets.matching { it.purpose == "production" }.each { println it.name }
    }
}

Output of gradle -q printProperties

> gradle -q printProperties
3.1.0.RELEASE
build@master.org
main
plugin

In this example, an ext block adds two extra properties to the project object. Additionally, a property named purpose is added to each source set by setting ext.purpose to null (null is a permissible value). Once the properties have been added, they can be read and set like predefined properties.

By requiring special syntax for adding a property, Gradle can fail fast when an attempt is made to set a (predefined or extra) property but the property is misspelled or does not exist. Extra properties can be accessed from anywhere their owning object can be accessed, giving them a wider scope than local variables. Extra properties on a project are visible from its subprojects.

For further details on extra properties and their API, see the ExtraPropertiesExtension class in the API documentation.

Configuring arbitrary objects

You can configure arbitrary objects in the following very readable way.

Example 68. Configuring arbitrary objects

build.gradle

task configure {
    doLast {
        def pos = configure(new java.text.FieldPosition(10)) {
            beginIndex = 1
            endIndex = 5
        }
        println pos.beginIndex
        println pos.endIndex
    }
}

Output of gradle -q configure

> gradle -q configure
1
5

Configuring arbitrary objects using an external script

You can also configure arbitrary objects using an external script.

Example 69. Configuring arbitrary objects using a script

build.gradle

task configure {
    doLast {
        def pos = new java.text.FieldPosition(10)
        // Apply the script
        apply from: 'other.gradle', to: pos
        println pos.beginIndex
        println pos.endIndex
    }
}

other.gradle

// Set properties.
beginIndex = 1
endIndex = 5

Output of gradle -q configure

> gradle -q configure
1
5

Some Groovy basics

The Groovy language provides plenty of features for creating DSLs, and the Gradle build language takes advantage of these. Understanding how the build language works will help you when you write your build script, and in particular, when you start to write custom plugins and tasks.

Groovy JDK

Groovy adds lots of useful methods to the standard Java classes. For example, Iterable gets an each method, which iterates over the elements of the Iterable:

Example 70. Groovy JDK methods

build.gradle

// Iterable gets an each() method
configurations.runtime.each { File f -> println f }

Have a look at http://groovy-lang.org/gdk.html for more details.

Property accessors

Groovy automatically converts a property reference into a call to the appropriate getter or setter method.

Example 71. Property accessors

build.gradle

// Using a getter method
println project.buildDir
println getProject().getBuildDir()

// Using a setter method
project.buildDir = 'target'
getProject().setBuildDir('target')

Optional parentheses on method calls

Parentheses are optional for method calls.

Example 72. Method call without parentheses

build.gradle

test.systemProperty 'some.prop', 'value'
test.systemProperty('some.prop', 'value')

List and map literals

Groovy provides some shortcuts for defining List and Map instances. Both kinds of literals are straightforward, but map literals have some interesting twists.

For instance, the “apply” method (where you typically apply plugins) actually takes a map parameter. However, when you have a line like “apply plugin:'java'”, you aren’t actually using a map literal, you’re actually using “named parameters”, which have almost exactly the same syntax as a map literal (without the wrapping brackets). That named parameter list gets converted to a map when the method is called, but it doesn’t start out as a map.

Example 73. List and map literals

build.gradle

// List literal
test.includes = ['org/gradle/api/**', 'org/gradle/internal/**']

List<String> list = new ArrayList<String>()
list.add('org/gradle/api/**')
list.add('org/gradle/internal/**')
test.includes = list

// Map literal.
Map<String, String> map = [key1:'value1', key2: 'value2']

// Groovy will coerce named arguments
// into a single map argument
apply plugin: 'java'

Closures as the last parameter in a method

The Gradle DSL uses closures in many places. You can find out more about closures here. When the last parameter of a method is a closure, you can place the closure after the method call:

Example 74. Closure as method parameter

build.gradle

repositories {
    println "in a closure"
}
repositories() { println "in a closure" }
repositories({ println "in a closure" })

Closure delegate

Each closure has a delegate object, which Groovy uses to look up variable and method references which are not local variables or parameters of the closure. Gradle uses this for configuration closures, where the delegate object is set to the object to be configured.

Example 75. Closure delegates

build.gradle

dependencies {
    assert delegate == project.dependencies
    testCompile('junit:junit:4.12')
    delegate.testCompile('junit:junit:4.12')
}

Default imports

To make build scripts more concise, Gradle automatically adds a set of import statements to the Gradle scripts. This means that instead of using throw new org.gradle.api.tasks.StopExecutionException() you can just type throw new StopExecutionException() instead.

Listed below are the imports added to each script:

Gradle default imports. 

import org.gradle.*
import org.gradle.api.*
import org.gradle.api.artifacts.*
import org.gradle.api.artifacts.component.*
import org.gradle.api.artifacts.dsl.*
import org.gradle.api.artifacts.ivy.*
import org.gradle.api.artifacts.maven.*
import org.gradle.api.artifacts.query.*
import org.gradle.api.artifacts.repositories.*
import org.gradle.api.artifacts.result.*
import org.gradle.api.artifacts.transform.*
import org.gradle.api.artifacts.type.*
import org.gradle.api.attributes.*
import org.gradle.api.capabilities.*
import org.gradle.api.component.*
import org.gradle.api.credentials.*
import org.gradle.api.distribution.*
import org.gradle.api.distribution.plugins.*
import org.gradle.api.dsl.*
import org.gradle.api.execution.*
import org.gradle.api.file.*
import org.gradle.api.initialization.*
import org.gradle.api.initialization.definition.*
import org.gradle.api.initialization.dsl.*
import org.gradle.api.invocation.*
import org.gradle.api.java.archives.*
import org.gradle.api.logging.*
import org.gradle.api.logging.configuration.*
import org.gradle.api.model.*
import org.gradle.api.plugins.*
import org.gradle.api.plugins.announce.*
import org.gradle.api.plugins.antlr.*
import org.gradle.api.plugins.buildcomparison.gradle.*
import org.gradle.api.plugins.osgi.*
import org.gradle.api.plugins.quality.*
import org.gradle.api.plugins.scala.*
import org.gradle.api.provider.*
import org.gradle.api.publish.*
import org.gradle.api.publish.ivy.*
import org.gradle.api.publish.ivy.plugins.*
import org.gradle.api.publish.ivy.tasks.*
import org.gradle.api.publish.maven.*
import org.gradle.api.publish.maven.plugins.*
import org.gradle.api.publish.maven.tasks.*
import org.gradle.api.publish.plugins.*
import org.gradle.api.publish.tasks.*
import org.gradle.api.reflect.*
import org.gradle.api.reporting.*
import org.gradle.api.reporting.components.*
import org.gradle.api.reporting.dependencies.*
import org.gradle.api.reporting.dependents.*
import org.gradle.api.reporting.model.*
import org.gradle.api.reporting.plugins.*
import org.gradle.api.resources.*
import org.gradle.api.specs.*
import org.gradle.api.tasks.*
import org.gradle.api.tasks.ant.*
import org.gradle.api.tasks.application.*
import org.gradle.api.tasks.bundling.*
import org.gradle.api.tasks.compile.*
import org.gradle.api.tasks.diagnostics.*
import org.gradle.api.tasks.incremental.*
import org.gradle.api.tasks.javadoc.*
import org.gradle.api.tasks.options.*
import org.gradle.api.tasks.scala.*
import org.gradle.api.tasks.testing.*
import org.gradle.api.tasks.testing.junit.*
import org.gradle.api.tasks.testing.junitplatform.*
import org.gradle.api.tasks.testing.testng.*
import org.gradle.api.tasks.util.*
import org.gradle.api.tasks.wrapper.*
import org.gradle.authentication.*
import org.gradle.authentication.aws.*
import org.gradle.authentication.http.*
import org.gradle.buildinit.plugins.*
import org.gradle.buildinit.tasks.*
import org.gradle.caching.*
import org.gradle.caching.configuration.*
import org.gradle.caching.http.*
import org.gradle.caching.local.*
import org.gradle.concurrent.*
import org.gradle.external.javadoc.*
import org.gradle.ide.visualstudio.*
import org.gradle.ide.visualstudio.plugins.*
import org.gradle.ide.visualstudio.tasks.*
import org.gradle.ide.xcode.*
import org.gradle.ide.xcode.plugins.*
import org.gradle.ide.xcode.tasks.*
import org.gradle.ivy.*
import org.gradle.jvm.*
import org.gradle.jvm.application.scripts.*
import org.gradle.jvm.application.tasks.*
import org.gradle.jvm.platform.*
import org.gradle.jvm.plugins.*
import org.gradle.jvm.tasks.*
import org.gradle.jvm.tasks.api.*
import org.gradle.jvm.test.*
import org.gradle.jvm.toolchain.*
import org.gradle.language.*
import org.gradle.language.assembler.*
import org.gradle.language.assembler.plugins.*
import org.gradle.language.assembler.tasks.*
import org.gradle.language.base.*
import org.gradle.language.base.artifact.*
import org.gradle.language.base.compile.*
import org.gradle.language.base.plugins.*
import org.gradle.language.base.sources.*
import org.gradle.language.c.*
import org.gradle.language.c.plugins.*
import org.gradle.language.c.tasks.*
import org.gradle.language.coffeescript.*
import org.gradle.language.cpp.*
import org.gradle.language.cpp.plugins.*
import org.gradle.language.cpp.tasks.*
import org.gradle.language.java.*
import org.gradle.language.java.artifact.*
import org.gradle.language.java.plugins.*
import org.gradle.language.java.tasks.*
import org.gradle.language.javascript.*
import org.gradle.language.jvm.*
import org.gradle.language.jvm.plugins.*
import org.gradle.language.jvm.tasks.*
import org.gradle.language.nativeplatform.*
import org.gradle.language.nativeplatform.tasks.*
import org.gradle.language.objectivec.*
import org.gradle.language.objectivec.plugins.*
import org.gradle.language.objectivec.tasks.*
import org.gradle.language.objectivecpp.*
import org.gradle.language.objectivecpp.plugins.*
import org.gradle.language.objectivecpp.tasks.*
import org.gradle.language.plugins.*
import org.gradle.language.rc.*
import org.gradle.language.rc.plugins.*
import org.gradle.language.rc.tasks.*
import org.gradle.language.routes.*
import org.gradle.language.scala.*
import org.gradle.language.scala.plugins.*
import org.gradle.language.scala.tasks.*
import org.gradle.language.scala.toolchain.*
import org.gradle.language.swift.*
import org.gradle.language.swift.plugins.*
import org.gradle.language.swift.tasks.*
import org.gradle.language.twirl.*
import org.gradle.maven.*
import org.gradle.model.*
import org.gradle.nativeplatform.*
import org.gradle.nativeplatform.platform.*
import org.gradle.nativeplatform.plugins.*
import org.gradle.nativeplatform.tasks.*
import org.gradle.nativeplatform.test.*
import org.gradle.nativeplatform.test.cpp.*
import org.gradle.nativeplatform.test.cpp.plugins.*
import org.gradle.nativeplatform.test.cunit.*
import org.gradle.nativeplatform.test.cunit.plugins.*
import org.gradle.nativeplatform.test.cunit.tasks.*
import org.gradle.nativeplatform.test.googletest.*
import org.gradle.nativeplatform.test.googletest.plugins.*
import org.gradle.nativeplatform.test.plugins.*
import org.gradle.nativeplatform.test.tasks.*
import org.gradle.nativeplatform.test.xctest.*
import org.gradle.nativeplatform.test.xctest.plugins.*
import org.gradle.nativeplatform.test.xctest.tasks.*
import org.gradle.nativeplatform.toolchain.*
import org.gradle.nativeplatform.toolchain.plugins.*
import org.gradle.normalization.*
import org.gradle.platform.base.*
import org.gradle.platform.base.binary.*
import org.gradle.platform.base.component.*
import org.gradle.platform.base.plugins.*
import org.gradle.play.*
import org.gradle.play.distribution.*
import org.gradle.play.platform.*
import org.gradle.play.plugins.*
import org.gradle.play.plugins.ide.*
import org.gradle.play.tasks.*
import org.gradle.play.toolchain.*
import org.gradle.plugin.devel.*
import org.gradle.plugin.devel.plugins.*
import org.gradle.plugin.devel.tasks.*
import org.gradle.plugin.management.*
import org.gradle.plugin.use.*
import org.gradle.plugins.ear.*
import org.gradle.plugins.ear.descriptor.*
import org.gradle.plugins.ide.*
import org.gradle.plugins.ide.api.*
import org.gradle.plugins.ide.eclipse.*
import org.gradle.plugins.ide.idea.*
import org.gradle.plugins.javascript.base.*
import org.gradle.plugins.javascript.coffeescript.*
import org.gradle.plugins.javascript.envjs.*
import org.gradle.plugins.javascript.envjs.browser.*
import org.gradle.plugins.javascript.envjs.http.*
import org.gradle.plugins.javascript.envjs.http.simple.*
import org.gradle.plugins.javascript.jshint.*
import org.gradle.plugins.javascript.rhino.*
import org.gradle.plugins.signing.*
import org.gradle.plugins.signing.signatory.*
import org.gradle.plugins.signing.signatory.pgp.*
import org.gradle.plugins.signing.type.*
import org.gradle.plugins.signing.type.pgp.*
import org.gradle.process.*
import org.gradle.swiftpm.*
import org.gradle.swiftpm.plugins.*
import org.gradle.swiftpm.tasks.*
import org.gradle.testing.base.*
import org.gradle.testing.base.plugins.*
import org.gradle.testing.jacoco.plugins.*
import org.gradle.testing.jacoco.tasks.*
import org.gradle.testing.jacoco.tasks.rules.*
import org.gradle.testkit.runner.*
import org.gradle.vcs.*
import org.gradle.vcs.git.*
import org.gradle.workers.*



[3] Any language element except for statement labels.

Authoring Tasks

In the introductory tutorial (Build Script Basics) you learned how to create simple tasks. You also learned how to add additional behavior to these tasks later on, and you learned how to create dependencies between tasks. This was all about simple tasks, but Gradle takes the concept of tasks further. Gradle supports enhanced tasks, which are tasks that have their own properties and methods. This is really different from what you are used to with Ant targets. Such enhanced tasks are either provided by you or built into Gradle.

Task outcomes

When Gradle executes a task, it can label the task with different outcomes in the console UI and via the Tooling API (see Embedding Gradle using the Tooling API). These labels are based on if a task has actions to execute, if it should execute those actions, if it did execute those actions and if those actions made any changes.

Table 3. Details about task outcomes

Outcome label Description of outcome Situations that have this outcome

(no label) or EXECUTED

Task executed its actions.

  • Used whenever a task has actions and Gradle has determined they should be executed as part of a build.

  • Used whenever a task has no actions and some dependencies, and any of the dependencies are executed. See also the section called “Lifecycle tasks”.

UP-TO-DATE

Task’s outputs did not change.

FROM-CACHE

Task’s outputs could be found from a previous execution.

  • Used when a task has outputs restored from the build cache. See Build Cache.

SKIPPED

Task did not execute its actions.

NO-SOURCE

Task did not need to execute its actions.

  • Used when a task has inputs and outputs, but no sources. For example, source files are .java files for JavaCompile.

Defining tasks

We have already seen how to define tasks using a keyword style in Build Script Basics. There are a few variations on this style, which you may need to use in certain situations. For example, the keyword style does not work in expressions.

Example 76. Defining tasks

build.gradle

task(hello) {
    doLast {
        println "hello"
    }
}

task(copy, type: Copy) {
    from(file('srcDir'))
    into(buildDir)
}

You can also use strings for the task names:

Example 77. Defining tasks - using strings for task names

build.gradle

task('hello') {
    doLast {
        println "hello"
    }
}

task('copy', type: Copy) {
    from(file('srcDir'))
    into(buildDir)
}

There is an alternative syntax for defining tasks, which you may prefer to use:

Example 78. Defining tasks with alternative syntax

build.gradle

tasks.create(name: 'hello') {
    doLast {
        println "hello"
    }
}

tasks.create(name: 'copy', type: Copy) {
    from(file('srcDir'))
    into(buildDir)
}

Here we add tasks to the tasks collection. Have a look at TaskContainer for more variations of the create() method.

Locating tasks

You often need to locate the tasks that you have defined in the build file, for example, to configure them or use them for dependencies. There are a number of ways of doing this. Firstly, each task is available as a property of the project, using the task name as the property name:

Example 79. Accessing tasks as properties

build.gradle

task hello

println hello.name
println project.hello.name

Tasks are also available through the tasks collection.

Example 80. Accessing tasks via tasks collection

build.gradle

task hello

println tasks.hello.name
println tasks['hello'].name

You can access tasks from any project using the task’s path using the tasks.getByPath() method. You can call the getByPath() method with a task name, or a relative path, or an absolute path.

Example 81. Accessing tasks by path

build.gradle

project(':projectA') {
    task hello
}

task hello

println tasks.getByPath('hello').path
println tasks.getByPath(':hello').path
println tasks.getByPath('projectA:hello').path
println tasks.getByPath(':projectA:hello').path

Output of gradle -q hello

> gradle -q hello
:hello
:hello
:projectA:hello
:projectA:hello

Have a look at TaskContainer for more options for locating tasks.

Configuring tasks

As an example, let’s look at the Copy task provided by Gradle. To create a Copy task for your build, you can declare in your build script:

Example 82. Creating a copy task

build.gradle

task myCopy(type: Copy)

This creates a copy task with no default behavior. The task can be configured using its API (see Copy). The following examples show several different ways to achieve the same configuration.

Just to be clear, realize that the name of this task is “myCopy”, but it is of typeCopy”. You can have multiple tasks of the same type, but with different names. You’ll find this gives you a lot of power to implement cross-cutting concerns across all tasks of a particular type.

Example 83. Configuring a task - various ways

build.gradle

Copy myCopy = task(myCopy, type: Copy)
myCopy.from 'resources'
myCopy.into 'target'
myCopy.include('**/*.txt', '**/*.xml', '**/*.properties')

This is similar to the way we would configure objects in Java. You have to repeat the context (myCopy) in the configuration statement every time. This is a redundancy and not very nice to read.

There is another way of configuring a task. It also preserves the context and it is arguably the most readable. It is usually our favorite.

Example 84. Configuring a task - with closure

build.gradle

task myCopy(type: Copy)

myCopy {
   from 'resources'
   into 'target'
   include('**/*.txt', '**/*.xml', '**/*.properties')
}

This works for any task. Line 3 of the example is just a shortcut for the tasks.getByName() method. It is important to note that if you pass a closure to the getByName() method, this closure is applied to configure the task, not when the task executes.

You can also use a configuration closure when you define a task.

Example 85. Defining a task with closure

build.gradle

task copy(type: Copy) {
   from 'resources'
   into 'target'
   include('**/*.txt', '**/*.xml', '**/*.properties')
}

Don’t forget about the build phases

A task has both configuration and actions. When using the doLast, you are simply using a shortcut to define an action. Code defined in the configuration section of your task will get executed during the configuration phase of the build regardless of what task was targeted. See Build Lifecycle for more details about the build lifecycle.

Adding dependencies to a task

There are several ways you can define the dependencies of a task. In the section called “Task dependencies” you were introduced to defining dependencies using task names. Task names can refer to tasks in the same project as the task, or to tasks in other projects. To refer to a task in another project, you prefix the name of the task with the path of the project it belongs to. The following is an example which adds a dependency from projectA:taskX to projectB:taskY:

Example 86. Adding dependency on task from another project

build.gradle

project('projectA') {
    task taskX(dependsOn: ':projectB:taskY') {
        doLast {
            println 'taskX'
        }
    }
}

project('projectB') {
    task taskY {
        doLast {
            println 'taskY'
        }
    }
}

Output of gradle -q taskX

> gradle -q taskX
taskY
taskX

Instead of using a task name, you can define a dependency using a Task object, as shown in this example:

Example 87. Adding dependency using task object

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}

task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.dependsOn taskY

Output of gradle -q taskX

> gradle -q taskX
taskY
taskX

For more advanced uses, you can define a task dependency using a closure. When evaluated, the closure is passed the task whose dependencies are being calculated. The closure should return a single Task or collection of Task objects, which are then treated as dependencies of the task. The following example adds a dependency from taskX to all the tasks in the project whose name starts with lib:

Example 88. Adding dependency using closure

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}

taskX.dependsOn {
    tasks.findAll { task -> task.name.startsWith('lib') }
}

task lib1 {
    doLast {
        println 'lib1'
    }
}

task lib2 {
    doLast {
        println 'lib2'
    }
}

task notALib {
    doLast {
        println 'notALib'
    }
}

Output of gradle -q taskX

> gradle -q taskX
lib1
lib2
taskX

For more information about task dependencies, see the Task API.

Ordering tasks

Task ordering is an incubating feature. Please be aware that this feature may change in later Gradle versions.

In some cases it is useful to control the order in which 2 tasks will execute, without introducing an explicit dependency between those tasks. The primary difference between a task ordering and a task dependency is that an ordering rule does not influence which tasks will be executed, only the order in which they will be executed.

Task ordering can be useful in a number of scenarios:

  • Enforce sequential ordering of tasks: e.g. 'build' never runs before 'clean'.

  • Run build validations early in the build: e.g. validate I have the correct credentials before starting the work for a release build.

  • Get feedback faster by running quick verification tasks before long verification tasks: e.g. unit tests should run before integration tests.

  • A task that aggregates the results of all tasks of a particular type: e.g. test report task combines the outputs of all executed test tasks.

There are two ordering rules available: “must run after” and “should run after”.

When you use the “must run after” ordering rule you specify that taskB must always run after taskA, whenever both taskA and taskB will be run. This is expressed as taskB.mustRunAfter(taskA). The “should run after” ordering rule is similar but less strict as it will be ignored in two situations. Firstly if using that rule introduces an ordering cycle. Secondly when using parallel execution and all dependencies of a task have been satisfied apart from the “should run after” task, then this task will be run regardless of whether its “should run after” dependencies have been run or not. You should use “should run after” where the ordering is helpful but not strictly required.

With these rules present it is still possible to execute taskA without taskB and vice-versa.

Example 89. Adding a 'must run after' task ordering

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
taskY.mustRunAfter taskX

Output of gradle -q taskY taskX

> gradle -q taskY taskX
taskX
taskY

Example 90. Adding a 'should run after' task ordering

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
taskY.shouldRunAfter taskX

Output of gradle -q taskY taskX

> gradle -q taskY taskX
taskX
taskY

In the examples above, it is still possible to execute taskY without causing taskX to run:

Example 91. Task ordering does not imply task execution

Output of gradle -q taskY

> gradle -q taskY
taskY

To specify a “must run after” or “should run after” ordering between 2 tasks, you use the Task.mustRunAfter(java.lang.Object[]) and Task.shouldRunAfter(java.lang.Object[]) methods. These methods accept a task instance, a task name or any other input accepted by Task.dependsOn(java.lang.Object[]).

Note that “B.mustRunAfter(A)” or “B.shouldRunAfter(A)” does not imply any execution dependency between the tasks:

  • It is possible to execute tasks A and B independently. The ordering rule only has an effect when both tasks are scheduled for execution.

  • When run with --continue, it is possible for B to execute in the event that A fails.

As mentioned before, the “should run after” ordering rule will be ignored if it introduces an ordering cycle:

Example 92. A 'should run after' task ordering is ignored if it introduces an ordering cycle

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
task taskZ {
    doLast {
        println 'taskZ'
    }
}
taskX.dependsOn taskY
taskY.dependsOn taskZ
taskZ.shouldRunAfter taskX

Output of gradle -q taskX

> gradle -q taskX
taskZ
taskY
taskX

Adding a description to a task

You can add a description to your task. This description is displayed when executing gradle tasks.

Example 93. Adding a description to a task

build.gradle

task copy(type: Copy) {
   description 'Copies the resource directory to the target directory.'
   from 'resources'
   into 'target'
   include('**/*.txt', '**/*.xml', '**/*.properties')
}

Replacing tasks

Sometimes you want to replace a task. For example, if you want to exchange a task added by the Java plugin with a custom task of a different type. You can achieve this with:

Example 94. Overwriting a task

build.gradle

task copy(type: Copy)

task copy(overwrite: true) {
    doLast {
        println('I am the new one.')
    }
}

Output of gradle -q copy

> gradle -q copy
I am the new one.

This will replace a task of type Copy with the task you’ve defined, because it uses the same name. When you define the new task, you have to set the overwrite property to true. Otherwise Gradle throws an exception, saying that a task with that name already exists.

Skipping tasks

Gradle offers multiple ways to skip the execution of a task.

Using a predicate

You can use the onlyIf() method to attach a predicate to a task. The task’s actions are only executed if the predicate evaluates to true. You implement the predicate as a closure. The closure is passed the task as a parameter, and should return true if the task should execute and false if the task should be skipped. The predicate is evaluated just before the task is due to be executed.

Example 95. Skipping a task using a predicate

build.gradle

task hello {
    doLast {
        println 'hello world'
    }
}

hello.onlyIf { !project.hasProperty('skipHello') }

Output of gradle hello -PskipHello

> gradle hello -PskipHello
> Task :hello SKIPPED

BUILD SUCCESSFUL in 0s

Using StopExecutionException

If the logic for skipping a task can’t be expressed with a predicate, you can use the StopExecutionException. If this exception is thrown by an action, the further execution of this action as well as the execution of any following action of this task is skipped. The build continues with executing the next task.

Example 96. Skipping tasks with StopExecutionException

build.gradle

task compile {
    doLast {
        println 'We are doing the compile.'
    }
}

compile.doFirst {
    // Here you would put arbitrary conditions in real life.
    // But this is used in an integration test so we want defined behavior.
    if (true) { throw new StopExecutionException() }
}
task myTask(dependsOn: 'compile') {
    doLast {
        println 'I am not affected'
    }
}

Output of gradle -q myTask

> gradle -q myTask
I am not affected

This feature is helpful if you work with tasks provided by Gradle. It allows you to add conditional execution of the built-in actions of such a task.[4]

Enabling and disabling tasks

Every task has an enabled flag which defaults to true. Setting it to false prevents the execution of any of the task’s actions. A disabled task will be labelled SKIPPED.

Example 97. Enabling and disabling tasks

build.gradle

task disableMe {
    doLast {
        println 'This should not be printed if the task is disabled.'
    }
}
disableMe.enabled = false

Output of gradle disableMe

> gradle disableMe
> Task :disableMe SKIPPED

BUILD SUCCESSFUL in 0s

Up-to-date checks (AKA Incremental Build)

An important part of any build tool is the ability to avoid doing work that has already been done. Consider the process of compilation. Once your source files have been compiled, there should be no need to recompile them unless something has changed that affects the output, such as the modification of a source file or the removal of an output file. And compilation can take a significant amount of time, so skipping the step when it’s not needed saves a lot of time.

Gradle supports this behavior out of the box through a feature it calls incremental build. You have almost certainly already seen it in action: it’s active nearly every time the UP-TO-DATE text appears next to the name of a task when you run a build. Task outcomes are described in the section called “Task outcomes”.

How does incremental build work? And what does it take to make use of it in your own tasks? Let’s take a look.

Task inputs and outputs

In the most common case, a task takes some inputs and generates some outputs. If we use the compilation example from earlier, we can see that the source files are the inputs and, in the case of Java, the generated class files are the outputs. Other inputs might include things like whether debug information should be included.

Figure 9. Example task inputs and outputs

Example task inputs and outputs

An important characteristic of an input is that it affects one or more outputs, as you can see from the previous figure. Different bytecode is generated depending on the content of the source files and the minimum version of the Java runtime you want to run the code on. That makes them task inputs. But whether compilation has 500MB or 600MB of maximum memory available, determined by the memoryMaximumSize property, has no impact on what bytecode gets generated. In Gradle terminology, memoryMaximumSize is just an internal task property.

As part of incremental build, Gradle tests whether any of the task inputs or outputs have changed since the last build. If they haven’t, Gradle can consider the task up to date and therefore skip executing its actions. Also note that incremental build won’t work unless a task has at least one task output, although tasks usually have at least one input as well.

What this means for build authors is simple: you need to tell Gradle which task properties are inputs and which are outputs. If a task property affects the output, be sure to register it as an input, otherwise the task will be considered up to date when it’s not. Conversely, don’t register properties as inputs if they don’t affect the output, otherwise the task will potentially execute when it doesn’t need to. Also be careful of non-deterministic tasks that may generate different output for exactly the same inputs: these should not be configured for incremental build as the up-to-date checks won’t work.

Let’s now look at how you can register task properties as inputs and outputs.

Custom task types

If you’re implementing a custom task as a class, then it takes just two steps to make it work with incremental build:

  1. Create typed properties (via getter methods) for each of your task inputs and outputs

  2. Add the appropriate annotation to each of those properties

Annotations must be placed on getters or on Groovy properties. Annotations placed on setters, or on a Java field without a corresponding annotated getter are ignored.

Gradle supports three main categories of inputs and outputs:

  • Simple values

    Things like strings and numbers. More generally, a simple value can have any type that implements Serializable.

  • Filesystem types

    These consist of the standard File class but also derivatives of Gradle’s FileCollection type and anything else that can be passed to either the Project.file(java.lang.Object) method - for single file/directory properties - or the Project.files(java.lang.Object[]) method.

  • Nested values

    Custom types that don’t conform to the other two categories but have their own properties that are inputs or outputs. In effect, the task inputs or outputs are nested inside these custom types.

As an example, imagine you have a task that processes templates of varying types, such as FreeMarker, Velocity, Moustache, etc. It takes template source files and combines them with some model data to generate populated versions of the template files.

This task will have three inputs and one output:

  • Template source files

  • Model data

  • Template engine

  • Where the output files are written

When you’re writing a custom task class, it’s easy to register properties as inputs or outputs via annotations. To demonstrate, here is a skeleton task implementation with some suitable inputs and outputs, along with their annotations:

Example 98. Custom task class

buildSrc/src/main/java/org/example/ProcessTemplates.java

package org.example;

import java.io.File;
import java.util.HashMap;
import org.gradle.api.*;
import org.gradle.api.file.*;
import org.gradle.api.tasks.*;

public class ProcessTemplates extends DefaultTask {
    private TemplateEngineType templateEngine;
    private FileCollection sourceFiles;
    private TemplateData templateData;
    private File outputDir;

    @Input
    public TemplateEngineType getTemplateEngine() {
        return this.templateEngine;
    }

    @InputFiles
    public FileCollection getSourceFiles() {
        return this.sourceFiles;
    }

    @Nested
    public TemplateData getTemplateData() {
        return this.templateData;
    }

    @OutputDirectory
    public File getOutputDir() { return this.outputDir; }

    // + setter methods for the above - assume we’ve defined them

    @TaskAction
    public void processTemplates() {
        // ...
    }
}

buildSrc/src/main/java/org/example/TemplateData.java

package org.example;

import java.util.HashMap;
import java.util.Map;
import org.gradle.api.tasks.Input;

public class TemplateData {
    private String name;
    private Map<String, String> variables;

    public TemplateData(String name, Map<String, String> variables) {
        this.name = name;
        this.variables = new HashMap<>(variables);
    }

    @Input
    public String getName() { return this.name; }

    @Input
    public Map<String, String> getVariables() {
        return this.variables;
    }
}

Output of gradle processTemplates

> gradle processTemplates
> Task :processTemplates


BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

Output of gradle processTemplates

> gradle processTemplates
> Task :processTemplates UP-TO-DATE

BUILD SUCCESSFUL in 0s
1 actionable task: 1 up-to-date

There’s plenty to talk about in this example, so let’s work through each of the input and output properties in turn:

  • templateEngine

    Represents which engine to use when processing the source templates, e.g. FreeMarker, Velocity, etc. You could implement this as a string, but in this case we have gone for a custom enum as it provides greater type information and safety. Since enums implement Serializable automatically, we can treat this as a simple value and use the @Input annotation, just as we would with a String property.

  • sourceFiles

    The source templates that the task will be processing. Single files and collections of files need their own special annotations. In this case, we’re dealing with a collection of input files and so we use the @InputFiles annotation. You’ll see more file-oriented annotations in a table later.

  • templateData

    For this example, we’re using a custom class to represent the model data. However, it does not implement Serializable, so we can’t use the @Input annotation. That’s not a problem as the properties within TemplateData - a string and a hash map with serializable type parameters - are serializable and can be annotated with @Input. We use @Nested on templateData to let Gradle know that this is a value with nested input properties.

  • outputDir

    The directory where the generated files go. As with input files, there are several annotations for output files and directories. A property representing a single directory requires @OutputDirectory. You’ll learn about the others soon.

These annotated properties mean that Gradle will skip the task if none of the source files, template engine, model data or generated files have changed since the previous time Gradle executed the task. This will often save a significant amount of time. You can learn how Gradle detects changes later.

This example is particularly interesting because it works with collections of source files. What happens if only one source file changes? Does the task process all the source files again or just the modified one? That depends on the task implementation. If the latter, then the task itself is incremental, but that’s a different feature to the one we’re discussing here. Gradle does help task implementers with this via its incremental task inputs feature.

Now that you have seen some of the input and output annotations in practice, let’s take a look at all the annotations available to you and when you should use them. The table below lists the available annotations and the corresponding property type you can use with each one.

Table 4. Incremental build property type annotations

Annotation Expected property type Description

@Input

Any serializable type

A simple input value

@InputFile

File*

A single input file (not directory)

@InputDirectory

File*

A single input directory (not file)

@InputFiles

Iterable<File>*

An iterable of input files and directories

@Classpath

Iterable<File>*

An iterable of input files and directories that represent a Java classpath. This allows the task to ignore irrelevant changes to the property, such as different names for the same files. It is similar to annotating the property @PathSensitive(RELATIVE) but it will ignore the names of JAR files directly added to the classpath, and it will consider changes in the order of the files as a change in the classpath. Gradle will inspect the contents of jar files on the classpath and ignore changes that do not affect the semantics of the classpath (such as file dates and entry order). See also the section called “Using the classpath annotations”.

The @Classpath annotation was introduced in Gradle 3.2. To stay compatible with earlier Gradle versions, classpath properties should also be annotated with @InputFiles.

@CompileClasspath

Iterable<File>*

An iterable of input files and directories that represent a Java compile classpath. This allows the task to ignore irrelevant changes that do not affect the API of the classes in classpath. See also the section called “Using the classpath annotations”.

The following kinds of changes to the classpath will be ignored:

  • Changes to the path of jar or top level directories.

  • Changes to timestamps and the order of entries in Jars.

  • Changes to resources and Jar manifests, including adding or removing resources.

  • Changes to private class elements, such as private fields, methods and inner classes.

  • Changes to code, such as method bodies, static initializers and field initializers (except for constants).

  • Changes to debug information, for example when a change to a comment affects the line numbers in class debug information.

  • Changes to directories, including directory entries in Jars.

The @CompileClasspath annotation was introduced in Gradle 3.4. To stay compatible with Gradle 3.3 and 3.2, compile classpath properties should also be annotated with @Classpath. For compatibility with Gradle versions before 3.2 the property should also be annotated with @InputFiles.

@OutputFile

File*

A single output file (not directory)

@OutputDirectory

File*

A single output directory (not file)

@OutputFiles

Map<String, File>** or Iterable<File>*

An iterable of output files (no directories). The task outputs can only be cached if a Map is provided.

@OutputDirectories

Map<String, File>** or Iterable<File>*

An iterable of output directories (no files). The task outputs can only be cached if a Map is provided.

@Destroys

File or Iterable<File>*

Specifies one or more files that are removed by this task. Note that a task can define either inputs/outputs or destroyables, but not both.

@LocalState

File or Iterable<File>*

Specifies one or more files that represent the local state of the task. These files are removed when the task is loaded from cache.

@Nested

Any custom type

A custom type that may not implement Serializable but does have at least one field or property marked with one of the annotations in this table. It could even be another @Nested.

@Console

Any type

Indicates that the property is neither an input nor an output. It simply affects the console output of the task in some way, such as increasing or decreasing the verbosity of the task.

@Internal

Any type

Indicates that the property is used internally but is neither an input nor an output.

*

In fact, File can be any type accepted by Project.file(java.lang.Object) and Iterable<File> can be any type accepted by Project.files(java.lang.Object[]). This includes instances of Callable, such as closures, allowing for lazy evaluation of the property values. Be aware that the types FileCollection and FileTree are Iterable<File>s.

**

Similar to the above, File can be any type accepted by Project.file(java.lang.Object). The Map itself can be wrapped in Callables, such as closures.

Table 5. Additional annotations used to further qualifying property type annotations

Annotation Description

@SkipWhenEmpty

Used with @InputFiles or @InputDirectory to tell Gradle to skip the task if the corresponding files or directory are empty, along with all other input files declared with this annotation. Tasks that have been skipped due to all of their input files that were declared with this annotation being empty will result in a distinct “no source” outcome. For example, NO-SOURCE will be emitted in the console output.

@Optional

Used with any of the property type annotations listed in the Optional API documentation. This annotation disables validation checks on the corresponding property. See the section on validation for more details.

@PathSensitive

Used with any input file property to tell Gradle to only consider the given part of the file paths as important. For example, if a property is annotated with @PathSensitive(PathSensitivity.NAME_ONLY), then moving the files around without changing their contents will not make the task out-of-date.

