Table of Contents

About Gradle

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 or Kotlin, 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 Apache License 2.0.

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.

What is Gradle?

Overview

Gradle is an open-source build automation tool that is designed to be flexible enough to build almost any type of software. The following is a high-level overview of some of its most important features:

High performance

Gradle avoids unnecessary work by only running the tasks that need to run because their inputs or outputs have changed. You can also use a build cache to enable the reuse of task outputs from previous runs or even from a different machine (with a shared build cache).

There are many other optimizations that Gradle implements and the development team continually work to improve Gradle’s performance.

JVM foundation

Gradle runs on the JVM and you must have a Java Development Kit (JDK) installed to use it. This is a bonus for users familiar with the Java platform as you can use the standard Java APIs in your build logic, such as custom task types and plugins. It also makes it easy to run Gradle on different platforms.

Note that Gradle isn’t limited to building just JVM projects, and it even comes packaged with support for building native projects.

Conventions

Gradle takes a leaf out of Maven’s book and makes common types of projects — such as Java projects — easy to build by implementing conventions. Apply the appropriate plugins and you can easily end up with slim build scripts for many projects. But these conventions don’t limit you: Gradle allows you to override them, add your own tasks, and make many other customizations to your convention-based builds.

Extensibility

You can readily extend Gradle to provide your own task types or even build model. See the Android build support for an example of this: it adds many new build concepts such as flavors and build types.

IDE support

Several major IDEs allow you to import Gradle builds and interact with them: Android Studio, IntelliJ IDEA, Eclipse, and NetBeans. Gradle also has support for generating the solution files required to load a project into Visual Studio.

Insight

Build scans provide extensive information about a build run that you can use to identify build issues. They are particularly good at helping you to identify problems with a build’s performance. You can also share build scans with others, which is particularly useful if you need ask for advice in fixing an issue with the build.

Five things you need to know about Gradle

Gradle is a flexible and powerful build tool that can easily feel intimidating when you first start. However, understanding the following core principles will make Gradle much more approachable and you will become adept with the tool before you know it.

1. Gradle is a general-purpose build tool

Gradle allows you to build any software, because it makes few assumptions about what you’re trying to build or how it should be done. The most notable restriction is that dependency management currently only supports Maven- and Ivy-compatible repositories and the filesystem.

This doesn’t mean you have to do a lot of work to create a build. Gradle makes it easy to build common types of project — say Java libraries — by adding a layer of conventions and prebuilt functionality through plugins. You can even create and publish custom plugins to encapsulate your own conventions and build functionality.

2. The core model is based on tasks

Gradle models its builds as Directed Acyclic Graphs (DAGs) of tasks (units of work). What this means is that a build essentially configures a set of tasks and wires them together — based on their dependencies — to create that DAG. Once the task graph has been created, Gradle determines which tasks need to be run in which order and then proceeds to execute them.

This diagram shows two example task graphs, one abstract and the other concrete, with the dependencies between the tasks represented as arrows:

Example task graphs
Figure 1. Two examples of Gradle task graphs

Almost any build process can be modeled as a graph of tasks in this way, which is one of the reasons why Gradle is so flexible. And that task graph can be defined by both plugins and your own build scripts, with tasks linked together via the task dependency mechanism.

Tasks themselves consist of:

  • Actions — pieces of work that do something, like copy files or compile source

  • Inputs — values, files and directories that the actions use or operate on

  • Outputs — files and directories that the actions modify or generate

In fact, all of the above are optional depending on what the task needs to do. Some tasks — such as the standard lifecycle tasks — don’t even have any actions. They simply aggregate multiple tasks together as a convenience.

Note
 You choose which task to run. Save time by specifying the task that does what you need, but no more than that. If you just want to run the unit tests, choose the task that does that — typically test. If you want to package an application, most builds have an assemble task for that.

One last thing: Gradle’s incremental build support is robust and reliable, so keep your builds running fast by avoiding the clean task unless you actually do want to perform a clean.

3. Gradle has several fixed build phases

It’s important to understand that Gradle evaluates and executes build scripts in three phases:

  1. Initialization

    Sets up the environment for the build and determine which projects will take part in it.

  2. Configuration

    Constructs and configures the task graph for the build and then determines which tasks need to run and in which order, based on the task the user wants to run.

  3. Execution

    Runs the tasks selected at the end of the configuration phase.

These phases form Gradle’s Build Lifecycle.

Note
Comparison to Apache Maven terminology

Gradle’s build phases are not like Maven’s phases. Maven uses its phases to divide the build execution into multiple stages. They serve a similar role to Gradle’s task graph, although less flexibly.

Maven’s concept of a build lifecycle is loosely similar to Gradle’s lifecycle tasks.

Well-designed build scripts consist mostly of declarative configuration rather than imperative logic. That configuration is understandably evaluated during the configuration phase. Even so, many such builds also have task actions — for example via doLast {} and doFirst {} blocks — which are evaluated during the execution phase. This is important because code evaluated during the configuration phase won’t see changes that happen during the execution phase.

Another important aspect of the configuration phase is that everything involved in it is evaluated every time the build runs. That is why it’s best practice to avoid expensive work during the configuration phase. Build scans can help you identify such hotspots, among other things.

4. Gradle is extensible in more ways than one

It would be great if you could build your project using only the build logic bundled with Gradle, but that’s rarely possible. Most builds have some special requirements that mean you need to add custom build logic.

Gradle provides several mechanisms that allow you to extend it, such as:

  • Custom task types.

    When you want the build to do some work that an existing task can’t do, you can simply write your own task type. It’s typically best to put the source file for a custom task type in the buildSrc directory or in a packaged plugin. Then you can use the custom task type just like any of the Gradle-provided ones.

  • Custom task actions.

    You can attach custom build logic that executes before or after a task via the Task.doFirst() and Task.doLast() methods.

  • Extra properties on projects and tasks.

    These allows you to add your own properties to a project or task that you can then use from your own custom actions or any other build logic. Extra properties can even be applied to tasks that aren’t explicitly created by you, such as those created by Gradle’s core plugins.

  • Custom conventions.

    Conventions are a powerful way to simplify builds so that users can understand and use them more easily. This can be seen with builds that use standard project structures and naming conventions, such as Java builds. You can write your own plugins that provide conventions — they just need to configure default values for the relevant aspects of a build.

  • A custom model.

    Gradle allows you to introduce new concepts into a build beyond tasks, files and dependency configurations. You can see this with most language plugins, which add the concept of source sets to a build. Appropriate modeling of a build process can greatly improve a build’s ease of use and its efficiency.

5. Build scripts operate against an API

It’s easy to view Gradle’s build scripts as executable code, because that’s what they are. But that’s an implementation detail: well-designed build scripts describe what steps are needed to build the software, not how those steps should do the work. That’s a job for custom task types and plugins.

Note

There is a common misconception that Gradle’s power and flexibility come from the fact that its build scripts are code. This couldn’t be further from the truth. It’s the underlying model and API that provide the power. As we recommend in our best practices, you should avoid putting much, if any, imperative logic in your build scripts.

Yet there is one area in which it is useful to view a build script as executable code: in understanding how the syntax of the build script maps to Gradle’s API. The API documentation — formed of the Groovy DSL Reference and the Javadocs — lists methods and properties, and refers to closures and actions. What do these mean within the context of a build script? Check out the Groovy Build Script Primer to learn the answer to that question so that you can make effective use of the API documentation.

Note
 As Gradle runs on the JVM, build scripts can also use the standard Java API. Groovy build scripts can additionally use the Groovy APIs, while Kotlin build scripts can use the Kotlin ones.

Getting Started

Getting Started

Everyone has to start somewhere and if you’re new to Gradle, this is where to begin.

Before you start

In order to use Gradle effectively, you need to know what it is and understand some of its fundamental concepts. So before you start using Gradle in earnest, we highly recommend you read What is Gradle?.

Even if you’re experienced with using Gradle, we suggest you read the section 5 things you need to know about Gradle as it clears up some common misconceptions.

Installation

If all you want to do is run an existing Gradle build, then you don’t need to install Gradle if the build has a Gradle Wrapper, identifiable via the gradlew and/or gradlew.bat files in the root of the build. You just need to make sure your system satisfies Gradle’s prerequisites.

Android Studio comes with a working installation of Gradle, so you don’t need to install Gradle separately in that case.

In order to create a new build or add a Wrapper to an existing build, you will need to install Gradle according to these instructions. Note that there may be other ways to install Gradle in addition to those described on that page, since it’s nearly impossible to keep track of all the package managers out there.

Try Gradle

Actively using Gradle is a great way to learn about it, so once you’ve installed Gradle, try one of the introductory hands-on tutorials:

There are also many other tutorials and guides available, which you can filter by category — for example Fundamentals.

Command line vs IDEs

Some folks are hard-core command-line users, while others prefer to never leave the comfort of their IDE. Many people happily use both and Gradle endeavors not to discriminate. Gradle is supported by several major IDEs and everything that can be done from the command line is available to IDEs via the Tooling API.

Android Studio and IntelliJ IDEA users should consider using Kotlin DSL build scripts for the superior IDE support when editing them.

Executing Gradle builds

If you follow any of the tutorials linked above, you will execute a Gradle build. But what do you do if you’re given a Gradle build without any instructions?

Here are some useful steps to follow:

  1. Determine whether the project has a Gradle wrapper and use it if it’s there — the main IDEs default to using the wrapper when it’s available.

  2. Discover the project structure.

    Either import the build with an IDE or run gradle projects from the command line. If only the root project is listed, it’s a single-project build. Otherwise it’s a multi-project build.

  3. Find out what tasks you can run.

    If you have imported the build into an IDE, you should have access to a view that displays all the available tasks. From the command line, run gradle tasks.

  4. Learn more about the tasks via gradle help --task <taskname>.

    The help task can display extra information about a task, including which projects contain that task and what options the task supports.

  5. Run the task that you are interested in.

    Many convention-based builds integrate with Gradle’s lifecycle tasks, so use those when you don’t have something more specific you want to do with the build. For example, most builds have clean, check, assemble and build tasks.

    From the command line, just run gradle <taskname> to execute a particular task. You can learn more about command-line execution in the corresponding user manual chapter. If you’re using an IDE, check its documentation to find out how to run a task.

Gradle builds often follow standard conventions on project structure and tasks, so if you’re familiar with other builds of the same type — such as Java, Android or native builds — then the file and directory structure of the build should be familiar, as well as many of the tasks and project properties.

For more specialized builds or those with significant customizations, you should ideally have access to documentation on how to run the build and what build properties you can configure.

Authoring Gradle builds

Learning to create and maintain Gradle builds is a process, and one that takes a little time. We recommend that you start with the appropriate core plugins and their conventions for your project, and then gradually incorporate customizations as you learn more about the tool.

Here are some useful first steps on your journey to mastering Gradle:

  1. Try one or two basic tutorials to see what a Gradle build looks like, particularly the ones that match the type of project you work with (Java, native, Android, etc.).

  2. Make sure you’ve read 5 things you need to know about Gradle!

  3. Learn about the fundamental elements of a Gradle build: projects, tasks, and the file API.

  4. If you are building software for the JVM, be sure to read about the specifics of those types of projects in Building Java & JVM projects and Testing in Java & JVM projects.

  5. Familiarize yourself with the core plugins that come packaged with Gradle, as they provide a lot of useful functionality out of the box.

  6. Learn how to author maintainable build scripts and best organize your Gradle projects.

The user manual contains a lot of other useful information and you can find more tutorials on various Gradle features among the Gradle Guides.

Integrating 3rd-party tools with Gradle

Gradle’s flexibility means that it readily works with other tools, such as those listed on our Gradle & Third-party Tools page.

There are two main modes of integration:

  • A tool drives Gradle — uses it to extract information about a build and run it — via the Tooling API

  • Gradle invokes or generates information for a tool via the 3rd-party tool’s APIs — this is usually done via plugins and custom task types

Tools that have existing Java-based APIs are generally straightforward to integrate. You can find many such integrations on Gradle’s plugin portal.

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.

Use of the Gradle Wrapper is the recommended way to upgrade Gradle.

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 Development Kit version 8 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

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-5.1-bin.zip
❯ ls /opt/gradle/gradle-5.1
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-5.1 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-5.1/bin
Microsoft Windows users

In File Explorer right-click on the This PC (or Computer) icon, then click Properties → Advanced System Settings → Environmental Variables.

Under System Variables select Path, then click Edit. Add an entry for C:\Gradle\gradle-5.1\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 5.1
------------------------------------------------------------

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

Groovy:       2.4.15
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:

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 8 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.

troubleshooting dependency management build scan
Figure 2. 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.

Note
 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.

Note
 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 in a searchable, visual way by going to the Timeline view and clicking on the task you want to inspect.

troubleshooting task execution build scan
Figure 3. 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.

troubleshooting refresh intellij
Figure 4. 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.

troubleshooting refresh eclipse
Figure 5. 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.

Upgrading and Migrating

Upgrading your build from Gradle 5.x

This chapter provides the information you need to migrate your Gradle 5.x builds to Gradle 5.1. For migrating from Gradle 4.x, complete the 4.x to 5.0 guide first.

We recommend the following steps for all users:

  1. Try running gradle help --scan and view the deprecations view of the generated build scan.

    Deprecations View of a Gradle Build Scan

    This is so that you can see any deprecation warnings that apply to your build.

    Alternatively, you could run gradle help --warning-mode=all to see the deprecations in the console, though it may not report as much detailed information.

  2. Update your plugins.

    Some plugins will break with this new version of Gradle, for example because they use internal APIs that have been removed or changed. The previous step will help you identify potential problems by issuing deprecation warnings when a plugin does try to use a deprecated part of the API.

  3. Run gradle wrapper --gradle-version 5.1 to update the project to 5.1.

  4. Try to run the project and debug any errors using the Troubleshooting Guide.

Upgrading from 5.0 and earlier

Potential breaking changes

The following changes were not previously deprecated:

Signing API changes

Input and output files of Sign tasks are now tracked via Signature.getToSign() and Signature.getFile(), respectively.

Collection properties default to empty collection

In Gradle 5.0, the collection property instances created using ObjectFactory would have no value defined, requiring plugin authors to explicitly set an initial value. This proved to be awkward and error prone so ObjectFactory now returns instances with an empty collection as their initial value.

Worker API: working directory of a worker can no longer be set

Since JDK 11 no longer supports changing the working directory of a running process, setting the working directory of a worker via its fork options is now prohibited. All workers now use the same working directory to enable reuse. Please pass files and directories as arguments instead. See examples in the Worker API documentation.

Changes to native linking tasks

To expand our idiomatic Provider API practices, the install name property from org.gradle.nativeplatform.tasks.LinkSharedLibrary is affected by this change.

  • getInstallName() was changed to return a Property.

  • setInstallName(String) was removed. Use Property.set() instead.

Passing arguments to Windows Resource Compiler

To expand our idiomatic Provider API practices, the WindowsResourceCompile task has been converted to use the Provider API.

Passing additional compiler arguments now follow the same pattern as the CppCompile and other tasks.

Copied configuration no longer shares a list of beforeResolve actions with original

The list of beforeResolve actions are no longer shared between a copied configuration and the original. Instead, a copied configuration receives a copy of the beforeResolve actions at the time the copy is made. Any beforeResolve actions added after copying (to either configuration) will not be shared between the original and the copy. This may break plugins that relied on the previous behaviour.

Changes to incubating POM customization types

  • The type of MavenPomDeveloper.properties has changed from Property<Map<String, String>> to MapProperty<String, String>.

  • The type of MavenPomContributor.properties has changed from Property<Map<String, String>> to MapProperty<String, String>.

Changes to specifying operating system for native projects

The incubating operatingSystems property on native components has been replaced with the targetMachines property.

Change in behavior for tasks extending AbstractArchiveTask or subtypes (Zip, Jar, War, Ear, Tar)

The AbstractArchiveTask has several new properties using the [Provider API](userguide/lazy_configuration.html). Plugins that extend these types and override methods from the base class may no longer behave the same way. Internally, AbstractArchiveTask prefers the new properties and methods like getArchiveName() are façades over the new properties.

If your plugin/build only uses these types (and does not extend them), nothing has changed.

Deprecated classes, methods and properties

Follow the API links to learn how to deal with these deprecations (if no extra information is provided here):

    validateTaskProperties.getClasses().setFrom(fileCollection)
    validateTaskProperties.getClasspath().setFrom(fileCollection)

Upgrading your build from Gradle 4.x to 5.0

This chapter provides the information you need to migrate your older Gradle 4.x builds to Gradle 5.0. In most cases, you will need to apply the changes from all versions that come after the one you’re upgrading from. For example, if you’re upgrading from Gradle 4.3, you will also need to apply the changes since 4.4, 4.5, etc.

Tip
 If you are using Gradle for Android, you need to move to version 3.3 or higher of both the Android Gradle Plugin and Android Studio.

For all users

  1. If you are not already on the latest 4.10.x release, read the sections below for help upgrading your project to the latest 4.10.x release. We recommend upgrading to the latest 4.10.x release to get the most useful warnings and deprecations information before moving to 5.0.

  2. Try running gradle help --scan and view the deprecations view of the generated build scan. If there are no warnings, the Deprecations tab will not appear.

    Deprecations View of a Gradle Build Scan

    This is so that you can see any deprecation warnings that apply to your build. Gradle 5.x will generate (potentially less obvious) errors if you try to upgrade directly to it.

    Alternatively, you could run gradle help --warning-mode=all to see the deprecations in the console, though it may not report as much detailed information.

  3. Update your plugins.

    Some plugins will break with this new version of Gradle, for example because they use internal APIs that have been removed or changed. The previous step will help you identify potential problems by issuing deprecation warnings when a plugin does try to use a deprecated part of the API.