Annotations are inherited from all parent types including implemented interfaces. Property type annotations override any other property type annotation declared in a parent type. This way an @InputFile property can be turned into an @InputDirectory property in a child task type.

Annotations on a property declared in a type override similar annotations declared by the superclass and in any implemented interfaces. Superclass annotations take precedence over annotations declared in implemented interfaces.

The Console and Internal annotations in the table are special cases as they don’t declare either task inputs or task outputs. So why use them? It’s so that you can take advantage of the Java Gradle Plugin Development plugin to help you develop and publish your own plugins. This plugin checks whether any properties of your custom task classes lack an incremental build annotation. This protects you from forgetting to add an appropriate annotation during development.

Using the classpath annotations

Besides @InputFiles, for JVM-related tasks Gradle understands the concept of classpath inputs. Both runtime and compile classpaths are treated differently when Gradle is looking for changes.

As opposed to input properties annotated with @InputFiles, for classpath properties the order of the entries in the file collection matter. On the other hand, the names and paths of the directories and jar files on the classpath itself are ignored. Timestamps and the order of class files and resources inside jar files on a classpath are ignored, too, thus recreating a jar file with different file dates will not make the task out of date.

Runtime classpaths are marked with @Classpath, and they offer further customization via classpath normalization.

Input properties annotated with @CompileClasspath are considered Java compile classpaths. Additionally to the aforementioned general classpath rules, compile classpaths ignore changes to everything but class files. Gradle uses the same class analysis described in the section called “Compile avoidance” to further filter changes that don’t affect the class' ABIs. This means that changes which only touch the implementation of classes do not make the task out of date.

Nested inputs

When analyzing @Nested task properties for declared input and output sub-properties Gradle uses the type of the actual value. Hence it can discover all sub-properties declared by a runtime sub-type.

When adding @Nested to a @Provider, the value of the Provider is treated as a nested input.

When adding @Nested to an iterable, each element is treated as a separate nested input. Each nested input in the iterable is assigned a name, which by default is the dollar sign followed by the index in the iterable, e.g. $2. If an element of the iterable implements Named, then the name is used as property name. The ordering of the elements in the iterable is crucial for for reliable up-to-date checks and caching if not all of the elements implement Named. Multiple elements which have the same name are not allowed.

When adding @Nested to a map, then for each value a nested input is added, using the key as name.

The type and classpath of nested inputs is tracked, too. This ensures that changes to the implementation of a nested input causes the build to be out of date. By this it is also possible to add user provided code as an input, e.g. by annotating an @Action property with @Nested. Note that any inputs to such actions should be tracked, either by annotated properties on the action or by manually registering them with the task.

Using nested inputs allows richer modeling and extensibility for tasks, as e.g. shown by Test.getJvmArgumentProviders().

This allows us to model the JaCoCo Java agent, thus declaring the necessary JVM arguments and providing the inputs and outputs to Gradle:

class JacocoAgent implements CommandLineArgumentProvider {
    private final JacocoTaskExtension jacoco;

    public JacocoAgent(JacocoTaskExtension jacoco) {
        this.jacoco = jacoco;
    }

    @Nested
    @Optional
    public JacocoTaskExtension getJacoco() {
        return jacoco.isEnabled() ? jacoco : null;
    }

    @Override
    public Iterable<String> asArguments() {
        return jacoco.isEnabled() ? ImmutableList.of(jacoco.getAsJvmArg()) : Collections.<String>emptyList();
    }
}

test.getJvmArgumentProviders().add(new JacocoAgent(extension))

For this to work, JacocoTaskExtension needs to have the correct input and output annotations.

The approach works for Test JVM arguments, since Test.getJvmArgumentProviders() is an Iterable annotated with @Nested.

There are other task types where this kind of nested inputs are available:

In the same way, this kind of modelling is available to custom tasks.

Runtime API

Custom task classes are an easy way to bring your own build logic into the arena of incremental build, but you don’t always have that option. That’s why Gradle also provides an alternative API that can be used with any tasks, which we look at next.

When you don’t have access to the source for a custom task class, there is no way to add any of the annotations we covered in the previous section. Fortunately, Gradle provides a runtime API for scenarios just like that. It can also be used for ad-hoc tasks, as you’ll see next.

Using it for ad-hoc tasks

This runtime API is provided through a couple of aptly named properties that are available on every Gradle task:

These objects have methods that allow you to specify files, directories and values which constitute the task’s inputs and outputs. In fact, the runtime API has almost feature parity with the annotations. All it lacks is validation of whether declared files are actually files and declared directories are directories. Nor will it create output directories if they don’t exist. But that’s it.

Let’s take the template processing example from before and see how it would look as an ad-hoc task that uses the runtime API:

Example 99. Ad-hoc task

build.gradle

task processTemplatesAdHoc {
    inputs.property("engine", TemplateEngineType.FREEMARKER)
    inputs.files(fileTree("src/templates"))
    inputs.property("templateData.name", "docs")
    inputs.property("templateData.variables", [year: 2013])
    outputs.dir("$buildDir/genOutput2")

    doLast {
        // Process the templates here
    }
}

Output of gradle processTemplatesAdHoc

> gradle processTemplatesAdHoc

> Task :processTemplatesAdHoc


BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

As before, there’s much to talk about. To begin with, you should really write a custom task class for this as it’s a non-trivial implementation that has several configuration options. In this case, there are no task properties to store the root source folder, the location of the output directory or any of the other settings. That’s deliberate to highlight the fact that the runtime API doesn’t require the task to have any state. In terms of incremental build, the above ad-hoc task will behave the same as the custom task class.

All the input and output definitions are done through the methods on inputs and outputs, such as property(), files(), and dir(). Gradle performs up-to-date checks on the argument values to determine whether the task needs to run again or not. Each method corresponds to one of the incremental build annotations, for example inputs.property() maps to @Input and outputs.dir() maps to @OutputDirectory. The only difference is that the file(), files(), dir() and dirs() methods don’t validate the type of file object at the given path (file or directory), unlike the annotations.

The files that a task removes can be specified through destroyables.register().

Example 100. Ad-hoc task declaring a destroyable

build.gradle

task removeTempDir {
    destroyables.register("$projectDir/tmpDir")
    doLast {
        delete("$projectDir/tmpDir")
    }
}

One notable difference between the runtime API and the annotations is the lack of a method that corresponds directly to @Nested. That’s why the example uses two property() declarations for the template data, one for each TemplateData property. You should utilize the same technique when using the runtime API with nested values. Any given task can either declare destroyables or inputs/outputs, but cannot declare both.

Using it for custom task types

Another type of example involves adding input and output definitions to instances of a custom task class that lacks the requisite annotations. For example, imagine that the ProcessTemplates task is provided by a plugin and that it’s missing the incremental build annotations. In order to make up for that deficiency, you can use the runtime API:

Example 101. Using runtime API with custom task type

build.gradle

task processTemplatesRuntime(type: ProcessTemplatesNoAnnotations) {
    templateEngine = TemplateEngineType.FREEMARKER
    sourceFiles = fileTree("src/templates")
    templateData = new TemplateData("test", [year: 2014])
    outputDir = file("$buildDir/genOutput3")

    inputs.property("engine",templateEngine)
    inputs.files(sourceFiles)
    inputs.property("templateData.name", templateData.name)
    inputs.property("templateData.variables", templateData.variables)
    outputs.dir(outputDir)
}

Output of gradle processTemplatesRuntime

> gradle processTemplatesRuntime
> Task :processTemplatesRuntime


BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

Output of gradle processTemplatesRuntime

> gradle processTemplatesRuntime
> Task :processTemplatesRuntime UP-TO-DATE


BUILD SUCCESSFUL in 0s
1 actionable task: 1 up-to-date

As you can see, we can both configure the tasks properties and use those properties as arguments to the incremental build runtime API. Using the runtime API like this is a little like using doLast() and doFirst() to attach extra actions to a task, except in this case we’re attaching information about inputs and outputs. Note that if the task type is already using the incremental build annotations, the runtime API will add inputs and outputs rather than replace them.

Fine-grained configuration

The runtime API methods only allow you to declare your inputs and outputs in themselves. However, the file-oriented ones return a builder - of type TaskInputFilePropertyBuilder - that lets you provide additional information about those inputs and outputs.

You can learn about all the options provided by the builder in its API documentation, but we’ll show you a simple example here to give you an idea of what you can do.

Let’s say we don’t want to run the processTemplates task if there are no source files, regardless of whether it’s a clean build or not. After all, if there are no source files, there’s nothing for the task to do. The builder allows us to configure this like so:

Example 102. Using skipWhenEmpty() via the runtime API

build.gradle

task processTemplatesRuntimeConf(type: ProcessTemplatesNoAnnotations) {
    // ...
    sourceFiles = fileTree("src/templates") {
        include "**/*.fm"
    }

    inputs.files(sourceFiles).skipWhenEmpty()
    // ...
}

Output of gradle clean processTemplatesRuntimeConf

> gradle clean processTemplatesRuntimeConf
> Task :processTemplatesRuntimeConf NO-SOURCE


BUILD SUCCESSFUL in 0s
1 actionable task: 1 up-to-date

The TaskInputs.files() method returns a builder that has a skipWhenEmpty() method. Invoking this method is equivalent to annotating to the property with @SkipWhenEmpty.

Prior to Gradle 3.0, you had to use the TaskInputs.source() and TaskInputs.sourceDir() methods to get the same behavior as with skipWhenEmpty(). These methods are now deprecated and should not be used with Gradle 3.0 and above.

Now that you have seen both the annotations and the runtime API, you may be wondering which API you should be using. Our recommendation is to use the annotations wherever possible, and it’s sometimes worth creating a custom task class just so that you can make use of them. The runtime API is more for situations in which you can’t use the annotations.

Important beneficial side effects

Once you declare a task’s formal inputs and outputs, Gradle can then infer things about those properties. For example, if an input of one task is set to the output of another, that means the first task depends on the second, right? Gradle knows this and can act upon it.

We’ll look at this feature next and also some other features that come from Gradle knowing things about inputs and outputs.

Inferred task dependencies

Consider an archive task that packages the output of the processTemplates task. A build author will see that the archive task obviously requires processTemplates to run first and so may add an explicit dependsOn. However, if you define the archive task like so:

Example 103. Inferred task dependency via task outputs

build.gradle

task packageFiles(type: Zip) {
    from processTemplates.outputs
}

Output of gradle clean packageFiles

> gradle clean packageFiles
> Task :processTemplates
> Task :packageFiles


BUILD SUCCESSFUL in 0s
3 actionable tasks: 2 executed, 1 up-to-date

Gradle will automatically make packageFiles depend on processTemplates. It can do this because it’s aware that one of the inputs of packageFiles requires the output of the processTemplates task. We call this an inferred task dependency.

The above example can also be written as

Example 104. Inferred task dependency via a task argument

build.gradle

task packageFiles2(type: Zip) {
    from processTemplates
}

Output of gradle clean packageFiles2

> gradle clean packageFiles2
> Task :processTemplates
> Task :packageFiles2


BUILD SUCCESSFUL in 0s
3 actionable tasks: 2 executed, 1 up-to-date

This is because the from() method can accept a task object as an argument. Behind the scenes, from() uses the project.files() method to wrap the argument, which in turn exposes the task’s formal outputs as a file collection. In other words, it’s a special case!

Input and output validation

The incremental build annotations provide enough information for Gradle to perform some basic validation on the annotated properties. In particular, it does the following for each property before the task executes:

  • @InputFile - verifies that the property has a value and that the path corresponds to a file (not a directory) that exists.

  • @InputDirectory - same as for @InputFile, except the path must correspond to a directory.

  • @OutputDirectory - verifies that the path doesn’t match a file and also creates the directory if it doesn’t already exist.

Such validation improves the robustness of the build, allowing you to identify issues related to inputs and outputs quickly.

You will occasionally want to disable some of this validation, specifically when an input file may validly not exist. That’s why Gradle provides the @Optional annotation: you use it to tell Gradle that a particular input is optional and therefore the build should not fail if the corresponding file or directory doesn’t exist.

Continuous build

Another benefit of defining task inputs and outputs is continuous build. Since Gradle knows what files a task depends on, it can automatically run a task again if any of its inputs change. By activating continuous build when you run Gradle - through the --continuous or -t options - you will put Gradle into a state in which it continually checks for changes and executes the requested tasks when it encounters such changes.

You can find out more about this feature in the section called “Continuous Build”.

Task parallelism

One last benefit of defining task inputs and outputs is that Gradle can use this information to make decisions about how to run tasks when the "--parallel" option is used. For instance, Gradle will inspect the outputs of tasks when selecting the next task to run and will avoid concurrent execution of tasks that write to the same output directory. Similarly, Gradle will use the information about what files a task destroys (e.g. specified by the Destroys annotation) and avoid running a task that removes a set of files while another task is running that consumes or creates those same files (and vice versa). It can also determine that a task that creates a set of files has already run and that a task that consumes those files has yet to run and will avoid running a task that removes those files in between. By providing task input and output information in this way, Gradle can infer creation/consumption/destruction relationships between tasks and can ensure that task execution does not violate those relationships.

How does it work?

Before a task is executed for the first time, Gradle takes a snapshot of the inputs. This snapshot contains the paths of input files and a hash of the contents of each file. Gradle then executes the task. If the task completes successfully, Gradle takes a snapshot of the outputs. This snapshot contains the set of output files and a hash of the contents of each file. Gradle persists both snapshots for the next time the task is executed.

Each time after that, before the task is executed, Gradle takes a new snapshot of the inputs and outputs. If the new snapshots are the same as the previous snapshots, Gradle assumes that the outputs are up to date and skips the task. If they are not the same, Gradle executes the task. Gradle persists both snapshots for the next time the task is executed.

Gradle also considers the code of the task as part of the inputs to the task. When a task, its actions, or its dependencies change between executions, Gradle considers the task as out-of-date.

Gradle understands if a file property (e.g. one holding a Java classpath) is order-sensitive. When comparing the snapshot of such a property, even a change in the order of the files will result in the task becoming out-of-date.

Note that if a task has an output directory specified, any files added to that directory since the last time it was executed are ignored and will NOT cause the task to be out of date. This is so unrelated tasks may share an output directory without interfering with each other. If this is not the behaviour you want for some reason, consider using TaskOutputs.upToDateWhen(groovy.lang.Closure)

The inputs for the task are also used to calculate the build cache key used to load task outputs when enabled. For more details see the section called “Task Output Caching”.

Advanced techniques

Everything you’ve seen so far in this section will cover most of the use cases you’ll encounter, but there are some scenarios that need special treatment. We’ll present a few of those next with the appropriate solutions.

Adding your own cached input/output methods

Have you ever wondered how the from() method of the Copy task works? It’s not annotated with @InputFiles and yet any files passed to it are treated as formal inputs of the task. What’s happening?

The implementation is quite simple and you can use the same technique for your own tasks to improve their APIs. Write your methods so that they add files directly to the appropriate annotated property. As an example, here’s how to add a sources() method to the custom ProcessTemplates class we introduced earlier:

Example 105. Declaring a method to add task inputs

buildSrc/src/main/java/org/example/ProcessTemplates.java

public class ProcessTemplates extends DefaultTask {
    // ...
    private FileCollection sourceFiles = getProject().files();

    @SkipWhenEmpty
    @InputFiles
    @PathSensitive(PathSensitivity.NONE)
    public FileCollection getSourceFiles() {
        return this.sourceFiles;
    }

    public void sources(FileCollection sourceFiles) {
        this.sourceFiles = this.sourceFiles.plus(sourceFiles);
    }

    // ...
}

build.gradle

task processTemplates(type: ProcessTemplates) {
    templateEngine = TemplateEngineType.FREEMARKER
    templateData = new TemplateData("test", [year: 2012])
    outputDir = file("$buildDir/genOutput")

    sources fileTree("src/templates")
}

Output of gradle processTemplates

> gradle processTemplates
> Task :processTemplates


BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

In other words, as long as you add values and files to formal task inputs and outputs during the configuration phase, they will be treated as such regardless from where in the build you add them.

If we want to support tasks as arguments as well and treat their outputs as the inputs, we can use the project.files() method like so:

Example 106. Declaring a method to add a task as an input

buildSrc/src/main/java/org/example/ProcessTemplates.java

// ...
public void sources(Task inputTask) {
    this.sourceFiles = this.sourceFiles.plus(getProject().files(inputTask));
}
// ...

build.gradle

task copyTemplates(type: Copy) {
    into "$buildDir/tmp"
    from "src/templates"
}

task processTemplates2(type: ProcessTemplates) {
    // ...
    sources copyTemplates
}

Output of gradle processTemplates2

> gradle processTemplates2
> Task :copyTemplates
> Task :processTemplates2


BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

This technique can make your custom task easier to use and result in cleaner build files. As an added benefit, our use of getProject().files() means that our custom method can set up an inferred task dependency.

One last thing to note: if you are developing a task that takes collections of source files as inputs, like this example, consider using the built-in SourceTask. It will save you having to implement some of the plumbing that we put into ProcessTemplates.

Linking an @OutputDirectory to an @InputFiles

When you want to link the output of one task to the input of another, the types often match and a simple property assignment will provide that link. For example, a File output property can be assigned to a File input.

Unfortunately, this approach breaks down when you want the files in a task’s @OutputDirectory (of type File) to become the source for another task’s @InputFiles property (of type FileCollection). Since the two have different types, property assignment won’t work.

As an example, imagine you want to use the output of a Java compilation task - via the destinationDir property - as the input of a custom task that instruments a set of files containing Java bytecode. This custom task, which we’ll call Instrument, has a classFiles property annotated with @InputFiles. You might initially try to configure the task like so:

Example 107. Failed attempt at setting up an inferred task dependency

build.gradle

apply plugin: "java"

task badInstrumentClasses(type: Instrument) {
    classFiles = fileTree(compileJava.destinationDir)
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean badInstrumentClasses

> gradle clean badInstrumentClasses
> Task :clean UP-TO-DATE
> Task :badInstrumentClasses NO-SOURCE


BUILD SUCCESSFUL in 0s
1 actionable task: 1 up-to-date

There’s nothing obviously wrong with this code, but you can see from the console output that the compilation task is missing. In this case you would need to add an explicit task dependency between instrumentClasses and compileJava via dependsOn. The use of fileTree() means that Gradle can’t infer the task dependency itself.

One solution is to use the TaskOutputs.files property, as demonstrated by the following example:

Example 108. Setting up an inferred task dependency between output dir and input files

build.gradle

task instrumentClasses(type: Instrument) {
    classFiles = compileJava.outputs.files
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean instrumentClasses

> gradle clean instrumentClasses
> Task :clean UP-TO-DATE
> Task :compileJava
> Task :instrumentClasses


BUILD SUCCESSFUL in 0s
3 actionable tasks: 2 executed, 1 up-to-date

Alternatively, you can get Gradle to access the appropriate property itself by using the project.files() method in place of project.fileTree():

Example 109. Setting up an inferred task dependency with files()

build.gradle

task instrumentClasses2(type: Instrument) {
    classFiles = files(compileJava)
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean instrumentClasses2

> gradle clean instrumentClasses2
> Task :clean UP-TO-DATE
> Task :compileJava
> Task :instrumentClasses2


BUILD SUCCESSFUL in 0s
3 actionable tasks: 2 executed, 1 up-to-date

Remember that files() can take tasks as arguments, whereas fileTree() cannot.

The downside of this approach is that all file outputs of the source task become the input files of the target - instrumentClasses in this case. That’s fine as long as the source task only has a single file-based output, like the JavaCompile task. But if you have to link just one output property among several, then you need to explicitly tell Gradle which task generates the input files using the builtBy method:

Example 110. Setting up an inferred task dependency with builtBy()

build.gradle

task instrumentClassesBuiltBy(type: Instrument) {
    classFiles = fileTree(compileJava.destinationDir) {
        builtBy compileJava
    }
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean instrumentClassesBuiltBy

> gradle clean instrumentClassesBuiltBy
> Task :clean UP-TO-DATE
> Task :compileJava
> Task :instrumentClassesBuiltBy


BUILD SUCCESSFUL in 0s
3 actionable tasks: 2 executed, 1 up-to-date

You can of course just add an explicit task dependency via dependsOn, but the above approach provides more semantic meaning, explaining why compileJava has to run beforehand.

Providing custom up-to-date logic

Gradle automatically handles up-to-date checks for output files and directories, but what if the task output is something else entirely? Perhaps it’s an update to a web service or a database table. Gradle has no way of knowing how to check whether the task is up to date in such cases.

That’s where the upToDateWhen() method on TaskOutputs comes in. This takes a predicate function that is used to determine whether a task is up to date or not. One use case is to disable up-to-date checks completely for a task, like so:

Example 111. Ignoring up-to-date checks

build.gradle

task alwaysInstrumentClasses(type: Instrument) {
    classFiles = files(compileJava)
    destinationDir = file("$buildDir/instrumented")
    outputs.upToDateWhen { false }
}

Output of gradle clean alwaysInstrumentClasses

> gradle clean alwaysInstrumentClasses
> Task :compileJava
> Task :alwaysInstrumentClasses


BUILD SUCCESSFUL in 0s
3 actionable tasks: 2 executed, 1 up-to-date

Output of gradle alwaysInstrumentClasses

> gradle alwaysInstrumentClasses
> Task :compileJava UP-TO-DATE
> Task :alwaysInstrumentClasses


BUILD SUCCESSFUL in 0s
2 actionable tasks: 1 executed, 1 up-to-date

The { false } closure ensures that copyResources will always perform the copy, irrespective of whether there is no change in the inputs or outputs.

You can of course put more complex logic into the closure. You could check whether a particular record in a database table exists or has changed for example. Just be aware that up-to-date checks should save you time. Don’t add checks that cost as much or more time than the standard execution of the task. In fact, if a task ends up running frequently anyway, because it’s rarely up to date, then it may not be worth having an up-to-date check at all. Remember that your checks will always run if the task is in the execution task graph.

One common mistake is to use upToDateWhen() instead of Task.onlyIf(). If you want to skip a task on the basis of some condition unrelated to the task inputs and outputs, then you should use onlyIf(). For example, in cases where you want to skip a task when a particular property is set or not set.

Configure input normalization

For up to date checks and the build cache Gradle needs to determine if two task input properties have the same value. In order to do so, Gradle first normalizes both inputs and then compares the result. For example, for a compile classpath, Gradle extracts the ABI signature from the classes on the classpath and then compares signatures between the last Gradle run and the current Gradle run as described in the section called “Compile avoidance”.

It is possible to customize Gradle’s built-in strategy for runtime classpath normalization. All inputs annotated with @Classpath are considered to be runtime classpaths.

Let’s say you want to add a file build-info.properties to all your produced jar files which contains information about the build, e.g. the timestamp when the build started or some ID to identify the CI job that published the artifact. This file is only for auditing purposes, and has no effect on the outcome of running tests. Nonetheless, this file is part of the runtime classpath for the test task and changes on every build invocation. Therefore, the test would be never up-to-date or pulled from the build cache. In order to benefit from incremental builds again, you are able tell Gradle to ignore this file on the runtime classpath at the project level by using Project.normalization(org.gradle.api.Action):

Example 112. Runtime classpath normalization

build.gradle

normalization {
    runtimeClasspath {
        ignore 'build-info.properties'
    }
}

The effect of this configuration would be that changes to build-info.properties would be ignored for up-to-date checks and build cache key calculations. Note that this will not change the runtime behavior of the test task - i.e. any test is still able to load build-info.properties and the runtime classpath is still the same as before.

Stale task outputs

When the Gradle version changes, Gradle detects that outputs from tasks that ran with older versions of Gradle need to be removed to ensure that the newest version of the tasks are starting from a known clean state.

Automatic clean-up of stale output directories has only been implemented for the output of source sets (Java/Groovy/Scala compilation).

Task rules

Sometimes you want to have a task whose behavior depends on a large or infinite number value range of parameters. A very nice and expressive way to provide such tasks are task rules:

Example 113. Task rule

build.gradle

tasks.addRule("Pattern: ping<ID>") { String taskName ->
    if (taskName.startsWith("ping")) {
        task(taskName) {
            doLast {
                println "Pinging: " + (taskName - 'ping')
            }
        }
    }
}

Output of gradle -q pingServer1

> gradle -q pingServer1
Pinging: Server1

The String parameter is used as a description for the rule, which is shown with gradle tasks.

Rules are not only used when calling tasks from the command line. You can also create dependsOn relations on rule based tasks:

Example 114. Dependency on rule based tasks

build.gradle

tasks.addRule("Pattern: ping<ID>") { String taskName ->
    if (taskName.startsWith("ping")) {
        task(taskName) {
            doLast {
                println "Pinging: " + (taskName - 'ping')
            }
        }
    }
}

task groupPing {
    dependsOn pingServer1, pingServer2
}

Output of gradle -q groupPing

> gradle -q groupPing
Pinging: Server1
Pinging: Server2

If you run “gradle -q tasks” you won’t find a task named “pingServer1” or “pingServer2”, but this script is executing logic based on the request to run those tasks.

Finalizer tasks

Finalizers tasks are an incubating feature (see the section called “Incubating”).

Finalizer tasks are automatically added to the task graph when the finalized task is scheduled to run.

Example 115. Adding a task finalizer

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.finalizedBy taskY

Output of gradle -q taskX

> gradle -q taskX
taskX
taskY

Finalizer tasks will be executed even if the finalized task fails.

Example 116. Task finalizer for a failing task

build.gradle

task taskX {
    doLast {
        println 'taskX'
        throw new RuntimeException()
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.finalizedBy taskY

Output of gradle -q taskX

> gradle -q taskX
taskX
taskY

BUILD FAILED in 0s

On the other hand, finalizer tasks are not executed if the finalized task didn’t do any work, for example if it is considered up to date or if a dependent task fails.

Finalizer tasks are useful in situations where the build creates a resource that has to be cleaned up regardless of the build failing or succeeding. An example of such a resource is a web container that is started before an integration test task and which should be always shut down, even if some of the tests fail.

To specify a finalizer task you use the Task.finalizedBy(java.lang.Object[]) method. This method accepts a task instance, a task name, or any other input accepted by Task.dependsOn(java.lang.Object[]).

Lifecycle tasks

Lifecycle tasks are tasks that do not do work themselves. They typically do not have any task actions. Lifecycle tasks can represent several concepts:

  • a work-flow step (e.g., run all checks with check)

  • a buildable thing (e.g., create a debug 32-bit executable for native components with debug32MainExecutable)

  • a convenience task to execute many of the same logical tasks (e.g., run all compilation tasks with compileAll)

Many Gradle plug-ins define their own lifecycle tasks to make it convenient to do specific things. When developing your own plugins, you should consider using your own lifecycle tasks or hooking into some of the tasks already provided by Gradle. See the Java plugin the section called “Tasks” for an example.

Unless a lifecycle task has actions, its outcome is determined by its dependencies. If any of the task’s dependencies are executed, the lifecycle task will be considered executed. If all of the task’s dependencies are up-to-date, skipped or from cache, the lifecycle task will be considered up-to-date.

Summary

If you are coming from Ant, an enhanced Gradle task like Copy seems like a cross between an Ant target and an Ant task. Although Ant’s tasks and targets are really different entities, Gradle combines these notions into a single entity. Simple Gradle tasks are like Ant’s targets, but enhanced Gradle tasks also include aspects of Ant tasks. All of Gradle’s tasks share a common API and you can create dependencies between them. These tasks are much easier to configure than an Ant task. They make full use of the type system, and are more expressive and easier to maintain.



[4] You might be wondering why there is neither an import for the StopExecutionException nor do we access it via its fully qualified name. The reason is, that Gradle adds a set of default imports to your script (see the section called “Default imports”).

Working With Files

Almost every Gradle build interacts with files in some way: think source files, file dependencies, reports and so on. That’s why Gradle comes with a comprehensive API that makes it simple to perform the file operations you need.

The API has two parts to it:

  • Specifying which files and directories to process

  • Specifying what to do with them

The File paths in depth section covers the first of these in detail, while subsequent sections, like File copying in depth, cover the second. To begin with, we’ll show you examples of the most common scenarios that users encounter.

Copying a single file

You copy a file by creating an instance of Gradle’s builtin Copy task and configuring it with the location of the file and where you want to put it. This example mimics copying a generated report into a directory that will be packed into an archive, such as a ZIP or TAR:

Example 117. How to copy a single file

build.gradle

task copyReport(type: Copy) {
    from file("${buildDir}/reports/my-report.pdf")
    into file("${buildDir}/toArchive")
}

The Project.file(java.lang.Object) method is used to create a file or directory path relative to the current project and is a common way to make build scripts work regardless of the project path. The file and directory paths are then used to specify what file to copy using Copy.from(java.lang.Object[]) and which directory to copy it to using Copy.into(java.lang.Object).

You can even use the path directly without the file() method, as explained early in the section File copying in depth:

Example 118. Using implicit string paths

build.gradle

task copyReport2(type: Copy) {
    from "${buildDir}/reports/my-report.pdf"
    into "${buildDir}/toArchive"
}

Although hard-coded paths make for simple examples, they also make the build brittle. It’s better to use a reliable, single source of truth, such as a task or shared project property. In the following modified example, we use a report task defined elsewhere that has the report’s location stored in its outputFile property:

Example 119. Prefer task/project properties over hard-coded paths

build.gradle

task copyReport3(type: Copy) {
    from myReportTask.outputFile
    into archiveReportsTask.dirToArchive
}

We have also assumed that the reports will be archived by archiveReportsTask, which provides us with the directory that will be archived and hence where we want to put the copies of the reports.

Copying multiple files

You can extend the previous examples to multiple files very easily by providing multiple arguments to from():

Example 120. Using multiple arguments with from()

build.gradle

task copyReportsForArchiving(type: Copy) {
    from "${buildDir}/reports/my-report.pdf", "src/docs/manual.pdf"
    into "${buildDir}/toArchive"
}

Two files are now copied into the archive directory. You can also use multiple from() statements to do the same thing, as shown in the first example of the section File copying in depth.

Now consider another example: what if you want to copy all the PDFs in a directory without having to specify each one? To do this, attach inclusion and/or exclusion patterns to the copy specification. Here we use a string pattern to include PDFs only:

Example 121. Using a flat filter

build.gradle

task copyPdfReportsForArchiving(type: Copy) {
    from "${buildDir}/reports"
    include "*.pdf"
    into "${buildDir}/toArchive"
}

One thing to note, as demonstrated in the following diagram, is that only the PDFs that reside directly in the reports directory are copied:

Figure 10. The effect of a flat filter on copying

The effect of a flat filter on copying

You can include files in subdirectories by using an Ant-style glob pattern (**/*), as done in this updated example:

Example 122. Using a deep filter

build.gradle

task copyAllPdfReportsForArchiving(type: Copy) {
    from "${buildDir}/reports"
    include "**/*.pdf"
    into "${buildDir}/toArchive"
}

This task has the following effect:

Figure 11. The effect of a deep filter on copying

The effect of a deep filter on copying

One thing to bear in mind is that a deep filter like this has the side effect of copying the directory structure below reports as well as the files. If you just want to copy the files without the directory structure, you need to use an explicit fileTree(dir) { includes }.files expression. We talk more about the difference between file trees and file collections in the File trees section.

This is just one of the variations in behavior you’re likely to come across when dealing with file operations in Gradle builds. Fortunately, Gradle provides elegant solutions to almost all those use cases. Read the in-depth sections later in the chapter for more detail on how the file operations work in Gradle and what options you have for configuring them.

Copying directory hierarchies

You may have a need to copy not just files, but the directory structure they reside in as well. This is the default behavior when you specify a directory as the from() argument, as demonstrated by the following example that copies everything in the reports directory, including all its subdirectories, to the destination:

Example 123. Copying an entire directory

build.gradle

task copyReportsDirForArchiving(type: Copy) {
    from "${buildDir}/reports"
    into "${buildDir}/toArchive"
}

The key aspect that users struggle with is controlling how much of the directory structure goes to the destination. In the above example, do you get a toArchive/reports directory or does everything in reports go straight into toArchive? The answer is the latter. If a directory is part of the from() path, then it won’t appear in the destination.

So how do you ensure that reports itself is copied across, but not any other directory in $buildDir? The answer is to add it as an include pattern:

Example 124. Copying an entire directory, including itself

build.gradle

task copyReportsDirForArchiving2(type: Copy) {
    from("${buildDir}") {
        include "reports/**"
    }
    into "${buildDir}/toArchive"
}

You’ll get the same behavior as before except with one extra level of directory in the destination, i.e. toArchive/reports.

One thing to note is how the include() directive applies only to the from(), whereas the directive in the previous section applied to the whole task. These different levels of granularity in the copy specification allow you to easily handle most requirements that you will come across. You can learn more about this in the section on child specifications.

Creating archives (zip, tar, etc.)

From the perspective of Gradle, packing files into an archive is effectively a copy in which the destination is the archive file rather than a directory on the file system. This means that creating archives looks a lot like copying, with all of the same features!

The simplest case involves archiving the entire contents of a directory, which this example demonstrates by creating a ZIP of the toArchive directory:

Example 125. Archiving a directory as a ZIP

build.gradle

task packageDistribution(type: Zip) {
    archiveName = "my-distribution.zip"
    destinationDir = file("${buildDir}/dist")

    from "${buildDir}/toArchive"
}

Notice how we specify the destination and name of the archive instead of an into(): both are required. You often won’t see them explicitly set, because most projects apply the Base Plugin. It provides some conventional values for those properties. The next example demonstrates this and you can learn more about the conventions in the archive naming section.

Each type of archive has its own task type, the most common ones being Zip, Tar and Jar. They all share most of the configuration options of Copy, including filtering and renaming.

One of the most common scenarios involves copying files into specified subdirectories of the archive. For example, let’s say you want to package all PDFs into a docs directory in the root of the archive. This docs directory doesn’t exist in the source location, so you have to create it as part of the archive. You do this by adding an into() declaration for just the PDFs:

Example 126. Using the Base Plugin for its archive name convention

build.gradle

plugins {
    id 'base'
}

version = "1.0.0"

task packageDistribution(type: Zip) {
    from("${buildDir}/toArchive") {
        exclude "**/*.pdf"
    }

    from("${buildDir}/toArchive") {
        include "**/*.pdf"
        into "docs"
    }
}

As you can see, you can have multiple from() declarations in a copy specification, each with its own configuration. See the section called “Using child specifications” for more information on this feature.

Unpacking archives

Archives are effectively self-contained file systems, so unpacking them is a case of copying the files from that file system onto the local file system — or even into another archive. Gradle enables this by providing some wrapper functions that make archives available as hierarchical collections of files (file trees).

The two functions of interest are Project.zipTree(java.lang.Object) and Project.tarTree(java.lang.Object), which produce a FileTree from a corresponding archive file. That file tree can then be used in a from() specification, like so:

Example 127. Unpacking a ZIP file

build.gradle

task unpackFiles(type: Copy) {
    from zipTree("src/resources/thirdPartyResources.zip")
    into "${buildDir}/resources"
}

As with a normal copy, you can control which files are unpacked via filters and even rename files as they are unpacked.

If you’re a Java developer and are wondering why there is no jarTree() method, that’s because zipTree() works perfectly well for JARs, WARs and EARs.

Creating "uber" or "fat" JARs

In the Java space, applications and their dependencies typically used to be packaged as separate JARs within a single distribution archive. That still happens, but there is another approach that is now common: placing the classes and resources of the dependencies directly into the application JAR, creating what is known as an uber or fat JAR.

Gradle makes this approach easy to accomplish. Consider the aim: to copy the contents of other JAR files into the application JAR. All you need for this is the Project.zipTree(java.lang.Object) method and the Jar task, as demonstrated by the uberJar task in the following example:

Example 128. Creating a Java uber or fat JAR

build.gradle

plugins {
    id 'java'
}

version = '1.0.0'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'commons-io:commons-io:2.6'
}

task uberJar(type: Jar) {
    appendix = 'uber'

    from sourceSets.main.output
    from configurations.runtimeClasspath.files.
                                         findAll { it.name.endsWith('jar') }.
                                         collect { zipTree(it) }
}

In this case, we’re taking the runtime dependencies of the project — configurations.runtimeClasspath.files — and wrapping each of the JAR files with the zipTree() method. The result is a collection of ZIP file trees, the contents of which are copied into the uber JAR alongside the application classes.

Creating directories

Many tasks need to create directories to store the files they generate, which is why Gradle automatically manages this aspect of tasks when they explicitly define file and directory outputs. You can learn about this feature in the incremental build section of the user guide. All core Gradle tasks ensure that any output directories they need are created if necessary using this mechanism.

In cases where you need to create a directory manually, you can use the Project.mkdir(java.lang.Object) method from within your build scripts or custom task implementations. Here’s a simple example that creates a single images directory in the project folder:

Example 129. Manually creating a directory

build.gradle

task ensureDirectory {
    doLast {
        mkdir "images"
    }
}

As described in the Apache Ant manual, the mkdir task will automatically create all necessary directories in the given path and will do nothing if the directory already exists.

Moving files and directories

Gradle has no API for moving files and directories around, but you can use the Apache Ant integration to easily do that, as shown in this example:

Example 130. Moving a directory using the Ant task

build.gradle

task moveReports {
    doLast {
        ant.move file: "${buildDir}/reports",
                 todir: "${buildDir}/toArchive"
    }
}

This is not a common requirement and should be used sparingly as you lose information and can easily break a build. It’s generally preferable to copy directories and files instead.

Renaming files on copy

The files used and generated by your builds sometimes don’t have names that suit, in which case you want to rename those files as you copy them. Gradle allows you to do this as part of a copy specification using the rename() configuration.

The following example removes the "-staging-" marker from the names of any files that have it:

Example 131. Renaming files as they are copied

build.gradle

task copyFromStaging(type: Copy) {
    from "src/main/webapp"
    into "${buildDir}/explodedWar"

    rename '(.+)-staging(.+)', '$1$2'
}

You can use regular expressions for this, as in the above example, or closures that use more complex logic to determine the target filename. For example, the following task truncates filenames:

Example 132. Truncating filenames as they are copied

build.gradle

task copyWithTruncate(type: Copy) {
    from "${buildDir}/reports"
    rename { String filename ->
        if (filename.size() > 10) {
            return filename[0..7] + "~" + filename.size()
        }
        else return filename
    }
    into "${buildDir}/toArchive"
}

As with filtering, you can also apply renaming to a subset of files by configuring it as part of a child specification on a from().

Deleting files and directories

You can easily delete files and directories using either the Delete task or the Project.delete(org.gradle.api.Action) method. In both cases, you specify which files and directories to delete in a way supported by the Project.files(java.lang.Object[]) method.

For example, the following task deletes the entire contents of a build’s output directory:

Example 133. Deleting a directory

build.gradle

task myClean(type: Delete) {
    delete buildDir
}

If you want more control over which files are deleted, you can’t use inclusions and exclusions in the same way as for copying files. Instead, you have to use the builtin filtering mechanisms of files() and fileTree(). The following example does just that to clear out temporary files from a source directory:

Example 134. Deleting files matching a specific pattern

build.gradle

task cleanTempFiles(type: Delete) {
    delete fileTree("src").matching {
        include "**/*.tmp"
    }
}

You’ll learn more about file collections and file trees in the next section.

File paths in depth

In order to perform some action on a file, you need to know where it is, and that’s the information provided by file paths. Gradle builds on the standard Java File class, which represents the location of a single file, and provides new APIs for dealing with collections of paths. This section shows you how to use the Gradle APIs to specify file paths for use in tasks and file operations.

But first, an important note on using hard-coded file paths in your builds.

On hard-coded file paths

Many examples in this chapter use hard-coded paths as string literals. This makes them easy to understand, but it’s not good practice for real builds. The problem is that paths often change and the more places you need to change them, the more likely you are to miss one and break the build.

Where possible, you should use tasks, task properties, and project properties — in that order of preference — to configure file paths. For example, if you were to create a task that packages the compiled classes of a Java application, you should aim for something like this:

Example 135. How to minimize the number of hard-coded paths in your build

build.gradle

ext {
    archivesDirPath = "${buildDir}/archives"
}

task packageClasses(type: Zip) {
    appendix = "classes"
    destinationDir = file(archivesDirPath)

    from compileJava
}

See how we’re using the compileJava task as the source of the files to package and we’ve created a project property archivesDirPath to store the location where we put archives, on the basis we’re likely to use it elsewhere in the build.

Using a task directly as an argument like this relies on it having defined outputs, so it won’t always be possible. In addition, this example could be improved further by relying on the Java plugin’s convention for destinationDir rather than overriding it, but it does demonstrate the use of project properties.

Single file paths

One of the great quandaries when developing a build is how to specify file locations when the build may be executed from an arbitrary directory — not necessarily in the project — and may be run on any number of different systems with incompatible directory layouts. The standard Java mechanism for specifying a file path runs into trouble in these situations:

  • new File(relative path) generates a path relative to the current working directory, which could be anywhere

  • new File(absolute path) will fail if the file system doesn’t have the requisite path.

Gradle solves this problem by providing the Project.file(java.lang.Object) method, which generates a path relative to the project directory (unless the given path is absolute, in which case it is used as is). Here are some examples of using the file() method with different types of argument:

Example 136. Locating files

build.gradle

// Using a relative path
File configFile = file('src/config.xml')

// Using an absolute path
configFile = file(configFile.absolutePath)

// Using a File object with a relative path
configFile = file(new File('src/config.xml'))

// Using a java.nio.file.Path object with a relative path
configFile = file(Paths.get('src', 'config.xml'))

// Using an absolute java.nio.file.Path object
configFile = file(Paths.get(System.getProperty('user.home')).resolve('global-config.xml'))

As you can see, you can pass strings, File instances and Path instances to the file() method, all of which result in an absolute File object. You can find other options for argument types in the reference guide, linked in the previous paragraph.

What happens in the case of multi-project builds? The file() method will always turn relative paths into paths that are relative to the current project directory, which may be a child project. If you want to use a path that’s relative to the root project directory, then you need to use the special Project.getRootDir() property to construct an absolute path, like so:

Example 137. Creating a path relative to a parent project

build.gradle

File configFile = file("${rootDir}/shared/config.xml")

Let’s say you’re working on a multi-project build in a dev/projects/AcmeHealth directory. You use the above example in the build of the library you’re fixing — at AcmeHealth/subprojects/AcmePatientRecordLib/build.gradle. The file path will resolve to the absolute version of dev/projects/AcmeHealth/shared/config.xml.

The file() method can be used to configure any task that has a property of type File. Many tasks, though, work on multiple files, so we look at how to specify sets of files next.

File collections

A file collection is simply a set of file paths that’s represented by the FileCollection interface. Any file paths. It’s important to understand that the file paths don’t have to be related in any way, so they don’t have to be in the same directory or even have a shared parent directory. You will also find that many parts of the Gradle API use FileCollection, such as the copying API discussed later in this chapter and dependency configurations.

The recommended way to specify a collection of files is to use the Project.files(java.lang.Object[]) method, which returns a FileCollection instance. This method is very flexible and allows you to pass multiple strings, File instances, collections of strings, collections of Files, and more. You can even pass in tasks as arguments if they have defined outputs. Learn about all the supported argument types in the reference guide.

As with the Project.file(java.lang.Object) method covered in the previous section, all relative paths are evaluated relative to the current project directory. The following example demonstrates some of the variety of argument types you can use — strings, File instances, a list and a Path:

Example 138. Creating a file collection

build.gradle

FileCollection collection = files('src/file1.txt',
                                  new File('src/file2.txt'),
                                  ['src/file3.csv', 'src/file4.csv'],
                                  Paths.get('src', 'file5.txt'))

File collections have some important attributes in Gradle. They can be:

  • created lazily

  • iterated over

  • filtered

  • combined

Lazy creation of a file collection is useful when you need to evaluate the files that make up a collection at the time a build runs. In the following example, we query the file system to find out what files exist in a particular directory and then make those into a file collection:

Example 139. Implementing a file collection

build.gradle

task list {
    doLast {
        File srcDir

        // Create a file collection using a closure
        collection = files { srcDir.listFiles() }

        srcDir = file('src')
        println "Contents of $srcDir.name"
        collection.collect { relativePath(it) }.sort().each { println it }

        srcDir = file('src2')
        println "Contents of $srcDir.name"
        collection.collect { relativePath(it) }.sort().each { println it }
    }
}

Output of gradle -q list

> gradle -q list
Contents of src
src/dir1
src/file1.txt
Contents of src2
src2/dir1
src2/dir2

The key to lazy creation is passing a closure to the files() method. Your closure simply needs to return a value of a type accepted by files(), such as List<File>, String, FileCollection, etc.

Iterating over a file collection can be done through the each() method on the collection or using the collection in a for loop. In both approaches, the file collection is treated as a set of File instances, i.e. your iteration variable will be of type File.

The following example demonstrates such iteration as well as how you can convert file collections to other types using the as operator or supported properties:

Example 140. Using a file collection

build.gradle

// Iterate over the files in the collection
collection.each { File file ->
    println file.name
}

// Convert the collection to various types
Set set = collection.files
Set set2 = collection as Set
List list = collection as List
String path = collection.asPath
File file = collection.singleFile
File file2 = collection as File

// Add and subtract collections
def union = collection + files('src/file2.txt')
def different = collection - files('src/file2.txt')


You can also see at the end of the example how to combine file collections using the + and - operators to merge and subtract them. An important feature of the resulting file collections is that they are live. In other words, when you combine file collections in this way, the result always reflects what’s currently in the source file collections, even if they change during the build.

For example, imagine collection in the above example gains an extra file or two after union is created. As long as you use union after those files are added to collection, union will also contain those additional files. The same goes for the different file collection.

Live collections are also important when it comes to filtering. If you want to use a subset of a file collection, you can take advantage of the FileCollection.filter(org.gradle.api.specs.Spec) method to determine which files to "keep". In the following example, we create a new collection that consists of only the files that end with .txt in the source collection:

Example 141. Filtering a file collection

build.gradle

FileCollection textFiles = collection.filter { File f ->
    f.name.endsWith(".txt")
}

Output of gradle -q filterTextFiles

> gradle -q filterTextFiles
src/file1.txt
src/file2.txt
src/file5.txt

If collection changes at any time, either by adding or removing files from itself, then textFiles will immediately reflect the change because it is also a live collection. Note that the closure you pass to filter() takes a File as an argument and should return a boolean.

File trees

A file tree is a file collection that retains the directory structure of the files it contains and has the type FileTree. This means that all the paths in a file tree must have a shared parent directory. The following diagram highlights the distinction between file trees and file collections in the common case of copying files:

Figure 12. The differences in how file trees and file collections behave when copying files

The differences in how file trees and file collections behave when copying files

Although FileTree extends FileCollection (an is-a relationship), their behaviors do differ. In other words, you can use a file tree wherever a file collection is required, but remember: a file collection is a flat list/set of files, while a file tree is a file and directory hierarchy. To convert a file tree to a flat collection, use the FileTree.getFiles() property.

The simplest way to create a file tree is to pass a file or directory path to the Project.fileTree(java.lang.Object) method. This will create a tree of all the files and directories in that base directory (but not the base directory itself). The following example demonstrates how to use the basic method and, in addition, how to filter the files and directories using Ant-style patterns:

Example 142. Creating a file tree

build.gradle

// Create a file tree with a base directory
FileTree tree = fileTree(dir: 'src/main')

// Add include and exclude patterns to the tree
tree.include '**/*.java'
tree.exclude '**/Abstract*'

// Create a tree using path
tree = fileTree('src').include('**/*.java')

// Create a tree using closure
tree = fileTree('src') {
    include '**/*.java'
}

// Create a tree using a map
tree = fileTree(dir: 'src', include: '**/*.java')
tree = fileTree(dir: 'src', includes: ['**/*.java', '**/*.xml'])
tree = fileTree(dir: 'src', include: '**/*.java', exclude: '**/*test*/**')

You can see more examples of supported patterns in the API docs for PatternFilterable. Also, see the API documentation for fileTree() to see what types you can pass as the base directory.

By default, fileTree() returns a FileTree instance that applies some default exclusion patterns for convenience — the same defaults as Ant in fact. For the complete default exclusion list, see the Ant manual.

If those default exclusions prove problematic, you can workaround the issue by using the defaultexcludes Ant task, as demonstrated in this example:

Example 143. Changing Ant default exclusions for a copy task

build.gradle

task forcedCopy(type: Copy) {
    into "${buildDir}/inPlaceApp"
    from 'src/main/webapp'

    doFirst {
        ant.defaultexcludes remove: "**/.git"
        ant.defaultexcludes remove: "**/.git/**"
        ant.defaultexcludes remove: "**/*~"
    }

    doLast {
        ant.defaultexcludes default: true
    }
}

In general, it’s best to ensure that the default exclusions are reset whenever you change them as modifications are visible to the entire build. The above example is performing such a reset in its doLast action.

You can do many of the same things with file trees that you can with file collections:

You can also traverse file trees using the FileTree.visit(org.gradle.api.Action) method. All of these techniques are demonstrated in the following example:

Example 144. Using a file tree

build.gradle

// Iterate over the contents of a tree
tree.each {File file ->
    println file
}

// Filter a tree
FileTree filtered = tree.matching {
    include 'org/gradle/api/**'
}

// Add trees together
FileTree sum = tree + fileTree(dir: 'src/test')

// Visit the elements of the tree
tree.visit {element ->
    println "$element.relativePath => $element.file"
}

We’ve discussed how to create your own file trees and file collections, but it’s also worth bearing in mind that many Gradle plugins provide their own instances of file trees, such as Java’s source sets. These can be used and manipulated in exactly the same way as the file trees you create yourself.

Another specific type of file tree that users commonly need is the archive, i.e. ZIP files, TAR files, etc. We look at those next.

Using archives as file trees

An archive is a directory and file hierarchy packed into a single file. In other words, it’s a special case of a file tree, and that’s exactly how Gradle treats archives. Instead of using the fileTree() method, which only works on normal file systems, you use the Project.zipTree(java.lang.Object) and Project.tarTree(java.lang.Object) methods to wrap archive files of the corresponding type (note that JAR, WAR and EAR files are ZIPs). Both methods return FileTree instances that you can then use in the same way as normal file trees. For example, you can extract some or all of the files of an archive by copying its contents to some directory on the file system. Or you can merge one archive into another.

Here are some simple examples of creating archive-based file trees:

Example 145. Using an archive as a file tree

build.gradle

// Create a ZIP file tree using path
FileTree zip = zipTree('someFile.zip')

// Create a TAR file tree using path
FileTree tar = tarTree('someFile.tar')

//tar tree attempts to guess the compression based on the file extension
//however if you must specify the compression explicitly you can:
FileTree someTar = tarTree(resources.gzip('someTar.ext'))


You can see a practical example of extracting an archive file in among the common scenarios we cover.

Understanding implicit conversion to file collections

Many objects in Gradle have properties which accept a set of input files. For example, the JavaCompile task has a source property that defines the source files to compile. You can set the value of this property using any of the types supported by the files() method, as mentioned in the api docs. This means you can, for example, set the property to a File, String, collection, FileCollection or even a closure.

This is a feature of specific tasks! That means implicit conversion will not happen for just any task that has a FileCollection or FileTree property. If you want to know whether implicit conversion happens in a particular situation, you will need to read the relevant documentation, such as the corresponding task’s API docs. Alternatively, you can remove all doubt by explicitly using Project.files(java.lang.Object[]) in your build.

Here are some examples of the different types of arguments that the source property can take:

Example 146. Specifying a set of files

build.gradle

task compile(type: JavaCompile)

// Use a File object to specify the source directory
compile {
    source = file('src/main/java')
}

// Use a String path to specify the source directory
compile {
    source = 'src/main/java'
}

// Use a collection to specify multiple source directories
compile {
    source = ['src/main/java', '../shared/java']
}

// Use a FileCollection (or FileTree in this case) to specify the source files
compile {
    source = fileTree(dir: 'src/main/java').matching { include 'org/gradle/api/**' }
}

// Using a closure to specify the source files.
compile {
    source = {
        // Use the contents of each zip file in the src dir
        file('src').listFiles().findAll {it.name.endsWith('.zip')}.collect { zipTree(it) }
    }
}

One other thing to note is that properties like source have corresponding methods in core Gradle tasks. Those methods follow the convention of appending to collections of values rather than replacing them. Again, this method accepts any of the types supported by the files() method, as shown here:

Example 147. Appending a set of files

build.gradle

compile {
    // Add some source directories use String paths
    source 'src/main/java', 'src/main/groovy'

    // Add a source directory using a File object
    source file('../shared/java')

    // Add some source directories using a closure
    source { file('src/test/').listFiles() }
}

As this is a common convention, we recommend that you follow it in your own custom tasks. Specifically, if you plan to add a method to configure a collection-based property, make sure the method appends rather than replaces values.

File copying in depth

The basic process of copying files in Gradle is a simple one:

  • Define a task of type Copy

  • Specify which files (and potentially directories) to copy

  • Specify a destination for the copied files

But this apparent simplicity hides a rich API that allows fine-grained control of which files are copied, where they go, and what happens to them as they are copied — renaming of the files and token substitution of file content are both possibilities, for example.

Let’s start with the last two items on the list, which form what is known as a copy specification. This is formally based on the CopySpec interface, which the Copy task implements, and offers:

CopySpec has several additional methods that allow you to control the copying process, but these two are the only required ones. into() is straightforward, requiring a directory path as its argument in any form supported by the Project.file(java.lang.Object) method. The from() configuration is far more flexible.

Not only does from() accept multiple arguments, it also allows several different types of argument. For example, some of the most common types are:

  • A String — treated as a file path or, if it starts with "file://", a file URI

  • A File — used as a file path

  • A FileCollection or FileTree — all files in the collection are included in the copy

  • A task — the files or directories that form a task’s defined outputs are included

In fact, from() accepts all the same arguments as Project.files(java.lang.Object[]), so see that method for a more detailed list of acceptable types.

Something else to consider is what type of thing a file path refers to:

  • A file — the file is copied as is

  • A directory — this is effectively treated as a file tree: everything in it, including subdirectories, is copied. However, the directory itself is not included in the copy.

  • A non-existent file — the path is ignored

Here is an example that uses multiple from() specifications, each with a different argument type. You will probably also notice that into() is configured lazily using a closure — a technique that also works with from():

Example 148. Specifying copy task source files and destination directory

build.gradle

task anotherCopyTask(type: Copy) {
    // Copy everything under src/main/webapp
    from 'src/main/webapp'
    // Copy a single file
    from 'src/staging/index.html'
    // Copy the output of a task
    from copyTask
    // Copy the output of a task using Task outputs explicitly.
    from copyTaskWithPatterns.outputs
    // Copy the contents of a Zip file
    from zipTree('src/main/assets.zip')
    // Determine the destination directory later
    into { getDestDir() }
}

Note that the lazy configuration of into() is different from a child specification, even though the syntax is similar. Keep an eye on the number of arguments to distinguish between them.

Filtering files

You’ve already seen that you can filter file collections and file trees directly in a Copy task, but you can also apply filtering in any copy specification through the CopySpec.include(java.lang.String[]) and CopySpec.exclude(java.lang.String[]) methods.

Both of these methods are normally used with Ant-style include or exclude patterns, as described in PatternFilterable. You can also perform more complex logic by using a closure that takes a FileTreeElement and returns true if the file should be included or false otherwise. The following example demonstrates both forms, ensuring that only .html and .jsp files are copied, except for those .html files with the word "DRAFT" in their content:

Example 149. Selecting the files to copy

build.gradle

task copyTaskWithPatterns(type: Copy) {
    from 'src/main/webapp'
    into "${buildDir}/explodedWar"
    include '**/*.html'
    include '**/*.jsp'
    exclude { FileTreeElement details ->
        details.file.name.endsWith('.html') &&
            details.file.text.contains('DRAFT')
    }
}

A question you may ask yourself at this point is what happens when inclusion and exclusion patterns overlap? Which pattern wins? Here are the basic rules:

  • If there are no explicit inclusions or exclusions, everything is included

  • If at least one inclusion is specified, only files and directories matching the patterns are included

  • Any exclusion pattern overrides any inclusions, so if a file or directory matches at least one exclusion pattern, it won’t be included, regardless of the inclusion patterns

Bear these rules in mind when creating combined inclusion and exclusion specifications so that you end up with the exact behavior you want.

Note that the inclusions and exclusions in the above example will apply to all from() configurations. If you want to apply filtering to a subset of the copied files, you’ll need to use child specifications.

Renaming files

The example of how to rename files on copy gives you most of the information you need to perform this operation. It demonstrates the two options for renaming:

  • Using a regular expression

  • Using a closure

Regular expressions are a flexible approach to renaming, particularly as Gradle supports regex groups that allow you to remove and replaces parts of the source filename. The following example shows how you can remove the string "-staging-" from any filename that contains it using a simple regular expression:

Example 150. Renaming files as they are copied

build.gradle

task rename(type: Copy) {
    from 'src/main/webapp'
    into "${buildDir}/explodedWar"
    // Use a closure to convert all file names to upper case
    rename { String fileName ->
        fileName.toUpperCase()
    }
    // Use a regular expression to map the file name
    rename '(.+)-staging-(.+)', '$1$2'
    rename(/(.+)-staging-(.+)/, '$1$2')
}

You can use any regular expression supported by the Java Pattern class and the substitution string (the second argument of rename() works on the same principles as the Matcher.appendReplacement() method.

Regular expressions in Groovy build scripts

There are two common issues people come across when using regular expressions in this context:

  1. If you use a slashy string (those delimited by '/') for the first argument, you must include the parentheses for rename() as shown in the above example.

  2. It’s safest to use single quotes for the second argument, otherwise you need to escape the '$' in group substitutions, i.e. "\$1\$2"

The first is a minor inconvenience, but slashy strings have the advantage that you don’t have to escape backslash ('\') characters in the regular expression. The second issue stems from Groovy’s support for embedded expressions using ${ } syntax in double-quoted and slashy strings.

The closure syntax for rename() is straightforward and can be used for any requirements that simple regular expressions can’t handle. You’re given the name of a file and you return a new name for that file, or null if you don’t want to change the name. Do be aware that the closure will be executed for every file that’s copied, so try to avoid expensive operations where possible.

Filtering file content (token substitution, templating, etc.)

Not to be confused with filtering which files are copied, file content filtering allows you to transform the content of files while they are being copied. This can involve basic templating that uses token substitution, removal of lines of text, or even more complex filtering using a full-blown template engine.

The following example demonstrates several forms of filtering, including token substitution using the CopySpec.expand(java.util.Map) method and another using CopySpec.filter(java.lang.Class) with an Ant filter:

Example 151. Filtering files as they are copied

build.gradle

import org.apache.tools.ant.filters.FixCrLfFilter
import org.apache.tools.ant.filters.ReplaceTokens

task filter(type: Copy) {
    from 'src/main/webapp'
    into "${buildDir}/explodedWar"
    // Substitute property tokens in files
    expand(copyright: '2009', version: '2.3.1')
    expand(project.properties)
    // Use some of the filters provided by Ant
    filter(FixCrLfFilter)
    filter(ReplaceTokens, tokens: [copyright: '2009', version: '2.3.1'])
    // Use a closure to filter each line
    filter { String line ->
        "[$line]"
    }
    // Use a closure to remove lines
    filter { String line ->
        line.startsWith('-') ? null : line
    }
    filteringCharset = 'UTF-8'
}

The filter() method has two variants, which behave differently:

  • one takes a FilterReader and is designed to work with Ant filters, such as ReplaceTokens

  • one takes a closure or Transformer that defines the transformation for each line of the source file

Note that both variants assume the source files are text based. When you use the ReplaceTokens class with filter(), the result is a template engine that replaces tokens of the form @tokenName@ (the Ant-style token) with values that you define.

The expand() method treats the source files as Groovy templates, which evaluate and expand expressions of the form ${expression}. You can pass in property names and values that are then expanded in the source files. expand() allows for more than basic token substitution as the embedded expressions are full-blown Groovy expressions.

It’s good practice to specify the character set when reading and writing the file, otherwise the transformations won’t work properly for non-ASCII text. You configure the character set with the CopySpec.getFilteringCharset() property. If it’s not specified, the JVM default character set is used, which is likely to be different from the one you want.

Using the CopySpec class

A copy specification (or copy spec for short) determines what gets copied to where, and what happens to files during the copy. You’ve alread seen many examples in the form of configuration for Copy and archiving tasks. But copy specs have two attributes that are worth covering in more detail:

  1. They can be independent of tasks

  2. They are hierarchical

The first of these attributes allows you to share copy specs within a build. The second provides fine-grained control within the overall copy specification.

Sharing copy specs

Consider a build that has several tasks that copy a project’s static website resources or add them to an archive. One task might copy the resources to a folder for a local HTTP server and another might package them into a distribution. You could manually specify the file locations and appropriate inclusions each time they are needed, but human error is more likely to creep in, resulting in inconsistencies between tasks.

One solution Gradle provides is the Project.copySpec(org.gradle.api.Action) method. This allows you to create a copy spec outside of a task, which can then be attached to an appropriate task using the CopySpec.with(org.gradle.api.file.CopySpec[]) method. The following example demonstrates how this is done:

Example 152. Sharing copy specifications

build.gradle

CopySpec webAssetsSpec = copySpec {
    from 'src/main/webapp'
    include '**/*.html', '**/*.png', '**/*.jpg'
    rename '(.+)-staging(.+)', '$1$2'
}

task copyAssets(type: Copy) {
    into "${buildDir}/inPlaceApp"
    with webAssetsSpec
}

task distApp(type: Zip) {
    archiveName = 'my-app-dist.zip'
    destinationDir = file("${buildDir}/dists")

    from appClasses
    with webAssetsSpec
}

Both the copyAssets and distApp tasks will process the static resources under src/main/webapp, as specified by webAssetsSpec.

The configuration defined by webAssetsSpec will not apply to the app classes included by the distApp task. That’s because from appClasses is its own child specification independent of with webAssetsSpec.

This can be confusing to understand, so it’s probably best to treat with() as an extra from() specification in the task. Hence it doesn’t make sense to define a standalone copy spec without at least one from() defined.

If you encounter a scenario in which you want to apply the same copy configuration to different sets of files, then you can share the configuration block directly without using copySpec(). Here’s an example that has two independent tasks that happen to want to process image files only:

Example 153. Sharing copy patterns only

build.gradle

def webAssetPatterns = {
    include '**/*.html', '**/*.png', '**/*.jpg'
}

task copyAppAssets(type: Copy) {
    into "${buildDir}/inPlaceApp"
    from 'src/main/webapp', webAssetPatterns
}

task archiveDistAssets(type: Zip) {
    archiveName = 'distribution-assets.zip'
    destinationDir = file("${buildDir}/dists")

    from 'distResources', webAssetPatterns
}

In this case, we assign the copy configuration to its own variable and apply it to whatever from() specification we want. This doesn’t just work for inclusions, but also exclusions, file renaming, and file content filtering.

Using child specifications

If you only use a single copy spec, the file filtering and renaming will apply to all the files that are copied. Sometimes this is what you want, but not always. Consider the following example that copies files into a directory structure that can be used by a Java Servlet container to deliver a website:

Figure 13. Creating an exploded WAR for a Servlet container

Creating an exploded WAR for a Servlet container

This is not a straightforward copy as the WEB-INF directory and its subdirectories don’t exist within the project, so they must be created during the copy. In addition, we only want HTML and image files going directly into the root folder — build/explodedWar — and only JavaScript files going into the js directory. So we need separate filter patterns for those two sets of files.

The solution is to use child specifications, which can be applied to both from() and into() declarations. The following task definition does the necessary work:

Example 154. Nested copy specs

build.gradle

task nestedSpecs(type: Copy) {
    into "${buildDir}/explodedWar"
    exclude '**/*staging*'
    from('src/dist') {
        include '**/*.html', '**/*.png', '**/*.jpg'
    }
    from(sourceSets.main.output) {
        into 'WEB-INF/classes'
    }
    into('WEB-INF/lib') {
        from configurations.runtimeClasspath
    }
}

Notice how the src/dist configuration has a nested inclusion specification: that’s the child copy spec. You can of course add content filtering and renaming here as required. A child copy spec is still a copy spec.

The above example also demonstrates how you can copy files into a subdirectory of the destination either by using a child into() on a from() or a child from() on an into(). Both approaches are acceptable, but you may want to create and follow a convention to ensure consistency across your build files.

Don’t get your into() specifications mixed up! For a normal copy — one to the filesystem rather than an archive — there should always be one "root" into() that simply specifies the overall destination directory of the copy. Any other into() should have a child spec attached and its path will be relative to the root into().

One final thing to be aware of is that a child copy spec inherits its destination path, include patterns, exclude patterns, copy actions, name mappings and filters from its parent. So be careful where you place your configuration.

Copying files in your own tasks

There might be occasions when you want to copy files or directories as part of a task. For example, a custom archiving task based on an unsupported archive format might want to copy files to a temporary directory before they are then archived. You still want to take advantage of Gradle’s copy API, but without introducing an extra Copy task.

The solution is to use the Project.copy(org.gradle.api.Action) method. It works the same way as the Copy task by configuring it with a copy spec. Here’s a trivial example:

Example 155. Copying files using the copy() method without up-to-date check

build.gradle

task copyMethod {
    doLast {
        copy {
            from 'src/main/webapp'
            into "${buildDir}/explodedWar"
            include '**/*.html'
            include '**/*.jsp'
        }
    }
}

The above example demonstrates the basic syntax and also highlights two major limitations of using the copy() method:

  1. The copy() method is not incremental. The example’s copyMethod task will always execute because it has no information about what files make up the task’s inputs. You have to manually define the task inputs and outputs.

  2. Using a task as a copy source, i.e. as an argument to from(), won’t set up an automatic task dependency between your task and that copy source. As such, if you are using the copy() method as part of a task action, you must explicitly declare all inputs and outputs in order to get the correct behavior.

The following example shows you how to workaround these limitations by using the dynamic API for task inputs and outputs:

Example 156. Copying files using the copy() method with up-to-date check

build.gradle

task copyMethodWithExplicitDependencies{
    // up-to-date check for inputs, plus add copyTask as dependency
    inputs.files copyTask
    outputs.dir 'some-dir' // up-to-date check for outputs
    doLast{
        copy {
            // Copy the output of copyTask
            from copyTask
            into 'some-dir'
        }
    }
}

These limitations make it preferable to use the Copy task wherever possible, because of its builtin support for incremental building and task dependency inference. That is why the copy() method is intended for use by custom tasks that need to copy files as part of their function. Custom tasks that use the copy() method should declare the necessary inputs and outputs relevant to the copy action.

Mirroring directories and file collections with the Sync task

The Sync task, which extends the Copy task, copies the source files into the destination directory and then removes any files from the destination directory which it did not copy. In other words, it synchronizes the contents of a directory with its source. This can be useful for doing things such as installing your application, creating an exploded copy of your archives, or maintaining a copy of the project’s dependencies.

Here is an example which maintains a copy of the project’s runtime dependencies in the build/libs directory.

Example 157. Using the Sync task to copy dependencies

build.gradle

task libs(type: Sync) {
    from configurations.runtime
    into "${buildDir}/libs"
}

You can also perform the same function in your own tasks with the Project.sync(org.gradle.api.Action) method.

Archive creation in depth

Archives are essentially self-contained file systems and Gradle treats them as such. This is why working with archives is very similar to working with files and directories, including such things as file permissions.

Out of the box, Gradle supports creation of both ZIP and TAR archives, and by extension Java’s JAR, WAR and EAR formats — Java’s archive formats are all ZIPs. Each of these formats has a corresponding task type to create them: Zip, Tar, Jar, War, and Ear. These all work the same way and are based on copy specifications, just like the Copy task.

Creating an archive file is essentially a file copy in which the destination is implicit, i.e. the archive file itself. Here’s a basic example that specifies the path and name of the target archive file:

Example 158. Archiving a directory as a ZIP

build.gradle

task packageDistribution(type: Zip) {
    archiveName = "my-distribution.zip"
    destinationDir = file("${buildDir}/dist")

    from "${buildDir}/toArchive"
}

In the next section you’ll learn about convention-based archive names, which can save you from always configuring the destination directory and archive name.

The full power of copy specifications are available to you when creating archives, which means you can do content filtering, file renaming or anything else that is covered in the previous section. A particularly common requirement is copying files into subdirectories of the archive that don’t exist in the source folders, something that can be achieved with into() child specifications.

Gradle does of course allow you create as many archive tasks as you want, but it’s worth bearing in mind that many convention-based plugins provide their own. For example, the Java plugin adds a jar task for packaging a project’s compiled classes and resources in a JAR. Many of these plugins provide sensible conventions for the names of archives as well as the copy specifications used. We recommend you use these tasks wherever you can, rather than overriding them with your own.

Archive naming

Gradle has several conventions around the naming of archives and where they are created based on the plugins your project uses. The main convention is provided by the Base Plugin, which defaults to creating archives in the $buildDir/distributions directory and typically uses archive names of the form [projectName]-[version].[type].

The following example comes from a project named 'zipProject', hence the myZip task creates an archive named 'zipProject-1.0.zip':

Example 159. Creation of ZIP archive

build.gradle

plugins {
    id 'base'
}

version = 1.0

task myZip(type: Zip) {
    from 'somedir'

    doLast {
        println archiveName
        println relativePath(destinationDir)
        println relativePath(archivePath)
    }
}

Output of gradle -q myZip

> gradle -q myZip
zipProject-1.0.zip
build/distributions
build/distributions/zipProject-1.0.zip

Note that the name of the archive does not derive from the name of the task that creates it.

If you want to change the name and location of a generated archive file, you can provide values for the archiveName and destinationDir properties of the corresponding task. These override any conventions that would otherwise apply.

Alternatively, you can make use of the default archive name pattern provided by AbstractArchiveTask.getArchiveName(): [baseName]-[appendix]-[version]-[classifier].[extension]. You can set each of these properties on the task separately if you wish. Note that the Base Plugin uses the convention of project name for baseName, project version for version and the archive type for extension. It does not provide values for the other properties.

This example — from the same project as the one above — configures just the baseName property, overriding the default value of the project name:

Example 160. Configuration of archive task - custom archive name

build.gradle

task myCustomZip(type: Zip) {
    baseName = 'customName'
    from 'somedir'

    doLast {
        println archiveName
    }
}

Output of gradle -q myCustomZip

> gradle -q myCustomZip
customName-1.0.zip

You can also override the default baseName value for all the archive tasks in your build by using the project property archivesBaseName, as demonstrated by the following example:

Example 161. Configuration of archive task - appendix & classifier

build.gradle

plugins {
    id 'base'
}

version = 1.0
archivesBaseName = "gradle"

task myZip(type: Zip) {
    from 'somedir'
}

task myOtherZip(type: Zip) {
    appendix = 'wrapper'
    classifier = 'src'
    from 'somedir'
}

task echoNames {
    doLast {
        println "Project name: ${project.name}"
        println myZip.archiveName
        println myOtherZip.archiveName
    }
}

Output of gradle -q echoNames

> gradle -q echoNames
Project name: zipProject
gradle-1.0.zip
gradle-wrapper-1.0-src.zip

You can find all the possible archive task properties in the API documentation for AbstractArchiveTask, but we have also summarized the main ones here:

archiveNameString, default: baseName-appendix-version-classifier.extension

The complete file name of the generated archive. If any of the properties in the default value are empty, their '-' separator is dropped.

archivePathFile, read-only, default: destinationDir/archiveName

The absolute file path of the generated archive.

destinationDirFile, default: depends on archive type

The target directory in which to put the generated archive. By default, JARs and WARs go into $buildDir/libs. ZIPs and TARs go into $buildDir/distributions.

baseNameString, default: project.name

The base name portion of the archive file name, typically a project name or some other descriptive name for what it contains.

appendixString, default: null

The appendix portion of the archive file name that comes immediately after the base name. It is typically used to distinguish between different forms of content, such as code and docs, or a minimal distribution versus a full or complete one.

versionString, default: project.version

The version portion of the archive file name, typically in the form of a normal project or product version.

classifierString, default: null

The classifier portion of the archive file name. Often used to distinguish between archives that target different platforms.

extensionString, default: depends on archive type and compression type

The filename extension for the archive. By default, this is set based on the archive task type and the compression type (if you’re creating a TAR). Will be one of: zip, jar, war, tar, tgz or tbz2. You can of course set this to a custom extension if you wish.

Sharing content between multiple archives

As described earlier, you can use the Project.copySpec(org.gradle.api.Action) method to share content between archives.

Reproducible archives

Sometimes it’s desirable to recreate archives exactly the same, byte for byte, on different machines. You want to be sure that building an artifact from source code produces the same result no matter when and where it is built. This is necessary for projects like reproducible-builds.org.

Reproducing the same byte-for-byte archive poses some challenges since the order of the files in an archive is influenced by the underlying file system. Each time a ZIP, TAR, JAR, WAR or EAR is built from source, the order of the files inside the archive may change. Files that only have a different timestamp also causes differences in archives from build to build. All AbstractArchiveTask (e.g. Jar, Zip) tasks shipped with Gradle include incubating support producing reproducible archives.

For example, to make a Zip task reproducible you need to set Zip.isReproducibleFileOrder() to true and Zip.isPreserveFileTimestamps() to false. In order to make all archive tasks in your build reproducible, consider adding the following configuration to your build file:

Example 162. Activating reproducible archives

build.gradle

tasks.withType(AbstractArchiveTask) {
    preserveFileTimestamps = false
    reproducibleFileOrder = true
}

Often you will want to publish an archive, so that it is usable from another project. This process is described in Publishing artifacts

Using Ant from Gradle

Gradle provides excellent integration with Ant. You can use individual Ant tasks or entire Ant builds in your Gradle builds. In fact, you will find that it’s far easier and more powerful using Ant tasks in a Gradle build script, than it is to use Ant’s XML format. You could even use Gradle simply as a powerful Ant task scripting tool.

Ant can be divided into two layers. The first layer is the Ant language. It provides the syntax for the build.xml file, the handling of the targets, special constructs like macrodefs, and so on. In other words, everything except the Ant tasks and types. Gradle understands this language, and allows you to import your Ant build.xml directly into a Gradle project. You can then use the targets of your Ant build as if they were Gradle tasks.

The second layer of Ant is its wealth of Ant tasks and types, like javac, copy or jar. For this layer Gradle provides integration simply by relying on Groovy, and the fantastic AntBuilder.