    In particular, you will need to use at least a 2.x version of the Shadow Plugin.

  4. Run gradle wrapper --gradle-version 5.0 to update the project to 5.0

  5. Move to Java 8 or higher if you haven’t already. Whereas Gradle 4.x requires Java 7, Gradle 5 requires Java 8 to run.

  6. Read the Upgrading from 4.10 section and make any necessary changes.

  7. Try to run the project and debug any errors using the Troubleshooting Guide.

In addition, Gradle has added several significant new and improved features that you should consider using in your builds:

Other notable changes to be aware of that may break your build include:

Upgrading from 4.10 and earlier

If you are not already on version 4.10, skip down to the section that applies to your current Gradle version and work your way up until you reach here. Then, apply these changes when moving from Gradle 4.10 to 5.0.

Other changes

  • The enableFeaturePreview('IMPROVED_POM_SUPPORT') and enableFeaturePreview('STABLE_PUBLISHING') flags are no longer necessary. These features are now enabled by default.

  • Gradle now bundles JAXB for Java 9 and above. You can remove the --add-modules java.xml.bind option from org.gradle.jvmargs, if set.

Potential breaking changes

The changes in this section have the potential to break your build, but the vast majority have been deprecated for quite some time and few builds will be affected by a large number of them. We strongly recommend upgrading to Gradle 4.10 first to get a report on what deprecations affect your build.

The following breaking changes are not from deprecations, but the result of changes in behavior:

The following breaking changes will appear as deprecation warnings with Gradle 4.10:

General

  • << for task definitions no longer works. In other words, you can not use the syntax task myTask << { …​ }.

    Use the Task.doLast() method instead, like this:

    task myTask {
    doLast {
        ...
    }
}

  • You can no longer use any of the following characters in domain object names, such as project and task names: <space> / \ : < > " ? * | . You should also not use . as a leading or trailing character.

Running Gradle & build environment

  • As mentioned before, Gradle can no longer be run on Java 7. However, you can still use forked compilation and testing to build and test software for Java 6 and above.

  • The -Dtest.single command-line option has been removed — use test filtering instead.

  • The -Dtest.debug command-line option has been removed — use the --debug-jvm option instead.

  • The -u/--no-search-upward command-line option has been removed — make sure all your builds have a settings.gradle file.

  • The --recompile-scripts command-line option has been removed.

  • You can no longer have a Gradle build nested in a subdirectory of another Gradle build unless the nested build has a settings.gradle file.

  • The DirectoryBuildCache.setTargetSizeInMB(long) method has been removed — use DirectoryBuildCache.removeUnusedEntriesAfterDays instead.

  • The org.gradle.readLoggingConfigFile system property no longer does anything — update affected tests to work with your java.util.logging settings.

Working with files

  • You can no longer cast FileCollection objects to other types using the as keyword or the asType() method.

  • You can no longer pass null as the configuration action of CopySpec.from(Object, Action).

  • For better compatibility with the Kotlin DSL, CopySpec.duplicatesStrategy is no longer nullable. The property setter no longer accepts null as a way to reset the property back to its default value. Use DuplicatesStrategy.INHERIT instead.

  • The FileCollection.stopExecutionIfEmpty() method has been removed — use the @SkipWhenEmpty annotation on FileCollection task properties instead.

  • The FileCollection.add() method has been removed — use Project.files() and Project.fileTree() to create configurable file collections/file trees and add to them via ConfigurableFileCollection.from().

  • SimpleFileCollection has been removed — use Project.files(Object…​) instead.

  • Don’t have your own classes extend AbstractFileCollection — use the Project.files() method instead. This problem may exhibit as a missing getBuildDependencies() method.

Java builds
Tasks & properties

  • The following legacy classes and methods related to lazy properties have been removed — use ObjectFactory.property() to create Property instances:

    • PropertyState

    • DirectoryVar

    • RegularFileVar

    • ProjectLayout.newDirectoryVar()

    • ProjectLayout.newFileVar()

    • Project.property(Class)

    • Script.property(Class)

    • ProviderFactory.property(Class)

  • Tasks configured and registered with the task configuration avoidance APIs have more restrictions on the other methods that can be called from a configuration action.

  • The internal @Option and @OptionValues annotations — package org.gradle.api.internal.tasks.options — have been removed. Use the public @Option and @OptionValues annotations instead.

  • The Task.deleteAllActions() method has been removed with no replacement.

  • The Task.dependsOnTaskDidWork() method has been removed — use declared inputs and outputs instead.

  • The following properties and methods of TaskInternal have been removed — use task dependencies, task rules, reusable utility methods, or the Worker API in place of executing a task directly.

    • execute()

    • executer

    • getValidators()

    • addValidator()

  • The TaskInputs.file(Object) method can no longer be called with an argument that resolves to anything other than a single regular file.

  • The TaskInputs.dir(Object) method can no longer be called with an argument that resolves to anything other than a single directory.

  • You can no longer register invalid inputs and outputs via TaskInputs and TaskOutputs.

  • The TaskDestroyables.file() and TaskDestroyables.files() methods have been removed — use TaskDestroyables.register() instead.

  • SimpleWorkResult has been removed — use WorkResult.didWork.

  • Overriding built-in tasks deprecated in 4.8 now produces an error.

    Attempting to replace a built-in task will produce an error similar to the following:

    > Cannot add task 'wrapper' as a task with that name already exists.
Scala & Play

  • Play 2.2 is no longer supported — please upgrade the version of Play you are using.

  • The ScalaDocOptions.styleSheet property has been removed — the Scaladoc Ant task in Scala 2.11.8 and later no longer supports this property.

Kotlin DSL

  • Artifact configuration accessors now have the type NamedDomainObjectProvider<Configuration> instead of Configuration

  • PluginAware.apply<T>(to) was renamed PluginAware.applyTo<T>(target).

Both changes could cause script compilation errors. See the Gradle Kotlin DSL release notes for more information and how to fix builds broken by the changes described above.

Miscellaneous

  • The ConfigurableReport.setDestination(Object) method has been removed — use ConfigurableReport.setDestination(File) instead.

  • The Signature.setFile(File) method has been removed — Gradle does not support changing the output file for the generated signature.

  • The read-only Signature.toSignArtifact property has been removed — it should never have been part of the public API.

  • The @DeferredConfigurable annotation has been removed.

  • The method isDeferredConfigurable() was removed from ExtensionSchema.

  • IdeaPlugin.performPostEvaluationActions() and EclipsePlugin.performPostEvaluationActions() have been removed.

  • The `BroadcastingCollectionEventRegister.getAddAction() method has been removed with no replacement.

  • The internal org.gradle.util package is no longer imported by default.

    Ideally you shouldn’t use classes from this package, but, as a quick fix, you can add explicit imports to your build scripts for those classes.

  • The gradlePluginPortal() repository no longer looks for JARs without a POM by default.

  • The Tooling API can no longer connect to builds using a Gradle version below Gradle 2.6. The same applies to builds run through TestKit.

  • Gradle 5.0 requires a minimum Tooling API client version of 3.0. Older client libraries can no longer run builds with Gradle 5.0.

  • The IdeaModule Tooling API model element contains methods to retrieve resources and test resources so those elements were removed from the result of IdeaModule.getSourceDirs() and IdeaModule.getTestSourceDirs().

  • In previous Gradle versions, the source field in SourceTask was accessible from subclasses. This is not the case anymore as the source field is now declared as private.

  • In the Worker API, the working directory of a worker can no longer be set.

  • A change in behavior related to dependency and version constraints may impact a small number of users.

  • There have been several changes to property factory methods on DefaultTask that may impact the creation of custom tasks.

Upgrading from 4.9 and earlier

If you are not already on version 4.9, skip down to the section that applies to your current Gradle version and work your way up until you reach here. Then, apply these changes when upgrading to Gradle 4.10.

Deprecated classes, methods and properties

Follow the API links to learn how to deal with these deprecations (if no extra information is provided here):

Potential breaking changes

Upgrading from 4.8 and earlier

Potential breaking changes

Upgrading from 4.7 and earlier

Potential breaking changes

  • Build will now fail if a specified init script is not found.

  • TaskContainer.remove() now actually removes the given task — some plugins may have accidentally relied on the old behavior.

  • Gradle now honors implicit wildcards in Maven POM exclusions.

  • The Kotlin DSL now respects JSR-305 package annotations.

    This will lead to some types annotated according to JSR-305 being treated as nullable where they were treated as non-nullable before. This may lead to compilation errors in the build script. See the relevant Kotlin DSL release notes for details.

  • Error messages will be directed to standard error rather than standard output now, unless a console is attached to both standard output and standard error. This may affect tools that scrape a build’s plain console output. Ignore this change if you’re upgrading from an earlier version of Gradle.

Deprecations

Prior to this release, builds were allowed to replace built-in tasks. This feature has been deprecated(https://docs.gradle.org/4.8/release-notes.html#overwriting-gradle’s-built-in-tasks).

The full list of built-in tasks that should not be replaced is: wrapper, init, help, tasks, projects, buildEnvironment, components, dependencies, dependencyInsight, dependentComponents, model, properties.

Upgrading from 4.6 and earlier

Potential breaking changes

  • Gradle will now, by convention, look for Checkstyle configuration files in the root project’s config/checkstyle directory.

    Checkstyle configuration files in subprojects — the old by-convention location — will be ignored unless you explicitly configure their path via checkstyle.configDir or checkstyle.config.

  • The structure of Gradle’s plain console output has changed, which may break tools that scrape that output.

  • The APIs of many native tasks related to compilation, linking and installation have changed in breaking ways.

  • [Kotlin DSL] Delegated properties used to access Gradle’s build properties — defined in gradle.properties for example — must now be explicitly typed.

  • [Kotlin DSL] Declaring a plugins {} block inside a nested scope now throws an exception.

  • [Kotlin DSL] Only one pluginManagement {} block is allowed now.

  • The cache control DSL provided by the org.gradle.api.artifacts.cache.* interfaces are no longer available.

  • getEnabledDirectoryReportDestinations(), getEnabledFileReportDestinations() and getEnabledReportNames() have all been removed from org.gradle.api.reporting.ReportContainer.

  • StartParameter.projectProperties and StartParameter.systemPropertiesArgs now return immutable maps.

Upgrading from 4.5 and earlier

Deprecations

  • You should not put annotation processors on the compile classpath or declare them with the -processorpath compiler argument.

    They should be added to the annotationProcessor configuration instead. If you don’t want any processing, but your compile classpath contains a processor unintentionally (e.g. as part of a library you depend on), use the -proc:none compiler argument to ignore it.

  • Use CommandLineArgumentProvider in place of CompilerArgumentProvider.

Potential breaking changes

  • The Java plugins now add a sourceSetAnnotationProcessor configuration for each source set, which might break if any of them match existing configurations you have. We recommend you remove your conflicting configuration declarations.

  • The StartParameter.taskOutputCacheEnabled property has been replaced by StartParameter.setBuildCacheEnabled(boolean).

  • The Visual Studio integration now only configures a single solution for all components in a build.

  • Gradle has replaced HttpClient 4.4.1 with version 4.5.5.

  • Gradle now bundles the kotlin-stdlib-jdk8 artifact instead of kotlin-stdlib-jre8. This may affect your build. Please see the Kotlin documentation for more details.

Upgrading from 4.4 and earlier

  • Make sure you have a settings.gradle file: it avoids a performance penalty and allows you to set the root project’s name.

  • Gradle now ignores the build cache configuration of included builds (composite builds) and instead uses the root build’s configuration for all the builds.

Potential breaking changes

  • Two overloaded ValidateTaskProperties.setOutputFile() methods were removed. They are replaced with auto-generated setters when the task is accessed from a build script, but that won’t be the case from plugins and other code outside of the build script.

  • The Maven Publish Plugin now produces more complete maven-metadata.xml files, including maintaining a list of <snapshotVersion> elements. Some older versions of Maven may not be able to consume this metadata.

  • HttpBuildCache no longer follows redirects.

  • The Depend task type has been removed.

  • Project.file(Object) no longer normalizes case for file paths on case-insensitive file systems. It now ignores case in such circumstances and does not touch the file system.

  • ListProperty no longer extends Property.

Upgrading from 4.3 and earlier

Potential breaking changes

  • AbstractTestTask is now extended by non-JVM test tasks as well as Test. Plugins should beware configuring all tasks of type AbstractTestTask because of this.

  • The default output location for EclipseClasspath.defaultOutputDir has changed from $projectDir/bin to $projectDir/bin/default.

  • The deprecated InstallExecutable.setDestinationDir(Provider) was removed — use InstallExecutable.installDirectory instead.

  • The deprecated InstallExecutable.setExecutable(Provider) was removed — use InstallExecutable.executableFile instead.

  • Gradle will no longer prefer a version of Visual Studio found on the path over other locations. It is now a last resort.

    You can bypass the toolchain discovery by specifying the installation directory of the version of Visual Studio you want via VisualCpp.setInstallDir(Object).

  • pluginManagement.repositories is now of type RepositoryHandler rather than PluginRepositoriesSpec, which has been removed.

  • 5xx HTTP errors during dependency resolution will now trigger exceptions in the build.

  • The embedded Apache Ant has been upgraded from 1.9.6 to 1.9.9.

  • Several third-party libraries used by Gradle have been upgraded to fix security issues.

Upgrading from 4.2 and earlier

Other deprecations
Potential breaking changes

  • DefaultTask.newOutputDirectory() now returns a DirectoryProperty instead of a DirectoryVar.

  • DefaultTask.newOutputFile() now returns a RegularFileProperty instead of a RegularFileVar.

  • DefaultTask.newInputFile() now returns a RegularFileProperty instead of a RegularFileVar.

  • ProjectLayout.buildDirectory now returns a DirectoryProperty instead of a DirectoryVar.

  • AbstractNativeCompileTask.compilerArgs is now of type ListProperty<String> instead of List<String>.

  • AbstractNativeCompileTask.objectFileDir is now of type DirectoryProperty instead of File.

  • AbstractLinkTask.linkerArgs is now of type ListProperty<String> instead of List<String>.

  • TaskDestroyables.getFiles() is no longer part of the public API.

  • Overlapping version ranges for a dependency now result in Gradle picking a version that satisfies all declared ranges.

    For example, if a dependency on some-module is found with a version range of [3,6] and also transitively with a range of [4,8], Gradle now selects version 6 instead of 8. The prior behavior was to select 8.

  • The order of elements in Iterable properties marked with either @OutputFiles or @OutputDirectories now matters. If the order changes, the property is no longer considered up to date.

    Prefer using separate properties with @OutputFile/@OutputDirectory annotations or use Map properties with @OutputFiles/@OutputDirectories instead.

  • Gradle will no longer ignore dependency resolution errors from a repository when there is another repository it can check. Dependency resolution will fail instead. This results in more deterministic behavior with respect to resolution results.

Upgrading from 4.1 and earlier

Potential breaking changes

Upgrading from 4.0

  • Consider using the new Worker API to enable units of work within your build to run in parallel.

Deprecated classes, methods and properties

Follow the API links to learn how to deal with these deprecations (if no extra information is provided here):

Potential breaking changes

  • Non-Java projects that have a project dependency on a Java project now consume the runtimeElements configuration by default instead of the default configuration.

    To override this behavior, you can explicitly declare the configuration to use in the project dependency. For example: project(path: ':myJavaProject', configuration: 'default').

  • Default Zinc compiler upgraded from 0.3.13 to 0.3.15.

  • [Kotlin DSL] Base package renamed from org.gradle.script.lang.kotlin to org.gradle.kotlin.dsl.

Changes in detail

[5.0] Default memory settings changed

The command line client now starts with 64MB of heap instead of 1GB. This may affect builds running directly inside the client VM using --no-daemon mode. We discourage the use of --no-daemon, but if you must use it, you can increase the available memory using the GRADLE_OPTS environment variable.

The Gradle daemon now starts with 512MB of heap instead of 1GB. Large projects may have to increase this setting using the org.gradle.jvmargs property.

All workers, including compilers and test executors, now start with 512MB of heap. The previous default was 1/4th of physical memory. Large projects may have to increase this setting on the relevant tasks, e.g. JavaCompile or Test.

[5.0] New default versions for code quality plugins

The default tool versions of the following code quality plugins have been updated:

  • The Checkstyle Plugin now uses 8.12 instead of 6.19 by default.

  • The CodeNarc Plugin now uses 1.2.1 instead of 1.1 by default.

  • The JaCoCo Plugin now uses 0.8.2 instead of 0.8.1 by default.

  • The PMD Plugin now uses 6.8.0 instead of 5.6.1 by default.

    In addition, the default ruleset was changed from the now deprecated java-basic to category/java/errorprone.xml.

    We recommend configuring a ruleset explicitly, though.

[5.0] Library upgrades

Several libraries that are used by Gradle have been upgraded:

  • Groovy was upgraded from 2.4.15 to 2.5.4.

  • Ant has been upgraded from 1.9.11 to 1.9.13.

  • The AWS SDK used to access S3-backed Maven/Ivy repositories has been upgraded from 1.11.267 to 1.11.407.

  • The BND library used by the OSGi Plugin has been upgraded from 3.4.0 to 4.0.0.

  • The Google Cloud Storage JSON API Client Library used to access Google Cloud Storage backed Maven/Ivy repositories has been upgraded from v1-rev116-1.23.0 to v1-rev136-1.25.0.

  • Ivy has been upgraded from 2.2.0 to 2.3.0.

  • The JUnit Platform libraries used by the Test task have been upgraded from 1.0.3 to 1.3.1.

  • The Maven Wagon libraries used to access Maven repositories have been upgraded from 2.4 to 3.0.0.

  • SLF4J has been upgraded from 1.7.16 to 1.7.25.