Finally, since build scripts are Groovy scripts, you can always execute an Ant build as an external process. Your build script may contain statements like: "ant clean compile".execute().[5]

You can use Gradle’s Ant integration as a path for migrating your build from Ant to Gradle. For example, you could start by importing your existing Ant build. Then you could move your dependency declarations from the Ant script to your build file. Finally, you could move your tasks across to your build file, or replace them with some of Gradle’s plugins. This process can be done in parts over time, and you can have a working Gradle build during the entire process.

Using Ant tasks and types in your build

In your build script, a property called ant is provided by Gradle. This is a reference to an AntBuilder instance. This AntBuilder is used to access Ant tasks, types and properties from your build script. There is a very simple mapping from Ant’s build.xml format to Groovy, which is explained below.

You execute an Ant task by calling a method on the AntBuilder instance. You use the task name as the method name. For example, you execute the Ant echo task by calling the ant.echo() method. The attributes of the Ant task are passed as Map parameters to the method. Below is an example of the echo task. Notice that we can also mix Groovy code and the Ant task markup. This can be extremely powerful.

Example 163. Using an Ant task

build.gradle

task hello {
    doLast {
        String greeting = 'hello from Ant'
        ant.echo(message: greeting)
    }
}

Output of gradle hello

> gradle hello

> Task :hello
[ant:echo] hello from Ant

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

You pass nested text to an Ant task by passing it as a parameter of the task method call. In this example, we pass the message for the echo task as nested text:

Example 164. Passing nested text to an Ant task

build.gradle

task hello {
    doLast {
        ant.echo('hello from Ant')
    }
}

Output of gradle hello

> gradle hello

> Task :hello
[ant:echo] hello from Ant

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

You pass nested elements to an Ant task inside a closure. Nested elements are defined in the same way as tasks, by calling a method with the same name as the element we want to define.

Example 165. Passing nested elements to an Ant task

build.gradle

task zip {
    doLast {
        ant.zip(destfile: 'archive.zip') {
            fileset(dir: 'src') {
                include(name: '**.xml')
                exclude(name: '**.java')
            }
        }
    }
}

You can access Ant types in the same way that you access tasks, using the name of the type as the method name. The method call returns the Ant data type, which you can then use directly in your build script. In the following example, we create an Ant path object, then iterate over the contents of it.

Example 166. Using an Ant type

build.gradle

task list {
    doLast {
        def path = ant.path {
            fileset(dir: 'libs', includes: '*.jar')
        }
        path.list().each {
            println it
        }
    }
}

More information about AntBuilder can be found in 'Groovy in Action' 8.4 or at the Groovy Wiki

Using custom Ant tasks in your build

To make custom tasks available in your build, you can use the taskdef (usually easier) or typedef Ant task, just as you would in a build.xml file. You can then refer to the custom Ant task as you would a built-in Ant task.

Example 167. Using a custom Ant task

build.gradle

task check {
    doLast {
        ant.taskdef(resource: 'checkstyletask.properties') {
            classpath {
                fileset(dir: 'libs', includes: '*.jar')
            }
        }
        ant.checkstyle(config: 'checkstyle.xml') {
            fileset(dir: 'src')
        }
    }
}

You can use Gradle’s dependency management to assemble the classpath to use for the custom tasks. To do this, you need to define a custom configuration for the classpath, then add some dependencies to the configuration. This is described in more detail in Declaring Dependencies.

Example 168. Declaring the classpath for a custom Ant task

build.gradle

configurations {
    pmd
}

dependencies {
    pmd group: 'pmd', name: 'pmd', version: '4.2.5'
}

To use the classpath configuration, use the asPath property of the custom configuration.

Example 169. Using a custom Ant task and dependency management together

build.gradle

task check {
    doLast {
        ant.taskdef(name: 'pmd',
                    classname: 'net.sourceforge.pmd.ant.PMDTask',
                    classpath: configurations.pmd.asPath)
        ant.pmd(shortFilenames: 'true',
                failonruleviolation: 'true',
                rulesetfiles: file('pmd-rules.xml').toURI().toString()) {
            formatter(type: 'text', toConsole: 'true')
            fileset(dir: 'src')
        }
    }
}

Importing an Ant build

You can use the ant.importBuild() method to import an Ant build into your Gradle project. When you import an Ant build, each Ant target is treated as a Gradle task. This means you can manipulate and execute the Ant targets in exactly the same way as Gradle tasks.

Example 170. Importing an Ant build

build.gradle

ant.importBuild 'build.xml'

build.xml

<project>
    <target name="hello">
        <echo>Hello, from Ant</echo>
    </target>
</project>

Output of gradle hello

> gradle hello

> Task :hello
[ant:echo] Hello, from Ant

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

You can add a task which depends on an Ant target:

Example 171. Task that depends on Ant target

build.gradle

ant.importBuild 'build.xml'

task intro(dependsOn: hello) {
    doLast {
        println 'Hello, from Gradle'
    }
}

Output of gradle intro

> gradle intro

> Task :hello
[ant:echo] Hello, from Ant

> Task :intro
Hello, from Gradle

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

Or, you can add behaviour to an Ant target:

Example 172. Adding behaviour to an Ant target

build.gradle

ant.importBuild 'build.xml'

hello {
    doLast {
        println 'Hello, from Gradle'
    }
}

Output of gradle hello

> gradle hello

> Task :hello
[ant:echo] Hello, from Ant
Hello, from Gradle

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

It is also possible for an Ant target to depend on a Gradle task:

Example 173. Ant target that depends on Gradle task

build.gradle

ant.importBuild 'build.xml'

task intro {
    doLast {
        println 'Hello, from Gradle'
    }
}

build.xml

<project>
    <target name="hello" depends="intro">
        <echo>Hello, from Ant</echo>
    </target>
</project>

Output of gradle hello

> gradle hello

> Task :intro
Hello, from Gradle

> Task :hello
[ant:echo] Hello, from Ant

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

Sometimes it may be necessary to “rename” the task generated for an Ant target to avoid a naming collision with existing Gradle tasks. To do this, use the AntBuilder.importBuild(java.lang.Object, org.gradle.api.Transformer) method.

Example 174. Renaming imported Ant targets

build.gradle

ant.importBuild('build.xml') { antTargetName ->
    'a-' + antTargetName
}

build.xml

<project>
    <target name="hello">
        <echo>Hello, from Ant</echo>
    </target>
</project>

Output of gradle a-hello

> gradle a-hello

> Task :a-hello
[ant:echo] Hello, from Ant

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

Note that while the second argument to this method should be a Transformer, when programming in Groovy we can simply use a closure instead of an anonymous inner class (or similar) due to Groovy’s support for automatically coercing closures to single-abstract-method types.

Ant properties and references

There are several ways to set an Ant property, so that the property can be used by Ant tasks. You can set the property directly on the AntBuilder instance. The Ant properties are also available as a Map which you can change. You can also use the Ant property task. Below are some examples of how to do this.

Example 175. Setting an Ant property

build.gradle

ant.buildDir = buildDir
ant.properties.buildDir = buildDir
ant.properties['buildDir'] = buildDir
ant.property(name: 'buildDir', location: buildDir)

build.xml

<echo>buildDir = ${buildDir}</echo>

Many Ant tasks set properties when they execute. There are several ways to get the value of these properties. You can get the property directly from the AntBuilder instance. The Ant properties are also available as a Map. Below are some examples.

Example 176. Getting an Ant property

build.xml

<property name="antProp" value="a property defined in an Ant build"/>

build.gradle

println ant.antProp
println ant.properties.antProp
println ant.properties['antProp']

There are several ways to set an Ant reference:

Example 177. Setting an Ant reference

build.gradle

ant.path(id: 'classpath', location: 'libs')
ant.references.classpath = ant.path(location: 'libs')
ant.references['classpath'] = ant.path(location: 'libs')

build.xml

<path refid="classpath"/>

There are several ways to get an Ant reference:

Example 178. Getting an Ant reference

build.xml

<path id="antPath" location="libs"/>

build.gradle

println ant.references.antPath
println ant.references['antPath']

Ant logging

Gradle maps Ant message priorities to Gradle log levels so that messages logged from Ant appear in the Gradle output. By default, these are mapped as follows:

Table 6. Ant message priority mapping

Ant Message Priority Gradle Log Level

VERBOSE

DEBUG

DEBUG

DEBUG

INFO

INFO

WARN

WARN

ERROR

ERROR

Fine tuning Ant logging

The default mapping of Ant message priority to Gradle log level can sometimes be problematic. For example, there is no message priority that maps directly to the LIFECYCLE log level, which is the default for Gradle. Many Ant tasks log messages at the INFO priority, which means to expose those messages from Gradle, a build would have to be run with the log level set to INFO, potentially logging much more output than is desired.

Conversely, if an Ant task logs messages at too high of a level, to suppress those messages would require the build to be run at a higher log level, such as QUIET. However, this could result in other, desirable output being suppressed.

To help with this, Gradle allows the user to fine tune the Ant logging and control the mapping of message priority to Gradle log level. This is done by setting the priority that should map to the default Gradle LIFECYCLE log level using the AntBuilder.setLifecycleLogLevel(java.lang.String) method. When this value is set, any Ant message logged at the configured priority or above will be logged at least at LIFECYCLE. Any Ant message logged below this priority will be logged at most at INFO.

For example, the following changes the mapping such that Ant INFO priority messages are exposed at the LIFECYCLE log level.

Example 179. Fine tuning Ant logging

build.gradle

ant.lifecycleLogLevel = "INFO"

task hello {
    doLast {
        ant.echo(level: "info", message: "hello from info priority!")
    }
}

Output of gradle hello

> gradle hello

> Task :hello
[ant:echo] hello from info priority!

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

On the other hand, if the lifecycleLogLevel was set to ERROR, Ant messages logged at the WARN priority would no longer be logged at the WARN log level. They would now be logged at the INFO level and would be suppressed by default.

API

The Ant integration is provided by AntBuilder.



[5] In Groovy you can execute Strings. To learn more about executing external processes with Groovy have a look in 'Groovy in Action' 9.3.2 or at the Groovy wiki

Build Lifecycle

We said earlier that the core of Gradle is a language for dependency based programming. In Gradle terms this means that you can define tasks and dependencies between tasks. Gradle guarantees that these tasks are executed in the order of their dependencies, and that each task is executed only once. These tasks form a Directed Acyclic Graph. There are build tools that build up such a dependency graph as they execute their tasks. Gradle builds the complete dependency graph before any task is executed. This lies at the heart of Gradle and makes many things possible which would not be possible otherwise.

Your build scripts configure this dependency graph. Therefore they are strictly speaking build configuration scripts.

Build phases

A Gradle build has three distinct phases.

Initialization

Gradle supports single and multi-project builds. During the initialization phase, Gradle determines which projects are going to take part in the build, and creates a Project instance for each of these projects.

Configuration

During this phase the project objects are configured. The build scripts of all projects which are part of the build are executed. Gradle 1.4 introduced an incubating opt-in feature called configuration on demand. In this mode, Gradle configures only relevant projects (see the section called “Configuration on demand”).

Execution

Gradle determines the subset of the tasks, created and configured during the configuration phase, to be executed. The subset is determined by the task name arguments passed to the gradle command and the current directory. Gradle then executes each of the selected tasks.

Settings file

Beside the build script files, Gradle defines a settings file. The settings file is determined by Gradle via a naming convention. The default name for this file is settings.gradle. Later in this chapter we explain how Gradle looks for a settings file.

The settings file is executed during the initialization phase. A multiproject build must have a settings.gradle file in the root project of the multiproject hierarchy. It is required because the settings file defines which projects are taking part in the multi-project build (see Authoring Multi-Project Builds). For a single-project build, a settings file is optional. Besides defining the included projects, you might need it to add libraries to your build script classpath (see Organizing Build Logic). Let’s first do some introspection with a single project build:

Example 180. Single project build

settings.gradle

println 'This is executed during the initialization phase.'

build.gradle

println 'This is executed during the configuration phase.'

task configured {
    println 'This is also executed during the configuration phase.'
}

task test {
    doLast {
        println 'This is executed during the execution phase.'
    }
}

task testBoth {
    doFirst {
      println 'This is executed first during the execution phase.'
    }
    doLast {
      println 'This is executed last during the execution phase.'
    }
    println 'This is executed during the configuration phase as well.'
}

Output of gradle test testBoth

> gradle test testBoth
This is executed during the initialization phase.

> Configure project :
This is executed during the configuration phase.
This is also executed during the configuration phase.
This is executed during the configuration phase as well.

> Task :test
This is executed during the execution phase.

> Task :testBoth
This is executed first during the execution phase.
This is executed last during the execution phase.

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

For a build script, the property access and method calls are delegated to a project object. Similarly property access and method calls within the settings file is delegated to a settings object. Look at the Settings class in the API documentation for more information.

Multi-project builds

A multi-project build is a build where you build more than one project during a single execution of Gradle. You have to declare the projects taking part in the multiproject build in the settings file. There is much more to say about multi-project builds in the chapter dedicated to this topic (see Authoring Multi-Project Builds).

Project locations

Multi-project builds are always represented by a tree with a single root. Each element in the tree represents a project. A project has a path which denotes the position of the project in the multi-project build tree. In most cases the project path is consistent with the physical location of the project in the file system. However, this behavior is configurable. The project tree is created in the settings.gradle file. By default it is assumed that the location of the settings file is also the location of the root project. But you can redefine the location of the root project in the settings file.

Building the tree

In the settings file you can use a set of methods to build the project tree. Hierarchical and flat physical layouts get special support.

Hierarchical layouts

Example 181. Hierarchical layout

settings.gradle

include 'project1', 'project2:child', 'project3:child1'

The include method takes project paths as arguments. The project path is assumed to be equal to the relative physical file system path. For example, a path 'services:api' is mapped by default to a folder 'services/api' (relative from the project root). You only need to specify the leaves of the tree. This means that the inclusion of the path 'services:hotels:api' will result in creating 3 projects: 'services', 'services:hotels' and 'services:hotels:api'. More examples of how to work with the project path can be found in the DSL documentation of Settings.include(java.lang.String[]).

Flat layouts

Example 182. Flat layout

settings.gradle

includeFlat 'project3', 'project4'

The includeFlat method takes directory names as an argument. These directories need to exist as siblings of the root project directory. The location of these directories are considered as child projects of the root project in the multi-project tree.

Modifying elements of the project tree

The multi-project tree created in the settings file is made up of so called project descriptors. You can modify these descriptors in the settings file at any time. To access a descriptor you can do:

Example 183. Lookup of elements of the project tree

settings.gradle

println rootProject.name
println project(':projectA').name

Using this descriptor you can change the name, project directory and build file of a project.

Example 184. Modification of elements of the project tree

settings.gradle

rootProject.name = 'main'
project(':projectA').projectDir = new File(settingsDir, '../my-project-a')
project(':projectA').buildFileName = 'projectA.gradle'

Look at the ProjectDescriptor class in the API documentation for more information.

Initialization

How does Gradle know whether to do a single or multiproject build? If you trigger a multiproject build from a directory with a settings file, things are easy. But Gradle also allows you to execute the build from within any subproject taking part in the build.[6] If you execute Gradle from within a project with no settings.gradle file, Gradle looks for a settings.gradle file in the following way:

  • It looks in a directory called master which has the same nesting level as the current dir.

  • If not found yet, it searches parent directories.

  • If not found yet, the build is executed as a single project build.

  • If a settings.gradle file is found, Gradle checks if the current project is part of the multiproject hierarchy defined in the found settings.gradle file. If not, the build is executed as a single project build. Otherwise a multiproject build is executed.

What is the purpose of this behavior? Gradle needs to determine whether the project you are in is a subproject of a multiproject build or not. Of course, if it is a subproject, only the subproject and its dependent projects are built, but Gradle needs to create the build configuration for the whole multiproject build (see Authoring Multi-Project Builds). You can use the -u command line option to tell Gradle not to look in the parent hierarchy for a settings.gradle file. The current project is then always built as a single project build. If the current project contains a settings.gradle file, the -u option has no meaning. Such a build is always executed as:

  • a single project build, if the settings.gradle file does not define a multiproject hierarchy

  • a multiproject build, if the settings.gradle file does define a multiproject hierarchy.

The automatic search for a settings.gradle file only works for multi-project builds with a physical hierarchical or flat layout. For a flat layout you must additionally follow the naming convention described above (“master”). Gradle supports arbitrary physical layouts for a multiproject build, but for such arbitrary layouts you need to execute the build from the directory where the settings file is located. For information on how to run partial builds from the root see the section called “Running tasks by their absolute path”.

Gradle creates a Project object for every project taking part in the build. For a multi-project build these are the projects specified in the Settings object (plus the root project). Each project object has by default a name equal to the name of its top level directory, and every project except the root project has a parent project. Any project may have child projects.

Configuration and execution of a single project build

For a single project build, the workflow of the after initialization phases are pretty simple. The build script is executed against the project object that was created during the initialization phase. Then Gradle looks for tasks with names equal to those passed as command line arguments. If these task names exist, they are executed as a separate build in the order you have passed them. The configuration and execution for multi-project builds is discussed in Authoring Multi-Project Builds.

Responding to the lifecycle in the build script

Your build script can receive notifications as the build progresses through its lifecycle. These notifications generally take two forms: You can either implement a particular listener interface, or you can provide a closure to execute when the notification is fired. The examples below use closures. For details on how to use the listener interfaces, refer to the API documentation.

Project evaluation

You can receive a notification immediately before and after a project is evaluated. This can be used to do things like performing additional configuration once all the definitions in a build script have been applied, or for some custom logging or profiling.

Below is an example which adds a test task to each project which has a hasTests property value of true.

Example 185. Adding of test task to each project which has certain property set

build.gradle

allprojects {
    afterEvaluate { project ->
        if (project.hasTests) {
            println "Adding test task to $project"
            project.task('test') {
                doLast {
                    println "Running tests for $project"
                }
            }
        }
    }
}

projectA.gradle

hasTests = true

Output of gradle -q test

> gradle -q test
Adding test task to project ':projectA'
Running tests for project ':projectA'

This example uses method Project.afterEvaluate() to add a closure which is executed after the project is evaluated.

It is also possible to receive notifications when any project is evaluated. This example performs some custom logging of project evaluation. Notice that the afterProject notification is received regardless of whether the project evaluates successfully or fails with an exception.

Example 186. Notifications

build.gradle

gradle.afterProject {project, projectState ->
    if (projectState.failure) {
        println "Evaluation of $project FAILED"
    } else {
        println "Evaluation of $project succeeded"
    }
}

Output of gradle -q test

> gradle -q test
Evaluation of root project 'buildProjectEvaluateEvents' succeeded
Evaluation of project ':projectA' succeeded
Evaluation of project ':projectB' FAILED

BUILD FAILED in 0s

You can also add a ProjectEvaluationListener to the Gradle to receive these events.

Task creation

You can receive a notification immediately after a task is added to a project. This can be used to set some default values or add behaviour before the task is made available in the build file.

The following example sets the srcDir property of each task as it is created.

Example 187. Setting of certain property to all tasks

build.gradle

tasks.whenTaskAdded { task ->
    task.ext.srcDir = 'src/main/java'
}

task a

println "source dir is $a.srcDir"

Output of gradle -q a

> gradle -q a
source dir is src/main/java

You can also add an Action to a TaskContainer to receive these events.

Task execution graph ready

You can receive a notification immediately after the task execution graph has been populated. We have seen this already in the section called “Configure by DAG”.

You can also add a TaskExecutionGraphListener to the TaskExecutionGraph to receive these events.

Task execution

You can receive a notification immediately before and after any task is executed.

The following example logs the start and end of each task execution. Notice that the afterTask notification is received regardless of whether the task completes successfully or fails with an exception.

Example 188. Logging of start and end of each task execution

build.gradle

task ok

task broken(dependsOn: ok) {
    doLast {
        throw new RuntimeException('broken')
    }
}

gradle.taskGraph.beforeTask { Task task ->
    println "executing $task ..."
}

gradle.taskGraph.afterTask { Task task, TaskState state ->
    if (state.failure) {
        println "FAILED"
    }
    else {
        println "done"
    }
}

Output of gradle -q broken

> gradle -q broken
executing task ':ok' ...
done
executing task ':broken' ...
FAILED

BUILD FAILED in 0s

You can also use a TaskExecutionListener to the TaskExecutionGraph to receive these events.



[6] Gradle supports partial multiproject builds (see Authoring Multi-Project Builds).

Logging

The log is the main 'UI' of a build tool. If it is too verbose, real warnings and problems are easily hidden by this. On the other hand you need relevant information for figuring out if things have gone wrong. Gradle defines 6 log levels, as shown in Table 7. There are two Gradle-specific log levels, in addition to the ones you might normally see. Those levels are QUIET and LIFECYCLE. The latter is the default, and is used to report build progress.

Table 7. Log levels

Level Used for

ERROR

Error messages

QUIET

Important information messages

WARNING

Warning messages

LIFECYCLE

Progress information messages

INFO

Information messages

DEBUG

Debug messages

The rich components of the console (build status and work in progress area) are displayed regardless of the log level used. Before Gradle 4.0 those rich components were only displayed at log level LIFECYCLE or below.

Choosing a log level

You can use the command line switches shown in Table 8 to choose different log levels. You can also configure the log level using gradle.properties, see the section called “Gradle properties”. In Table 9 you find the command line switches which affect stacktrace logging.

Table 8. Log level command-line options

Option Outputs Log Levels

no logging options

LIFECYCLE and higher

-q or --quiet

QUIET and higher

-w or --warn

WARN and higher

-i or --info

INFO and higher

-d or --debug

DEBUG and higher (that is, all log messages)

Table 9. Stacktrace command-line options

Option Meaning

No stacktrace options

No stacktraces are printed to the console in case of a build error (e.g. a compile error). Only in case of internal exceptions will stacktraces be printed. If the DEBUG log level is chosen, truncated stacktraces are always printed.

-s or --stacktrace

Truncated stacktraces are printed. We recommend this over full stacktraces. Groovy full stacktraces are extremely verbose (Due to the underlying dynamic invocation mechanisms. Yet they usually do not contain relevant information for what has gone wrong in your code.) This option renders stacktraces for deprecation warnings.

-S or --full-stacktrace

The full stacktraces are printed out. This option renders stacktraces for deprecation warnings.

Writing your own log messages

A simple option for logging in your build file is to write messages to standard output. Gradle redirects anything written to standard output to its logging system at the QUIET log level.

Example 189. Using stdout to write log messages

build.gradle

println 'A message which is logged at QUIET level'

Gradle also provides a logger property to a build script, which is an instance of Logger. This interface extends the SLF4J Logger interface and adds a few Gradle specific methods to it. Below is an example of how this is used in the build script:

Example 190. Writing your own log messages

build.gradle

logger.quiet('An info log message which is always logged.')
logger.error('An error log message.')
logger.warn('A warning log message.')
logger.lifecycle('A lifecycle info log message.')
logger.info('An info log message.')
logger.debug('A debug log message.')
logger.trace('A trace log message.')

Use the typical SLF4J pattern to replace a placeholder with an actual value as part of the log message.

Example 191. Writing a log message with placeholder

build.gradle

logger.info('A {} log message', 'info')

You can also hook into Gradle’s logging system from within other classes used in the build (classes from the buildSrc directory for example). Simply use an SLF4J logger. You can use this logger the same way as you use the provided logger in the build script.

Example 192. Using SLF4J to write log messages

build.gradle

import org.slf4j.Logger
import org.slf4j.LoggerFactory

Logger slf4jLogger = LoggerFactory.getLogger('some-logger')
slf4jLogger.info('An info log message logged using SLF4j')

Logging from external tools and libraries

Internally, Gradle uses Ant and Ivy. Both have their own logging system. Gradle redirects their logging output into the Gradle logging system. There is a 1:1 mapping from the Ant/Ivy log levels to the Gradle log levels, except the Ant/Ivy TRACE log level, which is mapped to Gradle DEBUG log level. This means the default Gradle log level will not show any Ant/Ivy output unless it is an error or a warning.

There are many tools out there which still use standard output for logging. By default, Gradle redirects standard output to the QUIET log level and standard error to the ERROR level. This behavior is configurable. The project object provides a LoggingManager, which allows you to change the log levels that standard out or error are redirected to when your build script is evaluated.

Example 193. Configuring standard output capture

build.gradle

logging.captureStandardOutput LogLevel.INFO
println 'A message which is logged at INFO level'

To change the log level for standard out or error during task execution, tasks also provide a LoggingManager.

Example 194. Configuring standard output capture for a task

build.gradle

task logInfo {
    logging.captureStandardOutput LogLevel.INFO
    doFirst {
        println 'A task message which is logged at INFO level'
    }
}

Gradle also provides integration with the Java Util Logging, Jakarta Commons Logging and Log4j logging toolkits. Any log messages which your build classes write using these logging toolkits will be redirected to Gradle’s logging system.

Changing what Gradle logs

You can replace much of Gradle’s logging UI with your own. You might do this, for example, if you want to customize the UI in some way - to log more or less information, or to change the formatting. You replace the logging using the Gradle.useLogger(java.lang.Object) method. This is accessible from a build script, or an init script, or via the embedding API. Note that this completely disables Gradle’s default output. Below is an example init script which changes how task execution and build completion is logged.

Example 195. Customizing what Gradle logs

init.gradle

useLogger(new CustomEventLogger())

class CustomEventLogger extends BuildAdapter implements TaskExecutionListener {

    public void beforeExecute(Task task) {
        println "[$task.name]"
    }

    public void afterExecute(Task task, TaskState state) {
        println()
    }
    
    public void buildFinished(BuildResult result) {
        println 'build completed'
        if (result.failure != null) {
            result.failure.printStackTrace()
        }
    }
}

Output of gradle -I init.gradle build

> gradle -I init.gradle build

> Task :compile
[compile]
compiling source


> Task :testCompile
[testCompile]
compiling test source


> Task :test
[test]
running unit tests


> Task :build
[build]

build completed
3 actionable tasks: 3 executed

Your logger can implement any of the listener interfaces listed below. When you register a logger, only the logging for the interfaces that it implements is replaced. Logging for the other interfaces is left untouched. You can find out more about the listener interfaces in the section called “Responding to the lifecycle in the build script”.

Authoring Multi-Project Builds

The powerful support for multi-project builds is one of Gradle’s unique selling points. This topic is also the most intellectually challenging.

A multi-project build in gradle consists of one root project, and one or more subprojects that may also have subprojects.

Cross project configuration

While each subproject could configure itself in complete isolation of the other subprojects, it is common that subprojects share common traits. It is then usually preferable to share configurations among projects, so the same configuration affects several subprojects.

Let’s start with a very simple multi-project build. Gradle is a general purpose build tool at its core, so the projects don’t have to be Java projects. Our first examples are about marine life.

Configuration and execution

the section called “Build phases” describes the phases of every Gradle build. Let’s zoom into the configuration and execution phases of a multi-project build. Configuration here means executing the build.gradle file of a project, which implies e.g. downloading all plugins that were declared using ‘apply plugin’. By default, the configuration of all projects happens before any task is executed. This means that when a single task, from a single project is requested, all projects of multi-project build are configured first. The reason every project needs to be configured is to support the flexibility of accessing and changing any part of the Gradle project model.

Configuration on demand

The Configuration injection feature and access to the complete project model are possible because every project is configured before the execution phase. Yet, this approach may not be the most efficient in a very large multi-project build. There are Gradle builds with a hierarchy of hundreds of subprojects. The configuration time of huge multi-project builds may become noticeable. Scalability is an important requirement for Gradle. Hence, starting from version 1.4 a new incubating 'configuration on demand' mode is introduced.

Configuration on demand mode attempts to configure only projects that are relevant for requested tasks, i.e. it only executes the build.gradle file of projects that are participating in the build. This way, the configuration time of a large multi-project build can be reduced. In the long term, this mode will become the default mode, possibly the only mode for Gradle build execution. The configuration on demand feature is incubating so not every build is guaranteed to work correctly. The feature should work very well for multi-project builds that have decoupled projects (the section called “Decoupled Projects”). In “configuration on demand” mode, projects are configured as follows:

  • The root project is always configured. This way the typical common configuration is supported (allprojects or subprojects script blocks).

  • The project in the directory where the build is executed is also configured, but only when Gradle is executed without any tasks. This way the default tasks behave correctly when projects are configured on demand.

  • The standard project dependencies are supported and makes relevant projects configured. If project A has a compile dependency on project B then building A causes configuration of both projects.

  • The task dependencies declared via task path are supported and cause relevant projects to be configured. Example: someTask.dependsOn(":someOtherProject:someOtherTask")

  • A task requested via task path from the command line (or Tooling API) causes the relevant project to be configured. For example, building 'projectA:projectB:someTask' causes configuration of projectB.

Eager to try out this new feature? To configure on demand with every build run see the section called “Gradle properties”. To configure on demand just for a given build, see the section called “Performance options”.

Defining common behavior

Let’s look at some examples with the following project tree. This is a multi-project build with a root project named water and a subproject named bluewhale.

Example 196. Multi-project tree - water & bluewhale projects

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/

Note: The code for this example can be found at samples/userguide/multiproject/firstExample/water in the ‘-all’ distribution of Gradle.

settings.gradle

include 'bluewhale'

And where is the build script for the bluewhale project? In Gradle build scripts are optional. Obviously for a single project build, a project without a build script doesn’t make much sense. For multiproject builds the situation is different. Let’s look at the build script for the water project and execute it:

Example 197. Build script of water (parent) project

build.gradle

Closure cl = { task -> println "I'm $task.project.name" }
task('hello').doLast(cl)
project(':bluewhale') {
    task('hello').doLast(cl)
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale

Gradle allows you to access any project of the multi-project build from any build script. The Project API provides a method called project(), which takes a path as an argument and returns the Project object for this path. The capability to configure a project build from any build script we call cross project configuration. Gradle implements this via configuration injection.

We are not that happy with the build script of the water project. It is inconvenient to add the task explicitly for every project. We can do better. Let’s first add another project called krill to our multi-project build.

Example 198. Multi-project tree - water, bluewhale & krill projects

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
  krill/

Note: The code for this example can be found at samples/userguide/multiproject/addKrill/water in the ‘-all’ distribution of Gradle.

settings.gradle

include 'bluewhale', 'krill'

Now we rewrite the water build script and boil it down to a single line.

Example 199. Water project build script

build.gradle

allprojects {
    task hello {
        doLast { task ->
            println "I'm $task.project.name"
        }
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
I'm krill

Is this cool or is this cool? And how does this work? The Project API provides a property allprojects which returns a list with the current project and all its subprojects underneath it. If you call allprojects with a closure, the statements of the closure are delegated to the projects associated with allprojects. You could also do an iteration via allprojects.each, but that would be more verbose.

Other build systems use inheritance as the primary means for defining common behavior. We also offer inheritance for projects as you will see later. But Gradle uses configuration injection as the usual way of defining common behavior. We think it provides a very powerful and flexible way of configuring multiproject builds.

Another possibility for sharing configuration is to use a common external script. See the section called “Configuring the project using an external build script” for more information.

Subproject configuration

The Project API also provides a property for accessing the subprojects only.

Defining common behavior

Example 200. Defining common behavior of all projects and subprojects

build.gradle

allprojects {
    task hello {
        doLast { task ->
            println "I'm $task.project.name"
        }
    }
}
subprojects {
    hello {
        doLast {
            println "- I depend on water"
        }
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
I'm krill
- I depend on water

You may notice that there are two code snippets referencing the “hello” task. The first one, which uses the “task” keyword, constructs the task and provides it’s base configuration. The second piece doesn’t use the “task” keyword, as it is further configuring the existing “hello” task. You may only construct a task once in a project, but you may add any number of code blocks providing additional configuration.

Adding specific behavior

You can add specific behavior on top of the common behavior. Usually we put the project specific behavior in the build script of the project where we want to apply this specific behavior. But as we have already seen, we don’t have to do it this way. We could add project specific behavior for the bluewhale project like this:

Example 201. Defining specific behaviour for particular project

build.gradle

allprojects {
    task hello {
        doLast { task ->
            println "I'm $task.project.name"
        }
    }
}
subprojects {
    hello {
        doLast {
            println "- I depend on water"
        }
    }
}
project(':bluewhale').hello {
    doLast {
        println "- I'm the largest animal that has ever lived on this planet."
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
I'm krill
- I depend on water

As we have said, we usually prefer to put project specific behavior into the build script of this project. Let’s refactor and also add some project specific behavior to the krill project.

Example 202. Defining specific behaviour for project krill

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
    build.gradle
  krill/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/spreadSpecifics/water in the ‘-all’ distribution of Gradle.

settings.gradle

include 'bluewhale', 'krill'

bluewhale/build.gradle

hello.doLast {
  println "- I'm the largest animal that has ever lived on this planet."
}

krill/build.gradle

hello.doLast {
  println "- The weight of my species in summer is twice as heavy as all human beings."
}

build.gradle

allprojects {
    task hello {
        doLast { task ->
            println "I'm $task.project.name"
        }
    }
}
subprojects {
    hello {
        doLast {
            println "- I depend on water"
        }
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
I'm krill
- I depend on water
- The weight of my species in summer is twice as heavy as all human beings.

Project filtering

To show more of the power of configuration injection, let’s add another project called tropicalFish and add more behavior to the build via the build script of the water project.

Filtering by name

Example 203. Adding custom behaviour to some projects (filtered by project name)

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
    build.gradle
  krill/
    build.gradle
  tropicalFish/

Note: The code for this example can be found at samples/userguide/multiproject/addTropical/water in the ‘-all’ distribution of Gradle.

settings.gradle

include 'bluewhale', 'krill', 'tropicalFish'

build.gradle

allprojects {
    task hello {
        doLast { task ->
            println "I'm $task.project.name"
        }
    }
}
subprojects {
    hello {
        doLast {
            println "- I depend on water"
        }
    }
}
configure(subprojects.findAll {it.name != 'tropicalFish'}) {
    hello {
        doLast {
            println '- I love to spend time in the arctic waters.'
        }
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I love to spend time in the arctic waters.
- I'm the largest animal that has ever lived on this planet.
I'm krill
- I depend on water
- I love to spend time in the arctic waters.
- The weight of my species in summer is twice as heavy as all human beings.
I'm tropicalFish
- I depend on water

The configure() method takes a list as an argument and applies the configuration to the projects in this list.

Filtering by properties

Using the project name for filtering is one option. Using extra project properties is another. (See the section called “Extra properties” for more information on extra properties.)

Example 204. Adding custom behaviour to some projects (filtered by project properties)

Build layout

water/
  build.gradle
  settings.gradle
  bluewhale/
    build.gradle
  krill/
    build.gradle
  tropicalFish/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/tropicalWithProperties/water in the ‘-all’ distribution of Gradle.

settings.gradle

include 'bluewhale', 'krill', 'tropicalFish'

bluewhale/build.gradle

ext.arctic = true
hello.doLast {
  println "- I'm the largest animal that has ever lived on this planet."
}

krill/build.gradle

ext.arctic = true
hello.doLast {
    println "- The weight of my species in summer is twice as heavy as all human beings."
}

tropicalFish/build.gradle

ext.arctic = false

build.gradle

allprojects {
    task hello {
        doLast { task ->
            println "I'm $task.project.name"
        }
    }
}
subprojects {
    hello {
        doLast {println "- I depend on water"}
        afterEvaluate { Project project ->
            if (project.arctic) { doLast {
                println '- I love to spend time in the arctic waters.' }
            }
        }
    }
}

Output of gradle -q hello

> gradle -q hello
I'm water
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
- I love to spend time in the arctic waters.
I'm krill
- I depend on water
- The weight of my species in summer is twice as heavy as all human beings.
- I love to spend time in the arctic waters.
I'm tropicalFish
- I depend on water

In the build file of the water project we use an afterEvaluate notification. This means that the closure we are passing gets evaluated after the build scripts of the subproject are evaluated. As the property arctic is set in those build scripts, we have to do it this way. You will find more on this topic in the section called “Dependencies - Which dependencies?”

Execution rules for multi-project builds

When we executed the hello task from the root project dir, things behaved in an intuitive way. All the hello tasks of the different projects were executed. Let’s switch to the bluewhale dir and see what happens if we execute Gradle from there.

Example 205. Running build from subproject

Output of gradle -q hello

> gradle -q hello
I'm bluewhale
- I depend on water
- I'm the largest animal that has ever lived on this planet.
- I love to spend time in the arctic waters.

The basic rule behind Gradle’s behavior is simple. Gradle looks down the hierarchy, starting with the current dir, for tasks with the name hello and executes them. One thing is very important to note. Gradle always evaluates every project of the multi-project build and creates all existing task objects. Then, according to the task name arguments and the current dir, Gradle filters the tasks which should be executed. Because of Gradle’s cross project configuration every project has to be evaluated before any task gets executed. We will have a closer look at this in the next section. Let’s now have our last marine example. Let’s add a task to bluewhale and krill.

Example 206. Evaluation and execution of projects

bluewhale/build.gradle

ext.arctic = true
hello {
    doLast {
        println "- I'm the largest animal that has ever lived on this planet."
    }
}

task distanceToIceberg {
    doLast {
        println '20 nautical miles'
    }
}

krill/build.gradle

ext.arctic = true
hello {
    doLast {
        println "- The weight of my species in summer is twice as heavy as all human beings."
    }
}

task distanceToIceberg {
    doLast {
        println '5 nautical miles'
    }
}

Output of gradle -q distanceToIceberg

> gradle -q distanceToIceberg
20 nautical miles
5 nautical miles

Here’s the output without the -q option:

Example 207. Evaluation and execution of projects

Output of gradle distanceToIceberg

> gradle distanceToIceberg

> Task :bluewhale:distanceToIceberg
20 nautical miles

> Task :krill:distanceToIceberg
5 nautical miles

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

The build is executed from the water project. Neither water nor tropicalFish have a task with the name distanceToIceberg. Gradle does not care. The simple rule mentioned already above is: Execute all tasks down the hierarchy which have this name. Only complain if there is no such task!

Running tasks by their absolute path

As we have seen, you can run a multi-project build by entering any subproject dir and execute the build from there. All matching task names of the project hierarchy starting with the current dir are executed. But Gradle also offers to execute tasks by their absolute path (see also the section called “Project and task paths”):

Example 208. Running tasks by their absolute path

Output of gradle -q :hello :krill:hello hello

> gradle -q :hello :krill:hello hello
I'm water
I'm krill
- I depend on water
- The weight of my species in summer is twice as heavy as all human beings.
- I love to spend time in the arctic waters.
I'm tropicalFish
- I depend on water

The build is executed from the tropicalFish project. We execute the hello tasks of the water, the krill and the tropicalFish project. The first two tasks are specified by their absolute path, the last task is executed using the name matching mechanism described above.

Project and task paths

A project path has the following pattern: It starts with an optional colon, which denotes the root project. The root project is the only project in a path that is not specified by its name. The rest of a project path is a colon-separated sequence of project names, where the next project is a subproject of the previous project.

The path of a task is simply its project path plus the task name, like “:bluewhale:hello”. Within a project you can address a task of the same project just by its name. This is interpreted as a relative path.

Dependencies - Which dependencies?

The examples from the last section were special, as the projects had no Execution Dependencies. They had only Configuration Dependencies. The following sections illustrate the differences between these two types of dependencies.

Execution dependencies

Dependencies and execution order

Example 209. Dependencies and execution order

Build layout

messages/
  build.gradle
  settings.gradle
  consumer/
    build.gradle
  producer/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/firstMessages/messages in the ‘-all’ distribution of Gradle.

build.gradle

ext.producerMessage = null

settings.gradle

include 'consumer', 'producer'

consumer/build.gradle

task action {
    doLast {
        println("Consuming message: ${rootProject.producerMessage}")
    }
}

producer/build.gradle

task action {
    doLast {
        println "Producing message:"
        rootProject.producerMessage = 'Watch the order of execution.'
    }
}

Output of gradle -q action

> gradle -q action
Consuming message: null
Producing message:

This didn’t quite do what we want. If nothing else is defined, Gradle executes the task in alphanumeric order. Therefore, Gradle will execute “:consumer:action” before “:producer:action”. Let’s try to solve this with a hack and rename the producer project to “aProducer”.

Example 210. Dependencies and execution order

Build layout

messages/
  build.gradle
  settings.gradle
  aProducer/
    build.gradle
  consumer/
    build.gradle

build.gradle

ext.producerMessage = null

settings.gradle

include 'consumer', 'aProducer'

aProducer/build.gradle

task action {
    doLast {
        println "Producing message:"
        rootProject.producerMessage = 'Watch the order of execution.'
    }
}

consumer/build.gradle

task action {
    doLast {
        println("Consuming message: ${rootProject.producerMessage}")
    }
}

Output of gradle -q action

> gradle -q action
Producing message:
Consuming message: Watch the order of execution.

We can show where this hack doesn’t work if we now switch to the consumer dir and execute the build.

Example 211. Dependencies and execution order

Output of gradle -q action

> gradle -q action
Consuming message: null

The problem is that the two “action” tasks are unrelated. If you execute the build from the “messages” project Gradle executes them both because they have the same name and they are down the hierarchy. In the last example only one “action” task was down the hierarchy and therefore it was the only task that was executed. We need something better than this hack.

Declaring dependencies

Example 212. Declaring dependencies

Build layout

messages/
  build.gradle
  settings.gradle
  consumer/
    build.gradle
  producer/
    build.gradle

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/messagesWithDependencies/messages in the ‘-all’ distribution of Gradle.

build.gradle

ext.producerMessage = null

settings.gradle

include 'consumer', 'producer'

consumer/build.gradle

task action(dependsOn: ":producer:action") {
    doLast {
        println("Consuming message: ${rootProject.producerMessage}")
    }
}

producer/build.gradle

task action {
    doLast {
        println "Producing message:"
        rootProject.producerMessage = 'Watch the order of execution.'
    }
}

Output of gradle -q action

> gradle -q action
Producing message:
Consuming message: Watch the order of execution.

Running this from the consumer directory gives:

Example 213. Declaring dependencies

Output of gradle -q action

> gradle -q action
Producing message:
Consuming message: Watch the order of execution.

This is now working better because we have declared that the “action” task in the “consumer” project has an execution dependency on the “action” task in the “producer” project.

The nature of cross project task dependencies

Of course, task dependencies across different projects are not limited to tasks with the same name. Let’s change the naming of our tasks and execute the build.

Example 214. Cross project task dependencies

consumer/build.gradle

task consume(dependsOn: ':producer:produce') {
    doLast {
        println("Consuming message: ${rootProject.producerMessage}")
    }
}

producer/build.gradle

task produce {
    doLast {
        println "Producing message:"
        rootProject.producerMessage = 'Watch the order of execution.'
    }
}

Output of gradle -q consume

> gradle -q consume
Producing message:
Consuming message: Watch the order of execution.

Configuration time dependencies

Let’s see one more example with our producer-consumer build before we enter Java land. We add a property to the “producer” project and create a configuration time dependency from “consumer” to “producer”.

Example 215. Configuration time dependencies

consumer/build.gradle

def message = rootProject.producerMessage

task consume {
    doLast {
        println("Consuming message: " + message)
    }
}

producer/build.gradle

rootProject.producerMessage = 'Watch the order of evaluation.'

Output of gradle -q consume

> gradle -q consume
Consuming message: null

The default evaluation order of projects is alphanumeric (for the same nesting level). Therefore the “consumer” project is evaluated before the “producer” project and the “producerMessage” value is set after it is read by the “consumer” project. Gradle offers a solution for this.

Example 216. Configuration time dependencies - evaluationDependsOn

consumer/build.gradle

evaluationDependsOn(':producer')

def message = rootProject.producerMessage

task consume {
    doLast {
        println("Consuming message: " + message)
    }
}

Output of gradle -q consume

> gradle -q consume
Consuming message: Watch the order of evaluation.

The use of the “evaluationDependsOn” command results in the evaluation of the “producer” project before the “consumer” project is evaluated. This example is a bit contrived to show the mechanism. In this case there would be an easier solution by reading the key property at execution time.

Example 217. Configuration time dependencies

consumer/build.gradle

task consume {
    doLast {
        println("Consuming message: ${rootProject.producerMessage}")
    }
}

Output of gradle -q consume

> gradle -q consume
Consuming message: Watch the order of evaluation.

Configuration dependencies are very different from execution dependencies. Configuration dependencies are between projects whereas execution dependencies are always resolved to task dependencies. Also note that all projects are always configured, even when you start the build from a subproject. The default configuration order is top down, which is usually what is needed.

To change the default configuration order to “bottom up”, use the “evaluationDependsOnChildren()” method instead.

On the same nesting level the configuration order depends on the alphanumeric position. The most common use case is to have multi-project builds that share a common lifecycle (e.g. all projects use the Java plugin). If you declare with dependsOn an execution dependency between different projects, the default behavior of this method is to also create a configuration dependency between the two projects. Therefore it is likely that you don’t have to define configuration dependencies explicitly.

Real life examples

Gradle’s multi-project features are driven by real life use cases. One good example consists of two web application projects and a parent project that creates a distribution including the two web applications.[7] For the example we use only one build script and do cross project configuration.

Example 218. Dependencies - real life example - crossproject configuration

Build layout

webDist/
  settings.gradle
  build.gradle
  date/
    src/main/java/
      org/gradle/sample/
        DateServlet.java
  hello/
    src/main/java/
      org/gradle/sample/
        HelloServlet.java

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/webDist in the ‘-all’ distribution of Gradle.

settings.gradle

include 'date', 'hello'

build.gradle

allprojects {
    apply plugin: 'java'
    group = 'org.gradle.sample'
    version = '1.0'
}

subprojects {
    apply plugin: 'war'
    repositories {
        mavenCentral()
    }
    dependencies {
        compile "javax.servlet:servlet-api:2.5"
    }
}

task explodedDist(type: Copy) {
    into "$buildDir/explodedDist"
    subprojects {
        from tasks.withType(War)
    }
}

We have an interesting set of dependencies. Obviously the date and hello projects have a configuration dependency on webDist, as all the build logic for the webapp projects is injected by webDist. The execution dependency is in the other direction, as webDist depends on the build artifacts of date and hello. There is even a third dependency. webDist has a configuration dependency on date and hello because it needs to know the archivePath. But it asks for this information at execution time. Therefore we have no circular dependency.

Such dependency patterns are daily bread in the problem space of multi-project builds. If a build system does not support these patterns, you either can’t solve your problem or you need to do ugly hacks which are hard to maintain and massively impair your productivity as a build master.

Project lib dependencies

What if one project needs the jar produced by another project in its compile path, and not just the jar but also the transitive dependencies of this jar? Obviously this is a very common use case for Java multi-project builds. As already mentioned in the section called “Project dependencies”, Gradle offers project lib dependencies for this.

Example 219. Project lib dependencies

Build layout

java/
  settings.gradle
  build.gradle
  api/
    src/main/java/
      org/gradle/sample/
        api/
          Person.java
        apiImpl/
          PersonImpl.java
  services/personService/
    src/
      main/java/
        org/gradle/sample/services/
          PersonService.java
      test/java/
        org/gradle/sample/services/
          PersonServiceTest.java
  shared/
    src/main/java/
      org/gradle/sample/shared/
        Helper.java

Note: The code for this example can be found at samples/userguide/multiproject/dependencies/java in the ‘-all’ distribution of Gradle.


We have the projects “shared”, “api” and “personService”. The “personService” project has a lib dependency on the other two projects. The “api” project has a lib dependency on the “shared” project. “services” is also a project, but we use it just as a container. It has no build script and gets nothing injected by another build script. We use the : separator to define a project path. Consult the DSL documentation of Settings.include(java.lang.String[]) for more information about defining project paths.

Example 220. Project lib dependencies

settings.gradle

include 'api', 'shared', 'services:personService'

build.gradle

subprojects {
    apply plugin: 'java'
    group = 'org.gradle.sample'
    version = '1.0'
    repositories {
        mavenCentral()
    }
    dependencies {
        testCompile "junit:junit:4.12"
    }
}

project(':api') {
    dependencies {
        compile project(':shared')
    }
}

project(':services:personService') {
    dependencies {
        compile project(':shared'), project(':api')
    }
}


All the build logic is in the “build.gradle” file of the root project.[8] A “lib” dependency is a special form of an execution dependency. It causes the other project to be built first and adds the jar with the classes of the other project to the classpath. It also adds the dependencies of the other project to the classpath. So you can enter the “api” directory and trigger a “gradle compile”. First the “shared” project is built and then the “api” project is built. Project dependencies enable partial multi-project builds.

If you come from Maven land you might be perfectly happy with this. If you come from Ivy land, you might expect some more fine grained control. Gradle offers this to you:

Example 221. Fine grained control over dependencies

build.gradle

subprojects {
    apply plugin: 'java'
    group = 'org.gradle.sample'
    version = '1.0'
}

project(':api') {
    configurations {
        spi
    }
    dependencies {
        compile project(':shared')
    }
    task spiJar(type: Jar) {
        baseName = 'api-spi'
        from sourceSets.main.output
        include('org/gradle/sample/api/**')
    }
    artifacts {
        spi spiJar
    }
}

project(':services:personService') {
    dependencies {
        compile project(':shared')
        compile project(path: ':api', configuration: 'spi')
        testCompile "junit:junit:4.12", project(':api')
    }
}

The Java plugin adds per default a jar to your project libraries which contains all the classes. In this example we create an additional library containing only the interfaces of the “api” project. We assign this library to a new dependency configuration. For the person service we declare that the project should be compiled only against the “api” interfaces but tested with all classes from “api”.

Disabling the build of dependency projects

Sometimes you don’t want depended on projects to be built when doing a partial build. To disable the build of the depended on projects you can run Gradle with the -a option.

Parallel project execution

With more and more CPU cores available on developer desktops and CI servers, it is important that Gradle is able to fully utilise these processing resources. More specifically, parallel execution attempts to:

  • Reduce total build time for a multi-project build where execution is IO bound or otherwise does not consume all available CPU resources.

  • Provide faster feedback for execution of small projects without awaiting completion of other projects.

Although Gradle already offers parallel test execution via Test.setMaxParallelForks(int) the feature described in this section is parallel execution at a project level. Parallel execution is an incubating feature. Please use it and let us know how it works for you.

Parallel project execution allows the separate projects in a decoupled multi-project build to be executed in parallel (see also: the section called “Decoupled Projects”). While parallel execution does not strictly require decoupling at configuration time, the long-term goal is to provide a powerful set of features that will be available for fully decoupled projects. Such features include:

How does parallel execution work? First, you need to tell Gradle to use parallel mode. You can use the --parallel command line argument or configure your build environment (the section called “Gradle properties”). Unless you provide a specific number of parallel threads, Gradle attempts to choose the right number based on available CPU cores. Every parallel worker exclusively owns a given project while executing a task. Task dependencies are fully supported and parallel workers will start executing upstream tasks first. Bear in mind that the alphabetical ordering of decoupled tasks, as can be seen during sequential execution, is not guaranteed in parallel mode. In other words, in parallel mode tasks will run as soon as their dependencies complete and a task worker is available to run them, which may be earlier than they would start during a sequential build. You should make sure that task dependencies and task inputs/outputs are declared correctly to avoid ordering issues.

Decoupled Projects

Gradle allows any project to access any other project during both the configuration and execution phases. While this provides a great deal of power and flexibility to the build author, it also limits the flexibility that Gradle has when building those projects. For instance, this effectively prevents Gradle from correctly building multiple projects in parallel, configuring only a subset of projects, or from substituting a pre-built artifact in place of a project dependency.

Two projects are said to be decoupled if they do not directly access each other’s project model. Decoupled projects may only interact in terms of declared dependencies: project dependencies (the section called “Project dependencies”) and/or task dependencies (the section called “Task dependencies”). Any other form of project interaction (i.e. by modifying another project object or by reading a value from another project object) causes the projects to be coupled. The consequence of coupling during the configuration phase is that if gradle is invoked with the 'configuration on demand' option, the result of the build can be flawed in several ways. The consequence of coupling during execution phase is that if gradle is invoked with the parallel option, one project task runs too late to influence a task of a project building in parallel. Gradle does not attempt to detect coupling and warn the user, as there are too many possibilities to introduce coupling.

A very common way for projects to be coupled is by using configuration injection (the section called “Cross project configuration”). It may not be immediately apparent, but using key Gradle features like the allprojects and subprojects keywords automatically cause your projects to be coupled. This is because these keywords are used in a build.gradle file, which defines a project. Often this is a “root project” that does nothing more than define common configuration, but as far as Gradle is concerned this root project is still a fully-fledged project, and by using allprojects that project is effectively coupled to all other projects. Coupling of the root project to subprojects does not impact 'configuration on demand', but using the allprojects and subprojects in any subproject’s build.gradle file will have an impact.

This means that using any form of shared build script logic or configuration injection (allprojects, subprojects, etc.) will cause your projects to be coupled. As we extend the concept of project decoupling and provide features that take advantage of decoupled projects, we will also introduce new features to help you to solve common use cases (like configuration injection) without causing your projects to be coupled.

In order to make good use of cross project configuration without running into issues for parallel and 'configuration on demand' options, follow these recommendations:

  • Avoid a subproject’s build.gradle referencing other subprojects; preferring cross configuration from the root project.

  • Avoid changing the configuration of other projects at execution time.

Multi-Project Building and Testing

The build task of the Java plugin is typically used to compile, test, and perform code style checks (if the CodeQuality plugin is used) of a single project. In multi-project builds you may often want to do all of these tasks across a range of projects. The buildNeeded and buildDependents tasks can help with this.

Look at Example 220. In this example, the “:services:personservice” project depends on both the “:api” and “:shared” projects. The “:api” project also depends on the “:shared” project.

Assume you are working on a single project, the “:api” project. You have been making changes, but have not built the entire project since performing a clean. You want to build any necessary supporting jars, but only perform code quality and unit tests on the project you have changed. The build task does this.

Example 222. Build and Test Single Project

Output of gradle :api:build

> gradle :api:build
> Task :shared:compileJava
> Task :shared:processResources
> Task :shared:classes
> Task :shared:jar
> Task :api:compileJava
> Task :api:processResources
> Task :api:classes
> Task :api:jar
> Task :api:assemble
> Task :api:compileTestJava
> Task :api:processTestResources
> Task :api:testClasses
> Task :api:test
> Task :api:check
> Task :api:build

BUILD SUCCESSFUL in 0s
9 actionable tasks: 9 executed

While you are working in a typical development cycle repeatedly building and testing changes to the “:api” project (knowing that you are only changing files in this one project), you may not want to even suffer the expense of building “:shared:compile” to see what has changed in the “:shared” project. Adding the “-a” option will cause Gradle to use cached jars to resolve any project lib dependencies and not try to re-build the depended on projects.

Example 223. Partial Build and Test Single Project

Output of gradle -a :api:build

> gradle -a :api:build
> Task :api:compileJava
> Task :api:processResources
> Task :api:classes
> Task :api:jar
> Task :api:assemble
> Task :api:compileTestJava
> Task :api:processTestResources
> Task :api:testClasses
> Task :api:test
> Task :api:check
> Task :api:build

BUILD SUCCESSFUL in 0s
6 actionable tasks: 6 executed

If you have just gotten the latest version of source from your version control system which included changes in other projects that “:api” depends on, you might want to not only build all the projects you depend on, but test them as well. The buildNeeded task also tests all the projects from the project lib dependencies of the testRuntime configuration.

Example 224. Build and Test Depended On Projects

Output of gradle :api:buildNeeded

> gradle :api:buildNeeded
> Task :shared:compileJava
> Task :shared:processResources
> Task :shared:classes
> Task :shared:jar
> Task :api:compileJava
> Task :api:processResources
> Task :api:classes
> Task :api:jar
> Task :api:assemble
> Task :api:compileTestJava
> Task :api:processTestResources
> Task :api:testClasses
> Task :api:test
> Task :api:check
> Task :api:build
> Task :shared:assemble
> Task :shared:compileTestJava
> Task :shared:processTestResources
> Task :shared:testClasses
> Task :shared:test
> Task :shared:check
> Task :shared:build
> Task :shared:buildNeeded
> Task :api:buildNeeded

BUILD SUCCESSFUL in 0s
12 actionable tasks: 12 executed

You also might want to refactor some part of the “:api” project that is used in other projects. If you make these types of changes, it is not sufficient to test just the “:api” project, you also need to test all projects that depend on the “:api” project. The buildDependents task also tests all the projects that have a project lib dependency (in the testRuntime configuration) on the specified project.

Example 225. Build and Test Dependent Projects

Output of gradle :api:buildDependents

> gradle :api:buildDependents
> Task :shared:compileJava
> Task :shared:processResources
> Task :shared:classes
> Task :shared:jar
> Task :api:compileJava
> Task :api:processResources
> Task :api:classes
> Task :api:jar
> Task :api:assemble
> Task :api:compileTestJava
> Task :api:processTestResources
> Task :api:testClasses
> Task :api:test
> Task :api:check
> Task :api:build
> Task :services:personService:compileJava
> Task :services:personService:processResources
> Task :services:personService:classes
> Task :services:personService:jar
> Task :services:personService:assemble
> Task :services:personService:compileTestJava
> Task :services:personService:processTestResources
> Task :services:personService:testClasses
> Task :services:personService:test
> Task :services:personService:check
> Task :services:personService:build
> Task :services:personService:buildDependents
> Task :api:buildDependents

BUILD SUCCESSFUL in 0s
17 actionable tasks: 17 executed

Finally, you may want to build and test everything in all projects. Any task you run in the root project folder will cause that same named task to be run on all the children. So you can just run “gradle build” to build and test all projects.

Multi Project and buildSrc

the section called “Build sources in the buildSrc project” tells us that we can place build logic to be compiled and tested in the special buildSrc directory. In a multi project build, there can only be one buildSrc directory which must be located in the root directory.

Property and method inheritance

Properties and methods declared in a project are inherited to all its subprojects. This is an alternative to configuration injection. But we think that the model of inheritance does not reflect the problem space of multi-project builds very well. In a future edition of this user guide we might write more about this.

Method inheritance might be interesting to use as Gradle’s Configuration Injection does not support methods yet (but will in a future release).

You might be wondering why we have implemented a feature we obviously don’t like that much. One reason is that it is offered by other tools and we want to have the check mark in a feature comparison :). And we like to offer our users a choice.

Summary

Writing this chapter was pretty exhausting and reading it might have a similar effect. Our final message for this chapter is that multi-project builds with Gradle are usually not difficult. There are five elements you need to remember: allprojects, subprojects, evaluationDependsOn, evaluationDependsOnChildren and project lib dependencies.[9] With those elements, and keeping in mind that Gradle has a distinct configuration and execution phase, you already have a lot of flexibility. But when you enter steep territory Gradle does not become an obstacle and usually accompanies and carries you to the top of the mountain.



[7] The real use case we had, was using http://lucene.apache.org/solr, where you need a separate war for each index you are accessing. That was one reason why we have created a distribution of webapps. The Resin servlet container allows us, to let such a distribution point to a base installation of the servlet container.

[8] We do this here, as it makes the layout a bit easier. We usually put the project specific stuff into the build script of the respective projects.

[9] So we are well in the range of the 7 plus 2 Rule :)

Using Gradle Plugins

Gradle at its core intentionally provides very little for real world automation. All of the useful features, like the ability to compile Java code, are added by plugins. Plugins add new tasks (e.g. JavaCompile), domain objects (e.g. SourceSet), conventions (e.g. Java source is located at src/main/java) as well as extending core objects and objects from other plugins.

In this chapter we discuss how to use plugins and the terminology and concepts surrounding plugins.

What plugins do

Applying a plugin to a project allows the plugin to extend the project’s capabilities. It can do things such as:

  • Extend the Gradle model (e.g. add new DSL elements that can be configured)

  • Configure the project according to conventions (e.g. add new tasks or configure sensible defaults)

  • Apply specific configuration (e.g. add organizational repositories or enforce standards)

By applying plugins, rather than adding logic to the project build script, we can reap a number of benefits. Applying plugins:

  • Promotes reuse and reduces the overhead of maintaining similar logic across multiple projects

  • Allows a higher degree of modularization, enhancing comprehensibility and organization

  • Encapsulates imperative logic and allows build scripts to be as declarative as possible

Types of plugins

There are two general types of plugins in Gradle, script plugins and binary plugins. Script plugins are additional build scripts that further configure the build and usually implement a declarative approach to manipulating the build. They are typically used within a build although they can be externalized and accessed from a remote location. Binary plugins are classes that implement the Plugin interface and adopt a programmatic approach to manipulating the build. Binary plugins can reside within a build script, within the project hierarchy or externally in a plugin jar.

A plugin often starts out as a script plugin (because they are easy to write) and then, as the code becomes more valuable, it’s migrated to a binary plugin that can be easily tested and shared between multiple projects or organizations.

Using plugins

To use the build logic encapsulated in a plugin, Gradle needs to perform two steps. First, it needs to resolve the plugin, and then it needs to apply the plugin to the target, usually a Project.

Resolving a plugin means finding the correct version of the jar which contains a given plugin and adding it the script classpath. Once a plugin is resolved, its API can be used in a build script. Script plugins are self-resolving in that they are resolved from the specific file path or URL provided when applying them. Core binary plugins provided as part of the Gradle distribution are automatically resolved.

Applying a plugin means actually executing the plugin’s Plugin.apply(T) on the Project you want to enhance with the plugin. Applying plugins is idempotent. That is, you can safely apply any plugin multiple times without side effects.

The most common use case for using a plugin is to both resolve the plugin and apply it to the current project. Since this is such a common use case, it’s recommended that build authors use the plugins DSL to both resolve and apply plugins in one step. The feature is technically still incubating, but it works well, and should be used by most users.

Script plugins

Example 226. Applying a script plugin

build.gradle

apply from: 'other.gradle'

Script plugins are automatically resolved and can be applied from a script on the local filesystem or at a remote location. Filesystem locations are relative to the project directory, while remote script locations are specified with an HTTP URL. Multiple script plugins (of either form) can be applied to a given target.

Binary plugins

You apply plugins by their plugin id, which is a globally unique identifier, or name, for plugins. Core Gradle plugins are special in that they provide short names, such as 'java' for the core JavaPlugin. All other binary plugins must use the fully qualified form of the plugin id (e.g. com.github.foo.bar), although some legacy plugins may still utilize a short, unqualified form. Where you put the plugin id depends on whether you are using the plugins DSL or the buildscript block.

Locations of binary plugins

A plugin is simply any class that implements the Plugin interface. Gradle provides the core plugins (e.g. JavaPlugin) as part of its distribution which means they are automatically resolved. However, non-core binary plugins need to be resolved before they can be applied. This can be achieved in a number of ways:

For more on defining your own plugins, see Writing Custom Plugins.

Applying plugins with the plugins DSL

The plugins DSL is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

The new plugins DSL provides a succinct and convenient way to declare plugin dependencies. It works with the Gradle plugin portal to provide easy access to both core and community plugins. The plugins DSL block configures an instance of PluginDependenciesSpec.

To apply a core plugin, the short name can be used:

Example 227. Applying a core plugin

build.gradle

plugins {
    id 'java'
}

To apply a community plugin from the portal, the fully qualified plugin id must be used:

Example 228. Applying a community plugin

build.gradle

plugins {
    id 'com.jfrog.bintray' version '0.4.1'
}

See PluginDependenciesSpec for more information on using the Plugin DSL.

Limitations of the plugins DSL

This way of adding plugins to a project is much more than a more convenient syntax. The plugins DSL is processed in a way which allows Gradle to determine the plugins in use very early and very quickly. This allows Gradle to do smart things such as:

  • Optimize the loading and reuse of plugin classes.

  • Allow different plugins to use different versions of dependencies.

  • Provide editors detailed information about the potential properties and values in the buildscript for editing assistance.

This requires that plugins be specified in a way that Gradle can easily and quickly extract, before executing the rest of the build script. It also requires that the definition of plugins to use be somewhat static.

There are some key differences between the new plugin mechanism and the “traditional” apply() method mechanism. There are also some constraints, some of which are temporary limitations while the mechanism is still being developed and some are inherent to the new approach.

Constrained Syntax

The new plugins {} block does not support arbitrary Groovy code. It is constrained, in order to be idempotent (produce the same result every time) and side effect free (safe for Gradle to execute at any time).

The form is:

plugins {
    id «plugin id» version «plugin version» [apply «false»]
}

Where «plugin version» and «plugin id» must be constant, literal, strings and the apply statement with a boolean can be used to disable the default behavior of applying the plugin immediately (e.g. you want to apply it only in subprojects). No other statements are allowed; their presence will cause a compilation error.

The plugins {} block must also be a top level statement in the buildscript. It cannot be nested inside another construct (e.g. an if-statement or for-loop).

Can only be used in build scripts

The plugins {} block can currently only be used in a project’s build script. It cannot be used in script plugins, the settings.gradle file or init scripts.

Future versions of Gradle will remove this restriction.

If the restrictions of the new syntax are prohibitive, the recommended approach is to apply plugins using the buildscript {} block.

Applying plugins to subprojects

If you have a multi-project build, you probably want to apply plugins to some or all of the subprojects in your build, but not to the root or master project. The default behavior of the plugins {} block is to immediately resolve and apply the plugins. But, you can use the apply false syntax to tell Gradle not to apply the plugin to the current project and then use apply plugin: «plugin id» in the subprojects block:

Example 229. Applying plugins only on certain subprojects.

settings.gradle

include 'helloA'
include 'helloB'
include 'goodbyeC'

build.gradle

plugins {
  id "org.gradle.sample.hello" version "1.0.0" apply false
  id "org.gradle.sample.goodbye" version "1.0.0" apply false
}

subprojects { subproject ->
    if (subproject.name.startsWith("hello")) {
        apply plugin: 'org.gradle.sample.hello'
    }
    if (subproject.name.startsWith("goodbye")) {
        apply plugin: 'org.gradle.sample.goodbye'
    }
}

If you then run gradle hello you’ll see that only the helloA and helloB subprojects had the hello plugin applied.

gradle/subprojects/docs/src/samples/plugins/multiproject $> gradle hello
Parallel execution is an incubating feature.
:helloA:hello
:helloB:hello
Hello!
Hello!

BUILD SUCCEEDED

Plugin Management

The pluginManagement {} DSL is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

Custom Plugin Repositories

By default, the plugins {} DSL resolves plugins from the public Gradle Plugin Portal. Many build authors would also like to resolve plugins from private Maven or Ivy repositories because the plugins contain proprietary implementation details, or just to have more control over what plugins are available to their builds.

To specify custom plugin repositories, use the repositories {} block inside pluginManagement {} in the settings.gradle file:

Example 230. Using plugins from custom plugin repositories.

settings.gradle

pluginManagement {
  repositories {
      maven {
        url 'maven-repo'
      }
      gradlePluginPortal()
      ivy {
        url 'ivy-repo'
      }
  }
}

This tells Gradle to first look in the Maven repository at maven-repo when resolving plugins and then to check the Gradle Plugin Portal if the plugins are not found in the Maven repository. If you don’t want the Gradle Plugin Portal to be searched, omit the gradlePluginPortal() line. Finally, the Ivy repository at ivy-repo will be checked.

Plugin Resolution Rules

Plugin resolution rules allow you to modify plugin requests made in plugins {} blocks, e.g. changing the requested version or explicitly specifying the implementation artifact coordinates.

To add resolution rules, use the resolutionStrategy {} inside the pluginManagement {} block:

Example 231. Plugin resolution strategy.

settings.gradle

pluginManagement {
  resolutionStrategy {
      eachPlugin {
          if (requested.id.namespace == 'org.gradle.sample') {
              useModule('org.gradle.sample:sample-plugins:1.0.0')
          }
      }
  }
  repositories {
      maven {
        url 'maven-repo'
      }
      gradlePluginPortal()
      ivy {
        url 'ivy-repo'
      }
  }
}

This tells Gradle to use the specified plugin implementation artifact instead of using its built-in default mapping from plugin ID to Maven/Ivy coordinates.

The pluginManagement {} block may only appear in the settings.gradle file, and must be the first block in the file. Custom Maven and Ivy plugin repositories must contain plugin marker artifacts in addition to the artifacts which actually implement the plugin. For more information on publishing plugins to custom repositories read Gradle Plugin Development Plugin.

See PluginManagementSpec for complete documentation for using the pluginManagement {} block.

Plugin Marker Artifacts

Since the plugins {} DSL block only allows for declaring plugins by their globally unique plugin id and version properties, Gradle needs a way to look up the coordinates of the plugin implementation artifact. To do so, Gradle will look for a Plugin Marker Artifact with the coordinates plugin.id:plugin.id.gradle.plugin:plugin.version. This marker needs to have a dependency on the actual plugin implementation. Publishing these markers is automated by the java-gradle-plugin.

For example, the following complete sample from the sample-plugins project shows how to publish a org.gradle.sample.hello plugin and a org.gradle.sample.goodbye plugin to both an Ivy and Maven repository using the combination of the java-gradle-plugin, the maven-publish plugin, and the ivy-publish plugin.

Example 232. Complete Plugin Publishing Sample

build.gradle

plugins {
  id 'java-gradle-plugin'
  id 'maven-publish'
  id 'ivy-publish'
}

group 'org.gradle.sample'
version '1.0.0'

gradlePlugin {
  plugins {
    hello {
      id = "org.gradle.sample.hello"
      implementationClass = "org.gradle.sample.hello.HelloPlugin"
    }
    goodbye {
      id = "org.gradle.sample.goodbye"
      implementationClass = "org.gradle.sample.goodbye.GoodbyePlugin"
    }
  }
}

publishing {
  repositories {
    maven {
      url "../consuming/maven-repo"
    }
    ivy {
      url "../consuming/ivy-repo"
    }
  }
}

Running gradle publish in the sample directory causes the following repo layouts to exist:

pluginMarkers

Legacy Plugin Application

With the introduction of the plugins DSL, users should have little reason to use the legacy method of applying plugins. It is documented here in case a build author cannot use the plugins DSL due to restrictions in how it currently works.

Applying Binary Plugins

Example 233. Applying a binary plugin

build.gradle

apply plugin: 'java'

Plugins can be applied using a plugin id. In the above case, we are using the short name ‘java’ to apply the JavaPlugin.

Rather than using a plugin id, plugins can also be applied by simply specifying the class of the plugin:

Example 234. Applying a binary plugin by type

build.gradle

apply plugin: JavaPlugin

The JavaPlugin symbol in the above sample refers to the JavaPlugin. This class does not strictly need to be imported as the org.gradle.api.plugins package is automatically imported in all build scripts (see the section called “Default imports”). Furthermore, it is not necessary to append .class to identify a class literal in Groovy as it is in Java.

Applying plugins with the buildscript block

Binary plugins that have been published as external jar files can be added to a project by adding the plugin to the build script classpath and then applying the plugin. External jars can be added to the build script classpath using the buildscript {} block as described in the section called “External dependencies for the build script”.

Example 235. Applying a plugin with the buildscript block

build.gradle

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath "com.jfrog.bintray.gradle:gradle-bintray-plugin:0.4.1"
    }
}

apply plugin: "com.jfrog.bintray"

Finding community plugins

Gradle has a vibrant community of plugin developers who contribute plugins for a wide variety of capabilities. The Gradle plugin portal provides an interface for searching and exploring community plugins.

More on plugins

This chapter aims to serve as an introduction to plugins and Gradle and the role they play. For more information on the inner workings of plugins, see Writing Custom Plugins.

Standard Gradle plugins

There are a number of plugins included in the Gradle distribution. These are listed below.

Language plugins

These plugins add support for various languages which can be compiled for and executed in the JVM.

Table 10. Language plugins

Plugin Id Automatically applies Works with Description

java

java-base

-

Adds Java compilation, testing and bundling capabilities to a project. It serves as the basis for many of the other Gradle plugins. See also Java Quickstart.

groovy

java, groovy-base

-

Adds support for building Groovy projects. See also Groovy Quickstart.

scala

java, scala-base

-

Adds support for building Scala projects.

antlr

java

-

Adds support for generating parsers using Antlr.

Incubating language plugins

These plugins add support for various languages:

Table 11. Language plugins

Plugin Id Automatically applies Works with Description

assembler

-

-

Adds native assembly language capabilities to a project.

c

-

-

Adds C source compilation capabilities to a project.

cpp

-

-

Adds C++ source compilation capabilities to a project.

objective-c

-

-

Adds Objective-C source compilation capabilities to a project.

objective-cpp

-

-

Adds Objective-C++ source compilation capabilities to a project.

windows-resources

-

-

Adds support for including Windows resources in native binaries.

Integration plugins

These plugins provide some integration with various runtime technologies.

Table 12. Integration plugins

Plugin Id Automatically applies Works with Description

application

java, distribution

-

Adds tasks for running and bundling a Java project as a command-line application.

ear

-

java

Adds support for building J2EE applications.

maven

-

java, war

Adds support for publishing artifacts to Maven repositories.

osgi

java-base

java

Adds support for building OSGi bundles.

war

java

-

Adds support for assembling web application WAR files. See also Web Application Quickstart.

Incubating integration plugins

These plugins provide some integration with various runtime technologies.

Table 13. Incubating integration plugins

Plugin Id Automatically applies Works with Description

distribution

-

-

Adds support for building ZIP and TAR distributions.

java-library-distribution

java, distribution

-

Adds support for building ZIP and TAR distributions for a Java library.

ivy-publish

-

java, war

This plugin provides a new DSL to support publishing artifacts to Ivy repositories, which improves on the existing DSL.

maven-publish

-

java, war

This plugin provides a new DSL to support publishing artifacts to Maven repositories, which improves on the existing DSL.

Software development plugins

These plugins provide help with your software development process.

Table 14. Software development plugins

Plugin Id Automatically applies Works with Description

announce

-

-

Publish messages to your favourite platforms, such as Twitter or Growl.

build-announcements

announce

-

Sends local announcements to your desktop about interesting events in the build lifecycle.

checkstyle

java-base

-

Performs quality checks on your project’s Java source files using Checkstyle and generates reports from these checks.

codenarc

groovy-base

-

Performs quality checks on your project’s Groovy source files using CodeNarc and generates reports from these checks.

eclipse

-

java,groovy, scala

Generates files that are used by Eclipse IDE, thus making it possible to import the project into Eclipse. See also Java Quickstart.

eclipse-wtp

-

ear, war

Does the same as the eclipse plugin plus generates eclipse WTP (Web Tools Platform) configuration files. After importing to eclipse your war/ear projects should be configured to work with WTP. See also Java Quickstart.

findbugs

java-base

-

Performs quality checks on your project’s Java source files using FindBugs and generates reports from these checks.

idea

-

java

Generates files that are used by Intellij IDEA IDE, thus making it possible to import the project into IDEA.

jdepend

java-base

-

Performs quality checks on your project’s source files using JDepend and generates reports from these checks.

pmd

java-base

-

Performs quality checks on your project’s Java source files using PMD and generates reports from these checks.

project-report

reporting-base

-

Generates reports containing useful information about your Gradle build.

signing

base

-

Adds the ability to digitally sign built files and artifacts.