[5.0] Improved support for dependency and version constraints

Through the Gradle 4.x release stream, new @Incubating features were added to the dependency resolution engine. These include sophisticated version constraints (prefer, strictly, reject), dependency constraints, and platform dependencies.

If you have been using the IMPROVED_POM_SUPPORT feature preview, playing with constraints or prefer, reject and other specific version indications, then make sure to take a good look at your dependency resolution results.

[5.0] BOM import

Gradle now provides support for importing bill of materials (BOM) files, which are effectively POM files that use <dependencyManagement> sections to control the versions of direct and transitive dependencies. All you need to do is declare the POM as a platform dependency.

The following example picks the versions of the gson and dom4j dependencies from the declared Spring Boot BOM:

dependencies {
    // import a BOM
    implementation platform('org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE')

    // define dependencies without versions
    implementation 'com.google.code.gson:gson'
    implementation 'dom4j:dom4j'
}
[5.0] Separation of compile and runtime dependencies when consuming POMs

Since Gradle 1.0, runtime-scoped dependencies have been included in the Java compilation classpath, which has some drawbacks:

  • The compilation classpath is much larger than it needs to be, slowing down compilation.

  • The compilation classpath includes runtime-scoped files that do not impact compilation, resulting in unnecessary re-compilation when those files change.

With this new behavior, the Java and Java Library plugins both honor the separation of compile and runtime scopes. This means that the compilation classpath only includes compile-scoped dependencies, while the runtime classpath adds the runtime-scoped dependencies as well. This is particularly useful if you develop and publish Java libraries with Gradle where the separation between api and implementation dependencies is reflected in the published scopes.

[5.0] Changes to property factory methods on DefaultTask
Property factory methods on DefaultTask are now final

The property factory methods such as newInputFile() are intended to be called from the constructor of a type that extends DefaultTask. These methods are now final to avoid subclasses overriding these methods and using state that is not initialized.

Inputs and outputs are not automatically registered

The Property instances that are returned by these methods are no longer automatically registered as inputs or outputs of the task. The Property instances need to be declared as inputs or outputs in the usual ways, such as attaching annotations such as @OutputFile or using the runtime API to register the property.

For example, you could previously use the following syntax and have both outputFile instances registered as declared outputs:

build.gradle
class MyTask extends DefaultTask {
    // note: no annotation here
    final RegularFileProperty outputFile = newOutputFile()
}

task myOtherTask {
    def outputFile = newOutputFile()
    doLast { ... }
}
build.gradle.kts
open class MyTask : DefaultTask() {
    // note: no annotation here
    val outputFile: RegularFileProperty = newOutputFile()
}

task("myOtherTask") {
    val outputFile = newOutputFile()
    doLast { ... }
}

Now you have to explicitly register outputFile, like this:

build.gradle
class MyTask extends DefaultTask {
    @OutputFile // property needs an annotation
    final RegularFileProperty outputFile = project.objects.fileProperty()
}

task myOtherTask {
    def outputFile = project.objects.fileProperty()
    outputs.file(outputFile) // or to be registered using the runtime API
    doLast { ... }
}
build.gradle.kts
open class MyTask : DefaultTask() {
    @OutputFile // property needs an annotation
    val outputFile: RegularFileProperty = project.objects.fileProperty()
}

task("myOtherTask") {
    val outputFile = project.objects.fileProperty()
    outputs.file(outputFile) // or to be registered using the runtime API
    doLast { ... }
}
[5.0] Gradle now bundles JAXB for Java 9 and above

In order to use S3 backed artifact repositories, you previously had to add --add-modules java.xml.bind to org.gradle.jvmargs when running on Java 9 and above.

Since Java 11 no longer contains the java.xml.bind module, Gradle now bundles JAXB 2.3.1 (com.sun.xml.bind:jaxb-impl) and uses it on Java 9 and above.

Please remove the --add-modules java.xml.bind option from org.gradle.jvmargs, if set.

[5.0] The gradlePluginPortal() repository no longer looks for JARs without a POM by default

With this new behavior, if a plugin or a transitive dependency of a plugin found in the gradlePluginPortal() repository has no Maven POM it will fail to resolve.

Artifacts published to a Maven repository without a POM should be fixed. If you encounter such artifacts, please ask the plugin or library author to publish a new version with proper metadata.

If you are stuck with a bad plugin, you can work around by re-enabling JARs as metadata source for the gradlePluginPortal() repository:

settings.gradle
pluginManagement {
    repositories {
        gradlePluginPortal().tap {
            metadataSources {
                mavenPom()
                artifact()
            }
        }
    }
}
settings.gradle.kts
pluginManagement {
    repositories {
        gradlePluginPortal().apply {
            (this as MavenArtifactRepository).metadataSources {
                mavenPom()
                artifact()
            }
        }
    }
}
Java Library Distribution Plugin utilizes Java Library Plugin

The Java Library Distribution Plugin is now based on the Java Library Plugin instead of the Java Plugin.

Additionally, the default distribution created by the plugin will contain all artifacts of the runtimeClasspath configuration instead of the deprecated runtime configuration.

Configuration Avoidance API disallows common configuration errors

The configuration avoidance API introduced in Gradle 4.9 allows you to avoid creating and configuring tasks that are never used.

With the existing API, this example adds two tasks (foo and bar):

build.gradle
tasks.create("foo") {
    tasks.create("bar")
}
build.gradle.kts
tasks.create("foo") {
    tasks.create("bar")
}

When converting this to use the new API, something surprising happens: bar doesn’t exist. The new API only executes configuration actions when necessary, so the register() for task bar only executes when foo is configured.

build.gradle
tasks.register("foo") {
    tasks.register("bar") // WRONG
}
build.gradle.kts
tasks.register("foo") {
    tasks.register("bar") // WRONG
}

To avoid this, Gradle now detects this and prevents modification to the underlying container (through create() or register()) when using the new API.

[5.0] Worker API: working directory of a worker can no longer be set

Since JDK 11 no longer supports changing the working directory of a running process, setting the working directory of a worker via its fork options is now prohibited.

All workers now use the same working directory to enable reuse.

Please pass files and directories as arguments instead.

[4.10] Publishing to AWS S3 requires new permissions

The S3 repository transport protocol allows Gradle to publish artifacts to AWS S3 buckets. Starting with this release, every artifact uploaded to an S3 bucket will be equipped with the bucket-owner-full-control canned ACL. Make sure that the AWS account used to publish artifacts has the s3:PutObjectAcl and s3:PutObjectVersionAcl permissions, otherwise the upload will fail.

{
    "Version":"2012-10-17",
    "Statement":[
        // ...
        {
            "Effect":"Allow",
            "Action":[
                "s3:PutObject", // necessary for uploading objects
                "s3:PutObjectAcl", // required starting with this release
                "s3:PutObjectVersionAcl" // if S3 bucket versioning is enabled
            ],
            "Resource":"arn:aws:s3:::myCompanyBucket/*"
        }
    ]
}

See AWS S3 Cross Account Access for more information.

[4.9] Consider trying the lazy API for task creation and configuration

Gradle 4.9 introduced a new way to create and configure tasks that works lazily. When you use this approach for tasks that are expensive to configure, or when you have many, many tasks, your build configuration time can drop significantly when those tasks don’t run.

You can learn more about lazily creating tasks in the Task Configuration Avoidance chapter. You can also read about the background to this new feature in this blog post.

[4.8] Switch to the Maven Publish and Ivy Publish Plugins

Now that the publishing plugins are stable, we recommend that you migrate from the legacy publishing mechanism for standard Java projects, i.e. those based on the Java Plugin. That includes projects that use any one of: Java Library Plugin, Application Plugin or War Plugin.

To use the new approach, simply replace any upload<Conf> configuration with a publishing {} block. See the publishing overview chapter for more information.

[4.8] Use deferred configuration for publishing plugins

Prior to Gradle 4.8, the publishing {} block was implicitly treated as if all the logic inside it was executed after the project was evaluated. This was confusing, because it was the only block that behaved that way. As part of the stabilization effort in Gradle 4.8, we are deprecating this behavior and asking all users to migrate their build.

The new, stable behavior can be switched on by adding the following to your settings file:

settings.gradle
enableFeaturePreview('STABLE_PUBLISHING')
settings.gradle.kts
enableFeaturePreview("STABLE_PUBLISHING")

We recommend doing a test run with a local repository to see whether all artifacts still have the expected coordinates. In most cases everything should work as before and you are done. However, your publishing block may rely on the implicit deferred configuration, particularly if it relies on values that may change during the configuration phase of the build.

For example, under the new behavior, the following logic assumes that jar.baseName doesn’t change after artifactId is set:

build.gradle
subprojects {
    publishing {
        publications {
            mavenJava {
                from components.java
                artifactId = jar.baseName
            }
        }
    }
}
build.gradle.kts
subprojects {
    publishing {
        publications {
            named<MavenPublication>("mavenJava") {
                from(components["java"])
                artifactId = tasks.jar.get().baseName
            }
        }
    }
}

If that assumption is incorrect or might possibly be incorrect in the future, the artifactId must be set within an afterEvaluate {} block, like so:

build.gradle
subprojects {
    publishing {
        publications {
            mavenJava {
                from components.java
                afterEvaluate {
                    artifactId = jar.baseName
                }
            }
        }
    }
}
build.gradle.kts
subprojects {
    publishing {
        publications {
            named<MavenPublication>("mavenJava") {
                from(components["java"])
                afterEvaluate {
                    artifactId = tasks.jar.get().baseName
                }
            }
        }
    }
}
[4.8] Configure existing wrapper and init tasks

You should no longer define your own wrapper and init tasks. Configure the existing tasks instead, for example by converting this:

build.gradle
task wrapper(type: Wrapper) {
    ...
}
build.gradle.kts
task<Wrapper>("wrapper") {
    ...
}

to this:

build.gradle
wrapper {
    ...
}
build.gradle.kts
tasks.wrapper {
    ...
}
[4.8] Gradle now honors implicit wildcards in Maven POM exclusions

If an exclusion in a Maven POM was missing either a groupId or artifactId, Gradle used to ignore the exclusion. Now the missing elements are treated as implicit wildcards — e.g. <groupId>*</groupId> — which means that some of your dependencies may now be excluded where they weren’t before.

You will need to explicitly declare any missing dependencies that you need.

[4.7] Changes to the structure of Gradle’s plain console output

The plain console mode now formats output consistently with the rich console, which means that the output format has changed. For example:

  • The output produced by a given task is now grouped together, even when other tasks execute in parallel with it.

  • Task execution headers are printed with a "> Task" prefix.

  • All output produced during build execution is written to the standard output file handle. This includes messages written to System.err unless you are redirecting standard error to a file or any other non-console destination.

This may break tools that scrape details from the plain console output.

[4.6] Changes to the APIs of native tasks related to compilation, linking and installation

Many tasks related to compiling, linking and installing native libraries and applications have been converted to the Provider API so that they support lazy configuration. This conversion has introduced some breaking changes to the APIs of the tasks so that they match the conventions of the Provider API.

The following tasks have been changed:

AbstractLinkTask and its subclasses

  • getDestinationDir() was replaced by getDestinationDirectory().

  • getBinaryFile(), getOutputFile() was replaced by getLinkedFile().

  • setOutputFile(File) was removed. Use Property.set() instead.

  • setOutputFile(Provider) was removed. Use Property.set() instead.

  • getTargetPlatform() was changed to return a Property.

  • setTargetPlatform(NativePlatform) was removed. Use Property.set() instead.

  • getToolChain() was changed to return a Property.

  • setToolChain(NativeToolChain) was removed. Use Property.set() instead.

CreateStaticLibrary

  • getOutputFile() was changed to return a Property.

  • setOutputFile(File) was removed. Use Property.set() instead.

  • setOutputFile(Provider) was removed. Use Property.set() instead.

  • getTargetPlatform() was changed to return a Property.

  • setTargetPlatform(NativePlatform) was removed. Use Property.set() instead.

  • getToolChain() was changed to return a Property.

  • setToolChain(NativeToolChain) was removed. Use Property.set() instead.

  • getStaticLibArgs() was changed to return a ListProperty.

  • setStaticLibArgs(List) was removed. Use ListProperty.set() instead.

InstallExecutable

  • getSourceFile() was replaced by getExecutableFile().

  • getPlatform() was replaced by getTargetPlatform().

  • setTargetPlatform(NativePlatform) was removed. Use Property.set() instead.

  • getToolChain() was changed to return a Property.

  • setToolChain(NativeToolChain) was removed. Use Property.set() instead.

The following have also seen similar changes:

[4.6] Visual Studio integration only supports a single solution file for all components of a build

VisualStudioExtension no longer has a solutions property. Instead, you configure a single solution via VisualStudioRootExtension in the root project, like so:

build.gradle
model {
    visualStudio {
        solution {
            solutionFile.location = "vs/${name}.sln"
        }
    }
}

In addition, there are no longer individual tasks to generate the solution files for each component, but rather a single visualStudio task that generates a solution file that encompasses all components in the build.

[4.5] HttpBuildCache no longer follows redirects

When connecting to an HTTP build cache backend via HttpBuildCache, Gradle does not follow redirects any more, treating them as errors instead. Getting a redirect from the build cache backend is mostly a configuration error — using an "http" URL instead of "https" for example — and has negative effects on performance.

[4.4] Third-party dependency upgrades

This version includes several upgrades of third-party dependencies:

  • jackson: 2.6.6 → 2.8.9

  • plexus-utils: 2.0.6 → 2.1

  • xercesImpl: 2.9.1 → 2.11.0

  • bsh: 2.0b4 → 2.0b6

  • bouncycastle: 1.57 → 1.58

This fix the following security issues:

Gradle does not expose public APIs for these 3rd-party dependencies, but those who customize Gradle will want to be aware.

Migrating Builds From Apache Maven

Apache Maven is a build tool for Java and other JVM-based projects that’s in widespread use, and so people that want to use Gradle often have to migrate an existing Maven build. This guide will help with such a migration by explaining the differences and similarities between the two tools' models and providing steps that you can follow to ease the process.

Converting a build can be scary, but you don’t have to do it alone. You can search docs, forums, and StackOverflow from help.gradle.org or reach out to the Gradle community on the forums if you get stuck.

Making a case for migration

The primary differences between Gradle and Maven are flexibility, performance, user experience, and dependency management. A visual overview of these aspects is available in the Maven vs Gradle feature comparison.

Since Gradle 3.0, Gradle has invested heavily in making Gradle builds much faster, with features such as build caching, compile avoidance, and an improved incremental Java compiler. Gradle is now 2-10x faster than Maven for the vast majority of projects, even without using a build cache. In-depth performance comparison and business cases for switching from Maven to Gradle can be found here.

General guidelines

Gradle and Maven have fundamentally different views on how to build a project. Gradle provides a flexible and extensible build model that delegates the actual work to a graph of task dependencies. Maven uses a model of fixed, linear phases to which you can attach goals (the things that do the work). This may make migrating between the two seem intimidating, but migrations can be surprisingly easy because Gradle follows many of the same conventions as Maven — such as the standard project structure — and its dependency management works in a similar way.

Here we lay out a series of steps for you to follow that will help facilitate the migration of any Maven build to Gradle:

  1. Keep the old Maven build and new Gradle build side by side

    You know the Maven build works, so you should keep it until you are confident that the Gradle build produces all the same artifacts and otherwise does what you need. This also means that users can try the Gradle build without getting a new copy of the source tree.

  2. Develop a mechanism to verify that the two builds produce the same artifacts

    This is a vitally important step to ensure that your deployments and tests don’t break. Even small changes, such as the contents of a manifest file in a JAR, can cause problems. If your Gradle build produces the same output as the Maven build, this will give you and others confidence in switching over and make it easier to implement the big changes that will provide the greatest benefits.

    This doesn’t mean that you need to verify every artifact at every stage, although doing so can help you quickly identify the source of a problem. You can just focus on the critical output such as final reports and the artifacts that are published or deployed.

    You will need to factor in some inherent differences in the build output that Gradle produces compared to Maven. Generated POMs will contain only the information needed for consumption and they will use <compile> and <runtime> scopes correctly for that scenario. You might also see differences in the order of files in archives and of files on classpaths. Most differences will be benign, but it’s worth identifying them and verifying that they are OK.

  3. Run an automatic conversion

    This will create all the Gradle build files you need, even for multi-module builds. For simpler Maven projects, the Gradle build will be ready to run!

  4. Create a build scan

    A build scan will make it easier to visualize what’s happening in the build. In particular, you’ll be able to see the project structure, the dependencies (regular and inter-project ones), what plugins are being used and the console output of the build.

    We recommend that you regularly generate build scans during the migration to help you identify and troubleshoot problems. If you want, you can then use them to identify opportunities to improve the performance of the build, and performance is a big reason for switching to Gradle in the first place.

  5. Verify your dependencies and fix any problems

  6. Configure integration and functional tests

    Many tests can simply be migrated by configuring an extra source set. If you are using a third-party library, such as FitNesse, look to see whether there is a suitable community plugin available on the Gradle Plugin Portal.

  7. Replace Maven plugins with Gradle equivalents

    In the case of popular plugins, Gradle often has an equivalent plugin that you can use. You might also find that you can replace a plugin with built-in Gradle functionality. As a last resort, you may need to reimplement a Maven plugin via your own custom plugins and task types.

The rest of this chapter looks in more detail at specific aspects of migrating a build from Maven to Gradle.

Understanding the build lifecycle

Maven builds are based around the concept of build lifecycles that consist of a set of fixed phases. This can prove an impediment for users migrating to Gradle because its build lifecycle is something different, although it’s important to understand how Gradle builds fit into the structure of initialization, configuration, and execution phases. Fortunately, Gradle has a feature that can mimic Maven’s phases: lifecycle tasks.