Incubating software development plugins

These plugins provide help with your software development process.

Table 15. Software development plugins

Plugin Id Automatically applies Works with Description

build-dashboard

reporting-base

-

Generates build dashboard report.

cunit

-

-

Adds support for running CUnit tests.

jacoco

reporting-base

java

Provides integration with the JaCoCo code coverage library for Java.

visual-studio

-

native language plugins

Adds integration with Visual Studio.

java-gradle-plugin

java

Assists with development of Gradle plugins by providing standard plugin build configuration and validation.

Base plugins

These plugins form the basic building blocks which the other plugins are assembled from. They are available for you to use in your build files, and are listed here for completeness. However, be aware that they are not yet considered part of Gradle’s public API. As such, these plugins are not documented in the user guide. You might refer to their API documentation to learn more about them.

Table 16. Base plugins

Plugin Id Description

base

Adds the standard lifecycle tasks and configures reasonable defaults for the archive tasks:

  • adds build ConfigurationName tasks. Those tasks assemble the artifacts belonging to the specified configuration.

  • adds upload ConfigurationName tasks. Those tasks assemble and upload the artifacts belonging to the specified configuration.

  • configures reasonable default values for all archive tasks (e.g. tasks that inherit from AbstractArchiveTask). For example, the archive tasks are tasks of types: Jar, Tar, Zip. Specifically, destinationDir, baseName and version properties of the archive tasks are preconfigured with defaults. This is extremely useful because it drives consistency across projects; the consistency regarding naming conventions of archives and their location after the build completed.

java-base

Adds the source sets concept to the project. Does not add any particular source sets.

groovy-base

Adds the Groovy source sets concept to the project.

scala-base

Adds the Scala source sets concept to the project.

reporting-base

Adds some shared convention properties to the project, relating to report generation.

Third party plugins

You can find a list of external plugins at the Gradle Plugins site.

The Project Report Plugin

The Project report plugin adds some tasks to your project which generate reports containing useful information about your build. These tasks generate the same content that you get by executing the tasks, dependencies, and properties tasks from the command line (see the section called “Project reporting”). In contrast to the command line reports, the report plugin generates the reports into a file. There is also an aggregating task that depends on all report tasks added by the plugin.

We plan to add much more to the existing reports and create additional ones in future releases of Gradle.

Usage

To use the Project report plugin, include the following in your build script:

apply plugin: 'project-report'

Tasks

The project report plugin defines the following tasks:

Table 17. Project report plugin - tasks

Task name Depends on Type Description

dependencyReport

-

DependencyReportTask

Generates the project dependency report.

htmlDependencyReport

-

HtmlDependencyReportTask

Generates an HTML dependency and dependency insight report for the project or a set of projects.

propertyReport

-

PropertyReportTask

Generates the project property report.

taskReport

-

TaskReportTask

Generates the project task report.

projectReport

dependencyReport, propertyReport, taskReport, htmlDependencyReport

Task

Generates all project reports.

Project layout

The project report plugin does not require any particular project layout.

Dependency management

The project report plugin does not define any dependency configurations.

Convention properties

The project report defines the following convention properties:

Table 18. Project report plugin - convention properties

Property name Type Default value Description

reportsDirName

String

reports

The name of the directory to generate reports into, relative to the build directory.

reportsDir

File (read-only)

buildDir/reportsDirName

The directory to generate reports into.

projects

Set<Project>

A one element set with the project the plugin was applied to.

The projects to generate the reports for.

projectReportDirName

String

project

The name of the directory to generate the project report into, relative to the reports directory.

projectReportDir

File (read-only)

reportsDir/projectReportDirName

The directory to generate the project report into.

These convention properties are provided by a convention object of type ProjectReportsPluginConvention.

The Build Dashboard Plugin

The build dashboard plugin is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

The Build Dashboard plugin can be used to generate a single HTML dashboard that provides a single point of access to all of the reports generated by a build.

Usage

To use the Build Dashboard plugin, include the following in your build script:

Example 236. Using the Build Dashboard plugin

build.gradle

apply plugin: 'build-dashboard'

Applying the plugin adds the buildDashboard task to your project. The task aggregates the reports for all tasks that implement the Reporting interface from all projects in the build. It is typically only applied to the root project.

The buildDashboard task does not depend on any other tasks. It will only aggregate the reporting tasks that are independently being executed as part of the build run. To generate the build dashboard, simply include this task in the list of tasks to execute. For example, “gradle buildDashboard build” will generate a dashboard for all of the reporting tasks that are dependents of the build task.

Tasks

The Build Dashboard plugin adds the following task to the project:

Table 19. Build Dashboard plugin - tasks

Task name Depends on Type Description

buildDashboard

-

GenerateBuildDashboard

Generates build dashboard report.

Project layout

The Build Dashboard plugin does not require any particular project layout.

Dependency management

The Build Dashboard plugin does not define any dependency configurations.

Configuration

You can influence the location of build dashboard plugin generation via ReportingExtension.

Comparing Builds

Build comparison support is an incubating feature. This means that it is incomplete and not yet at regular Gradle production quality. This also means that this Gradle User Guide chapter is a work in progress.

Gradle provides support for comparing the outcomes (e.g. the produced binary archives) of two builds. There are several reasons why you may want to compare the outcomes of two builds. You may want to compare:

  • A build with a newer version of Gradle than it’s currently using (i.e. upgrading the Gradle version).

  • A Gradle build with a build executed by another tool such as Apache Ant, Apache Maven or something else (i.e. migrating to Gradle).

  • The same Gradle build, with the same version, before and after a change to the build (i.e. testing build changes).

By comparing builds in these scenarios you can make an informed decision about the Gradle upgrade, migration to Gradle or build change by understanding the differences in the outcomes. The comparison process produces a HTML report outlining which outcomes were found to be identical and identifying the differences between non-identical outcomes.

Definition of terms

The following are the terms used for build comparison and their definitions.

“Build”

In the context of build comparison, a build is not necessarily a Gradle build. It can be any invokable “process” that produces observable “outcomes”. At least one of the builds in a comparison will be a Gradle build.

“Build Outcome”

Something that happens in an observable manner during a build, such as the creation of a zip file or test execution. These are the things that are compared.

“Source Build”

The build that comparisons are being made against, typically the build in its “current” state. In other words, the left hand side of the comparison.

“Target Build”

The build that is being compared to the source build, typically the “proposed” build. In other words, the right hand side of the comparison.

“Host Build”

The Gradle build that executes the comparison process. It may be the same project as either the “target” or “source” build or may be a completely separate project. It does not need to be the same Gradle version as the “source” or “target” builds. The host build must be run with Gradle 1.2 or newer.

“Compared Build Outcome”

Build outcomes that are intended to be logically equivalent in the “source” and “target” builds, and are therefore meaningfully comparable.

“Uncompared Build Outcome”

A build outcome is uncompared if a logical equivalent from the other build cannot be found (e.g. a build produces a zip file that the other build does not).

“Unknown Build Outcome”

A build outcome that cannot be understood by the host build. This can occur when the source or target build is a newer Gradle version than the host build and that Gradle version exposes new outcome types. Unknown build outcomes can be compared in so far as they can be identified to be logically equivalent to an unknown build outcome in the other build, but no meaningful comparison of what the build outcome actually is can be performed. Using the latest Gradle version for the host build will avoid encountering unknown build outcomes.

Current Capabilities

As this is an incubating feature, a limited set of the eventual functionality has been implemented at this time.

Supported builds

Only support for comparing Gradle builds is available at this time. Both the source and target build must execute with Gradle newer or equal to version 1.0. The host build must be at least version 1.2. If the host build is run with version 3.0 or newer, source and target builds must be at least version 1.2. If the host build is run with a version older than 2.0, source and target builds must be older than version 3.0. So if you for example want to compare a build under version 1.1 with a build under version 3.0, you have to execute the host build with a 2.x version.

Future versions will provide support for executing builds from other build systems such as Apache Ant or Apache Maven, as well as support for executing arbitrary processes (e.g. shell script based builds)

Supported build outcomes

Only support for comparing build outcomes that are zip archives is supported at this time. This includes jar, war and ear archives.

Future versions will provide support for comparing outcomes such as test execution (i.e. which tests were executed, which tests failed, etc.)

Comparing Gradle Builds

The compare-gradle-builds plugin can be used to facilitate a comparison between two Gradle builds. The plugin adds a CompareGradleBuilds task named “compareGradleBuilds” to the project. The configuration of this task specifies what is to be compared. By default, it is configured to compare the current build with itself using the current Gradle version by executing the tasks: “clean assemble”.

apply plugin: 'compare-gradle-builds'

This task can be configured to change what is compared.

compareGradleBuilds {
    sourceBuild {
        projectDir "/projects/project-a"
        gradleVersion "1.1"
    }
    targetBuild {
        projectDir "/projects/project-b"
        gradleVersion "1.2"
    }
}

The example above specifies a comparison between two different projects using two different Gradle versions.

Trying Gradle upgrades

You can use the build comparison functionality to very quickly try a new Gradle version with your build.

To try your current build with a different Gradle version, simply add the following to the build.gradle of the root project.

apply plugin: 'compare-gradle-builds'

compareGradleBuilds {
    targetBuild.gradleVersion = "«gradle version»"
}

Then simply execute the compareGradleBuilds task. You will see the console output of the “source” and “target” builds as they are executing.

The comparison “result”

If there are any differences between the compared outcomes, the task will fail. The location of the HTML report providing insight into the comparison will be given. If all compared outcomes are found to be identical, and there are no uncompared outcomes, and there are no unknown build outcomes, the task will succeed.

You can configure the task to not fail on compared outcome differences by setting the ignoreFailures property to true.

compareGradleBuilds {
    ignoreFailures = true
}

Which archives are compared?

For an archive to be a candidate for comparison, it must be added as an artifact of the archives configuration. Take a look at Publishing artifacts for more information on how to configure and add artifacts.

The archive must also have been produced by a Zip, Jar, War, Ear task. Future versions of Gradle will support increased flexibility in this area.

Publishing artifacts

This chapter describes the original publishing mechanism available in Gradle 1.0: in Gradle 1.3 a new mechanism for publishing was introduced. While this new mechanism is incubating and not yet complete, it introduces some new concepts and features that do (and will) make Gradle publishing even more powerful.

You can read about the new publishing plugins in Ivy Publishing (new) and Maven Publishing (new). Please try them out and give us feedback.

Introduction

This chapter is about how you declare the outgoing artifacts of your project, and how to work with them (e.g. upload them). We define the artifacts of the projects as the files the project provides to the outside world. This might be a library or a ZIP distribution or any other file. A project can publish as many artifacts as it wants.

Artifacts and configurations

Like dependencies, artifacts are grouped by configurations. In fact, a configuration can contain both artifacts and dependencies at the same time.

For each configuration in your project, Gradle provides the tasks uploadConfigurationName and buildConfigurationName.[10] Execution of these tasks will build or upload the artifacts belonging to the respective configuration.

the section called “Dependency configurations” shows the configurations added by the Java plugin. Two of the configurations are relevant for the usage with artifacts. The archives configuration is the standard configuration to assign your artifacts to. The Java plugin automatically assigns the default jar to this configuration. We will talk more about the runtime configuration in the section called “More about project libraries”. As with dependencies, you can declare as many custom configurations as you like and assign artifacts to them.

Declaring artifacts

Archive task artifacts

You can use an archive task to define an artifact:

Example 237. Defining an artifact using an archive task

build.gradle

task myJar(type: Jar)

artifacts {
    archives myJar
}

It is important to note that the custom archives you are creating as part of your build are not automatically assigned to any configuration. You have to explicitly do this assignment.

File artifacts

You can also use a file to define an artifact:

Example 238. Defining an artifact using a file

build.gradle

def someFile = file('build/somefile.txt')

artifacts {
    archives someFile
}

Gradle will figure out the properties of the artifact based on the name of the file. You can customize these properties:

Example 239. Customizing an artifact

build.gradle

task myTask(type:  MyTaskType) {
    destFile = file('build/somefile.txt')
}

artifacts {
    archives(myTask.destFile) {
        name 'my-artifact'
        type 'text'
        builtBy myTask
    }
}

There is a map-based syntax for defining an artifact using a file. The map must include a file entry that defines the file. The map may include other artifact properties:

Example 240. Map syntax for defining an artifact using a file

build.gradle

task generate(type:  MyTaskType) {
    destFile = file('build/somefile.txt')
}

artifacts {
    archives file: generate.destFile, name: 'my-artifact', type: 'text', builtBy: generate
}

Publishing artifacts

We have said that there is a specific upload task for each configuration. Before you can do an upload, you have to configure the upload task and define where to publish the artifacts to. The repositories you have defined (as described in Declaring Repositories) are not automatically used for uploading. In fact, some of those repositories only allow downloading artifacts, not uploading. Here is an example of how you can configure the upload task of a configuration:

Example 241. Configuration of the upload task

build.gradle

repositories {
    flatDir {
        name "fileRepo"
        dirs "repo"
    }
}

uploadArchives {
    repositories {
        add project.repositories.fileRepo
        ivy {
            credentials {
                username "username"
                password "pw"
            }
            url "http://repo.mycompany.com"
        }
    }
}

As you can see, you can either use a reference to an existing repository or create a new repository.

If an upload repository is defined with multiple patterns, Gradle must choose a pattern to use for uploading each file. By default, Gradle will upload to the pattern defined by the url parameter, combined with the optional layout parameter. If no url parameter is supplied, then Gradle will use the first defined artifactPattern for uploading, or the first defined ivyPattern for uploading Ivy files, if this is set.

Uploading to a Maven repository is described in the section called “Interacting with Maven repositories”.

More about project libraries

If your project is supposed to be used as a library, you need to define what are the artifacts of this library and what are the dependencies of these artifacts. The Java plugin adds a runtime configuration for this purpose, with the implicit assumption that the runtime dependencies are the dependencies of the artifact you want to publish. Of course this is fully customizable. You can add your own custom configuration or let the existing configurations extend from other configurations. You might have a different group of artifacts which have a different set of dependencies. This mechanism is very powerful and flexible.

If someone wants to use your project as a library, she simply needs to declare which configuration of the dependency to depend on. A Gradle dependency offers the configuration property to declare this. If this is not specified, the default configuration is used (see Managing Dependency Configurations). Using your project as a library can either happen from within a multi-project build or by retrieving your project from a repository. In the latter case, an ivy.xml descriptor in the repository is supposed to contain all the necessary information. If you work with Maven repositories you don’t have the flexibility as described above. For how to publish to a Maven repository, see the section the section called “Interacting with Maven repositories”.



[10] To be exact, the Base plugin provides those tasks. This plugin is automatically applied if you use the Java plugin.

The Maven Plugin

This chapter is a work in progress

The Maven plugin adds support for deploying artifacts to Maven repositories.

Usage

To use the Maven plugin, include the following in your build script:

Example 242. Using the Maven plugin

build.gradle

apply plugin: 'maven'

Tasks

The Maven plugin defines the following tasks:

Table 20. Maven plugin - tasks

Task name Depends on Type Description

install

All tasks that build the associated archives.

Upload

Installs the associated artifacts to the local Maven cache, including Maven metadata generation. By default the install task is associated with the archives configuration. This configuration has by default only the default jar as an element. To learn more about installing to the local repository, see: the section called “Installing to the local repository”

Dependency management

The Maven plugin does not define any dependency configurations.

Convention properties

The Maven plugin defines the following convention properties:

Table 21. Maven plugin - properties

Property name Type Default value Description

mavenPomDir

File

${project.buildDir}/poms

The directory where the generated POMs are written to.

conf2ScopeMappings

Conf2ScopeMappingContainer

n/a

Instructions for mapping Gradle configurations to Maven scopes. See the section called “Dependency mapping”.

These properties are provided by a MavenPluginConvention convention object.

Convention methods

The maven plugin provides a factory method for creating a POM. This is useful if you need a POM without the context of uploading to a Maven repo.

Example 243. Creating a standalone pom.

build.gradle

task writeNewPom {
    doLast {
        pom {
            project {
                inceptionYear '2008'
                licenses {
                    license {
                        name 'The Apache Software License, Version 2.0'
                        url 'http://www.apache.org/licenses/LICENSE-2.0.txt'
                        distribution 'repo'
                    }
                }
            }
        }.writeTo("$buildDir/newpom.xml")
    }
}

Amongst other things, Gradle supports the same builder syntax as polyglot Maven. To learn more about the Gradle Maven POM object, see MavenPom. See also: MavenPluginConvention

Interacting with Maven repositories

Introduction

With Gradle you can deploy to remote Maven repositories or install to your local Maven repository. This includes all Maven metadata manipulation and works also for Maven snapshots. In fact, Gradle’s deployment is 100 percent Maven compatible as we use the native Maven Ant tasks under the hood.

Deploying to a Maven repository is only half the fun if you don’t have a POM. Fortunately Gradle can generate this POM for you using the dependency information it has.

Deploying to a Maven repository

Let’s assume your project produces just the default jar file. Now you want to deploy this jar file to a remote Maven repository.

Example 244. Upload of file to remote Maven repository

build.gradle

apply plugin: 'maven'

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://localhost/tmp/myRepo/")
        }
    }
}

That is all. Calling the uploadArchives task will generate the POM and deploys the artifact and the POM to the specified repository.

There is more work to do if you need support for protocols other than file. In this case the native Maven code we delegate to needs additional libraries. Which libraries are needed depends on what protocol you plan to use. The available protocols and the corresponding libraries are listed in Table 22 (those libraries have transitive dependencies which have transitive dependencies).[11] For example, to use the ssh protocol you can do:

Example 245. Upload of file via SSH

build.gradle

configurations {
    deployerJars
}

repositories {
    mavenCentral()
}

dependencies {
    deployerJars "org.apache.maven.wagon:wagon-ssh:2.2"
}

uploadArchives {
    repositories.mavenDeployer {
        configuration = configurations.deployerJars
        repository(url: "scp://repos.mycompany.com/releases") {
            authentication(userName: "me", password: "myPassword")
        }
    }
}

There are many configuration options for the Maven deployer. The configuration is done via a Groovy builder. All the elements of this tree are Java beans. To configure the simple attributes you pass a map to the bean elements. To add bean elements to its parent, you use a closure. In the example above repository and authentication are such bean elements. Table 23 lists the available bean elements and a link to the Javadoc of the corresponding class. In the Javadoc you can see the possible attributes you can set for a particular element.

In Maven you can define repositories and optionally snapshot repositories. If no snapshot repository is defined, releases and snapshots are both deployed to the repository element. Otherwise snapshots are deployed to the snapshotRepository element.

Table 22. Protocol jars for Maven deployment

Protocol Library

http

org.apache.maven.wagon:wagon-http:2.2

ssh

org.apache.maven.wagon:wagon-ssh:2.2

ssh-external

org.apache.maven.wagon:wagon-ssh-external:2.2

ftp

org.apache.maven.wagon:wagon-ftp:2.2

webdav

org.apache.maven.wagon:wagon-webdav:1.0-beta-2

file

-

Installing to the local repository

The Maven plugin adds an install task to your project. This task depends on all the archives task of the archives configuration. It installs those archives to your local Maven repository. If the default location for the local repository is redefined in a Maven settings.xml, this is considered by this task.

Maven POM generation

When deploying an artifact to a Maven repository, Gradle automatically generates a POM for it. The groupId, artifactId, version and packaging elements used for the POM default to the values shown in the table below. The dependency elements are created from the project’s dependency declarations.

Table 24. Default Values for Maven POM generation

Maven Element Default Value

groupId

project.group

artifactId

uploadTask.repositories.mavenDeployer.pom.artifactId (if set) or archiveTask.baseName.

version

project.version

packaging

archiveTask.extension

Here, uploadTask and archiveTask refer to the tasks used for uploading and generating the archive, respectively (for example uploadArchives and jar). archiveTask.baseName defaults to project.archivesBaseName which in turn defaults to project.name.

When you set the “archiveTask.baseName” property to a value other than the default, you’ll also have to set uploadTask.repositories.mavenDeployer.pom.artifactId to the same value. Otherwise, the project at hand may be referenced with the wrong artifact ID from generated POMs for other projects in the same build.

Generated POMs can be found in <buildDir>/poms. They can be further customized via the MavenPom API. For example, you might want the artifact deployed to the Maven repository to have a different version or name than the artifact generated by Gradle. To customize these you can do:

Example 246. Customization of pom

build.gradle

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://localhost/tmp/myRepo/")
            pom.version = '1.0Maven'
            pom.artifactId = 'myMavenName'
        }
    }
}

To add additional content to the POM, the pom.project builder can be used. With this builder, any element listed in the Maven POM reference can be added.

Example 247. Builder style customization of pom

build.gradle

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://localhost/tmp/myRepo/")
            pom.project {
                licenses {
                    license {
                        name 'The Apache Software License, Version 2.0'
                        url 'http://www.apache.org/licenses/LICENSE-2.0.txt'
                        distribution 'repo'
                    }
                }
            }
        }
    }
}

Note: groupId, artifactId, version, and packaging should always be set directly on the pom object.

Example 248. Modifying auto-generated content

build.gradle

def installer = install.repositories.mavenInstaller
def deployer = uploadArchives.repositories.mavenDeployer

[installer, deployer]*.pom*.whenConfigured {pom ->
    pom.dependencies.find {dep -> dep.groupId == 'group3' && dep.artifactId == 'runtime' }.optional = true
}

If you have more than one artifact to publish, things work a little bit differently. See the section called “Multiple artifacts per project”.

To customize the settings for the Maven installer (see the section called “Installing to the local repository”), you can do:

Example 249. Customization of Maven installer

build.gradle

install {
    repositories.mavenInstaller {
        pom.version = '1.0Maven'
        pom.artifactId = 'myName'
    }
}

Multiple artifacts per project

Maven can only deal with one artifact per project. This is reflected in the structure of the Maven POM. We think there are many situations where it makes sense to have more than one artifact per project. In such a case you need to generate multiple POMs. In such a case you have to explicitly declare each artifact you want to publish to a Maven repository. The MavenDeployer and the MavenInstaller both provide an API for this:

Example 250. Generation of multiple poms

build.gradle

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://localhost/tmp/myRepo/")
            addFilter('api') {artifact, file ->
                artifact.name == 'api'
            }
            addFilter('service') {artifact, file ->
                artifact.name == 'service'
            }
            pom('api').version = 'mySpecialMavenVersion'
        }
    }
}

You need to declare a filter for each artifact you want to publish. This filter defines a boolean expression for which Gradle artifact it accepts. Each filter has a POM associated with it which you can configure. To learn more about this have a look at PomFilterContainer and its associated classes.

Dependency mapping

The Maven plugin configures the default mapping between the Gradle configurations added by the Java and War plugin and the Maven scopes. Most of the time you don’t need to touch this and you can safely skip this section. The mapping works like the following. You can map a configuration to one and only one scope. Different configurations can be mapped to one or different scopes. You can also assign a priority to a particular configuration-to-scope mapping. Have a look at Conf2ScopeMappingContainer to learn more. To access the mapping configuration you can say:

Example 251. Accessing a mapping configuration

build.gradle

task mappings {
    doLast {
        println conf2ScopeMappings.mappings
    }
}

Gradle exclude rules are converted to Maven excludes if possible. Such a conversion is possible if in the Gradle exclude rule the group as well as the module name is specified (as Maven needs both in contrast to Ivy). Per-configuration excludes are also included in the Maven POM, if they are convertible.



[11] It is planned for a future release to provide out-of-the-box support for this

The Signing Plugin

The signing plugin adds the ability to digitally sign built files and artifacts. These digital signatures can then be used to prove who built the artifact the signature is attached to as well as other information such as when the signature was generated.

The signing plugin currently only provides support for generating OpenPGP signatures (which is the signature format required for publication to the Maven Central Repository).

Usage

To use the Signing plugin, include the following in your build script:

Example 252. Using the Signing plugin

build.gradle

apply plugin: 'signing'

Signatory credentials

In order to create OpenPGP signatures, you will need a key pair (instructions on creating a key pair using the GnuPG tools can be found in the GnuPG HOWTOs). You need to provide the signing plugin with your key information, which means three things:

  • The public key ID (The last 8 symbols of the keyId. You can use gpg -K to get it).

  • The absolute path to the secret key ring file containing your private key. (Since gpg 2.1, you need to export the keys with command gpg --keyring secring.gpg --export-secret-keys > ~/.gnupg/secring.gpg).

  • The passphrase used to protect your private key.

These items must be supplied as the values of properties signing.keyId, signing.secretKeyRingFile, and signing.password respectively. Given the personal and private nature of these values, a good practice is to store them in the user gradle.properties file (described in the section called “System properties”).

signing.keyId=24875D73
signing.password=secret
signing.secretKeyRingFile=/Users/me/.gnupg/secring.gpg

If specifying this information (especially signing.password) in the user gradle.properties file is not feasible for your environment, you can source the information however you need to and set the project properties manually.

import org.gradle.plugins.signing.Sign

gradle.taskGraph.whenReady { taskGraph ->
    if (taskGraph.allTasks.any { it instanceof Sign }) {
        // Use Java 6's console to read from the console (no good for
        // a CI environment)
        Console console = System.console()
        console.printf "\n\nWe have to sign some things in this build." +
                       "\n\nPlease enter your signing details.\n\n"

        def id = console.readLine("PGP Key Id: ")
        def file = console.readLine("PGP Secret Key Ring File (absolute path): ")
        def password = console.readPassword("PGP Private Key Password: ")

        allprojects { ext."signing.keyId" = id }
        allprojects { ext."signing.secretKeyRingFile" = file }
        allprojects { ext."signing.password" = password }

        console.printf "\nThanks.\n\n"
    }
}

Note that the presence of a null value for any these three properties will cause an exception.

Using OpenPGP subkeys

OpenPGP supports subkeys, which are like the normal keys, except they’re bound to a master key pair. One feature of OpenPGP subkeys is that they can be revoked independently of the master keys which makes key management easier. A practical case study of how subkeys can be leveraged in software development can be read on the Debian wiki.

The signing plugin supports OpenPGP subkeys out of the box. Just specify a subkey ID as the value in the signing.keyId property.

Using gpg-agent

By default the signing plugin uses a Java-based implementation of PGP for signing. This implementation cannot use the gpg-agent program for managing private keys, though. If you want to use the gpg-agent, you can change the signatory implementation used by the signing plugin:

Example 253. Sign with GnuPG

build.gradle

signing {
    useGpgCmd()
    sign configurations.archives
}

This tells the signing plugin to use the GnupgSignatory instead of the default PgpSignatory. The GnupgSignatory relies on the gpg2 program to sign the artifacts. Of course, this requires that GnuPG is installed.

Without any further configuration the gpg2 (on Windows: gpg2.exe) executable found on the PATH will be used. The password is supplied by the gpg-agent and the default key is used for signing.

Gnupg signatory configuration

The GnupgSignatory supports a number of configuration options for controlling how gpg is invoked. These are typically set in gradle.properties:

Example 254. Configure the GnupgSignatory

gradle.properties

signing.gnupg.executable=gpg
signing.gnupg.useLegacyGpg=true
signing.gnupg.homeDir=gnupg-home
signing.gnupg.optionsFile=gnupg-home/gpg.conf
signing.gnupg.keyName=24875D73
signing.gnupg.passphrase=gradle

signing.gnupg.executable

The gpg executable that is invoked for signing. The default value of this property depends on useLegacyGpg. If that is true then the default value of executable is "gpg" otherwise it is "gpg2".

signing.gnupg.useLegacyGpg

Must be true if GnuPG version 1 is used and false otherwise. The default value of the property is false.

signing.gnupg.homeDir

Sets the home directory for GnuPG. If not given the default home directory of GnuPG is used.

signing.gnupg.optionsFile

Sets a custom options file for GnuPG. If not given GnuPG’s default configuration file is used.

signing.gnupg.keyName

The id of the key that should be used for signing. If not given then the default key configured in GnuPG will be used.

signing.gnupg.passphrase

The passphrase for unlocking the secret key. If not given then the gpg-agent program is used for getting the passphrase.

All configuration properties are optional.

Specifying what to sign

As well as configuring how things are to be signed (i.e. the signatory configuration), you must also specify what is to be signed. The Signing plugin provides a DSL that allows you to specify the tasks and/or configurations that should be signed.

Signing Configurations

It is common to want to sign the artifacts of a configuration. For example, the Java plugin configures a jar to build and this jar artifact is added to the archives configuration. Using the Signing DSL, you can specify that all of the artifacts of this configuration should be signed.

Example 255. Signing a configuration

build.gradle

signing {
    sign configurations.archives
}

This will create a task (of type Sign) in your project named “signArchives”, that will build any archives artifacts (if needed) and then generate signatures for them. The signature files will be placed alongside the artifacts being signed.

Example 256. Signing a configuration output

Output of gradle signArchives

> gradle signArchives
> Task :compileJava
> Task :processResources
> Task :classes
> Task :jar
> Task :signArchives

BUILD SUCCESSFUL in 0s
4 actionable tasks: 4 executed

Signing Tasks

In some cases the artifact that you need to sign may not be part of a configuration. In this case you can directly sign the task that produces the artifact to sign.

Example 257. Signing a task

build.gradle

task stuffZip (type: Zip) {
    baseName = "stuff"
    from "src/stuff"
}

signing {
    sign stuffZip
}

This will create a task (of type Sign) in your project named “signStuffZip”, that will build the input task’s archive (if needed) and then sign it. The signature file will be placed alongside the artifact being signed.

Example 258. Signing a task output

Output of gradle signStuffZip

> gradle signStuffZip
> Task :stuffZip
> Task :signStuffZip

BUILD SUCCESSFUL in 0s
2 actionable tasks: 2 executed

For a task to be “signable”, it must produce an archive of some type. Tasks that do this are the Tar, Zip, Jar, War and Ear tasks.

Conditional Signing

A common usage pattern is to only sign build artifacts under certain conditions. For example, you may not wish to sign artifacts for non-release versions. To achieve this, you can specify that signing is only required under certain conditions.

Example 259. Conditional signing

build.gradle

version = '1.0-SNAPSHOT'
ext.isReleaseVersion = !version.endsWith("SNAPSHOT")

signing {
    required { isReleaseVersion && gradle.taskGraph.hasTask("uploadArchives") }
    sign configurations.archives
}

In this example, we only want to require signing if we are building a release version and we are going to publish it. Because we are inspecting the task graph to determine if we are going to be publishing, we must set the signing.required property to a closure to defer the evaluation. See SigningExtension.setRequired(java.lang.Object) for more information.

Publishing the signatures

When specifying what is to be signed via the Signing DSL, the resultant signature artifacts are automatically added to the signatures and archives dependency configurations. This means that if you want to upload your signatures to your distribution repository along with the artifacts you simply execute the uploadArchives task as normal.

Signing POM files

Signing the generated POM file generated by the Maven Publishing plugin is currently not supported. Future versions of Gradle might add this functionality.

When deploying signatures for your artifacts to a Maven repository, you will also want to sign the published POM file. The signing plugin adds a signing.signPom() (see: SigningExtension.signPom(org.gradle.api.artifacts.maven.MavenDeployment, groovy.lang.Closure)) method that can be used in the beforeDeployment() block in your upload task configuration.

Example 260. Signing a POM for deployment

build.gradle

uploadArchives {
    repositories {
        mavenDeployer {
            beforeDeployment { MavenDeployment deployment -> signing.signPom(deployment) }
        }
    }
}

When signing is not required and the POM cannot be signed due to insufficient configuration (i.e. no credentials for signing) then the signPom() method will silently do nothing.

Ivy Publishing (new)

This chapter describes the new incubating Ivy publishing support provided by the “ivy-publish” plugin. Eventually this new publishing support will replace publishing via the Upload task.

If you are looking for documentation on the original Ivy publishing support using the Upload task please see Publishing artifacts.

This chapter describes how to publish build artifacts in the Apache Ivy format, usually to a repository for consumption by other builds or projects. What is published is one or more artifacts created by the build, and an Ivy module descriptor (normally ivy.xml) that describes the artifacts and the dependencies of the artifacts, if any.

A published Ivy module can be consumed by Gradle (see Declaring Dependencies) and other tools that understand the Ivy format.

The “ivy-publish” Plugin

The ability to publish in the Ivy format is provided by the “ivy-publish” plugin.

The “publishing” plugin creates an extension on the project named “publishing” of type PublishingExtension. This extension provides a container of named publications and a container of named repositories. The “ivy-publish” plugin works with IvyPublication publications and IvyArtifactRepository repositories.

Example 261. Applying the “ivy-publish” plugin

build.gradle

apply plugin: 'ivy-publish'

Applying the “ivy-publish” plugin does the following:

Publications

If you are not familiar with project artifacts and configurations, you should read Publishing artifacts, which introduces these concepts. This chapter also describes “publishing artifacts” using a different mechanism than what is described in this chapter. The publishing functionality described here will eventually supersede that functionality.

Publication objects describe the structure/configuration of a publication to be created. Publications are published to repositories via tasks, and the configuration of the publication object determines exactly what is published. All of the publications of a project are defined in the PublishingExtension.getPublications() container. Each publication has a unique name within the project.

For the “ivy-publish” plugin to have any effect, an IvyPublication must be added to the set of publications. This publication determines which artifacts are actually published as well as the details included in the associated Ivy module descriptor file. A publication can be configured by adding components, customizing artifacts, and by modifying the generated module descriptor file directly.

Publishing a Software Component

The simplest way to publish a Gradle project to an Ivy repository is to specify a SoftwareComponent to publish. The components presently available for publication are:

Table 25. Software Components

Name Provided By Artifacts Dependencies

java

Java Plugin

Generated jar file

Dependencies from 'runtime' configuration

web

War Plugin

Generated war file

No dependencies

In the following example, artifacts and runtime dependencies are taken from the java component, which is added by the Java Plugin.

Example 262. Publishing a Java module to Ivy

build.gradle

publications {
    ivyJava(IvyPublication) {
        from components.java
    }
}

Publishing custom artifacts

It is also possible to explicitly configure artifacts to be included in the publication. Artifacts are commonly supplied as raw files, or as instances of AbstractArchiveTask (e.g. Jar, Zip).

For each custom artifact, it is possible to specify the name, extension, type, classifier and conf values to use for publication. Note that each artifacts must have a unique name/classifier/extension combination.

Configure custom artifacts as follows:

Example 263. Publishing additional artifact to Ivy

build.gradle

task sourceJar(type: Jar) {
    from sourceSets.main.java
    classifier "source"
}
publishing {
    publications {
        ivy(IvyPublication) {
            from components.java
            artifact(sourceJar) {
                type "source"
                conf "compile"
            }
        }
    }
}

See the IvyPublication class in the API documentation for more detailed information on how artifacts can be customized.

Identity values for the published project

The generated Ivy module descriptor file contains an <info> element that identifies the module. The default identity values are derived from the following:

Overriding the default identity values is easy: simply specify the organisation, module or revision attributes when configuring the IvyPublication. The status and branch attributes can be set via the descriptor property (see IvyModuleDescriptorSpec). The descriptor property can also be used to add additional custom elements as children of the <info> element.

Example 264. customizing the publication identity

build.gradle

publishing {
    publications {
        ivy(IvyPublication) {
            organisation 'org.gradle.sample'
            module 'project1-sample'
            revision '1.1'
            descriptor.status = 'milestone'
            descriptor.branch = 'testing'
            descriptor.extraInfo 'http://my.namespace', 'myElement', 'Some value'

            from components.java
        }
    }
}

Certain repositories are not able to handle all supported characters. For example, the ':' character cannot be used as an identifier when publishing to a filesystem-backed repository on Windows.

Gradle will handle any valid Unicode character for organisation, module and revision (as well as artifact name, extension and classifier). The only values that are explicitly prohibited are ‘\’, ‘/’ and any ISO control character. The supplied values are validated early during publication.

Modifying the generated module descriptor

At times, the module descriptor file generated from the project information will need to be tweaked before publishing. The “ivy-publish” plugin provides a hook to allow such modification.

Example 265. Customizing the module descriptor file

build.gradle

publications {
    ivyCustom(IvyPublication) {
        descriptor.withXml {
            asNode().info[0].appendNode('description',
                                        'A demonstration of ivy descriptor customization')
        }
    }
}

In this example we are simply adding a 'description' element to the generated Ivy dependency descriptor, but this hook allows you to modify any aspect of the generated descriptor. For example, you could replace the version range for a dependency with the actual version used to produce the build.

See IvyModuleDescriptorSpec.withXml(org.gradle.api.Action) in the API documentation for more information.

It is possible to modify virtually any aspect of the created descriptor should you need to. This means that it is also possible to modify the descriptor in such a way that it is no longer a valid Ivy module descriptor, so care must be taken when using this feature.

The identifier (organisation, module, revision) of the published module is an exception; these values cannot be modified in the descriptor using the withXML hook.

Publishing multiple modules

Sometimes it’s useful to publish multiple modules from your Gradle build, without creating a separate Gradle subproject. An example is publishing a separate API and implementation jar for your library. With Gradle this is simple:

Example 266. Publishing multiple modules from a single project

build.gradle

task apiJar(type: Jar) {
    baseName "publishing-api"
    from sourceSets.main.output
    exclude '**/impl/**'
}
publishing {
    publications {
        impl(IvyPublication) {
            organisation 'org.gradle.sample.impl'
            module 'project2-impl'
            revision '2.3'

            from components.java
        }
        api(IvyPublication) {
            organisation 'org.gradle.sample'
            module 'project2-api'
            revision '2'
        }
    }
}

If a project defines multiple publications then Gradle will publish each of these to the defined repositories. Each publication must be given a unique identity as described above.

Repositories

Publications are published to repositories. The repositories to publish to are defined by the PublishingExtension.getRepositories() container.

Example 267. Declaring repositories to publish to

build.gradle

repositories {
    ivy {
        // change to point to your repo, e.g. http://my.org/repo
        url "$buildDir/repo"
    }
}

The DSL used to declare repositories for publishing is the same DSL that is used to declare repositories for dependencies (RepositoryHandler). However, in the context of Ivy publication only the repositories created by the ivy() methods can be used as publication destinations. You cannot publish an IvyPublication to a Maven repository for example.

Performing a publish

The “ivy-publish” plugin automatically creates a PublishToIvyRepository task for each IvyPublication and IvyArtifactRepository combination in the publishing.publications and publishing.repositories containers respectively.

The created task is named “publish«PUBNAME»PublicationTo«REPONAME»Repository”, which is “publishIvyJavaPublicationToIvyRepository” for this example. This task is of type PublishToIvyRepository.

Example 268. Choosing a particular publication to publish

build.gradle

apply plugin: 'java'
apply plugin: 'ivy-publish'

group = 'org.gradle.sample'
version = '1.0'

publishing {
    publications {
        ivyJava(IvyPublication) {
            from components.java
        }
    }
    repositories {
        ivy {
            // change to point to your repo, e.g. http://my.org/repo
            url "$buildDir/repo"
        }
    }
}

Output of gradle publishIvyJavaPublicationToIvyRepository

> gradle publishIvyJavaPublicationToIvyRepository
> Task :generateDescriptorFileForIvyJavaPublication
> Task :compileJava NO-SOURCE
> Task :processResources NO-SOURCE
> Task :classes UP-TO-DATE
> Task :jar
> Task :publishIvyJavaPublicationToIvyRepository

BUILD SUCCESSFUL in 0s
3 actionable tasks: 3 executed

The “publish” lifecycle task

The “publish” plugin (that the “ivy-publish” plugin implicitly applies) adds a lifecycle task that can be used to publish all publications to all applicable repositories named “publish”.

In more concrete terms, executing this task will execute all PublishToIvyRepository tasks in the project. This is usually the most convenient way to perform a publish.

Example 269. Publishing all publications via the “publish” lifecycle task

Output of gradle publish

> gradle publish
> Task :generateDescriptorFileForIvyJavaPublication
> Task :compileJava NO-SOURCE
> Task :processResources NO-SOURCE
> Task :classes UP-TO-DATE
> Task :jar
> Task :publishIvyJavaPublicationToIvyRepository
> Task :publish

BUILD SUCCESSFUL in 0s
3 actionable tasks: 3 executed

Generating the Ivy module descriptor file without publishing

At times it is useful to generate the Ivy module descriptor file (normally ivy.xml) without publishing your module to an Ivy repository. Since descriptor file generation is performed by a separate task, this is very easy to do.

The “ivy-publish” plugin creates one GenerateIvyDescriptor task for each registered IvyPublication, named “generateDescriptorFileFor«PUBNAME»Publication”, which will be “generateDescriptorFileForIvyJavaPublication” for the previous example of the “ivyJava” publication.

You can specify where the generated Ivy file will be located by setting the destination property on the generated task. By default this file is written to “build/publications/«PUBNAME»/ivy.xml”.

Example 270. Generating the Ivy module descriptor file

build.gradle

model {
    tasks.generateDescriptorFileForIvyCustomPublication {
        destination = file("$buildDir/generated-ivy.xml")
    }
}

Output of gradle generateDescriptorFileForIvyCustomPublication

> gradle generateDescriptorFileForIvyCustomPublication
> Task :generateDescriptorFileForIvyCustomPublication

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

The “ivy-publish” plugin leverages some experimental support for late plugin configuration, and the GenerateIvyDescriptor task will not be constructed until the publishing extension is configured. The simplest way to ensure that the publishing plugin is configured when you attempt to access the GenerateIvyDescriptor task is to place the access inside a model block, as the example above demonstrates.

The same applies to any attempt to access publication-specific tasks like PublishToIvyRepository. These tasks should be referenced from within a model block.

Complete example

The following example demonstrates publishing with a multi-project build. Each project publishes a Java component and a configured additional source artifact. The descriptor file is customized to include the project description for each project.

Example 271. Publishing a Java module

build.gradle

subprojects {
    apply plugin: 'java'
    apply plugin: 'ivy-publish'

    version = '1.0'
    group = 'org.gradle.sample'

    repositories {
        mavenCentral()
    }
    task sourceJar(type: Jar) {
        from sourceSets.main.java
        classifier "source"
    }
}

project(":project1") {
    description = "The first project"

    dependencies {
       compile 'junit:junit:4.12', project(':project2')
    }
}

project(":project2") {
    description = "The second project"

    dependencies {
       compile 'commons-collections:commons-collections:3.2.2'
    }
}

subprojects {
    publishing {
        repositories {
            ivy {
                // change to point to your repo, e.g. http://my.org/repo
                url "${rootProject.buildDir}/repo"
            }
        }
        publications {
            ivy(IvyPublication) {
                from components.java
                artifact(sourceJar) {
                    type "source"
                    conf "compile"
                }
                descriptor.withXml {
                    asNode().info[0].appendNode('description', description)
                }
            }
        }
    }
}

The result is that the following artifacts will be published for each project:

  • The Ivy module descriptor file: “ivy-1.0.xml”.

  • The primary “jar” artifact for the Java component: “project1-1.0.jar”.

  • The source “jar” artifact that has been explicitly configured: “project1-1.0-source.jar”.

When project1 is published, the module descriptor (i.e. the ivy.xml file) that is produced will look like:

Note that «PUBLICATION-TIME-STAMP» in this example Ivy module descriptor will be the timestamp of when the descriptor was generated.

Example 272. Example generated ivy.xml

output-ivy.xml

<?xml version="1.0" encoding="UTF-8"?>
<ivy-module version="2.0">
  <info organisation="org.gradle.sample" module="project1" revision="1.0" status="integration" publication="«PUBLICATION-TIME-STAMP»">
    <description>The first project</description>
  </info>
  <configurations>
    <conf name="compile" visibility="public"/>
    <conf name="default" visibility="public" extends="compile,runtime"/>
    <conf name="runtime" visibility="public"/>
  </configurations>
  <publications>
    <artifact name="project1" type="jar" ext="jar" conf="compile"/>
    <artifact name="project1" type="source" ext="jar" conf="compile" m:classifier="source" xmlns:m="http://ant.apache.org/ivy/maven"/>
  </publications>
  <dependencies>
    <dependency org="junit" name="junit" rev="4.12" conf="compile-&gt;default"/>
    <dependency org="org.gradle.sample" name="project2" rev="1.0" conf="compile-&gt;default"/>
  </dependencies>
</ivy-module>

Future features

The “ivy-publish” plugin functionality as described above is incomplete, as the feature is still incubating. In upcoming Gradle releases, the functionality will be expanded to include (but not limited to):

  • Convenient customization of module attributes (module, organisation etc.)

  • Convenient customization of dependencies reported in module descriptor.

  • Multiple discrete publications per project

Maven Publishing (new)

This chapter describes the new incubating Maven publishing support provided by the “maven-publish” plugin. Eventually this new publishing support will replace publishing via the Upload task.

Note: Signing the generated POM file generated by this plugin is currently not supported. Future versions of Gradle might add this functionality. Please use the Maven plugin for the purpose of publishing your artifacts to Maven Central.

If you are looking for documentation on the original Maven publishing support using the Upload task please see Publishing artifacts.

This chapter describes how to publish build artifacts to an Apache Maven Repository. A module published to a Maven repository can be consumed by Maven, Gradle (see Declaring Dependencies) and other tools that understand the Maven repository format.

The “maven-publish” Plugin

The ability to publish in the Maven format is provided by the “maven-publish” plugin.

The “publishing” plugin creates an extension on the project named “publishing” of type PublishingExtension. This extension provides a container of named publications and a container of named repositories. The “maven-publish” plugin works with MavenPublication publications and MavenArtifactRepository repositories.

Example 273. Applying the 'maven-publish' plugin

build.gradle

apply plugin: 'maven-publish'

Applying the “maven-publish” plugin does the following:

Publications

If you are not familiar with project artifacts and configurations, you should read the Publishing artifacts that introduces these concepts. This chapter also describes “publishing artifacts” using a different mechanism than what is described in this chapter. The publishing functionality described here will eventually supersede that functionality.

Publication objects describe the structure/configuration of a publication to be created. Publications are published to repositories via tasks, and the configuration of the publication object determines exactly what is published. All of the publications of a project are defined in the PublishingExtension.getPublications() container. Each publication has a unique name within the project.

For the “maven-publish” plugin to have any effect, a MavenPublication must be added to the set of publications. This publication determines which artifacts are actually published as well as the details included in the associated POM file. A publication can be configured by adding components, customizing artifacts, and by modifying the generated POM file directly.

Publishing a Software Component

The simplest way to publish a Gradle project to a Maven repository is to specify a SoftwareComponent to publish. The components presently available for publication are:

Table 26. Software Components

Name Provided By Artifacts Dependencies

java

The Java Plugin

Generated jar file

Dependencies from 'runtime' configuration

web

The War Plugin

Generated war file

No dependencies

In the following example, artifacts and runtime dependencies are taken from the java component, which is added by the Java Plugin.

Example 274. Adding a MavenPublication for a Java component

build.gradle

publishing {
    publications {
        mavenJava(MavenPublication) {
            from components.java
        }
    }
}

Publishing custom artifacts

It is also possible to explicitly configure artifacts to be included in the publication. Artifacts are commonly supplied as raw files, or as instances of AbstractArchiveTask (e.g. Jar, Zip).

For each custom artifact, it is possible to specify the extension and classifier values to use for publication. Note that only one of the published artifacts can have an empty classifier, and all other artifacts must have a unique classifier/extension combination.

Configure custom artifacts as follows:

Example 275. Adding additional artifact to a MavenPublication

build.gradle

task sourceJar(type: Jar) {
    from sourceSets.main.allJava
}

publishing {
    publications {
        mavenJava(MavenPublication) {
            from components.java

            artifact sourceJar {
                classifier "sources"
            }
        }
    }
}

See the MavenPublication class in the API documentation for more information about how artifacts can be customized.

Identity values in the generated POM

The attributes of the generated POM file will contain identity values derived from the following project properties:

Overriding the default identity values is easy: simply specify the groupId, artifactId or version attributes when configuring the MavenPublication.

Example 276. customizing the publication identity

build.gradle

publishing {
    publications {
        maven(MavenPublication) {
            groupId 'org.gradle.sample'
            artifactId 'project1-sample'
            version '1.1'

            from components.java
        }
    }
}

Certain repositories will not be able to handle all supported characters. For example, the ':' character cannot be used as an identifier when publishing to a filesystem-backed repository on Windows.

Maven restricts 'groupId' and 'artifactId' to a limited character set ([A-Za-z0-9_\\-.]+) and Gradle enforces this restriction. For 'version' (as well as artifact 'extension' and 'classifier'), Gradle will handle any valid Unicode character.

The only Unicode values that are explicitly prohibited are ‘\’, ‘/’ and any ISO control character. Supplied values are validated early in publication.

Modifying the generated POM

The generated POM file may need to be tweaked before publishing. The “maven-publish” plugin provides a hook to allow such modification.

Example 277. Modifying the POM file

build.gradle

publications {
    mavenCustom(MavenPublication) {
        pom.withXml {
            asNode().appendNode('description',
                                'A demonstration of maven POM customization')
        }
    }
}

In this example we are adding a 'description' element for the generated POM. With this hook, you can modify any aspect of the POM. For example, you could replace the version range for a dependency with the actual version used to produce the build.

See MavenPom.withXml(org.gradle.api.Action) in the API documentation for more information.

It is possible to modify virtually any aspect of the created POM. This means that it is also possible to modify the POM in such a way that it is no longer a valid Maven POM, so care must be taken when using this feature.

The identifier (groupId, artifactId, version) of the published module is an exception; these values cannot be modified in the POM using the withXML hook.

Publishing multiple modules

Sometimes it’s useful to publish multiple modules from your Gradle build, without creating a separate Gradle subproject. An example is publishing a separate API and implementation jar for your library. With Gradle this is simple:

Example 278. Publishing multiple modules from a single project

build.gradle

task apiJar(type: Jar) {
    baseName "publishing-api"
    from sourceSets.main.output
    exclude '**/impl/**'
}

publishing {
    publications {
        impl(MavenPublication) {
            groupId 'org.gradle.sample.impl'
            artifactId 'project2-impl'
            version '2.3'

            from components.java
        }
        api(MavenPublication) {
            groupId 'org.gradle.sample'
            artifactId 'project2-api'
            version '2'

            artifact apiJar
        }
    }
}

If a project defines multiple publications then Gradle will publish each of these to the defined repositories. Each publication must be given a unique identity as described above.

Repositories

Publications are published to repositories. The repositories to publish to are defined by the PublishingExtension.getRepositories() container.

Example 279. Declaring repositories to publish to

build.gradle

publishing {
    repositories {
        maven {
            // change to point to your repo, e.g. http://my.org/repo
            url "$buildDir/repo"
        }
    }
}

The DSL used to declare repositories for publication is the same DSL that is used to declare repositories to consume dependencies from, RepositoryHandler. However, in the context of Maven publication only MavenArtifactRepository repositories can be used for publication.

Performing a publish

The “maven-publish” plugin automatically creates a PublishToMavenRepository task for each MavenPublication and MavenArtifactRepository combination in the publishing.publications and publishing.repositories containers respectively.

The created task is named “publish«PUBNAME»PublicationTo«REPONAME»Repository”.

Example 280. Publishing a project to a Maven repository

build.gradle

apply plugin: 'java'
apply plugin: 'maven-publish'

group = 'org.gradle.sample'
version = '1.0'

publishing {
    publications {
        mavenJava(MavenPublication) {
            from components.java
        }
    }
}
publishing {
    repositories {
        maven {
            // change to point to your repo, e.g. http://my.org/repo
            url "$buildDir/repo"
        }
    }
}

Output of gradle publish

> gradle publish
> Task :generatePomFileForMavenJavaPublication
> Task :compileJava
> Task :processResources NO-SOURCE
> Task :classes
> Task :jar
> Task :publishMavenJavaPublicationToMavenRepository
> Task :publish

BUILD SUCCESSFUL in 0s
4 actionable tasks: 4 executed

In this example, a task named “publishMavenJavaPublicationToMavenRepository” is created, which is of type PublishToMavenRepository. This task is wired into the publish lifecycle task. Executing “gradle publish” builds the POM file and all of the artifacts to be published, and transfers them to the repository.

Publishing to Maven Local

For integration with a local Maven installation, it is sometimes useful to publish the module into the local .m2 repository. In Maven parlance, this is referred to as 'installing' the module. The “maven-publish” plugin makes this easy to do by automatically creating a PublishToMavenLocal task for each MavenPublication in the publishing.publications container. Each of these tasks is wired into the publishToMavenLocal lifecycle task. You do not need to have mavenLocal in your publishing.repositories section.

The created task is named “publish«PUBNAME»PublicationToMavenLocal”.

Example 281. Publish a project to the Maven local repository

Output of gradle publishToMavenLocal

> gradle publishToMavenLocal
> Task :generatePomFileForMavenJavaPublication
> Task :compileJava
> Task :processResources NO-SOURCE
> Task :classes
> Task :jar
> Task :publishMavenJavaPublicationToMavenLocal
> Task :publishToMavenLocal

BUILD SUCCESSFUL in 0s
4 actionable tasks: 4 executed

The resulting task in this example is named “publishMavenJavaPublicationToMavenLocal”. This task is wired into the publishToMavenLocal lifecycle task. Executing “gradle publishToMavenLocal” builds the POM file and all of the artifacts to be published, and “installs” them into the local Maven repository.

Generating the POM file without publishing

At times it is useful to generate a Maven POM file for a module without actually publishing. Since POM generation is performed by a separate task, it is very easy to do so.

The task for generating the POM file is of type GenerateMavenPom, and it is given a name based on the name of the publication: “generatePomFileFor«PUBNAME»Publication”. So in the example below, where the publication is named “mavenCustom”, the task will be named “generatePomFileForMavenCustomPublication”.

Example 282. Generate a POM file without publishing

build.gradle

model {
    tasks.generatePomFileForMavenCustomPublication {
        destination = file("$buildDir/generated-pom.xml")
    }
}

Output of gradle generatePomFileForMavenCustomPublication

> gradle generatePomFileForMavenCustomPublication
> Task :generatePomFileForMavenCustomPublication

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

All details of the publishing model are still considered in POM generation, including components, custom artifacts, and any modifications made via pom.withXml.

The “maven-publish” plugin leverages some experimental support for late plugin configuration, and any GenerateMavenPom tasks will not be constructed until the publishing extension is configured. The simplest way to ensure that the publishing plugin is configured when you attempt to access the GenerateMavenPom task is to place the access inside a model block, as the example above demonstrates.

The same applies to any attempt to access publication-specific tasks like PublishToMavenRepository. These tasks should be referenced from within a model block.

The Distribution Plugin

The distribution plugin is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

The distribution plugin facilitates building archives that serve as distributions of the project. Distribution archives typically contain the executable application and other supporting files, such as documentation.

Usage

To use the distribution plugin, include the following in your build script:

Example 283. Using the distribution plugin

build.gradle

apply plugin: 'distribution'

The plugin adds an extension named “distributions” of type DistributionContainer to the project. It also creates a single distribution in the distributions container extension named “main”. If your build only produces one distribution you only need to configure this distribution (or use the defaults).

You can run “gradle distZip” to package the main distribution as a ZIP, or “gradle distTar” to create a TAR file. To build both types of archives just run gradle assembleDist. The files will be created at “$buildDir/distributions/$project.name-$project.version.«ext»”.

You can run “gradle installDist” to assemble the uncompressed distribution into “$buildDir/install/main”.

Tasks

The Distribution plugin adds the following tasks to the project:

Table 27. Distribution plugin - tasks

Task name Depends on Type Description

distZip

-

Zip

Creates a ZIP archive of the distribution contents

distTar

-

Tar

Creates a TAR archive of the distribution contents

assembleDist

distTar, distZip

Task

Creates ZIP and TAR archives with the distribution contents

installDist

-

Sync

Assembles the distribution content and installs it on the current machine

For each extra distribution set you add to the project, the distribution plugin adds the following tasks:

Table 28. Multiple distributions - tasks

Task name Depends on Type Description

${distribution.name}DistZip

-

Zip

Creates a ZIP archive of the distribution contents

${distribution.name}DistTar

-

Tar

Creates a TAR archive of the distribution contents

assemble${distribution.name.capitalize()}Dist

${distribution.name}DistTar, ${distribution.name}DistZip

Task

Assembles all distribution archives

install${distribution.name.capitalize()}Dist

-

Sync

Assembles the distribution content and installs it on the current machine

Example 284. Adding extra distributions

build.gradle

apply plugin: 'distribution'

version = '1.2'
distributions {
    custom {}
}

This will add following tasks to the project:

  • customDistZip

  • customDistTar

  • assembleCustomDist

  • installCustomDist

Given that the project name is “myproject” and version “1.2”, running “gradle customDistZip” will produce a ZIP file named “myproject-custom-1.2.zip”.

Running “gradle installCustomDist” will install the distribution contents into “$buildDir/install/custom”.

Distribution contents

All of the files in the “src/$distribution.name/dist” directory will automatically be included in the distribution. You can add additional files by configuring the Distribution object that is part of the container.

Example 285. Configuring the main distribution

build.gradle

apply plugin: 'distribution'

distributions {
    main {
        baseName = 'someName'
        contents {
            from { 'src/readme' }
        }
    }
}

apply plugin:'maven'

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://some/repo")
        }
    }
}


In the example above, the content of the “src/readme” directory will be included in the distribution (along with the files in the “src/main/dist” directory which are added by default).

The “baseName” property has also been changed. This will cause the distribution archives to be created with a different name.

Publishing distributions

The distribution plugin adds the distribution archives as candidate for default publishing artifacts. With the maven plugin applied the distribution zip file will be published when running uploadArchives if no other default artifact is configured

Example 286. publish main distribution

build.gradle

apply plugin:'maven'

uploadArchives {
    repositories {
        mavenDeployer {
            repository(url: "file://some/repo")
        }
    }
}

The Announce Plugin

The Gradle announce plugin allows you to send custom announcements during a build. The following notification systems are supported:

Usage

To use the announce plugin, apply it to your build script:

Example 287. Applying the announce plugin

build.gradle

apply plugin: 'announce'

Next, configure your notification service(s) of choice (see table below for which configuration properties are available):

Example 288. Configure the announce plugin

build.gradle

announce {  
  username = 'myId'
  password = 'myPassword'
}

Finally, send announcements with the announce method:

Example 289. Using the announce plugin

build.gradle

task helloWorld {
    doLast {
        println "Hello, world!"
    }
}  

helloWorld.doLast {  
    announce.announce("helloWorld completed!", "twitter")
    announce.announce("helloWorld completed!", "local")
}

The announce method takes two String arguments: The message to be sent, and the notification service to be used. The following table lists supported notification services and their configuration properties.

Table 29. Announce Plugin Notification Services

Notification Service Operating System Configuration Properties Further Information

twitter

Any

username, password

snarl

Windows

growl

macOS

notify-send

Ubuntu

Requires the notify-send package to be installed. Use sudo apt-get install libnotify-bin to install it.

local

Windows, macOS, Ubuntu

Automatically chooses between snarl, growl, and notify-send depending on the current operating system.

Configuration

See the AnnouncePluginExtension class in the API documentation.

The Build Announcements Plugin

The build announcements plugin is currently incubating. Please be aware that the DSL and other configuration may change in later Gradle versions.

The build announcements plugin uses the announce plugin to send local announcements on important events in the build.

Usage

To use the build announcements plugin, include the following in your build script:

Example 290. Using the build announcements plugin

build.gradle

apply plugin: 'build-announcements'

That’s it. If you want to tweak where the announcements go, you can configure the announce plugin to change the local announcer.

You can also apply the plugin from an init script:

Example 291. Using the build announcements plugin from an init script

init.gradle

rootProject {
    apply plugin: 'build-announcements'
}

Dependency management

Introduction to Dependency Management

What is dependency management?

Software projects rarely work in isolation. In most cases, a project relies on reusable functionality in the form of libraries or is broken up into individual components to compose a modularized system. Dependency management is a technique for declaring, resolving and using dependencies required by the project in an automated fashion.

For a general overview on the terms used throughout the user guide, refer to Dependency Management Terminology.

Dependency management in Gradle

Gradle has built-in support for dependency management and lives up the task of fulfilling typical scenarios encountered in modern software projects. We’ll explore the main concepts with the help of an example project. The illustration below should give you an rough overview on all the moving parts.

Figure 14. Dependency management big picture

Dependency management big picture

The example project builds Java source code. Some of the Java source files import classes from Google Guava, a open-source library providing a wealth of utility functionality. In addition to Guava, the project needs the JUnit libraries for compiling and executing test code.

Guava and JUnit represent the dependencies of this project. A build script developer can declare dependencies for different scopes e.g. just for compilation of source code or for executing tests. In Gradle, the scope of a dependency is called a configuration. For a full overview, see the reference material on dependency types.

Often times dependencies come in the form of modules. You’ll need to tell Gradle where to find those modules so they can be consumed by the build. The location for storing modules is called a repository. By declaring repositories for a build, Gradle will know how to find and retrieve modules. Repositories can come in different forms: as local directory or a remote repository. The reference on repository types provides a broad coverage on this topic.

At runtime, Gradle will locate the declared dependencies if needed for operating a specific task. The dependencies might need to be downloaded from a remote repository, retrieved from a local directory or requires another project to be built in a multi-project setting. This process is called dependency resolution. You can find a detailed discussion in the section called “How dependency resolution works”.

Once resolved, the resolution mechanism stores the underlying files of a dependency in a local cache, also referred to as the dependency cache. Future builds reuse the files stored in the cache to avoid unnecessary network calls.

Modules can provide additional metadata. Metadata is the data that describes the module in more detail e.g. the coordinates for finding it in a repository, information about the project, or its authors. As part of the metadata, a module can define that other modules are needed for it to work properly. For example, the JUnit 5 platform module also requires the platform commons module. Gradle automatically resolves those additional modules, so called transitive dependencies. If needed, you can customize the behavior the handling of transitive dependencies to your project’s requirements.

Projects with tens or hundreds of declared dependencies can easily suffer from dependency hell. Gradle provides sufficient tooling to visualize, navigate and analyze the dependency graph of a project either with the help of a build scan or built-in tasks. Learn more in Inspecting Dependencies.

Figure 15. Build scan dependencies report

Build scan dependencies report

How dependency resolution works

Gradle takes your dependency declarations and repository definitions and attempts to download all of your dependencies by a process called dependency resolution. Below is a brief outline of how this process works.

  • Given a required dependency, Gradle attempts to resolve the dependency by searching for the module the dependency points at. Each repository is inspected in order. Depending on the type of repository, Gradle looks for metadata files describing the module (.module, .pom or ivy.xml file) or directly for artifact files.

    • If the dependency is declared as a dynamic version (like 1.+), Gradle will resolve this to the highest available concrete version (like 1.2) in the repository. For Maven repositories, this is done using the maven-metadata.xml file, while for Ivy repositories this is done by directory listing.

    • If the module metadata is a POM file that has a parent POM declared, Gradle will recursively attempt to resolve each of the parent modules for the POM.

  • Once each repository has been inspected for the module, Gradle will choose the 'best' one to use. This is done using the following criteria:

    • For a dynamic version, a 'higher' concrete version is preferred over a 'lower' version.

    • Modules declared by a module metadata file (.module, .pom or ivy.xml file) are preferred over modules that have an artifact file only.

    • Modules from earlier repositories are preferred over modules in later repositories.

    • When the dependency is declared by a concrete version and a module metadata file is found in a repository, there is no need to continue searching later repositories and the remainder of the process is short-circuited.

  • All of the artifacts for the module are then requested from the same repository that was chosen in the process above.

The dependency resolution process is highly customizable to meet enterprise requirements. For more information, see the chapter on Customizing Dependency Resolution Behavior.

Declaring Dependencies

Gradle builds can declare dependencies on modules hosted in repositories, files and other Gradle projects. You can find examples for common scenarios in this section. For more information, see the full reference on all types of dependencies.

Every dependency needs to be assigned to a configuration when declared in a build script. For more information on the purpose and syntax of configurations, see Managing Dependency Configurations.

Declaring a dependency to a module

Modern software projects rarely build code in isolation. Projects reference modules for the purpose of reusing existing and proven functionality. Upon resolution, selected versions of modules are downloaded from dedicated repositories and stored in the dependency cache to avoid unnecessary network traffic.

Figure 16. Resolving dependencies from remote repositories

Resolving dependencies from remote repositories

Declaring a concrete version of a dependency

A typical example for such a library in a Java project is the Spring framework. The following code snippet declares a compile-time dependency on the Spring web module by its coordinates: org.springframework:spring-web:5.0.2.RELEASE. Gradle resolves the module including its transitive dependencies from the Maven Central repository and uses it to compile Java source code. The version attribute of the dependency coordinates points to a concrete version indicating that the underlying artifacts do not change over time. The use of concrete versions ensure reproducibility for the aspect of dependency resolution.

Example 292. Declaring a dependency with a concrete version

build.gradle

apply plugin: 'java-library'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework:spring-web:5.0.2.RELEASE'
}

A Gradle project can define other types of repositories hosting modules. You can learn more about the syntax and API in the section on declaring repositories. Refer to The Java Plugin for a deep dive on declaring dependencies for a Java project. The resolution behavior for dependencies is highly customizable.

Declaring a dependency without version

A recommended practice for larger projects is to declare dependencies without versions and use dependency constraints for version declaration. The advantage is that dependency constrains allow you to manage versions of all dependencies, including transitive ones, in one place.

Example 293. Declaring a dependency without version

build.gradle

dependencies {
    implementation 'org.springframework:spring-web'
}

dependencies {
    constraints {
        implementation 'org.springframework:spring-web:5.0.2.RELEASE'
    }
}

Declaring a dynamic version

Projects might adopt a more aggressive approach for consuming dependencies to modules. For example you might want to always integrate the latest version of a dependency to consume cutting edge features at any given time. A dynamic version allows for resolving the latest version or the latest version of a version range for a given module.

Using dynamic versions in a build bears the risk of potentially breaking it. As soon as a new version of the dependency is released that contains an incompatible API change your source code might stop compiling.

Example 294. Declaring a dependency with a dynamic version

build.gradle

apply plugin: 'java-library'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'org.springframework:spring-web:5.+'
}

A build scan can effectively visualize dynamic dependency versions and their respective, selected versions.

Figure 17. Dynamic dependencies in build scan

Dynamic dependencies in build scan

By default, Gradle caches dynamic versions of dependencies for 24 hours. Within this time frame, Gradle does not try to resolve newer versions from the declared repositories. The threshold can be configured as needed for example if you want to resolve new versions earlier.

Declaring a changing version

A team might decide to implement a series of features before releasing a new version of the application or library. A common strategy to allow consumers to integrate an unfinished version of their artifacts early and often is to release a module with a so-called changing version. A changing version indicates that the feature set is still under active development and hasn’t released a stable version for general availability yet.

In Maven repositories, changing versions are commonly referred to as snapshot versions. Snapshot versions contain the suffix -SNAPSHOT. The following example demonstrates how to declare a snapshot version on the Spring dependency.

Example 295. Declaring a dependencies with a changing version

build.gradle

apply plugin: 'java-library'

repositories {
    mavenCentral()
    maven {
        url 'https://repo.spring.io/snapshot/'
    }
}

dependencies {
    implementation 'org.springframework:spring-web:5.0.3.BUILD-SNAPSHOT'
}

By default, Gradle caches changing versions of dependencies for 24 hours. Within this time frame, Gradle does not try to resolve newer versions from the declared repositories. The threshold can be configured as needed for example if you want to resolve new snapshot versions earlier.

Gradle is flexible enough to treat any version as changing version e.g. if you wanted to model snapshot behavior for an Ivy module. All you need to do is to set the property ExternalModuleDependency.setChanging(boolean) to true.

Declaring a file dependency

Projects sometimes do not rely on a binary repository product e.g. JFrog Artifactory or Sonatype Nexus for hosting and resolving external dependencies. It’s common practice to host those dependencies on a shared drive or check them into version control alongside the project source code. Those dependencies are referred to as file dependencies, the reason being that they represent a file without any metadata (like information about transitive dependencies, the origin or its author) attached to them.

Figure 18. Resolving file dependencies from the local file system and a shared drive

Resolving file dependencies from the local file system and a shared drive

The following example resolves file dependencies from the directories ant, libs and tools.

Example 296. Declaring multiple file dependencies

build.gradle

configurations {
    antContrib
    externalLibs
    deploymentTools
}

dependencies {
    antContrib files('ant/antcontrib.jar')
    externalLibs files('libs/commons-lang.jar', 'libs/log4j.jar')
    deploymentTools fileTree(dir: 'tools', include: '*.exe')
}

As you can see in the code example, every dependency has to define its exact location in the file system. The most prominent methods for creating a file reference are Project.files(java.lang.Object[]) and Project.fileTree(java.lang.Object). Alternatively, you can also define the source directory of one or many file dependencies in the form of a flat directory repository.

Declaring a project dependency

Software projects often break up software components into modules to improve maintainability and prevent strong coupling. Modules can define dependencies between each other to reuse code within the same project.

Gradle can model dependencies between modules. Those dependencies are called project dependencies because each module is represented by a Gradle project. At runtime, the build automatically ensures that project dependencies are built in the correct order and added to the classpath for compilation. The chapter Authoring Multi-Project Builds discusses how to set up and configure multi-project builds in more detail.

Figure 19. Dependencies between projects

Dependencies between projects

The following example declares the dependencies on the utils and api project from the web-service project. The method Project.project(java.lang.String) creates a reference to a specific subproject by path.

Example 297. Declaring project dependencies

build.gradle

project(':web-service') {
    dependencies {
        implementation project(':utils')
        implementation project(':api')
    }
}

Resolving specific artifacts from a module dependency

Whenever Gradle tries to resolve a module from a Maven or Ivy repository, it looks for a metadata file and the default artifact file, a JAR. The build fails if none of these artifact files can be resolved. Under certain conditions, you might want to tweak the way Gradle resolves artifacts for a dependency.

  • The dependency only provides a non-standard artifact without any metadata e.g. a ZIP file.

  • The module metadata declares more than one artifact e.g. as part of an Ivy dependency descriptor.

  • You only want to download a specific artifact without any of the transitive dependencies declared in the metadata.

Gradle is a polyglot build tool and not limited to just resolving Java libraries. Let’s assume you wanted to build a web application using JavaScript as the client technology. Most projects check in external JavaScript libraries into version control. An external JavaScript library is no different than a reusable Java library so why not download it from a repository instead?

Google Hosted Libraries is a distribution platform for popular, open-source JavaScript libraries. With the help of the artifact-only notation you can download a JavaScript library file e.g. JQuery. The @ character separates the dependency’s coordinates from the artifact’s file extension.

Example 298. Resolving a JavaScript artifact for a declared dependency

build.gradle

repositories {
    ivy {
        url 'https://ajax.googleapis.com/ajax/libs'
        layout 'pattern', {
            artifact '[organization]/[revision]/[module].[ext]'
        }
    }
}

configurations {
    js
}

dependencies {
    js 'jquery:jquery:3.2.1@js'
}

Some modules ship different "flavors" of the same artifact or they publish multiple artifacts that belong to a specific module version but have a different purpose. It’s common for a Java library to publish the artifact with the compiled class files, another one with just the source code in it and a third one containing the Javadocs.

In JavaScript, a library may exist as uncompressed or minified artifact. In Gradle, a specific artifact identifier is called classifier, a term generally used in Maven and Ivy dependency management.

Let’s say we wanted to download the minified artifact of the JQuery library instead of the uncompressed file. You can provide the classifier min as part of the dependency declaration.

Example 299. Resolving a JavaScript artifact with classifier for a declared dependency

build.gradle

repositories {
    ivy {
        url 'https://ajax.googleapis.com/ajax/libs'
        layout 'pattern', {
            artifact '[organization]/[revision]/[module](.[classifier]).[ext]'
        }
    }
}

configurations {
    js
}

dependencies {
    js 'jquery:jquery:3.2.1:min@js'
}

Managing Dependency Configurations

What is a configuration?

Every dependency declared for a Gradle project applies to a specific scope. For example some dependencies should be used for compiling source code whereas others only need to be available at runtime. Gradle represents the scope of a dependency with the help of a Configuration. Every configuration can be identified by a unique name.

Many Gradle plugins add pre-defined configurations to your project. The Java plugin, for example, adds configurations to represent the various classpaths it needs for source code compilation, executing tests and the like. See the Java plugin chapter for an example. The sections above demonstrate how to declare dependencies for different use cases.

Figure 20. Configurations use declared dependencies for specific purposes

Configurations use declared dependencies for specific purposes

For more examples on the usage of configurations to navigate, inspect and post-process metadata and artifacts of assigned dependencies, see Working with Dependencies.

Defining custom configurations

You can define configurations yourself, so-called custom configurations. A custom configuration is useful for separating the scope of dependencies needed for a dedicated purpose.

Let’s say you wanted to declare a dependency on the Jasper Ant task for the purpose of pre-compiling JSP files that should not end up in the classpath for compiling your source code. It’s fairly simple to achieve that goal by introducing a custom configuration and using it in a task.

Example 300. Declaring and using a custom configuration

build.gradle

configurations {
    jasper
}

repositories {
    mavenCentral()
}

dependencies {
    jasper 'org.apache.tomcat.embed:tomcat-embed-jasper:9.0.2'
}

task preCompileJsps {
    doLast {
        ant.taskdef(classname: 'org.apache.jasper.JspC',
                    name: 'jasper',
                    classpath: configurations.jasper.asPath)
        ant.jasper(validateXml: false,
                   uriroot: file('src/main/webapp'),
                   outputDir: file("$buildDir/compiled-jsps"))
    }
}

A project’s configurations are managed by a configurations object. Configurations have a name and can extend each other. To learn more about this API have a look at ConfigurationContainer.