These allow you to define your own "lifecycles" by creating no-action tasks that simply depend on the tasks you’re interested in. And to make the transition to Gradle easier for Maven users, the Base Plugin — applied by all the JVM language plugins like the Java Library Plugin — provides a set of lifecycle tasks that correspond to the main Maven phases.

Here is a list of some of the main Maven phases and the Gradle tasks that they map to:

clean

Use the clean task provided by the Base Plugin.

compile

Use the classes task provided by the Java Plugin and other JVM language plugins. This compiles all classes for all source files of all languages and also performs resource filtering via the processResources task.

test

Use the test task provided by the Java Plugin. It runs just the unit tests, or more specifically, the tests that make up the test source set.

package

Use the assemble task provided by the Base Plugin. This builds whatever is the appropriate package for the project, for example a JAR for Java libraries or a WAR for traditional Java webapps.

verify

Use the check task provided by the Base Plugin. This runs all verification tasks that are attached to it, which typically includes the unit tests, any static analysis tasks — such as Checkstyle — and others. If you want to include integration tests, you will have to configure these manually, which is a simple process.

install

Use the publishToMavenLocal task provided by the Maven Publish Plugin.

Note that Gradle builds don’t require you to "install" artifacts as you have access to more appropriate features like inter-project dependencies and composite builds. You should only use publishToMavenLocal for interoperating with Maven builds.

Gradle also allows you to resolve dependencies against the local Maven cache, as described in the Declaring repositories section.

deploy

Use the publish task provided by the Maven Publish Plugin — making sure you switch from the older Maven Plugin (ID: maven) if your build is using that one. This will publish your package to all configured publication repositories. There are also other tasks that allow you to publish to a single repository even when multiple ones are defined.

Note that the Maven Publish Plugin does not publish source and Javadoc JARs by default, but this can easily be configured as explained elsewhere in the user manual.

Performing an automatic conversion

Gradle’s init task is typically used to create a new skeleton project, but you can also use it to convert an existing Maven build to Gradle automatically. Once Gradle is installed on your system, all you have to do is run the command

> gradle init

from the root project directory and let Gradle do its thing. That basically consists of parsing the existing POMs and generating the corresponding Gradle build scripts. Gradle will also create a settings script if you’re migrating a multi-project build.

You’ll find that the new Gradle build includes the following:

  • All the custom repositories that are specified in the POM

  • Your external and inter-project dependencies

  • The appropriate plugins to build the project (limited to one or more of the Maven, Java and War Plugins)

See the Build Init Plugin chapter for a complete list of the automatic conversion features.

Note

You should replace the Maven Plugin (id 'maven') in the build script (build.gradle) with the Maven Publish Plugin.

One thing to bear in mind is that assemblies are not automatically converted. They aren’t necessarily problematic to convert, but you will need to do some manual work. Options include:

If you’re lucky and don’t have many plugins or much in the way of customisation in your Maven build, you can simply run

> gradle build

once the migration has completed. This will run the tests and produce the required artifacts without any extra intervention on your part.

Migrating dependencies

Gradle’s dependency management system is more flexible than Maven’s, but it still supports the same concepts of repositories, declared dependencies, scopes (dependency configurations in Gradle), and transitive dependencies. In fact, Gradle works perfectly with Maven-compatible repositories, which makes it easy to migrate your dependencies.

Note
 One notable difference between the two tools is in how they manage version conflicts. Maven uses a "closest" match algorithm, whereas Gradle picks the newest. Don’t worry though, you have a lot of control over which versions are selected, as documented in Managing Transitive Dependencies.

Over the following sections, we will show you how to migrate the most common elements of a Maven build’s dependency management information.

Declaring dependencies

Gradle uses the same dependency identifier components as Maven: group ID, artifact ID and version. It also supports classifiers. So all you need to do is substitute the identifier information for a dependency into Gradle’s syntax, which is described in the Declaring Dependencies chapter.

For example, consider this Maven-style dependency on Log4J:

<dependencies>
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
        <version>1.2.12</version>
    </dependency>
</dependencies>

This dependency would look like the following in a Gradle build script:

Example 1. Declaring a simple compile-time dependency
build.gradle
dependencies {
    implementation 'log4j:log4j:1.2.12'  (1)
}
build.gradle.kts
dependencies {
    implementation("log4j:log4j:1.2.12")  (1)
}

  1. Attaches version 1.2.12 of Log4J to the implementation configuration (scope)

The string identifier takes the form "<groupId>:<artifactId>:<version>", although Gradle refers to them as "group", "module" and "version".

The above example raises an obvious question: what is that implementation configuration? It’s one of the standard dependency configurations provided by the Java Plugin and is often used as a substitute for Maven’s default compile scope.

Several of the differences between Maven’s scopes and Gradle’s standard configurations come down to Gradle distinguishing between the dependencies required to build a module and the dependencies required to build a module that depends on it. Maven makes no such distinction, so published POMs typically include dependencies that consumers of a library don’t actually need.

Here are the main Maven dependency scopes and how you should deal with their migration:

compile

Gradle has two configurations that can be used in place of the compile scope: implementation and api. The former is available to any project that applies the Java Plugin, while api is only available to projects that specifically apply the Java Library Plugin.

In most cases you should simply use the implementation configuration, particularly if you’re building an application or webapp. But if you’re building a library, you can learn about which dependencies should be declared using api in the section on Building Java libraries. Even more information on the differences between api and implementation is provided in the Java Library Plugin chapter linked above.

runtime

Use the runtimeOnly configuration.

test

Gradle distinguishes between those dependencies that are required to compile a project’s tests and those that are only needed to run them.

Dependencies required for test compilation should be declared against the testImplementation configuration. Those that are only required for running the tests should use testRuntimeOnly.

provided

Use the compileOnly configuration.

Note that the War Plugin adds providedCompile and providedRuntime dependency configurations. These behave slightly differently from compileOnly and simply ensure that those dependencies aren’t packaged in the WAR file. However, the dependencies are included on runtime and test runtime classpaths, so use these configurations if that’s the behavior you need.

import

The import scope is mostly used within <dependencyManagement> blocks and applies solely to POM-only publications. Read the section on Using bills of materials to learn more about how to replicate this behavior.

You can also specify a regular dependency on a POM-only publication. In this case, the dependencies declared in that POM are treated as normal transitive dependencies of the build.

For example, imagine you want to use the groovy-all POM for your tests. It’s a POM-only publication that has its own dependencies listed inside a <dependencies> block. The appropriate configuration in the Gradle build looks like this:

Example 2. Consuming a POM-only dependency
build.gradle
dependencies {
    testImplementation 'org.codehaus.groovy:groovy-all:2.5.4'
}
build.gradle.kts
dependencies {
    testImplementation("org.codehaus.groovy:groovy-all:2.5.4")
}

The result of this will be that all compile and runtime scope dependencies in the groovy-all POM get added to the test runtime classpath, while only the compile scope dependencies get added to the test compilation classpath. Dependencies with other scopes will be ignored.

Declaring repositories

Gradle allows you to retrieve declared dependencies from any Maven-compatible or Ivy-compatible repository. Unlike Maven, it has no default repository and so you have to declare at least one. In order to have the same behavior as your Maven build, just configure Maven Central in your Gradle build, like this:

Example 3. Configuring the build to use Maven Central
build.gradle
repositories {
    mavenCentral()
}
build.gradle.kts
repositories {
    mavenCentral()
}

You can also use the repositories {} block to configure custom repositories, as described in the Repository Types chapter.

Lastly, Gradle allows you to resolve dependencies against the local Maven cache/repository. This helps Gradle builds interoperate with Maven builds, but it shouldn’t be a technique that you use if you don’t need that interoperability. If you want to share published artifacts via the filesystem, consider configuring a custom Maven repository with a file:// URL.

You might also be interested in learning about Gradle’s own dependency cache, which behaves more reliably than Maven’s and can be used safely by multiple concurrent Gradle processes.

Excluding transitive dependencies

Maven builds use exclusions to keep unwanted dependencies — or unwanted versions of dependencies — out of the dependency graph. You can do the same thing with Gradle, but that’s not necessarily the right thing to do. Gradle provides other options that may be more appropriate for a given situation, so you really need to understand why an exclusion is in place to migrate it properly.

If you want to exclude a dependency for reasons unrelated to versions, then check out the section on Excluding transitive module dependencies. It shows you how to attach an exclusion either to an entire configuration (often the most appropriate solution) or to a dependency. You can even easily apply an exclusion to all configurations.

If you’re more interested in controlling which version of a dependency is actually resolved, then Gradle has better options than exclusions. Consider using dependency constraints or forcing a particular version.

Handling optional dependencies

There are two sides to optional dependencies:

  • How the build treats them as transitive dependencies

  • How declared dependencies are published as optional

For the first scenario, Gradle behaves the same way as Maven and simply ignores any transitive dependencies that are declared as optional. They are not resolved and have no impact on the versions selected if the same dependencies appear elsewhere in the dependency graph as non-optional.

As for publishing dependencies as optional, Gradle has no built-in support at this time. However, you can use the Nebula Optional Base Plugin to get the required behavior. Alternatively, you can use the MavenPom.withXML() method to customize the generated POM.

Using bills of materials (BOMs)

Maven allows you to share dependency constraints by defining dependencies inside a <dependencyManagement> section of a POM file that has a packaging type of pom. This special type of POM (a BOM) can then be imported into other POMs so that you have consistent library versions across your projects.

Gradle can use such BOMs for the same purpose, using a special dependency syntax based on platform() and enforcedPlatform() methods. You simply declare the dependency in the normal way, but wrap the dependency identifier in the appropriate method, as shown in this example that "imports" the Spring Boot Dependencies BOM:

Example 4. Importing a BOM in a Gradle build
build.gradle
dependencies {
    implementation platform('org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE') (1)

    implementation 'com.google.code.gson:gson' (2)
    implementation 'dom4j:dom4j'
}
build.gradle.kts
dependencies {
    implementation(platform("org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE"))  (1)

    implementation("com.google.code.gson:gson")  (2)
    implementation("dom4j:dom4j")
}

  1. Applies the Spring Boot Dependencies BOM

  2. Adds a dependency whose version is defined by that BOM

You can learn more about this feature and the difference between platform() and enforcedPlatform() in the section on importing version recommendations from a Maven BOM.

Note
 You can use this feature to apply the <dependencyManagement> information from any dependency’s POM to the Gradle build, even those that don’t have a packaging type of pom. However, platform() and enforcedPlatform() will ignore any dependencies declared in the <dependencies> block.

Migrating multi-module builds (project aggregation)

Maven’s multi-module builds map nicely to Gradle’s multi-project builds. Try the corresponding tutorial to see how a basic multi-project Gradle build is set up.

To migrate a multi-module Maven build, simply follow these steps:

  1. Create a settings script that matches the <modules> block of the root POM.

    For example, this <modules> block:

    <modules>
    <module>simple-weather</module>
    <module>simple-webapp</module>
</modules>

    can be migrated by adding the following line to the settings script:

    Example 5. Declaring which projects are part of the build
    settings.gradle
    rootProject.name = 'simple-multi-module'  (1)

include 'simple-weather', 'simple-webapp'  (2)
    settings.gradle.kts
    rootProject.name = "simple-multi-module"  (1)

include("simple-weather", "simple-webapp")  (2)

    1. Sets the name of the overall project

    2. Configures two subprojects as part of this build

    Output of gradle projects
    > gradle projects

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

Root project 'simple-multi-module'
+--- Project ':simple-weather'
\--- Project ':simple-webapp'

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

  2. Replace cross-module dependencies with project dependencies.

  3. Replicate project inheritance with cross-project configuration.

    This basically involves creating a root project build script that injects shared configuration into the appropriate subprojects.

One notable feature that’s missing is a standard way to share dependency versions across projects in a multi-project build. A common approach is to use extra properties in the root build script to store the versions, since those properties are visible from subprojects as well.

Migrating Maven profiles and properties

Maven allows you parameterize builds using properties of various sorts. Some are read-only properties of the project model, others are user-defined in the POM. It even allows you to treat system properties as project properties.

Gradle has a similar system of project properties, although it differentiates between those and system properties. You can, for example, define properties in:

  • the build script

  • a gradle.properties file in the root project directory

  • a gradle.properties file in the $HOME/.gradle directory

Those aren’t the only options, so if you are interested in finding out more about how and where you can define properties, check out the Build Environment chapter.

One important piece of behavior you need to be aware of is what happens when the same property is defined in both the build script and one of the external properties files: the build script value takes precedence. Always. Fortunately, you can mimic the concept of profiles to provide overridable default values.

Which brings us on to Maven profiles. These are a way to enable and disable different configurations based on environment, target platform, or any other similar factor. Logically, they are nothing more than limited ‘if' statements. And since Gradle has much more powerful ways to declare conditions, it does not need to have formal support for profiles (except in the POMs of dependencies). You can easily get the same behavior by combining conditions with secondary build scripts, as you’ll see.

Let’s say you have different deployment settings depending on the environment: local development (the default), a test environment, and production. To add profile-like behavior, you first create build scripts for each environment in the project root: profile-default.gradle, profile-test.gradle, and profile-prod.gradle. You can then conditionally apply one of those profile scripts based on a project property of your own choice.

The following example demonstrates the basic technique using a project property called buildProfile and profile scripts that simply initialize an extra project property called message:

Example 6. Mimicing the behavior of Maven profiles in Gradle
build.gradle
if (!hasProperty('buildProfile')) ext.buildProfile = 'default'  (1)

apply from: "profile-${buildProfile}.gradle"  (2)

task greeting {
    doLast {
        println message  (3)
    }
}
profile-default.gradle
ext.message = 'foobar'  (4)
profile-test.gradle
ext.message = 'testing 1 2 3'  (4)
profile-prod.gradle
ext.message = 'Hello, world!'  (4)
build.gradle.kts
val buildProfile: String? by project  (1)

apply(from = "profile-${buildProfile ?: "default"}.gradle.kts")  (2)

tasks.register("greeting") {
    val message: String by project.extra
    doLast {
        println(message)  (3)
    }
}
profile-default.gradle.kts
val message by extra("foobar")  (4)
profile-test.gradle.kts
val message by extra("testing 1 2 3")  (4)
profile-prod.gradle.kts
val message by extra("Hello, world!")  (4)

  1. Checks for the existence of (Groovy) or binds (Kotlin) the buildProfile project property

  2. Applies the appropriate profile script, using the value of buildProfile in the script filename

  3. Prints out the value of the message extra project property

  4. Initializes the message extra project property, whose value can then be used in the main build script

With this setup in place, you can activate one of the profiles by passing a value for the project property you’re using — buildProfile in this case:

Output of gradle greeting
> gradle greeting
foobar
Output of gradle -PbuildProfile=test greeting
> gradle -PbuildProfile=test greeting
testing 1 2 3

You’re not limited to checking project properties. You could also check environment variables, the JDK version, the OS the build is running on, or anything else you can imagine.

One thing to bear in mind is that high level condition statements make builds harder to understand and maintain, similar to the way they complicate object-oriented code. The same applies to profiles. Gradle offers you many better ways to avoid the extensive use of profiles that Maven often requires, for example by configuring multiple tasks that are variants of one another. See the publishPubNamePublicationToRepoNameRepository tasks created by the Maven Publish Plugin.

For a lengthier discussion on working with Maven profiles in Gradle, look no further than this blog post.

Filtering resources

Maven has a phase called process-resources that has the goal resources:resources bound to it by default. This gives the build author an opportunity to perform variable substitution on various files, such as web resources, packaged properties files, etc.

The Java plugin for Gradle provides a processResources task to do the same thing. This is a Copy task that copies files from the configured resources directory — src/main/resources by default — to an output directory. And as with any Copy task, you can configure it to perform file filtering, renaming, and content filtering.

As an example, here’s a configuration that treats the source files as Groovy SimpleTemplateEngine templates, providing version and buildNumber properties to those templates:

Example 7. Filtering the content of resources via the processResources task
build.gradle
processResources {
    expand(version: version, buildNumber: currentBuildNumber)
}
build.gradle.kts
tasks {
    processResources {
        expand("version" to version, "buildNumber" to currentBuildNumber)
    }
}

See the API docs for CopySpec to see all the options available to you.

Configuring integration tests

Many Maven builds incorporate integration tests of some sort, which Maven supports through an extra set of phases: pre-integration-test, integration-test, post-integration-test, and verify. It also uses the Failsafe plugin in place of Surefire so that failed integration tests don’t automatically fail the build (because you may need to clean up resources, such as a running application server).

This behavior is easy to replicate in Gradle with source sets, as explained in our chapter on Testing in Java & JVM projects. You can then configure a clean-up task, such as one that shuts down a test server for example, to always run after the integration tests regardless of whether they succeed or fail using Task.finalizedBy().

If you really don’t want your integration tests to fail the build, then you can use the Test.ignoreFailures setting described in the Test execution section of the Java testing chapter.

Source sets also give you a lot of flexibility on where you place the source files for your integration tests. You can easily keep them in the same directory as the unit tests or, more preferably, in a separate source directory like src/integTest/java. To support other types of tests, you just add more source sets and Test tasks!

Migrating common plugins

Maven and Gradle share a common approach of extending the build through plugins. Although the plugin systems are very different beneath the surface, they share many feature-based plugins, such as:

  • Shade/Shadow

  • Jetty

  • Checkstyle

  • JaCoCo

  • AntRun (see further down)

Why does this matter? Because many plugins rely on standard Java conventions, so migration is just a matter of replicating the configuration of the Maven plugin in Gradle. As an example, here’s a simple Maven Checkstyle plugin configuration:

...
<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-checkstyle-plugin</artifactId>
  <version>2.17</version>
  <executions>
    <execution>
      <id>validate</id>
      <phase>validate</phase>
      <configuration>
        <configLocation>checkstyle.xml</configLocation>
        <encoding>UTF-8</encoding>
        <consoleOutput>true</consoleOutput>
        <failsOnError>true</failsOnError>
        <linkXRef>false</linkXRef>
      </configuration>
      <goals>
        <goal>check</goal>
      </goals>
    </execution>
  </executions>
</plugin>
...