Inheriting dependencies from other configurations

A configuration can extend other configurations to form an inheritance hierarchy. Child configurations inherit the whole set of dependencies declared for any of its superconfigurations.

Configuration inheritance is heavily used by Gradle core plugins like the Java plugin. For example the testImplementation configuration extends the implementation configuration. The configuration hierarchy has a practical purpose: compiling tests requires the dependencies of the source code under test on top of the dependencies needed write the test class. A Java project that uses JUnit to write and execute test code also needs Guava if its classes are imported in the production source code.

Figure 21. Configuration inheritance provided by the Java plugin

Configuration inheritance provided by the Java plugin

Under the covers the testImplementation and implementation configurations form an inheritance hierarchy by calling the method Configuration.extendsFrom(org.gradle.api.artifacts.Configuration[]). A configuration can extend any other configuration irrespective of its definition in the build script or a plugin.

Let’s say you wanted to write a suite of smoke tests. Each smoke test makes a HTTP call to verify a web service endpoint. As the underlying test framework the project already uses JUnit. You can define a new configuration named smokeTest that extends from the testImplementation configuration to reuse the existing test framework dependency.

Example 301. Extending a configuration from another configuration

build.gradle

configurations {
    smokeTest.extendsFrom testImplementation
}

dependencies {
    testImplementation 'junit:junit:4.12'
    smokeTest 'org.apache.httpcomponents:httpclient:4.5.5'
}

Declaring Repositories

Gradle can resolve dependencies from one or many repositories based on Maven, Ivy or flat directory formats. Check out the full reference on all types of repositories for more information.

Declaring a publicly-available repository

Organizations building software may want to leverage public binary repositories to download and consume open source dependencies. Popular public repositories include Maven Central, Bintray JCenter and the Google Android repository. Gradle provides built-in shortcut methods for the most widely-used repositories.

Figure 22. Declaring a repository with the help of shortcut methods

Declaring a repository with the help of shortcut methods

To declare JCenter as repository, add this code to your build script:

Example 302. Declaring JCenter repository as source for resolving dependencies

build.gradle

repositories {
    jcenter()
}

Under the covers Gradle resolves dependencies from the respective URL of the public repository defined by the shortcut method. All shortcut methods are available via the RepositoryHandler API. Alternatively, you can spell out the URL of the repository for more fine-grained control.

Declaring a custom repository by URL

Most enterprise projects set up a binary repository available only within an intranet. In-house repositories enable teams to publish internal binaries, setup user management and security measure and ensure uptime and availability. Specifying a custom URL is also helpful if you want to declare a less popular, but publicly-available repository.

Add the following code to declare an in-house repository for your build reachable through a custom URL.

Example 303. Declaring a custom repository by URL

build.gradle

repositories {
    maven {
        url "http://repo.mycompany.com/maven2"
    }
}

Repositories with custom URLs can be specified as Maven or Ivy repositories by calling the corresponding methods available on the RepositoryHandler API. Gradle supports other protocols than http or https as part of the custom URL e.g. file, sftp or s3. For a full coverage see the reference manual on supported transport protocols.

You can also define your own repository layout by using ivy { } repositories as they are very flexible in terms of how modules are organised in a repository.

Declaring multiple repositories

You can define more than one repository for resolving dependencies. Declaring multiple repositories is helpful if some dependencies are only available in one repository but not the other. You can mix any type of repository described in the reference section.

This example demonstrates how to declare various shortcut and custom URL repositories for a project:

Example 304. Declaring multiple repositories

build.gradle

repositories {
    jcenter()
    maven {
        url "https://maven.springframework.org/release"
    }
    maven {
        url "https://maven.restlet.org"
    }
}

The order of declaration determines how Gradle will check for dependencies at runtime. If Gradle finds a module descriptor in a particular repository, it will attempt to download all of the artifacts for that module from the same repository. You can learn more about the inner workings of Gradle’s resolution mechanism.

Inspecting Dependencies

Gradle provides sufficient tooling to navigate large dependency graphs and mitigate situations that can lead to dependency hell. Users can chose to render the full graph of dependencies as well as identify the selection reason and origin for a dependency. The origin of a dependency can be a declared dependency in the build script or a transitive dependency in graph plus their corresponding configuration. Gradle offers both capabilities through visual representation via build scans and as command line tooling.

Listing dependencies in a project

A project can declare one or more dependencies. Gradle can visualize the whole dependency tree for every configuration available in the project.

Rendering the dependency tree is particularly useful if you’d like to identify which dependencies have been resolved at runtime. It also provides you with information about any dependency conflict resolution that occurred in the process and clearly indicates the selected version. The dependency report always contains declared and transitive dependencies.

Let’s say you’d want to create tasks for your project that use the JGit library to execute SCM operations e.g. to model a release process. You can declare dependencies for any external tooling with the help of a custom configuration so that it doesn’t doesn’t pollute other contexts like the compilation classpath for your production source code.

Example 305. Declaring the JGit dependency with a custom configuration

build.gradle

repositories {
    jcenter()
}

configurations {
    scm
}

dependencies {
    scm 'org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r'
}

A build scan can visualize dependencies as a navigable, searchable tree. Additional context information can be rendered by clicking on a specific dependency in the graph.

Figure 23. Dependency tree in a build scan

Dependency tree in a build scan

Every Gradle project provides the task dependencies to render the so-called dependency report from the command line. By default the dependency report renders dependencies for all configurations. To pair down on the information provide the optional parameter --configuration.

Example 306. Rendering the dependency report for a custom configuration

Output of gradle -q dependencies --configuration scm

> gradle -q dependencies --configuration scm

------------------------------------------------------------
Root project
------------------------------------------------------------

scm
\--- org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r
     +--- com.jcraft:jsch:0.1.54
     +--- com.googlecode.javaewah:JavaEWAH:1.1.6
     +--- org.apache.httpcomponents:httpclient:4.3.6
     |    +--- org.apache.httpcomponents:httpcore:4.3.3
     |    +--- commons-logging:commons-logging:1.1.3
     |    \--- commons-codec:commons-codec:1.6
     \--- org.slf4j:slf4j-api:1.7.2

A web-based, searchable dependency report is available by adding the --scan option.

The dependencies report provides detailed information about the dependencies available in the graph. Any dependency that could not be resolved is marked with FAILED in red color. Dependencies with the same coordinates that can occur multiple times in the graph are omitted and indicated by an asterisk. Dependencies that had to undergo conflict resolution render the requested and selected version separated by a right arrow character.

Identifying which dependency version was selected and why

Large software projects inevitably deal with an increased number of dependencies either through direct or transitive dependencies. The dependencies report provides you with the raw list of dependencies but does not explain why they have been selected or which dependency is responsible for pulling them into the graph.

Let’s have a look at a concrete example. A project may request two different versions of the same dependency either as direct or transitive dependency. Gradle applies version conflict resolution to ensure that only one version of the dependency exists in the dependency graph. In this example the conflicting dependency is represented by commons-codec:commons-codec.

Example 307. Declaring the JGit dependency and a conflicting dependency

build.gradle

repositories {
    jcenter()
}

configurations {
    scm
}

dependencies {
    scm 'org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r'
    scm 'commons-codec:commons-codec:1.7'
}

The dependency tree in a build scan renders the selection reason (conflict resolution) as well as the origin of a dependency if you click on a dependency and select the "Required By" tab.

Figure 24. Dependency insight capabilities in a build scan

Dependency insight capabilities in a build scan

Every Gradle project provides the task dependencyInsight to render the so-called dependency insight report from the command line. Given a dependency in the dependency graph you can identify the selection reason and track down the origin of the dependency selection. You can think of the dependency insight report as the inverse representation of the dependency report for a given dependency. When executing the task you have to provide the mandatory parameter --dependency to specify the coordinates of the dependency under inspection. The parameter --configuration is optional but helps with filtering the output.

Example 308. Using the dependency insight report for a given dependency

Output of gradle -q dependencyInsight --dependency commons-codec --configuration scm

> gradle -q dependencyInsight --dependency commons-codec --configuration scm
commons-codec:commons-codec:1.7 (conflict resolution)
   variant "default+runtime"
\--- scm

commons-codec:commons-codec:1.6 -> 1.7
   variant "default+runtime"
\--- org.apache.httpcomponents:httpclient:4.3.6
     \--- org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r
          \--- scm

A web-based, searchable dependency report is available by adding the --scan option.

Justifying dependency declarations with custom reasons

When you declare a dependency or a dependency constraint, you can provide a custom reason for the declaration. This makes the dependency declarations in your build script and the dependency insight report easier to interpret.

Example 309. Giving a reason for choosing a certain module version in a dependency declaration

build.gradle

apply plugin: 'java-library'

repositories {
    jcenter()
}

dependencies {
    implementation('org.ow2.asm:asm:6.0') {
        because 'we require a JDK 9 compatible bytecode generator'
    }
}

Example 310. Using the dependency insight report with custom reasons

Output of gradle -q dependencyInsight --dependency asm

> gradle -q dependencyInsight --dependency asm
org.ow2.asm:asm:6.0 (we require a JDK 9 compatible bytecode generator)
   variant "compile" [
      org.gradle.usage = java-api
   ]
\--- compileClasspath

A web-based, searchable dependency report is available by adding the --scan option.

Managing Transitive Dependencies

Resolution behavior for transitive dependencies can be customized to a high degree to meet enterprise requirements.

Managing versions of transitive dependencies with dependency constraints

Dependency constraints allow you to define the version or the version range of both dependencies declared in the build script and transitive dependencies. It is the preferred method to express constraints that should be applied to all dependencies of a configuration. When Gradle attempts to resolve a dependency to a module version, all dependency declarations with version, all transitive dependencies and all dependency constraints for that module are taken into consideration. The highest version that matches all conditions is selected. If no such version is found, Gradle fails with an error showing the conflicting declarations. If this happens you can adjust your dependencies or dependency constraints declarations, or make other adjustments to the transitive dependencies if needed. Similar to dependency declarations, dependency constraint declarations are scoped by configurations and can therefore be selectively defined for parts of a build. If a dependency constraint influenced the resolution result, any type of dependency resolve rules may still be applied afterwards.

Example 311. Define dependency constraints

build.gradle

dependencies {
    implementation 'org.apache.httpcomponents:httpclient'
    constraints {
        implementation('org.apache.httpcomponents:httpclient:4.5.3') {
            because 'previous versions have a bug impacting this application'
        }
        implementation('commons-codec:commons-codec:1.11') {
            because 'version 1.9 pulled from httpclient has bugs affecting this application'
        }
    }
}

In the example, all versions are omitted from the dependency declaration. Instead, the versions are defined in the constraints block. The version definition for commons-codec:1.11 is only taken into account if commons-codec is brought in as transitive dependency, since commons-codec is not defined as dependency in the project. Otherwise, the constraint has no effect.

Dependency constraints are not yet published, but that will be added in a future release. This means that their use currently only targets builds that do not publish artifacts to maven or ivy repositories.

Dependency constraints themselves can also be added transitively. If a modules’s metadata is defined in a .pom file that contains dependency entries with <optional>true</optional>, Gradle will create a dependency constraint for each of these so-called optional dependencies. This leads to a similar resolution behavior as provided by Maven: if the corresponding module is brought in by another, non-optional dependency declaration, then the constraint will apply when choosing the version for that module (e.g., if the optional dependency defines a higher version, that one is chosen).

Support for optional dependencies from pom files is active by default with Gradle 5.0+. For using it in Gradle 4.6+, you need to activate it by adding enableFeaturePreview('IMPROVED_POM_SUPPORT') in settings.gradle.

Excluding transitive module dependencies

Declared dependencies in a build script can pull in a lot of transitive dependencies. You might decide that you do not want a particular transitive dependency as part of the dependency graph for a good reason.

  • The dependency is undesired due to licensing constraints.

  • The dependency is not available in any of the declared repositories.

  • The metadata for the dependency exists but the artifact does not.

  • The metadata provides incorrect coordinates for a transitive dependency.

Transitive dependencies can be excluded on the level of a declared dependency or a configuration. Let’s demonstrate both use cases. In the following two examples the build script declares a dependency on Log4J, a popular logging framework in the Java world. The metadata of the particular version of Log4J also defines transitive dependencies.

Example 312. Unresolved artifacts for transitive dependencies

build.gradle

apply plugin: 'java'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'log4j:log4j:1.2.15'
}

If resolved from Maven Central some of the transitive dependencies provide metadata but not the corresponding binary artifact. As a result any task requiring the binary files will fail e.g. a compilation task.

> gradle -q compileJava

* What went wrong:
Could not resolve all files for configuration ':compileClasspath'.
> Could not find jms.jar (javax.jms:jms:1.1).
  Searched in the following locations:
      https://repo.maven.apache.org/maven2/javax/jms/jms/1.1/jms-1.1.jar
> Could not find jmxtools.jar (com.sun.jdmk:jmxtools:1.2.1).
  Searched in the following locations:
      https://repo.maven.apache.org/maven2/com/sun/jdmk/jmxtools/1.2.1/jmxtools-1.2.1.jar
> Could not find jmxri.jar (com.sun.jmx:jmxri:1.2.1).
  Searched in the following locations:
      https://repo.maven.apache.org/maven2/com/sun/jmx/jmxri/1.2.1/jmxri-1.2.1.jar

The situation can be fixed by adding a repository containing those dependencies. In the given example project, the source code does not actually use any of Log4J’s functionality that require the JMS (e.g. JMSAppender) or JMX libraries. It’s safe to exclude them from the dependency declaration.

Exclusions need to spelled out as a key/value pair via the attributes group and/or module. For more information, refer to ModuleDependency.exclude(java.util.Map).

Example 313. Excluding transitive dependency for a particular dependency declaration

build.gradle

dependencies {
    implementation('log4j:log4j:1.2.15') {
        exclude group: 'javax.jms', module: 'jms'
        exclude group: 'com.sun.jdmk', module: 'jmxtools'
        exclude group: 'com.sun.jmx', module: 'jmxri'
    }
}

You may find that other dependencies will want to pull in the same transitive dependency that misses the artifacts. Alternatively, you can exclude the transitive dependencies for a particular configuration by calling the method Configuration.exclude(java.util.Map).

Example 314. Excluding transitive dependency for a particular configuration

build.gradle

configurations {
    implementation {
        exclude group: 'javax.jms', module: 'jms'
        exclude group: 'com.sun.jdmk', module: 'jmxtools'
        exclude group: 'com.sun.jmx', module: 'jmxri'
    }
}

dependencies {
    implementation 'log4j:log4j:1.2.15'
}

As a build script author you often times know that you want to exclude a dependency for all configurations available in the project. You can use the method DomainObjectCollection.all(org.gradle.api.Action) to define a global rule.

You might encounter other use cases that don’t quite fit the bill of an exclude rule. For example you want to automatically select a version for a dependency with a specific requested version or you want to select a different group for a requested dependency to react to a relocation. Those use cases are better solved by the ResolutionStrategy API. Some of these use cases are covered in Customizing Dependency Resolution Behavior.

Enforcing a particular dependency version

Gradle resolves any dependency version conflicts by selecting the latest version found in the dependency graph. Some projects might need to divert from the default behavior and enforce an earlier version of a dependency e.g. if the source code of the project depends on an older API of a dependency than some of the external libraries.

Enforcing a version of a dependency requires a conscious decision. Changing the version of a transitive dependency might lead to runtime errors if external libraries do not properly function without them. Consider upgrading your source code to use a newer version of the library as an alternative approach.

Let’s say a project uses the HttpClient library for performing HTTP calls. HttpClient pulls in Commons Codec as transitive dependency with version 1.10. However, the production source code of the project requires an API from Commons Codec 1.9 which is not available in 1.10 anymore. A dependency version can be enforced by declaring it in the build script and setting ExternalDependency.setForce(boolean) to true.

Example 315. Enforcing a dependency version

build.gradle

dependencies {
    implementation 'org.apache.httpcomponents:httpclient:4.5.4'
    implementation('commons-codec:commons-codec:1.9') {
        force = true
    }
}

If the project requires a specific version of a dependency on a configuration-level then it can be achieved by calling the method ResolutionStrategy.force(java.lang.Object[]).

Example 316. Enforcing a dependency version on the configuration-level

build.gradle

configurations {
    compileClasspath {
        resolutionStrategy.force 'commons-codec:commons-codec:1.9'
    }
}

dependencies {
    implementation 'org.apache.httpcomponents:httpclient:4.5.4'
}

Disabling resolution of transitive dependencies

By default Gradle resolves all transitive dependencies specified by the dependency metadata. Sometimes this behavior may not be desirable e.g. if the metadata is incorrect or defines a large graph of transitive dependencies. You can tell Gradle to disable transitive dependency management for a dependency by setting ModuleDependency.setTransitive(boolean) to true. As a result only the main artifact will be resolved for the declared dependency.

Example 317. Disabling transitive dependency resolution for a declared dependency

build.gradle

dependencies {
    implementation('com.google.guava:guava:23.0') {
        transitive = false
    }
}

Disabling transitive dependency resolution will likely require you to declare the necessary runtime dependencies in your build script which otherwise would have been resolved automatically. Not doing so might lead to runtime classpath issues.

A project can decide to disable transitive dependency resolution completely. You either don’t want to rely on the metadata published to the consumed repositories or you want to gain full control over the dependencies in your graph. For more information, see Configuration.setTransitive(boolean).

Example 318. Disabling transitive dependency resolution on the configuration-level

build.gradle

configurations.all {
    transitive = false
}

dependencies {
    implementation 'com.google.guava:guava:23.0'
}

Importing version recommendations from a Maven BOM

Gradle provides support for importing bill of materials (BOM) files, which are effectively .pom files that use <dependencyManagement> to control the dependency versions of direct and transitive dependencies. The BOM support in Gradle works similar to using <scope>import</scope> when depending on a BOM in Maven. In Gradle however, it is done via a regular dependency declaration on the BOM:

Example 319. Depending on a BOM to import its dependency constraints

build.gradle

dependencies {
    // import a BOM
    implementation 'org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE'

    // define dependencies without versions
    implementation 'com.google.code.gson:gson'
    implementation 'dom4j:dom4j'
}

In the example, the versions of gson and dom4j are provided by the Spring Boot BOM. This way, if you are developing for a platform like Spring Boot, you do not have to declare any versions yourself but can rely on the versions the platform provides.

Gradle treats all entries in the <dependencyManagement> block of a BOM similar to Gradle’s dependency constraints. This means that any version defined in the <dependencyManagement> block can impact the dependency resolution result. In order to qualify as a BOM, a .pom file needs to have <packaging>pom</packaging> set.

Importing dependency constraints from Maven BOMs is active by default with Gradle 5.0+. For using it in Gradle 4.6+, you need to activate it by adding enableFeaturePreview('IMPROVED_POM_SUPPORT') in settings.gradle.

Working with Dependencies

Gradle provides an extensive API for navigating, inspecting and post-processing metadata and artifacts of resolved dependencies.

The main entry point for this functionality is the Configuration API. To learn more about the fundamentals of configurations, see Managing Dependency Configurations.

Iterating over dependencies assigned to a configuration

Sometimes you’ll want to implement logic based on the dependencies declared in the build script of a project e.g. to inspect them in a Gradle plugin. You can iterate over the set of dependencies assigned to a configuration with the help of the method Configuration.getDependencies(). Alternatively, you can also use Configuration.getAllDependencies() to include the dependencies declared in superconfigurations. These APIs only return the declared dependencies and do not trigger dependency resolution. Therefore, the dependency sets do not include transitive dependencies. Calling the APIs during the configuration phase of the build lifecycle does not result in a significant performance impact.

Example 320. Iterating over the dependencies assigned to a configuration

build.gradle

task iterateDeclaredDependencies {
    doLast {
        DependencySet dependencySet = configurations.scm.dependencies

        dependencySet.each {
            logger.quiet "$it.group:$it.name:$it.version"
        }
    }
}

Iterating over artifacts resolved for a module

None of the dependency reporting helps you with inspecting or further processing the underlying, resolved artifacts of a module. A typical use case for accessing the artifacts is to copy them into a specific directory or filter out files of interest based on a specific file extension.

You can iterate over the complete set of artifacts resolved for a module with the help of the method FileCollection.getFiles(). Every file instance returned from the method points to its location in the dependency cache. Using this method on a Configuration instance is possible as the interface extends FileCollection.

Example 321. Iterating over the artifacts resolved for a module

build.gradle

task iterateResolvedArtifacts {
    dependsOn configurations.scm

    doLast {
        Set<File> files = configurations.scm.files

        files.each {
            logger.quiet it.absolutePath
        }
    }
}

Iterating over the artifacts of a module automatically resolves the configuration. A resolved configuration becomes immutable and cannot add or remove dependencies. If needed you can copy a configuration for further modification via Configuration.copy().

Navigating the dependency graph

As a plugin developer, you may want to navigate the full graph of dependencies assigned to a configuration e.g. for turning the dependency graph into a visualization. You can access the full graph of dependencies for a configuration with the help of the ResolutionResult.

The resolution result provides various methods for accessing the resolved and unresolved dependencies. For demonstration purposes the sample code uses ResolutionResult.getRoot() to access the root node the resolved dependency graph. Each dependency of this component returns an instance of ResolvedDependencyResult or UnresolvedDependencyResult providing detailed information about the node.

Example 322. Walking the resolved and unresolved dependencies of a configuration

build.gradle

task walkDependencyGraph(type: DependencyGraphWalk) {
    dependsOn configurations.scm
}

class DependencyGraphWalk extends DefaultTask {
    @TaskAction
    void walk() {
        Configuration configuration = project.configurations.scm
        ResolutionResult resolutionResult = configuration.incoming.resolutionResult
        ResolvedComponentResult root = resolutionResult.root
        logger.quiet configuration.name
        traverseDependencies(0, root.dependencies)
    }

    private void traverseDependencies(int level, Set<? extends DependencyResult> results) {
        for (DependencyResult result : results) {
            if (result instanceof ResolvedDependencyResult) {
                ResolvedComponentResult componentResult = result.selected
                ComponentIdentifier componentIdentifier = componentResult.id
                String node = calculateIndentation(level) + "- $componentIdentifier.displayName ($componentResult.selectionReason)"
                logger.quiet node
                traverseDependencies(level + 1, componentResult.dependencies)
            } else if (result instanceof UnresolvedDependencyResult) {
                ComponentSelector componentSelector = result.attempted
                String node = calculateIndentation(level) + "- $componentSelector.displayName (failed)"
                logger.quiet node
            }
        }
    }

    private String calculateIndentation(int level) {
        '     ' * level
    }
}

Accessing a module’s metadata file

As part of the dependency resolution process, Gradle downloads the metadata file of a module and stores it in the dependency cache. Some organizations enforce strong restrictions on accessing repositories outside of internal network. Instead of downloading artifacts, those organizations prefer to provide an "installable" Gradle cache with all artifacts contained in it to fulfill the build’s dependency requirements.

The artifact query API provides access to the raw files of a module. Currently, it allows getting a handle to the metadata file and some selected, additional artifacts (e.g. a JVM-based module’s source and Javadoc files). The main API entry point is ArtifactResolutionQuery.

Let’s say you wanted to post-process the metadata file of a Maven module. The group, name and version of the module component serve as input to the artifact resolution query. After executing the query, you get a handle to all components that match the criteria and their underlying files. Additionally, it’s very easy to post-process the metadata file. The example code uses Groovy’s XmlSlurper to ask for POM element values.

Example 323. Accessing a Maven module's metadata artifact

build.gradle

apply plugin: 'java-library'

repositories {
    mavenCentral()
}

dependencies {
    implementation 'com.google.guava:guava:18.0'
}

task printGuavaMetadata {
    dependsOn configurations.compileClasspath

    doLast {
        ArtifactResolutionQuery query = dependencies.createArtifactResolutionQuery()
            .forModule('com.google.guava', 'guava', '18.0')
            .withArtifacts(MavenModule, MavenPomArtifact)
        ArtifactResolutionResult result = query.execute()

        for(component in result.resolvedComponents) {
            Set<ArtifactResult> mavenPomArtifacts = component.getArtifacts(MavenPomArtifact)
            ArtifactResult guavaPomArtifact = mavenPomArtifacts.find { it.file.name == 'guava-18.0.pom' }
            def xml = new XmlSlurper().parse(guavaPomArtifact.file)
            println guavaPomArtifact.file
            println xml.name
            println xml.description
        }
    }
}

Customizing Dependency Resolution Behavior

There are a number of ways that you can influence how Gradle resolves dependencies. All of these mechanisms offer an API to define a reason for why they are used. Providing reasons makes dependency resolution results more understandable. If any customization influenced the resolution result, the provided reason will show up in dependency insight report.

Using dependency resolve rules

A dependency resolve rule is executed for each resolved dependency, and offers a powerful api for manipulating a requested dependency prior to that dependency being resolved. The feature currently offers the ability to change the group, name and/or version of a requested dependency, allowing a dependency to be substituted with a completely different module during resolution.

Dependency resolve rules provide a very powerful way to control the dependency resolution process, and can be used to implement all sorts of advanced patterns in dependency management. Some of these patterns are outlined below. For more information and code samples see the ResolutionStrategy class in the API documentation.

Modelling releasable units

Often an organisation publishes a set of libraries with a single version; where the libraries are built, tested and published together. These libraries form a "releasable unit", designed and intended to be used as a whole. It does not make sense to use libraries from different releasable units together.

But it is easy for transitive dependency resolution to violate this contract. For example:

  • module-a depends on releasable-unit:part-one:1.0

  • module-b depends on releasable-unit:part-two:1.1

A build depending on both module-a and module-b will obtain different versions of libraries within the releasable unit.

Dependency resolve rules give you the power to enforce releasable units in your build. Imagine a releasable unit defined by all libraries that have org.gradle group. We can force all of these libraries to use a consistent version:

Example 324. Forcing a consistent version for a group of libraries

build.gradle

configurations.all {
    resolutionStrategy.eachDependency { DependencyResolveDetails details ->
        if (details.requested.group == 'org.gradle') {
            details.useVersion '1.4'
            details.because 'API breakage in higher versions'
        }
    }
}

Implementing a custom versioning scheme

In some corporate environments, the list of module versions that can be declared in Gradle builds is maintained and audited externally. Dependency resolve rules provide a neat implementation of this pattern:

  • In the build script, the developer declares dependencies with the module group and name, but uses a placeholder version, for example: default.

  • The default version is resolved to a specific version via a dependency resolve rule, which looks up the version in a corporate catalog of approved modules.

This rule implementation can be neatly encapsulated in a corporate plugin, and shared across all builds within the organisation.

Example 325. Using a custom versioning scheme

build.gradle

configurations.all {
    resolutionStrategy.eachDependency { DependencyResolveDetails details ->
        if (details.requested.version == 'default') {
            def version = findDefaultVersionInCatalog(details.requested.group, details.requested.name)
            details.useVersion version.version
            details.because version.because
        }
    }
}

def findDefaultVersionInCatalog(String group, String name) {
    //some custom logic that resolves the default version into a specific version
    [version: "1.0", because: 'tested by QA']
}

Blacklisting a particular version with a replacement

Dependency resolve rules provide a mechanism for blacklisting a particular version of a dependency and providing a replacement version. This can be useful if a certain dependency version is broken and should not be used, where a dependency resolve rule causes this version to be replaced with a known good version. One example of a broken module is one that declares a dependency on a library that cannot be found in any of the public repositories, but there are many other reasons why a particular module version is unwanted and a different version is preferred.

In example below, imagine that version 1.2.1 contains important fixes and should always be used in preference to 1.2. The rule provided will enforce just this: any time version 1.2 is encountered it will be replaced with 1.2.1. Note that this is different from a forced version as described above, in that any other versions of this module would not be affected. This means that the 'newest' conflict resolution strategy would still select version 1.3 if this version was also pulled transitively.

Example 326. Blacklisting a version with a replacement

build.gradle

configurations.all {
    resolutionStrategy.eachDependency { DependencyResolveDetails details ->
        if (details.requested.group == 'org.software' && details.requested.name == 'some-library' && details.requested.version == '1.2') {
            details.useVersion '1.2.1'
            details.because 'fixes critical bug in 1.2'
        }
    }
}

Substituting a dependency module with a compatible replacement

At times a completely different module can serve as a replacement for a requested module dependency. Examples include using groovy in place of groovy-all, or using log4j-over-slf4j instead of log4j. You can perform these substitutions using dependency resolve rules:

Example 327. Changing dependency group and/or name during resolution

build.gradle

configurations.all {
    resolutionStrategy.eachDependency { DependencyResolveDetails details ->
        if (details.requested.name == 'groovy-all') {
            details.useTarget group: details.requested.group, name: 'groovy', version: details.requested.version
            details.because "prefer 'groovy' over 'groovy-all'"
        }
        if (details.requested.name == 'log4j') {
            details.useTarget "org.slf4j:log4j-over-slf4j:1.7.10"
            details.because "prefer 'log4j-over-slf4j' 1.7.10 over any version of 'log4j'"
        }
    }
}

Using dependency substitution rules

Dependency substitution rules work similarly to dependency resolve rules. In fact, many capabilities of dependency resolve rules can be implemented with dependency substitution rules. They allow project and module dependencies to be transparently substituted with specified replacements. Unlike dependency resolve rules, dependency substitution rules allow project and module dependencies to be substituted interchangeably.

Adding a dependency substitution rule to a configuration changes the timing of when that configuration is resolved. Instead of being resolved on first use, the configuration is instead resolved when the task graph is being constructed. This can have unexpected consequences if the configuration is being further modified during task execution, or if the configuration relies on modules that are published during execution of another task.

To explain:

  • A Configuration can be declared as an input to any Task, and that configuration can include project dependencies when it is resolved.

  • If a project dependency is an input to a Task (via a configuration), then tasks to build the project artifacts must be added to the task dependencies.

  • In order to determine the project dependencies that are inputs to a task, Gradle needs to resolve the Configuration inputs.

  • Because the Gradle task graph is fixed once task execution has commenced, Gradle needs to perform this resolution prior to executing any tasks.

In the absence of dependency substitution rules, Gradle knows that an external module dependency will never transitively reference a project dependency. This makes it easy to determine the full set of project dependencies for a configuration through simple graph traversal. With this functionality, Gradle can no longer make this assumption, and must perform a full resolve in order to determine the project dependencies.

Substituting an external module dependency with a project dependency

One use case for dependency substitution is to use a locally developed version of a module in place of one that is downloaded from an external repository. This could be useful for testing a local, patched version of a dependency.

The module to be replaced can be declared with or without a version specified.

Example 328. Substituting a module with a project

build.gradle

configurations.all {
    resolutionStrategy.dependencySubstitution {
        substitute module("org.utils:api") with project(":api") because "we work with the unreleased development version"
        substitute module("org.utils:util:2.5") with project(":util")
    }
}

Note that a project that is substituted must be included in the multi-project build (via settings.gradle). Dependency substitution rules take care of replacing the module dependency with the project dependency and wiring up any task dependencies, but do not implicitly include the project in the build.

Substituting a project dependency with a module replacement

Another way to use substitution rules is to replace a project dependency with a module in a multi-project build. This can be useful to speed up development with a large multi-project build, by allowing a subset of the project dependencies to be downloaded from a repository rather than being built.

The module to be used as a replacement must be declared with a version specified.

Example 329. Substituting a project with a module

build.gradle

configurations.all {
    resolutionStrategy.dependencySubstitution {
        substitute project(":api") with module("org.utils:api:1.3") because "we use a stable version of utils"
    }
}

When a project dependency has been replaced with a module dependency, that project is still included in the overall multi-project build. However, tasks to build the replaced dependency will not be executed in order to build the resolve the depending Configuration.

Conditionally substituting a dependency

A common use case for dependency substitution is to allow more flexible assembly of sub-projects within a multi-project build. This can be useful for developing a local, patched version of an external dependency or for building a subset of the modules within a large multi-project build.

The following example uses a dependency substitution rule to replace any module dependency with the group org.example, but only if a local project matching the dependency name can be located.

Example 330. Conditionally substituting a dependency

build.gradle

configurations.all {
    resolutionStrategy.dependencySubstitution.all { DependencySubstitution dependency ->
        if (dependency.requested instanceof ModuleComponentSelector && dependency.requested.group == "org.example") {
            def targetProject = findProject(":${dependency.requested.module}")
            if (targetProject != null) {
                dependency.useTarget targetProject
            }
        }
    }
}

Note that a project that is substituted must be included in the multi-project build (via settings.gradle). Dependency substitution rules take care of replacing the module dependency with the project dependency, but do not implicitly include the project in the build

Using component metadata rules

Each module has metadata associated with it, such as its group, name, version, dependencies, and so on. This metadata typically originates in the module’s descriptor. Metadata rules allow certain parts of a module’s metadata to be manipulated from within the build script. They take effect after a module’s descriptor has been downloaded, but before it has been selected among all candidate versions. This makes metadata rules another instrument for customizing dependency resolution.

One piece of module metadata that Gradle understands is a module’s status scheme. This concept, also known from Ivy, models the different levels of maturity that a module transitions through over time. The default status scheme, ordered from least to most mature status, is integration, milestone, release. Apart from a status scheme, a module also has a (current) status, which must be one of the values in its status scheme. If not specified in the (Ivy) descriptor, the status defaults to integration for Ivy modules and Maven snapshot modules, and release for Maven modules that aren’t snapshots.

A module’s status and status scheme are taken into consideration when a latest version selector is resolved. Specifically, latest.someStatus will resolve to the highest module version that has status someStatus or a more mature status. For example, with the default status scheme in place, latest.integration will select the highest module version regardless of its status (because integration is the least mature status), whereas latest.release will select the highest module version with status release. Here is what this looks like in code:

Example 331. 'Latest' version selector

build.gradle

dependencies {
    config1 "org.sample:client:latest.integration"
    config2 "org.sample:client:latest.release"
}

task listConfigs {
    doLast {
        configurations.config1.each { println it.name }
        println()
        configurations.config2.each { println it.name }
    }
}

Output of gradle -q listConfigs

> gradle -q listConfigs
client-1.5.jar

client-1.4.jar

The next example demonstrates latest selectors based on a custom status scheme declared in a component metadata rule that applies to all modules:

Example 332. Custom status scheme

build.gradle

dependencies {
    config3 "org.sample:api:latest.silver"
    components {
        all { ComponentMetadataDetails details ->
            if (details.id.group == "org.sample" && details.id.name == "api") {
                details.statusScheme = ["bronze", "silver", "gold", "platinum"]
            }
        }
    }
}

Component metadata rules can be applied to a specified module. Modules must be specified in the form of group:module.

Example 333. Custom status scheme by module

build.gradle

dependencies {
    config4 "org.sample:lib:latest.prod"
    components {
        withModule('org.sample:lib') { ComponentMetadataDetails details ->
            details.statusScheme = ["int", "rc", "prod"]
        }
    }
}

Gradle can also create component metadata rules utilizing Ivy-specific metadata for modules resolved from an Ivy repository. Values from the Ivy descriptor are made available via the IvyModuleDescriptor interface.

Example 334. Ivy component metadata rule

build.gradle

dependencies {
    config6 "org.sample:lib:latest.rc"
    components {
        withModule("org.sample:lib") { ComponentMetadataDetails details, IvyModuleDescriptor ivyModule ->
            if (ivyModule.branch == 'testing') {
                details.status = "rc"
            }
        }
    }
}

Note that any rule that declares specific arguments must always include a ComponentMetadataDetails argument as the first argument. The second Ivy metadata argument is optional.

Component metadata rules can also be defined using a rule source object. A rule source object is any object that contains exactly one method that defines the rule action and is annotated with @Mutate.

This method:

Example 335. Rule source component metadata rule

build.gradle

dependencies {
    config5 "org.sample:api:latest.gold"
    components {
        withModule('org.sample:api', new CustomStatusRule())
    }
}

class CustomStatusRule {
    @Mutate
    void setStatusScheme(ComponentMetadataDetails details) {
        details.statusScheme = ["bronze", "silver", "gold", "platinum"]
    }
}

Using component selection rules

Component selection rules may influence which component instance should be selected when multiple versions are available that match a version selector. Rules are applied against every available version and allow the version to be explicitly rejected by rule. This allows Gradle to ignore any component instance that does not satisfy conditions set by the rule. Examples include:

  • For a dynamic version like 1.+ certain versions may be explicitly rejected from selection.

  • For a static version like 1.4 an instance may be rejected based on extra component metadata such as the Ivy branch attribute, allowing an instance from a subsequent repository to be used.

Rules are configured via the ComponentSelectionRules object. Each rule configured will be called with a ComponentSelection object as an argument which contains information about the candidate version being considered. Calling ComponentSelection.reject(java.lang.String) causes the given candidate version to be explicitly rejected, in which case the candidate will not be considered for the selector.

The following example shows a rule that disallows a particular version of a module but allows the dynamic version to choose the next best candidate.

Example 336. Component selection rule

build.gradle

configurations {
    rejectConfig {
        resolutionStrategy {
            componentSelection {
                // Accept the highest version matching the requested version that isn't '1.5'
                all { ComponentSelection selection ->
                    if (selection.candidate.group == 'org.sample' && selection.candidate.module == 'api' && selection.candidate.version == '1.5') {
                        selection.reject("version 1.5 is broken for 'org.sample:api'")
                    }
                }
            }
        }
    }
}

dependencies {
    rejectConfig "org.sample:api:1.+"
}