Everything outside of the configuration block can safely be ignored when migrating to Gradle. In this case, the corresponding Gradle configuration looks like the following:

Example 8. Configuring the Gradle Checkstyle Plugin
build.gradle
checkstyle {
    config = resources.text.fromFile('checkstyle.xml', 'UTF-8')
    showViolations = true
    ignoreFailures = false
}
build.gradle.kts
checkstyle {
    config = resources.text.fromFile("checkstyle.xml", "UTF-8")
    isShowViolations = true
    isIgnoreFailures = false
}

The Checkstyle tasks are automatically added as dependencies of the check task, which also includes test. If you want to ensure that Checkstyle runs before the tests, then just specify an ordering with the mustRunAfter() method:

Example 9. Controlling when the checkstyle task runs
build.gradle
test.mustRunAfter checkstyleMain, checkstyleTest
build.gradle.kts
tasks {
    test {
        mustRunAfter(checkstyleMain, checkstyleTest)
    }
}

As you can see, the Gradle configuration is often much shorter than the Maven equivalent. You also have a much more flexible execution model since you are no longer constrained by Maven’s fixed phases.

While migrating a project from Maven, don’t forget about source sets. These often provide a more elegant solution for handling integration tests or generated sources than Maven can provide, so you should factor them into your migration plans.

Ant goals

Many Maven builds rely on the AntRun plugin to customize the build without the overhead of implementing a custom Maven plugin. Gradle has no equivalent plugin because Ant is a first-class citizen in Gradle builds, via the ant object. For example, you can use Ant’s Echo task like this:

Example 10. Invoking Ant tasks
build.gradle
task sayHello {
    doLast {
        ant.echo message: 'Hello!'
    }
}
build.gradle.kts
tasks.register("sayHello") {
    doLast {
        ant.withGroovyBuilder {
            "echo"("message" to "Hello!")
        }
    }
}

Even Ant properties and filesets are supported natively. To learn more, see Using Ant from Gradle.

Tip

It may be simpler and cleaner to just create custom task types to replace the work that Ant is doing for you. You can then more readily benefit from incremental build and other useful Gradle features.

Understanding which plugins you don’t need

It’s worth remembering that Gradle builds are typically easier to extend and customize than Maven ones. In this context, that means you may not need a Gradle plugin to replace a Maven one. For example, the Maven Enforcer plugin allows you to control dependency versions and environmental factors, but these things can easily be configured in a normal Gradle build script.

Dealing with uncommon and custom plugins

You may come across Maven plugins that have no counterpart in Gradle, particularly if you or someone in your organisation has written a custom plugin. Such cases rely on you understanding how Gradle (and potentially Maven) works, because you will usually have to write your own plugin.

For the purposes of migration, there are two key types of Maven plugins:

  • Those that use the Maven project object.

  • Those that don’t.

Why is this important? Because if you use one of the latter, you can trivially reimplement it as a custom Gradle task type. Simply define task inputs and outputs that correspond to the mojo parameters and convert the execution logic into a task action.

If a plugin depends on the Maven project, then you will have to rewrite it. Don’t start by considering how the Maven plugin works, but look at what problem it is trying to solve. Then try to work out how to solve that problem in Gradle. You’ll probably find that the two build models are different enough that "transcribing" Maven plugin code into a Gradle plugin just won’t be effective. On the plus side, the plugin is likely to be much easier to write than the original Maven one because Gradle has a much richer build model and API.

If you do need to implement custom logic, either via build scripts or plugins, check out the Guides related to plugin development. Also be sure to familiarize yourself with Gradle’s Groovy DSL Reference, which provides comprehensive documentation on the API that you’ll be working with. It details the standard configuration blocks (and the objects that back them), the core types in the system (Project, Task, etc.), and the standard set of task types. The main entry point is the Project interface as that’s the top-level object that backs the build scripts.

Further reading

This chapter has covered the major topics that are specific to migrating Maven builds to Gradle. All that remain are a few other areas that may be useful during or after a migration:

As a final note, this guide has only touched on a few of Gradle’s features and we encourage you to learn about the rest from the other chapters of the user manual and from our tutorial-style Gradle Guides.

Migrating Builds From Apache Ant

Apache Ant is a build tool with a long history in the Java world that is still widely used, albeit by a decreasing number of teams. While flexible, it lacks conventions and many of the powerful features that Gradle can provide. Migrating to Gradle is worthwhile so that your builds can become slimmer, simpler and faster, while still retaining the flexibility you enjoy with Ant. You’ll also benefit from robust support for multi-project builds and easy-to-use, flexible dependency management.

The biggest challenge in migrating from Ant to Gradle is that there is no such thing as a standard Ant build. That makes it difficult to provide specific instructions. Fortunately, Gradle has some great integration features with Ant that can make the process relatively smooth. And even migrating from Ivy-based dependency management isn’t particularly hard because Gradle has a similar model based on dependency configurations that works with Ivy-compatible repositories.

We will start by outlining the things you should consider at the outset of migrating a build from Ant to Gradle and offer some general guidelines on how to proceed.

General guidelines

When you undertake to migrate a build from Ant to Gradle, you should keep in mind the nature of both what you already have and where you would like to end up. Do you want a Gradle build that mirrors the structure of the existing Ant build? Or do you want to move to something that is more idiomatic to Gradle? What are the main benefits you are looking for?

To understand the implications, consider the two extreme endpoints that you could aim for:

  • An imported build via ant.importBuild()

    This approach is quick, simple and works for many Ant-based builds. You end up with a build that’s effectively identical to the original Ant build, except your Ant targets become Gradle tasks. Even the dependencies between targets are retained.

    The downside is that you’re still using the Ant build, which you must continue to maintain. You also lose the advantages of Gradle’s conventions, many of its plugins, its dependency management, and so on. You can still enhance the build with incremental build information, but it’s more effort than would be the case for a normal Gradle build.

  • An idiomatic Gradle build

    If you want to future proof your build, this is where you want to end up. Making use of Gradle’s conventions and plugins will result in a smaller, easier-to-maintain build, with a structure that is familiar to many Java developers. You will also find it easier to take advantage of Gradle’s power features to improve build performance.

    The main downside is the extra work required to perform the migration, particularly if the existing build is complex and has many inter-project dependencies. But such builds often benefit the most from a switch to idomatic Gradle. In addition, Gradle provides many features that can ease the migration, such as the ability to use core and custom Ant tasks directly from a Gradle build.

You ideally want to end up somewhere close to the second option in the long term, but you don’t have to get there in one fell swoop.

What follows is a series of steps to help you decide the approach you want to take and how to go about it:

  1. Keep the old Ant build and new Gradle build side by side

    You know the Ant build works, so you should keep it until you are confident that the Gradle build produces all the same artifacts and otherwise does what you need. This also means that users can try the Gradle build without getting a new copy of the source tree.

    Don’t try to change the directory and file structure of the build until after you’re ready to make the switch.

  2. Develop a mechanism to verify that the two builds produce the same artifacts

    This is a vitally important step to ensure that your deployments and tests don’t break. Even small changes, such as the contents of a manifest file in a JAR, can cause problems. If your Gradle build produces the same output as the Ant build, this will give you and others confidence in switching over and make it easier to implement the big changes that will provide the greatest benefits.

  3. Decide whether you have a multi-project build or not

    Multi-project builds are generally harder to migrate and require more work than single-project ones. We have provided some dedicated advice to help with the process in the Migrating multi-project builds section.

  4. Work out what plugins to use for each project

    We expect that the vast majority of Ant builds are for JVM-based projects, for which there are a wealth of plugins that provide a lot of the functionality you need. Not only are there the core plugins that come packaged with Gradle, but you can also find many useful plugins on the Plugin Portal.

    Even if the Java Plugin or one of its derivatives (such as the Java Library Plugin) aren’t a good match for your build, you should at least consider the Base Plugin for its lifecycle tasks.

  5. Import the Ant build or create a Gradle build from scratch

    This step very much depends on the requirements of your build. If a selection of Gradle plugins can do the vast majority of the work your Ant build does, then it probably makes sense to create a fresh Gradle build script that doesn’t depend on the Ant build and either implements the missing pieces itself or utilizes existing Ant tasks.

    The alternative approach is to import the Ant build into the Gradle build script and gradually replace the Ant build functionality. This allows you to have a working Gradle build at each stage, but it requires a bit of work to get the Gradle tasks working properly with the Ant ones. You can learn more about this approach in Working with an imported build.

  6. Configure your build for the existing directory and file structure

    Gradle makes use of conventions to eliminate much of the boilerplate associated with older builds and to make it easier for users to work with new builds once they are familiar with those conventions. But that doesn’t mean you have to follow them.

    Gradle provides many configuration options that allow for a good degree of customization. Those options are typically made available through the plugins that provide the conventions. For example, the standard source directory structure for production Java code — src/main/java — is provided by the Java Plugin, which allows you to configure a different source path. Many paths can be modified via properties on the Project object.

  7. Migrate to standard Gradle conventions if you wish

    Once you’re confident that the Gradle build is producing the same artifacts and other resources as the Ant build, you can consider migrating to the standard conventions, such as for source directory paths. Doing so will allow you to remove the extra configuration that was required to override those conventions. New team members will also find it easier to work with the build after the change.

    It’s up to you to decide whether this step is worth the time, energy and potential disruption that it might incur, which in turn depends on your specific build and team.

The rest of the chapter covers some common scenarios you will likely deal with during the migration, such as dependency management and working with Ant tasks.

Working with an imported build

The first step of many migrations will involve importing an Ant build using ant.importBuild(). If you do that, how do you then move towards a standard Gradle build without replacing everything at once?

The important thing to remember is that the Ant targets become real Gradle tasks, meaning you can do things like modify their task dependencies, attach extra task actions, and so on. This allows you to substitute native Gradle tasks for the equivalent Ant ones, maintaining any links to other existing tasks.

As an example, imagine that you have a Java library project that you want to migrate from Ant to Gradle. The Gradle build script has the line that imports the Ant build and now want to use the standard Gradle mechanism for compiling the Java source files. However, you want to keep using the existing package task that creates the library’s JAR file.

In diagrammatic form, the scenario looks like the following, where each box represents a target/task:

ant task migration

The idea is to substitute the standard Gradle compileJava task for the Ant build task. There are several steps involved in this substitution:

  1. Applying the Java Library Plugin

    This provides the compileJava task shown in the diagram.

  2. Renaming the old build task

    The name build conflicts with the standard build task provided by the Base Plugin (via the Java Library Plugin).

  3. Configuring the compilation to use the existing directory structure

    There’s a good chance the Ant build does not conform to the standard Gradle directory structure, so you need to tell Gradle where to find the source files and where to place the compiled classes so package can find them.

  4. Updating task dependencies

    compileJava must depend on prepare, package must depend on compileJava rather than ant_build, and assemble must depend on package rather than the standard Gradle jar task.

Applying the plugin is as simple as inserting a plugins {} block at the beginning of the Gradle build script, i.e. before ant.importBuild(). Here’s how to apply the Java Library Plugin:

Example 11. Applying the Java Library Plugin
build.gradle
plugins {
    id 'java-library'
}
build.gradle.kts
plugins {
    `java-library`
}

To rename the build task, use the variant of AntBuilder.importBuild() that accepts a transformer, like this:

Example 12. Renaming targets on import
build.gradle
ant.importBuild('build.xml') { String oldTargetName ->
    return oldTargetName == 'build' ? 'ant_build' : oldTargetName  (1)
}
build.gradle.kts
ant.importBuild("build.xml") { oldTargetName ->
    if (oldTargetName == "build") "ant_build" else oldTargetName  (1)
}

  1. Renames the build target to ant_build and leaves all other targets unchanged

Configuring a different path for the sources is described in the Building Java & JVM projects chapter, while you can change the output directory for the compiled classes in a similar way.

Let’s say the original Ant build stores these paths in Ant properties, src.dir for the Java source files and classes.dir for the output. Here’s how you would configure Gradle to use those paths:

Example 13. Configuring the source sets
build.gradle
sourceSets {
    main {
        java {
            srcDirs = [ ant.properties['src.dir'] ]
            outputDir = file(ant.properties['classes.dir'])
        }
    }
}
build.gradle.kts
sourceSets {
    main {
        java.setSrcDirs(listOf(ant.properties["src.dir"]))
        java.outputDir = file(ant.properties["classes.dir"] ?: "$buildDir/classes")
    }
}

You should eventually aim to switch the standard directory structure for your type of project if possible and then you’ll be able to remove this customization.

The last step is also straightforward and involves using the Task.dependsOn property and Task.dependsOn() method to detach and link tasks. The property is appropriate for replacing dependencies, while the method is the preferred way to add to the existing dependencies.

Here is the required task dependency configuration required by the example scenario, which should come after the Ant build import:

Example 14. Configuring the task dependencies
build.gradle
compileJava.dependsOn 'prepare'  (1)
package.dependsOn = [ 'compileJava' ]  (2)
assemble.dependsOn = [ 'package' ]  (3)
build.gradle.kts
tasks {
    compileJava {
        dependsOn("prepare")  (1)
    }
    named("package") {
        setDependsOn(listOf(compileJava))  (2)
    }
    assemble {
        setDependsOn(listOf("package"))  (3)
    }
}

  1. Makes compilation depend on the prepare task

  2. Detaches package from the ant_build task and makes it depend on compileJava

  3. Detaches assemble from the standard Gradle jar task and makes it depend on package instead

That’s it! These four steps will successfully replace the old Ant compilation with the Gradle implementation. Even this small migration will be a big help because you’ll be able to take advantage of Gradle’s incremental Java compilation for faster builds.

Tip

This is just a demonstration of how to go about performing a migration in stages. It may make more sense to include resource processing — like with properties files — and packaging with the compilation in this stage, since all three aspects are well integrated in Gradle.

One important question you will have to ask yourself is how many tasks to migrate in each stage. The larger the chunks you can migrate in one go the better, but this must be offset against how many custom steps within the Ant build will be affected by the changes.

For example, if the Ant build follows a fairly standard approach for compilation, static resources, packaging and unit tests, then it is probably worth migrating all those together. But if the build performs some extra processing on the compiled classes, or does something unique when processing the static resources, it is probably worth splitting those tasks into separate stages.

Managing dependencies

Ant builds typically take one of two approaches to dealing with binary dependencies (such as libraries):

  • Storing them with the project in a local "lib" directory

  • Using Apache Ivy to manage them

They each require a different technique for the migration to Gradle, but you will find the process straightforward in either case. We look at the details of each scenario in the following sections.

Serving dependencies from a directory

When you are attempting to migrate a build that stores its dependencies on the filesystem, either locally or on the network, you should consider whether you want to eventually move to managed dependencies using remote repositories. That’s because you can incorporate filesystem dependencies into a Gradle build in one of two ways:

It’s easier to migrate to managed dependencies served from Maven- or Ivy-compatible repositories if you take the first approach, but doing so requires all your files to conform to the naming convention "<moduleName>-<version>.<extension>".

Note

If you store your dependencies in the standard Maven repository layout — <repoDir>/<group>/<module>/<version> — then you can define a custom Maven repository with a "file://" URL.

To demonstrate the two techniques, consider a project that has the following library JARs in its libs directory:

libs
├── our-custom.jar
├── log4j-1.2.8.jar
└── commons-io-2.1.jar

The file our-custom.jar lacks a version number, so it has to be added as a file dependency. But the other two JARs match the required naming convention and so can be declared as normal module dependencies that are retrieved from a flat-directory repository.

The following sample build script demonstrates how you can incorporate all of these libraries into a build:

Example 15. Declaring dependencies served from the filesystem
build.gradle
repositories {
    flatDir {
        name = 'libs dir'
        dir file('libs')  (1)
    }
}

dependencies {
    implementation files('libs/our-custom.jar')  (2)
    implementation ':log4j:1.2.8', ':commons-io:2.1'  (3)
}
build.gradle.kts
repositories {
    flatDir {
        name = "libs dir"
        dir(file("libs"))  (1)
    }
}

dependencies {
    implementation(files("libs/our-custom.jar"))  (2)
    implementation(":log4j:1.2.8")     (3)
    implementation(":commons-io:2.1")  (3)
}

  1. Specifies the path to the directory containing the JAR files

  2. Declares a file dependency for the unversioned JAR

  3. Declares dependencies using standard dependency coordinates — note that no group is specified, but each identifier has a leading :, implying an empty group

The above sample will add our-custom.jar, log4j-1.2.8.jar and commons-io-2.1.jar to the implementation configuration, which is used to compile the project’s code.

Note

You can also specify a group in these module dependencies, even though they don’t actually have a group. That’s because the flat-directory repository simply ignores the information.

If you then add a normal Maven- or Ivy-compatible repository at a later date, Gradle will preferentially download the module dependencies that are declared with a group from that repository rather than the flat-directory one.
Migrating Ivy dependencies

Apache Ivy is a standalone dependency management tool that is widely used with Ant. It works in a similar fashion to Gradle. In fact, they both allow you to

  • Define your own configurations

  • Extend configurations from one another

  • Attach dependencies to configurations

  • Resolve dependencies from Ivy-compatible repositories

  • Publish artifacts to Ivy-compatible repositories

The most notable difference is that Gradle has standard configurations for specific types of projects. For example, the Java Plugin defines configurations like implementation, testImplementation and runtimeOnly. You can still define your own dependency configurations, though.

This similarity means that it’s usually quite straightforward to migrate from Ivy to Gradle:

  • Transcribe the dependency declarations from your module descriptors into the dependencies {} block of your Gradle build script, ideally using the standard configurations provided by any plugins you apply.

  • Transcribe any configuration declarations from your module descriptors into the configurations {} block of the build script for any custom configurations that can’t be replaced by Gradle’s standard ones.

  • Transcribe the resolvers from your Ivy settings file into the repositories {} block of the build script.

See the chapters on Declaring Dependencies, Managing Dependency Configurations and Declaring Repositories for more information.

Ivy provides several Ant tasks that handle Ivy’s process for fetching dependencies. The basic steps of that process consist of:

  1. Configure — applies the configuration defined in the Ivy settings file

  2. Resolve — locates the declared dependencies and downloads them to the cache if necessary

  3. Retrieve — copies the cached dependencies to another directory

Gradle’s process is similar, but you don’t have to explicitly invoke the first two steps as it performs them automatically. The third step doesn’t happen at all — unless you create a task to do it — because Gradle typically uses the files in the dependency cache directly in classpaths and as the source for assembling application packages.

Let’s look in more detail at how Ivy’s steps map to Gradle:

Configuration

Most of Gradle’s dependency-related configuration is baked into the build script, as you’ve seen with elements like the dependencies {} block. Another particularly important configuration element is resolutionStrategy, which can be accessed from dependency configurations. This provides many of the features you might get from Ivy’s conflict managers and is a powerful way to control transitive dependencies and caching.

Some Ivy configuration options have no equivalent in Gradle. For example, there are no lock strategies because Gradle ensures that its dependency cache is concurrency safe, period. Nor are there "latest strategies" because it’s simpler to have a reliable, single strategy for conflict resolution. If the "wrong" version is picked, you can easily override it using forced versions or other resolution strategy options.

See the chapters on Managing Transitive Dependencies and Customizing Dependency Resolution Behavior for more information on this aspect of Gradle.

Resolution

At the beginning of the build, Gradle will automatically resolve any dependencies that you have declared and download them to its cache. It searches the repositories for those dependencies, with the search order defined by the order in which the repositories are declared.

It’s worth noting that Gradle supports the same dynamic version syntax as Ivy, so you can still use versions like 1.0.+. You can also use the special latest.integration and latest.release labels if you wish. If you decide to use such dynamic and changing dependencies, you can configure the caching behavior for them via resolutionStrategy.

You might also want to consider dependency locking if you’re using dynamic and/or changing dependencies. It’s a way to make the build more reliable and allows for reproducible builds.

Retrieval

As mentioned, Gradle does not automatically copy files from the dependency cache. Its standard tasks typically use the files directly. If you want to copy the dependencies to a local directory, you can use a Copy task like this in your build script:

Example 16. Copying dependencies to a local directory
build.gradle
task retrieveRuntimeDependencies(type: Copy) {
    into "$buildDir/libs"
    from configurations.runtimeClasspath
}
build.gradle.kts
tasks {
    register<Copy>("retrieveRuntimeDependencies") {
        into("$buildDir/libs")
        from(configurations.runtimeClasspath)
    }
}

A configuration is also a file collection, hence why it can be used in the from() configuration. You can use a similar technique to attach a configuration to a compilation task or one that produces documentation. See the chapter on Working with Files for more examples and information on Gradle’s file API.

Publishing artifacts

Projects that use Ivy to manage dependencies often also use it for publishing JARs and other artifacts to repositories. If you’re migrating such a build, then you’ll be glad to know that Gradle has built-in support for publishing artifacts to Ivy-compatible repositories.

Before you attempt to migrate this particular aspect of your build, read the Publishing chapter to learn about Gradle’s publishing model. That chapter’s examples are based on Maven repositories, but the same model is used for Ivy repositories as well.

The basic migration process looks like this:

Once that’s all done, you’ll be able to generate an Ivy module descriptor for each publication and publish them to one or more repositories.

Let’s say you have defined a publication named "myLibrary" and a repository named "myRepo". Ivy’s Ant tasks would then map to the Gradle tasks like this:

  • <deliver> → generateDescriptorFileForMyLibraryPublication

  • <publish> → publishMyLibraryPublicationToMyRepoRepository

There is also a convenient publish task that publishes all publications to all repositories. If you’d prefer to limit which publications go to which repositories, check out the relevant section of the Publishing chapter.

Note
On dependency versions

Ivy will, by default, automatically replace dynamic versions of dependencies with the resolved "static" versions when it generates the module descriptor. Gradle does not mimic this behavior: declared dependency versions are left unchanged.

You can replicate the default Ivy behavior by using the Nebula Ivy Resolved Plugin. Alternatively, you can customize the descriptor file so that it contains the versions you want.

Dealing with custom Ant tasks

One of the advantages of Ant is that it’s fairly easy to create a custom task and incorporate it into a build. If you have such tasks, then there are two main options for migrating them to a Gradle build:

The first option is usually quick and easy, but not always. And if you want to integrate the task into incremental build, you must use the incremental build runtime API. You also often have to work with Ant paths and filesets, which are clunky.

The second option is preferable in the long term, if you have the time. Gradle task types tend to be simpler than Ant tasks because they don’t have to work with an XML-based interface. You also gain access to Gradle’s rich APIs. Lastly, this approach can make use of the type-safe incremental build API based on typed properties.

Working with files

Ant has many tasks for working with files, most of which have Gradle equivalents. As with other areas of Ant to Gradle migration, you can use those Ant tasks from within your Gradle build. However, we strongly recommend migrating to native Gradle constructs where possible so that the build benefits from:

  • Incremental build

  • Easier integration with other parts of the build, such as dependency configurations

  • More idiomatic build scripts

That said, it can be convenient to use those Ant tasks that have no direct equivalents, such as <checksum> and <chown>. Even then, in the long run it may be better to convert these to native Gradle task types that make use of standard Java APIs or third-party libraries to achieve the same thing.

Here are the most common file-related elements used by Ant builds, along with the Gradle equivalents:

  • <copy> — prefer the Gradle Copy task type

  • <zip> (plus Java variants) — prefer the Zip task type (plus Jar, War, and Ear)

  • <unzip> — prefer using the Project.zipTree() method with a Copy task

You can see several examples of Gradle’s file API and learn more about it in the Working with Files chapter.

Note
On paths and filesets

Ant makes use of the concepts of path-like structures and filesets to enable users to work with collections of files and directories. Gradle has a simpler, more powerful model based on FileCollections and FileTrees that can be treated as objects from within the build. Both types allow filtering based on Ant’s glob syntax, e.g. **/books_*. Learn more about these types and other aspects of Gradle’s file API in the Working with Files chapter.

You can still construct Ant paths and filesets from within your build via the ant object if you need to interact with an Ant task that requires them. The chapter on Ant integration has examples that use both <path> and <fileset>. There is even a method on FileCollection that will convert a file collection to a fileset or similar Ant type.

Migrating Ant properties

Ant makes use of a properties map to store values that can be reused throughout the build. The big downsides to this approach are that property values are all strings and the properties themselves behave like global variables.

Tip
Interacting with Ant properties in Gradle

Sometimes you will want to make use of an Ant task directly from your Gradle build and that task requires one or more Ant properties to be set. If that’s the case, you can easily set those properties via the ant object, as described in the Using Ant from Gradle chapter.

Gradle does use something similar in the form of project properties, which are a reasonable way to parameterize a build. These can be set from the command line, in a gradle.properties file, or even via specially named system properties and environment variables.

If you have existing Ant properties files, you can copy their contents into the project’s gradle.properties file. Just be aware of two important points:

  • Properties set in gradle.properties do not override extra project properties defined in the build script with the same name

  • Imported Ant tasks will not automatically "see" the Gradle project properties — you must copy them into the Ant properties map for that to happen

Another important factor to understand is that a Gradle build script works with an object-oriented API and it’s often best to use the properties of tasks, source sets and other objects where possible. For example, this build script fragment creates tasks for packaging Javadoc documentation as a JAR and unpacking it, linking tasks via their properties:

Example 17. Using task properties in place of project properties
build.gradle
ext {
    tmpDistDir = file("$buildDir/dist")
}

task javadocJar(type: Jar) {
    from javadoc  (1)
    classifier = 'javadoc'
}

task unpackJavadocs(type: Copy) {
    from zipTree(javadocJar.archivePath)  (2)
    into tmpDistDir  (3)
}
build.gradle.kts
val tmpDistDir by extra { file("$buildDir/dist") }

tasks {
    register<Jar>("javadocJar") {
        from(javadoc)  (1)
        classifier = "javadoc"
    }

    register<Copy>("unpackJavadocs") {
        from(zipTree(named<Jar>("javadocJar").get().archivePath))  (2)
        into(tmpDistDir)  (3)
    }
}

  1. Packages all javadoc's output files — equivalent to from javadoc.destinationDir

  2. Uses the location of the Javadoc JAR held by the javadocJar task

  3. Uses an extra project property called tmpDistDir to define the location of the 'dist' directory

As you can see from the example with tmpDistDir, there is often still a need to define paths and the like through properties, which is why Gradle also provides extra properties that can be attached to the project, tasks and some other types of objects.

Migrating multi-project builds

Multi-project builds are a particular challenge to migrate because there is no standard approach in Ant for either structuring them or handling inter-project dependencies. Most of them likely use the <ant> task in some way, but that’s about all that one can say.

Fortunately, Gradle’s multi-project support can handle fairly diverse project structures and it provides much more robust and helpful support than Ant for constructing and maintaining multi-project builds. The ant.importBuild() method also handles <ant> and <antcall> tasks transparently, which allows for a phased migration.

We will suggest one process for migration here and hope that it either works for your case or at least gives you some ideas. It breaks down like this:

  1. Start by learning how Gradle configures multi-project builds.

  2. Create a Gradle build script in each project of the build, setting their contents to this line:

    ant.importBuild 'build.xml'
    ant.importBuild("build.xml")

    Replace build.xml with the path to the actual Ant build file that corresponds to the project. If there is no corresponding Ant build file, leave the Gradle build script empty. Your build may not be suitable in that case for this migration approach, but continue with these steps to see whether there is still a way to do a phased migration.

  3. Create a settings file that includes all the projects that now have a Gradle build script.

  4. Implement inter-project dependencies.

    Some projects in your multi-project build will depend on artifacts produced by one or more other projects in that build. Such projects need to ensure that those projects they depend on have produced their artifacts and that they know the paths to those artifacts.

    Ensuring the production of the required artifacts typically means calling into other projects' builds via the <ant> task. This unfortunately bypasses the Gradle build, negating any changes you make to the Gradle build scripts. You will need to replace targets that use <ant> tasks with Gradle task dependencies.

    For example, imagine you have a web project that depends on a "util" library that’s part of the same build. The Ant build file for "web" might have a target like this:

    web/build.xml
        <target name="buildRequiredProjects">
        <ant dir="${root.dir}/util" target="build"/>  <!-- <1> -->
    </target>

    1. root.dir would have to be defined by the build

    This can be replaced by an inter-project task dependency in the corresponding Gradle build script, as demonstrated in the following example that assumes the "web" project’s "compile" task is the thing that requires "util" to be built beforehand:

    web/build.gradle
    ant.importBuild 'build.xml'

compile.dependsOn = [ ':util:build' ]
    web/build.gradle.kts
    ant.importBuild("build.xml")

tasks {
    named<Task>("compile") {
        setDependsOn(listOf(":util:build"))
    }
}

    This is not as robust or powerful as Gradle’s project dependencies, but it solves the immediate problem without big changes to the build. Just be careful to remove or override any dependencies on tasks that delegate to other subprojects, like the buildRequiredProjects task.

  5. Identify the projects that have no dependencies on other projects and migrate them to idiomatic Gradle builds scripts.

    Just follow the advice in the rest of this guide to migrate individual project builds. As mentioned elsewhere, you should ideally use Gradle standard plugins where possible. This may mean that you need to add an extra copy task to each build that copies the generated artifacts to the location expected by the rest of the Ant builds.

  6. Migrate projects as and when they depend solely on projects with fully migrated Gradle builds.

    At this point, you should be able to switch to using proper project dependencies attached to the appropriate dependency configurations.

  7. Clean up projects once no part of the Ant build depends on them.

    We mentioned in step 5 that you might need to add copy tasks to satisfy the requirements of dependent Ant builds. Once those builds have been migrated, such build logic will no longer be needed and should be removed.

At the end of the process you should have a Gradle build that you are confident works as it should, with much less build logic than before.

Further reading

This chapter has covered the major topics that are specific to migrating Ant builds to Gradle. All that remain are a few other areas that may be useful during or after a migration:

As a final note, this guide has only touched on a few of Gradle’s features and we encourage you to learn about the rest from the other chapters of the user manual and from our tutorial-style Gradle Guides.

Running Gradle 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 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 Command-line 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.

org.gradle.priority=(low,normal)

Specifies the scheduling priority for the Gradle daemon and all processes launched by it. Default is normal. See also performance command-line options.

The following example demonstrates usage of various properties.

Example 18. Setting properties with a gradle.properties file
gradle.properties
gradlePropertiesProp=gradlePropertiesValue
sysProp=shouldBeOverWrittenBySysProp
systemProp.system=systemValue
build.gradle
task printProps {
    doLast {
        println commandLineProjectProp
        println gradlePropertiesProp
        println systemProjectProp
        println System.properties['system']
    }
}
build.gradle.kts
// Project properties can be accessed via delegation
val commandLineProjectProp: String by project
val gradlePropertiesProp: String by project
val systemProjectProp: String by project

tasks.register("printProps") {
    doLast {
        println(commandLineProjectProp)
        println(gradlePropertiesProp)
        println(systemProjectProp)
        println(System.getProperty("system"))
    }
}
$ gradle -q -PcommandLineProjectProp=commandLineProjectPropValue -Dorg.gradle.project.systemProjectProp=systemPropertyValue printProps
commandLineProjectPropValue
gradlePropertiesValue
systemPropertyValue
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.

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 Authenticated wrapper downloads.

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 JVM arguments to use when starting the Gradle client VM. The client VM only handles command line input/output, so it is rare that one would need to change its VM options. The actual build is run by the Gradle daemon, which is not affected by this environment variable.

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 for the client VM. This VM is also used for the daemon, unless a different one is specified in a Gradle properties file.

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".

Setting a project property via a system property
org.gradle.project.foo=bar
Setting a project property via an environment variable
ORG_GRADLE_PROJECT_foo=bar
Note

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.

Note

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

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

The org.gradle.jvmargs Gradle property controls the VM running the build. It defaults to -Xmx512m "-XX:MaxMetaspaceSize=256m"

Changing JVM settings for the build VM
org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=512m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8

The JAVA_OPTS environment variable controls the command line client, which is only used to display console output. It defaults to -Xmx64m

Changing JVM settings for the client VM
JAVA_OPTS="-Xmx64m -XX:MaxPermSize=64m -XX:+HeapDumpOnOutOfMemoryError -Dfile.encoding=UTF-8"
Note

There is one case where the client VM can also serve as the build VM: If you deactivate the Gradle Daemon and the client VM has the same settings as required for the build VM, the client VM will run the build directly. Otherwise the client VM will fork a new VM to run the actual build in order to honor the different settings.

Certain tasks, like the test task, also fork additional JVM processes. You can configure these through the tasks themselves. They all use -Xmx512m by default.

Example 19. Set Java compile options for JavaCompile tasks
build.gradle
plugins {
    id 'java'
}

tasks.withType(JavaCompile) {
    options.compilerArgs += ['-Xdoclint:none', '-Xlint:none', '-nowarn']
}
build.gradle.kts
plugins {
    java
}

tasks.withType<JavaCompile>().configureEach {
    options.compilerArgs = listOf("-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 20. 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")
        }
    }
}
build.gradle.kts
tasks.register("performRelease") {
    doLast {
        if (project.hasProperty("isCI")) {
            println("Performing release actions")
        } else {
            throw InvalidUserDataException("Cannot perform release outside of CI")
        }
    }
}
$ 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.

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.

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.

The Gradle Daemon

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

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 Daemon 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.

Tip
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

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 8 runtime, but the requested environment calls for Java 10, 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 512MB of heap. It will use the JVM’s default minimum heap size. 512MB 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 that is used by IDEs and other tools to integrate with Gradle always uses the Gradle Daemon to execute builds. If you are executing Gradle builds from within your 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.

Initialization Scripts

Gradle provides a powerful mechanism to allow customizing the build based on the current environment. This mechanism also supports tools that wish to integrate with Gradle.

Note that this is completely different from the “init” task provided by the “build-init” plugin (see Build Init Plugin).

Basic usage

Initialization scripts (a.k.a. init scripts) are similar to other scripts in Gradle. These scripts, however, are run before the build starts. Here are several possible uses:

  • Set up enterprise-wide configuration, such as where to find custom plugins.

  • Set up properties based on the current environment, such as a developer’s machine vs. a continuous integration server.

  • Supply personal information about the user that is required by the build, such as repository or database authentication credentials.

  • Define machine specific details, such as where JDKs are installed.

  • Register build listeners. External tools that wish to listen to Gradle events might find this useful.

  • Register build loggers. You might wish to customize how Gradle logs the events that it generates.

One main limitation of init scripts is that they cannot access classes in the buildSrc project (see Using buildSrc to extract imperative logic for details of this feature).

Using an init script

There are several ways to use an init script:

  • Specify a file on the command line. The command line option is -I or --init-script followed by the path to the script. The command line option can appear more than once, each time adding another init script. The build will fail if any of the files specified on the command line does not exist.

  • Put a file called init.gradle (or init.gradle.kts for Kotlin) in the USER_HOME/.gradle/ directory.

  • Put a file that ends with .gradle (or .init.gradle.kts for Kotlin) in the USER_HOME/.gradle/init.d/ directory.

  • Put a file that ends with .gradle (or .init.gradle.kts for Kotlin) in the GRADLE_HOME/init.d/ directory, in the Gradle distribution. This allows you to package up a custom Gradle distribution containing some custom build logic and plugins. You can combine this with the Gradle wrapper as a way to make custom logic available to all builds in your enterprise.

If more than one init script is found they will all be executed, in the order specified above. Scripts in a given directory are executed in alphabetical order. This allows, for example, a tool to specify an init script on the command line and the user to put one in their home directory for defining the environment and both scripts will run when Gradle is executed.

Writing an init script

Similar to a Gradle build script, an init script is a Groovy or Kotlin script. Each init script has a Gradle instance associated with it. Any property reference and method call in the init script will delegate to this Gradle instance.

Each init script also implements the Script interface.

Configuring projects from an init script

You can use an init script to configure the projects in the build. This works in a similar way to configuring projects in a multi-project build. The following sample shows how to perform extra configuration from an init script before the projects are evaluated. This sample uses this feature to configure an extra repository to be used only for certain environments.

Example 21. Using init script to perform extra configuration before projects are evaluated
build.gradle
repositories {
    mavenCentral()
}

task showRepos {
    doLast {
        println "All repos:"
        println repositories.collect { it.name }
    }
}
init.gradle
allprojects {
    repositories {
        mavenLocal()
    }
}
build.gradle.kts
repositories {
    mavenCentral()
}

tasks.register("showRepos") {
    doLast {
        println("All repos:")
        //TODO:kotlin-dsl remove filter once we're no longer on a kotlin eap
        println(repositories.map { it.name }.filter { it != "maven" })
    }
}
init.gradle.kts
allprojects {
    repositories {
        mavenLocal()
    }
}
Output when applying the init script
> gradle --init-script init.gradle -q showRepos
All repos:
[MavenLocal, MavenRepo]
> gradle --init-script init.gradle.kts -q showRepos
All repos:
[MavenLocal, MavenRepo]

External dependencies for the init script

In External dependencies for the build script it was explained how to add external dependencies to a build script. Init scripts can also declare dependencies. You do this with the initscript() method, passing in a closure which declares the init script classpath.

Example 22. Declaring external dependencies for an init script
init.gradle
initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.apache.commons:commons-math:2.0'
    }
}
init.gradle.kts
initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.apache.commons:commons-math:2.0")
    }
}

The closure passed to the initscript() method configures a ScriptHandler instance. You declare the init script classpath by adding dependencies to the classpath configuration. This is the same way you declare, for example, the Java compilation classpath. You can use any of the dependency types described in Declaring Dependencies, except project dependencies.

Having declared the init script classpath, you can use the classes in your init script as you would any other classes on the classpath. The following example adds to the previous example, and uses classes from the init script classpath.

Example 23. An init script with external dependencies
init.gradle
import org.apache.commons.math.fraction.Fraction

initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath 'org.apache.commons:commons-math:2.0'
    }
}

println Fraction.ONE_FIFTH.multiply(2)
init.gradle.kts
import org.apache.commons.math.fraction.Fraction

initscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.apache.commons:commons-math:2.0")
    }
}

println(Fraction.ONE_FIFTH.multiply(2))

Output when applying the init script

> gradle --init-script init.gradle -q doNothing
2 / 5
> gradle --init-script init.gradle.kts -q doNothing
2 / 5

Init script plugins

Similar to a Gradle build script or a Gradle settings file, plugins can be applied on init scripts.

Example 24. Using plugins in init scripts
init.gradle
apply plugin: EnterpriseRepositoryPlugin

class EnterpriseRepositoryPlugin implements Plugin<Gradle> {

    private static String ENTERPRISE_REPOSITORY_URL = "https://repo.gradle.org/gradle/repo"

    void apply(Gradle gradle) {
        // ONLY USE ENTERPRISE REPO FOR DEPENDENCIES
        gradle.allprojects { project ->
            project.repositories {

                // Remove all repositories not pointing to the enterprise repository url
                all { ArtifactRepository repo ->
                    if (!(repo instanceof MavenArtifactRepository) ||
                          repo.url.toString() != ENTERPRISE_REPOSITORY_URL) {
                        project.logger.lifecycle "Repository ${repo.url} removed. Only $ENTERPRISE_REPOSITORY_URL is allowed"
                        remove repo
                    }
                }

                // add the enterprise repository
                maven {
                    name "STANDARD_ENTERPRISE_REPO"
                    url ENTERPRISE_REPOSITORY_URL
                }
            }
        }
    }
}
build.gradle
/*
 * Copyright 2013 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

// tag::show-repos-task[]
repositories{
    mavenCentral()
}

 task showRepositories {
     doLast {
         repositories.each {
             println "repository: ${it.name} ('${it.url}')"
         }
     }
}
// end::show-repos-task[]
init.gradle.kts
apply<EnterpriseRepositoryPlugin>()

class EnterpriseRepositoryPlugin : Plugin<Gradle> {
    companion object {
        const val ENTERPRISE_REPOSITORY_URL = "https://repo.gradle.org/gradle/repo"
    }

    override fun apply(gradle: Gradle) {
        // ONLY USE ENTERPRISE REPO FOR DEPENDENCIES
        gradle.allprojects {
            repositories {

                // Remove all repositories not pointing to the enterprise repository url
                all {
                    if (this !is MavenArtifactRepository || url.toString() != ENTERPRISE_REPOSITORY_URL) {
                        project.logger.lifecycle("Repository ${(this as? MavenArtifactRepository)?.url ?: name} removed. Only $ENTERPRISE_REPOSITORY_URL is allowed")
                        remove(this)
                    }
                }

                // add the enterprise repository
                add(maven {
                    name = "STANDARD_ENTERPRISE_REPO"
                    url = uri(ENTERPRISE_REPOSITORY_URL)
                })
            }
        }
    }
}
build.gradle.kts
// tag::show-repos-task[]
repositories{
    mavenCentral()
}

tasks.register("showRepositories") {
    doLast {
        repositories.map { it as MavenArtifactRepository }.forEach {
            println("repository: ${it.name} ('${it.url}')")
        }
    }
}
// end::show-repos-task[]

Output when applying the init script

> gradle --init-script init.gradle -q showRepositories
repository: STANDARD_ENTERPRISE_REPO ('https://repo.gradle.org/gradle/repo')
> gradle --init-script init.gradle.kts -q showRepositories
repository: STANDARD_ENTERPRISE_REPO ('https://repo.gradle.org/gradle/repo')

The plugin in the init script ensures that only a specified repository is used when running the build.

When applying plugins within the init script, Gradle instantiates the plugin and calls the plugin instance’s Plugin.apply(T) method. The gradle object is passed as a parameter, which can be used to configure all aspects of a build. Of course, the applied plugin can be resolved as an external dependency as described in External dependencies for the init script

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: 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.

Build Cache

Note
 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.

Tip
 For a hands-on approach to learning how to use the build cache, try the Using the Build Cache guide. It covers the different scenarios that caching can improve and has 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 Configure the Build Cache.

Task Output Caching

Beyond incremental builds described in up-to-date checks, 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 load 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 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 store new outputs in the build cache. If the previous build outputs have been already stored in the cache 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.)

Note

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.

Enable caching of non-cacheable tasks

As we have seen, built-in tasks, or tasks provided by plugins, are cacheable if their class is annotated with the Cacheable annotation. But what if you want to make cacheable a task whose class is not cacheable? Let’s take a concrete example: your build script uses a generic NpmTask task to create a JavaScript bundle by delegating to NPM (and running npm run bundle). This process is similar to a complex compilation task, but NpmTask is too generic to be cacheable by default: it just takes arguments and runs npm with those arguments.

The inputs and outputs of this task are simple to figure out. The inputs are the directory containing the JavaScript files, and the NPM configuration files. The output is the bundle file generated by this task.

Using annotations

We create a subclass of the NpmTask and use annotations to declare the inputs and outputs.

When possible, it is better to use delegation instead of creating a subclass. That is the case for the built in JavaExec, Exec, Copy and Sync tasks, which have a method on Project to do the actual work.

If you’re a modern JavaScript developer, you know that bundling can be quite long, and is worth caching. To achieve that, we need to tell Gradle that it’s allowed to cache the output of that task, using the @CacheableTask annotation.

This is sufficient to make the task cacheable on your own machine. However, input files are identified by default by their absolute path. So if the cache needs to be shared between several developers or machines using different paths, that won’t work as expected. So we also need to set the path sensitivity. In this case, the relative path of the input files can be used to identify them.

Note that it is possible to override property annotations from the base class by overriding the getter of the base class and annotating that method.

Example 25. Custom cacheable BundleTask
build.gradle
@CacheableTask                                       (1)
class BundleTask extends NpmTask {

    @Override @Internal                              (2)
    ListProperty<String> getArgs() {
        super.getArgs()
    }

    @InputDirectory
    @SkipWhenEmpty
    @PathSensitive(PathSensitivity.RELATIVE)         (3)
    final DirectoryProperty scripts = project.objects.directoryProperty()

    @InputFiles
    @PathSensitive(PathSensitivity.RELATIVE)         (4)
    final ConfigurableFileCollection configFiles = project.files()

    @OutputFile
    final RegularFileProperty bundle = project.objects.fileProperty()

    BundleTask() {
        args.addAll("run", "bundle")
        bundle.set(project.layout.buildDirectory.file("bundle.js"))
        scripts.set(project.layout.projectDirectory.dir("scripts"))
        configFiles.from(project.layout.projectDirectory.file("package.json"))
        configFiles.from(project.layout.projectDirectory.file("package-lock.json"))
    }
}

task bundle(type: BundleTask)
build.gradle.kts
@CacheableTask                                       (1)
open class BundleTask : NpmTask() {

    @get:Internal                                    (2)
    override val args
        get() = super.args


    @get:InputDirectory
    @get:SkipWhenEmpty
    @get:PathSensitive(PathSensitivity.RELATIVE)     (3)
    val scripts: DirectoryProperty = project.objects.directoryProperty()

    @get:InputFiles
    @get:PathSensitive(PathSensitivity.RELATIVE)     (4)
    val configFiles: ConfigurableFileCollection = project.files()

    @get:OutputFile
    val bundle: RegularFileProperty = project.objects.fileProperty()

    init {
        args.addAll("run", "bundle")
        bundle.set(project.layout.buildDirectory.file("bundle.js"))
        scripts.set(project.layout.projectDirectory.dir("scripts"))
        configFiles.from(project.layout.projectDirectory.file("package.json"))
        configFiles.from(project.layout.projectDirectory.file("package-lock.json"))
    }
}

tasks.register<BundleTask>("bundle")

  • (1) Add @Cacheable to enable caching for the task.

  • (2) Override the getter of a property of the base class to change the input annotation to @Internal.

  • (3) (4) Declare the path sensitivity.

Using the runtime API

If for some reason you cannot create a new custom task class, it is also possible to make a task cacheable using the runtime API to declare the inputs and outputs.

For enabling caching for the task you need to use the TaskOutputs.cacheIf() method.

The declarations via the runtime API have the same effect as the annotations described above. Note that you cannot override file inputs and outputs via the runtime API. Input properties can be overridden by specifying the same property name.

Example 26. Make the bundle task cacheable
build.gradle
task bundle(type: NpmTask) {
    args = ['run', 'bundle']

    outputs.cacheIf { true }

    inputs.dir(file("scripts"))
        .withPropertyName("scripts")
        .withPathSensitivity(PathSensitivity.RELATIVE)

    inputs.files("package.json", "package-lock.json")
        .withPropertyName("configFiles")
        .withPathSensitivity(PathSensitivity.RELATIVE)

    outputs.file("$buildDir/bundle.js")
        .withPropertyName("bundle")
}
build.gradle.kts
tasks.register<NpmTask>("bundle") {
    args.set(listOf("run", "bundle"))

    outputs.cacheIf { true }

    inputs.dir(file("scripts"))
        .withPropertyName("scripts")
        .withPathSensitivity(PathSensitivity.RELATIVE)

    inputs.files("package.json", "package-lock.json")
        .withPropertyName("configFiles")
        .withPathSensitivity(PathSensitivity.RELATIVE)

    outputs.file("$buildDir/bundle.js")
        .withPropertyName("bundle")
}

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 stores ("pushes") build outputs in 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 27. Configure the local cache
settings.gradle
buildCache {
    local(DirectoryBuildCache) {
        directory = new File(rootDir, 'build-cache')
        removeUnusedEntriesAfterDays = 30
    }
}
settings.gradle.kts
buildCache {
    local<DirectoryBuildCache> {
        directory = 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 28. Load from HttpBuildCache
settings.gradle
buildCache {
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
    }
}
settings.gradle.kts
buildCache {
    remote<HttpBuildCache> {
        url = uri("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 29. Configure remote HTTP cache
settings.gradle
buildCache {
    remote(HttpBuildCache) {
        url = 'http://example.com:8123/cache/'
        credentials {
            username = 'build-cache-user'
            password = 'some-complicated-password'
        }
    }
}
settings.gradle.kts
buildCache {
    remote<HttpBuildCache> {
        url = uri("http://example.com:8123/cache/")
        credentials {
            username = "build-cache-user"
            password = "some-complicated-password"
        }
    }
}
Note

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.

This is a convenient workaround, but you shouldn’t use it as a long-term solution.
Example 30. Allow untrusted cache server
settings.gradle
buildCache {
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
        allowUntrustedServer = true
    }
}
settings.gradle.kts
buildCache {
    remote<HttpBuildCache> {
        url = uri("https://example.com:8123/cache/")
        isAllowUntrustedServer = true
    }
}
Configuration use cases

The recommended use case for the build cache is that your continuous integration server populates the remote build cache from clean builds while developers load from the remote build cache and store in the local build cache. The configuration would then look as follows.

Example 31. Recommended setup for CI push use case
settings.gradle
boolean isCiServer = System.getenv().containsKey("CI")

buildCache {
    local {
        enabled = !isCiServer
    }
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
        push = isCiServer
    }
}
settings.gradle.kts
val isCiServer = System.getenv().containsKey("CI")

buildCache {
    local {
        isEnabled = !isCiServer
    }
    remote<HttpBuildCache> {
        url = uri("https://example.com:8123/cache/")
        isPush = 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 32. Consistent setup for buildSrc and main build
settings.gradle
apply from: new File(settingsDir, 'gradle/buildCacheSettings.gradle')
gradle/buildCacheSettings.gradle
boolean isCiServer = System.getenv().containsKey("CI")

buildCache {
    local {
        enabled = !isCiServer
    }
    remote(HttpBuildCache) {
        url = 'https://example.com:8123/cache/'
        push = isCiServer
    }
}
buildSrc/settings.gradle
apply from: new File(settingsDir, '../gradle/buildCacheSettings.gradle')
settings.gradle.kts
apply(from = File(settingsDir, "gradle/buildCacheSettings.gradle.kts"))
gradle/buildCacheSettings.gradle.kts
val isCiServer = System.getenv().containsKey("CI")

buildCache {
    local {
        isEnabled = !isCiServer
    }
    remote<HttpBuildCache> {
        url = uri("https://example.com:8123/cache/")
        isPush = isCiServer
    }
}
buildSrc/settings.gradle.kts
apply(from = File(settingsDir, "../gradle/buildCacheSettings.gradle.kts"))

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 33. 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
    }
}
init.gradle.kts
gradle.settingsEvaluated {
    buildCache {
        // vvv Your custom configuration goes here
        remote<HttpBuildCache> {
            url = uri("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).

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

Composite builds

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 Declaring substitutions.

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 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 34. Dependencies of my-app
my-app/build.gradle
plugins {
    id 'java'
    id 'application'
    id 'idea'
}

group "org.sample"
version "1.0"

application {
    mainClassName = "org.sample.myapp.Main"
}

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

repositories {
    jcenter()
}
my-app/build.gradle.kts
plugins {
    java
    application
    idea
}

group = "org.sample"
version = "1.0"

application {
    mainClassName = "org.sample.myapp.Main"
}

dependencies {
    implementation("org.sample:number-utils:1.0")
    implementation("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: 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 the settings file

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 (or settings.gradle.kts in Kotlin) file. The settings 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 35. Declaring a separate composite
composite/settings.gradle
rootProject.name = 'adhoc'

includeBuild '../my-app'
includeBuild '../my-utils'
composite/settings.gradle.kts
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 36. Depending on task from included build
composite/build.gradle
task run {
    dependsOn gradle.includedBuild('my-app').task(':run')
}
composite/build.gradle.kts
tasks.register("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, including other composite builds. However there are some limitations.

Every included 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.

Note

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 'anonymous-library', that produces a java utility library but does not declare a value for the group attribute:

Example 37. Build that does not declare group attribute
build.gradle
plugins {
    id 'java'
}
build.gradle.kts
plugins {
    java
}

When this build is included in a composite, it will attempt to substitute for the dependency module "undefined:anonymous-library" ("undefined" being the default value for project.group, and "anonymous-library" 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 38. Declaring the substitutions for an included build
settings.gradle
rootProject.name = 'app'

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

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 39. Depending on a single task from an included build
composite/build.gradle
task run {
    dependsOn gradle.includedBuild('my-app').task(':run')
}
composite/build.gradle.kts
tasks.register("run") {
    dependsOn(gradle.includedBuild("my-app").task(":run"))
}
Example 40. Depending on a task with path in all included builds
build.gradle
task publishDeps {
    dependsOn gradle.includedBuilds*.task(':uploadArchives')
}
build.gradle.kts
tasks.register("publishDeps") {
    dependsOn(gradle.includedBuilds.map { it.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:

  • No support for included builds that have publications that don’t mirror the project default configuration. See Cases where composite builds won’t work.

  • Software model based native builds are not supported. (Binary dependencies are not yet supported for native builds).

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.

Authoring Gradle Builds

Build Script Basics

This chapter introduces you to the basics of writing Gradle build scripts. For a quick hands-on introduction, try the Creating New Gradle Builds guide.

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.

You run a Gradle build using the gradle command. The gradle command looks for a file called build.gradle.kts in the current directory.[3] We call this build.gradle.kts 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.kts.

Example 41. Your first build script
build.gradle
task hello {
    doLast {
        println 'Hello world!'
    }
}
build.gradle.kts
tasks.register("hello") {
    doLast {
        println("Hello world!")
    }
}

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

Tip
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 42. 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 block containing some 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.

Build scripts are code

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

Example 43. Using Groovy or Kotlin in Gradle’s tasks
build.gradle
task upper {
    doLast {
        String someString = 'mY_nAmE'
        println "Original: $someString"
        println "Upper case: ${someString.toUpperCase()}"
    }
}
build.gradle.kts
tasks.register("upper") {
    doLast {
        val 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 44. Using Groovy or Kotlin in Gradle’s tasks
build.gradle
task count {
    doLast {
        4.times { print "$it " }
    }
}
build.gradle.kts
tasks.register("count") {
    doLast {
        repeat(4) { print("$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 45. 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"
    }
}
build.gradle.kts
tasks.register("hello") {
    doLast {
        println("Hello world!")
    }
}
tasks.register("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 46. Lazy dependsOn - the other task does not exist (yet)
build.gradle
task taskX {
    dependsOn 'taskY'
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
build.gradle.kts
tasks.register("taskX") {
    dependsOn("taskY")
    doLast {
        println("taskX")
    }
}
tasks.register("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 Adding dependencies to a task.

Please notice that you can’t use shortcut notation when referring to a task that is not yet defined.

Dynamic tasks

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

Example 47. Dynamic creation of a task
build.gradle
4.times { counter ->
    task "task$counter" {
        doLast {
            println "I'm task number $counter"
        }
    }
}
build.gradle.kts
repeat(4) { counter ->
    tasks.register("task$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 48. Accessing a task via API - adding a dependency
build.gradle
4.times { counter ->
    task "task$counter" {
        doLast {
            println "I'm task number $counter"
        }
    }
}
task0.dependsOn task2, task3
build.gradle.kts
repeat(4) { counter ->
    tasks.register("task$counter") {
        doLast {
            println("I'm task number $counter")
        }
    }
}
tasks.named("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 49. Accessing a task via API - adding behaviour
build.gradle
task hello {
    doLast {
        println 'Hello Earth'
    }
}
hello.doFirst {
    println 'Hello Venus'
}
hello.configure {
    doLast {
        println 'Hello Mars'
    }
}
hello.configure {
    doLast {
        println 'Hello Jupiter'
    }
}
build.gradle.kts
val hello by tasks.registering {
    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.

Groovy DSL shortcut notations

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

Example 50. 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 51. Adding extra properties to a task
build.gradle
task myTask {
    ext.myProperty = "myValue"
}

task printTaskProperties {
    doLast {
        println myTask.myProperty
    }
}
build.gradle.kts
tasks.register("myTask") {
    extra["myProperty"] = "myValue"
}

tasks.register("printTaskProperties") {
    doLast {
        println(tasks["myTask"].extra["myProperty"])
    }
}
Output of gradle -q printTaskProperties
> gradle -q printTaskProperties
myValue

Extra properties aren’t limited to tasks. You can read more about them in 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. And it is usable from Kotlin too. From the example below, you can learn how to execute Ant tasks and how to access Ant properties:

Example 52. 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]}"
            }
        }
    }
}
build.gradle.kts
tasks.register("loadfile") {
    doLast {
        val files = file("./antLoadfileResources").listFiles().sorted()
        files.forEach { file ->
            if (file.isFile) {
                ant.withGroovyBuilder {
                    "loadfile"("srcFile" to file, "property" to 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 Ant.

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 53. 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()
}
build.gradle.kts
tasks.register("checksum") {
    doLast {
        fileList("./antLoadfileResources").forEach { file ->
            ant.withGroovyBuilder {
                "checksum"("file" to file, "property" to "cs_${file.name}")
            }
            println("$file.name Checksum: ${ant.properties["cs_${file.name}"]}")
        }
    }
}

tasks.register("loadfile") {
    doLast {
        fileList("./antLoadfileResources").forEach { file ->
            ant.withGroovyBuilder {
                "loadfile"("srcFile" to file, "property" to file.name)
            }
            println("I'm fond of ${file.name}")
        }
    }
}

fun fileList(dir: String): List<File> =
    file(dir).listFiles { file: File -> file.isFile }.sorted()
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 Gradle Projects.

Default tasks

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

Example 54. 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!"
    }
}
build.gradle.kts
defaultTasks("clean", "run")

task("clean") {
    doLast {
        println("Default Cleaning!")
    }
}

tasks.register("run") {
    doLast {
        println("Default Running!")
    }
}

tasks.register("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 55. 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'
    }
}
build.gradle.kts
tasks.register("distribution") {
    doLast {
        println("We build the zip with version=$version")
    }
}

tasks.register("release") {
    dependsOn("distribution")
    doLast {
        println("We release now")
    }
}

gradle.taskGraph.whenReady {
    version =
        if (hasTask(":release")) "1.0"
        else "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).

External dependencies for the build script

If your build script needs to use external libraries, you can add them to the script’s classpath in the build script itself. You do this using the buildscript() method, passing in a block which declares the build script classpath.

Example 56. Declaring external dependencies for the build script
build.gradle
buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath group: 'commons-codec', name: 'commons-codec', version: '1.2'
    }
}
build.gradle.kts
buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        "classpath"(group = "commons-codec", name = "commons-codec", version = "1.2")
    }
}

The block passed to the buildscript() method configures a ScriptHandler instance. You declare the build script classpath by adding dependencies to the classpath configuration. This is the same way you declare, for example, the Java compilation classpath. You can use any of the dependency types except project dependencies.

Having declared the build script classpath, you can use the classes in your build script as you would any other classes on the classpath. The following example adds to the previous example, and uses classes from the build script classpath.

Example 57. A build script with external dependencies
build.gradle
import org.apache.commons.codec.binary.Base64

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath group: 'commons-codec', name: 'commons-codec', version: '1.2'
    }
}

task encode {
    doLast {
        def byte[] encodedString = new Base64().encode('hello world\n'.getBytes())
        println new String(encodedString)
    }
}
build.gradle.kts
import org.apache.commons.codec.binary.Base64

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        "classpath"(group = "commons-codec", name = "commons-codec", version = "1.2")
    }
}

tasks.register("encode") {
    doLast {
        val encodedString = Base64().encode("hello world\n".toByteArray())
        println(String(encodedString))
    }
}
Output of gradle -q encode
> gradle -q encode
aGVsbG8gd29ybGQK

For multi-project builds, the dependencies declared with a project’s buildscript() method are available to the build scripts of all its sub-projects.

Build script dependencies may be Gradle plugins. Please consult Using Gradle Plugins for more information on Gradle plugins.

Every project automatically has a buildEnvironment task of type BuildEnvironmentReportTask that can be invoked to report on the resolution of the build script dependencies.

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 More About Tasks.

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

Authoring Tasks

In the introductory tutorial 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. 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.

(no label) or EXECUTED

Task executed its actions.

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

  • Task has no actions and some dependencies, and any of the dependencies are executed. See also Lifecycle Tasks.

UP-TO-DATE

Task’s outputs did not change.

  • Task has outputs and inputs and they have not changed. See Incremental Builds.

  • Task has actions, but the task tells Gradle it did not change its outputs.

  • Task has no actions and some dependencies, but all of the dependencies are up-to-date, skipped or from cache. See also Lifecycle Tasks.

  • Task has no actions and no dependencies.

FROM-CACHE

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

  • 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.

  • 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 strings for task names in this chapter. There are a few variations on this style, which you may need to use in certain situations.

Note

The task configuration APIs are described in more detail in the task configuration avoidance chapter.
Example 58. Defining tasks using strings for task names
build.gradle
task('hello') {
    doLast {
        println "hello"
    }
}

task('copy', type: Copy) {
    from(file('srcDir'))
    into(buildDir)
}
build.gradle.kts
tasks.register("hello") {
    doLast {
        println("hello")
    }
}

tasks.register<Copy>("copy") {
    from(file("srcDir"))
    into(buildDir)
}

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

Example 59. Defining tasks using the tasks container
build.gradle
tasks.create('hello') {
    doLast {
        println "hello"
    }
}

tasks.create('copy', Copy) {
    from(file('srcDir'))
    into(buildDir)
}
build.gradle.kts
tasks.register("hello") {
    doLast {
        println("hello")
    }
}

tasks {
    register<Copy>("copy") {
        from(file("srcDir"))
        into(buildDir)
    }
}

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

And finally, there are language specific syntaxes for the Groovy and Kotlin DSL:

Example 60. Defining tasks using a DSL specific syntax
build.gradle
// Using Groovy dynamic keywords

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

task(copy, type: Copy) {
    from(file('srcDir'))
    into(buildDir)
}
build.gradle.kts
// Using Kotlin delegated properties

val hello by tasks.registering {
    doLast {
        println("hello")
    }
}

val copy by tasks.registering(Copy::class) {
    from(file("srcDir"))
    into(buildDir)
}

Note that the Kotlin delegated properties syntax is particularly useful if you need the created task for further reference.

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, just like with defining tasks there are language specific syntaxes for the Groovy and Kotlin DSL:

Example 61. Accessing tasks using a DSL specific syntax
build.gradle
task hello
task copy(type: Copy)

// Access tasks using Groovy dynamic properties on Project

println hello.name
println project.hello.name

println copy.destinationDir
println project.copy.destinationDir
build.gradle.kts
task("hello")
task<Copy>("copy")

// Access tasks using Kotlin delegated properties

val hello by tasks.getting
println(hello.name)

val copy by tasks.getting(Copy::class)
println(copy.destinationDir)

Tasks are also available through the tasks collection.

Example 62. Accessing tasks via tasks collection
build.gradle
task hello
task copy(type: Copy)

println tasks.hello.name
println tasks.named('hello').get().name

println tasks.copy.destinationDir
println tasks.named('copy').get().destinationDir
build.gradle.kts
tasks.register("hello")
tasks.register<Copy>("copy")

println(tasks["hello"].name)
println(tasks.named("hello").get().name)

println(tasks.getByName<Copy>("copy").destinationDir)
println(tasks.named<Copy>("copy").get().destinationDir)

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 63. 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
build.gradle.kts
project(":projectA") {
    tasks.register("hello")
}

tasks.register("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 64. Creating a copy task
build.gradle
task myCopy(type: Copy)
build.gradle.kts
tasks.register<Copy>("myCopy")

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 type “Copy”. 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 65. Configuring a task using the API
build.gradle
Copy myCopy = tasks.getByName("myCopy")
myCopy.from 'resources'
myCopy.into 'target'
myCopy.include('**/*.txt', '**/*.xml', '**/*.properties')
build.gradle.kts
val myCopy = tasks.named<Copy>("myCopy")
myCopy {
    from("resources")
    into("target")
    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 66. Configuring a task using a DSL specific syntax
build.gradle
// Configure task using Groovy dynamic task configuration block
myCopy {
   from 'resources'
   into 'target'
}
myCopy.include('**/*.txt', '**/*.xml', '**/*.properties')
build.gradle.kts
// Configure task using Kotlin delegated properties and a lambda
val myCopy by tasks.existing(Copy::class) {
    from("resources")
    into("target")
}
myCopy { include("**/*.txt", "**/*.xml", "**/*.properties") }

This works for any task. Task access is just a shortcut for the tasks.named() method. It is important to note that if you pass a block to the named() method, this block is applied to configure the task, not when the task executes.

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

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

Example 67. Defining a task with a configuration block
build.gradle
task copy(type: Copy) {
   from 'resources'
   into 'target'
   include('**/*.txt', '**/*.xml', '**/*.properties')
}
build.gradle.kts
tasks.register<Copy>("copy") {
   from("resources")
   into("target")
   include("**/*.txt", "**/*.xml", "**/*.properties")
}
Tip
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.

Passing arguments to a task constructor

As opposed to configuring the mutable properties of a Task after creation, you can pass argument values to the Task class’s constructor. In order to pass values to the Task constructor, you must annotate the relevant constructor with @javax.inject.Inject.

Example 68. Task class with @Inject constructor
build.gradle
class CustomTask extends DefaultTask {
    final String message
    final int number

    @Inject
    CustomTask(String message, int number) {
        this.message = message
        this.number = number
    }
}
build.gradle.kts
open class CustomTask @Inject constructor(
    private val message: String,
    private val number: Int
) : DefaultTask()

You can then create a task, passing the constructor arguments at the end of the parameter list.

Example 69. Creating a task with constructor arguments using TaskContainer
build.gradle
tasks.create('myTask', CustomTask, 'hello', 42)
build.gradle.kts
tasks.register<CustomTask>("myTask", "hello", 42)

You can also create the task using a constructorArgs Map argument using the Project API:

Example 70. Creating a task with constructor arguments using Map
build.gradle
task myTask(type: CustomTask, constructorArgs: ['hello', 42])
build.gradle.kts
task("myTask", "type" to CustomTask::class.java, "constructorArgs" to listOf("hello", 42))
Note
Prefer creating a task with constructor arguments using the TaskContainer

It’s recommended to use the Task Configuration Avoidance APIs to improve configuration time.

In all circumstances, the values passed as constructor arguments must be non-null. If you attempt to pass a null value, Gradle will throw a NullPointerException indicating which runtime value is null.

Adding dependencies to a task

There are several ways you can define the dependencies of a task. In 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 71. 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'
        }
    }
}
build.gradle.kts
project("projectA") {
    tasks.register("taskX") {
        dependsOn(":projectB:taskY")
        doLast {
            println("taskX")
        }
    }
}

project("projectB") {
    tasks.register("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 72. Adding dependency using task object
build.gradle
task taskX {
    doLast {
        println 'taskX'
    }
}

task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.dependsOn taskY
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
    }
}

val taskY by tasks.registering {
    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 lazy block. When evaluated, the block is passed the task whose dependencies are being calculated. The lazy block 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 73. Adding dependency using a lazy block
build.gradle
task taskX {
    doLast {
        println 'taskX'
    }
}

// Using a Groovy Closure
taskX.dependsOn {
    tasks.findAll { task -> task.name.startsWith('lib') }
}

task lib1 {
    doLast {
        println 'lib1'
    }
}

task lib2 {
    doLast {
        println 'lib2'
    }
}

task notALib {
    doLast {
        println 'notALib'
    }
}
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
    }
}

// Using a Gradle Provider
taskX {
    dependsOn(provider {
        tasks.filter { task -> task.name.startsWith("lib") }
    })
}

tasks.register("lib1") {
    doLast {
        println("lib1")
    }
}

tasks.register("lib2") {
    doLast {
        println("lib2")
    }
}

tasks.register("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

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 74. Adding a 'must run after' task ordering
build.gradle
task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
taskY.mustRunAfter taskX
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
    }
}
val taskY by tasks.registering {
    doLast {
        println("taskY")
    }
}
taskY { mustRunAfter(taskX) }
Output of gradle -q taskY taskX
> gradle -q taskY taskX
taskX
taskY
Example 75. Adding a 'should run after' task ordering
build.gradle
task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
taskY.shouldRunAfter taskX
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
    }
}
val taskY by tasks.registering {
    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 76. 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 77. 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
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
    }
}
val taskY by tasks.registering {
    doLast {
        println("taskY")
    }
}
val taskZ by tasks.registering {
    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 78. 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')
}
build.gradle.kts
tasks.register<Copy>("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 79. Overwriting a task
build.gradle
task copy(type: Copy)

task copy(overwrite: true) {
    doLast {
        println('I am the new one.')
    }
}
build.gradle.kts
tasks.register<Copy>("copy")

task("copy", "overwrite" to true).apply {
    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 80. Skipping a task using a predicate
build.gradle
task hello {
    doLast {
        println 'hello world'
    }
}

hello.onlyIf { !project.hasProperty('skipHello') }
build.gradle.kts
val hello by tasks.registering {
    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 81. 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'
    }
}
build.gradle.kts
val compile by tasks.registering {
    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 StopExecutionException()
        }
    }
}
tasks.register("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 82. Enabling and disabling tasks
build.gradle
task disableMe {
    doLast {
        println 'This should not be printed if the task is disabled.'
    }
}
disableMe.enabled = false
build.gradle.kts
val disableMe by tasks.registering {
    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
Task timeouts

Every task has a timeout property which can be used to limit its execution time. When a task reaches its timeout, its task execution thread is interrupted. The task will be marked as failed. Finalizer tasks will still be run. If --continue is used, other tasks can continue running after it. Tasks that don’t respond to interrupts can’t be timed out. All of Gradle’s built-in tasks respond to timeouts in a timely manner.

Example 83. Specifying task timeouts
build.gradle
task hangingTask() {
    doLast {
        Thread.sleep(100000)
    }
    timeout = Duration.ofMillis(500)
}
build.gradle.kts
import java.time.Duration

tasks {
    register("hangingTask") {
        doLast {
            Thread.sleep(100000)
        }
        timeout.set(Duration.ofMillis(500))
    }
}

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 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.

taskInputsOutputs
Figure 6. 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

Note

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:

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 84. 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 (run again)
> 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 1. 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 Using the classpath annotations.

Note: 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 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.

Note

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 or map of output files. Using a file tree turns caching off for the task.

@OutputDirectories

Map<String, File>** or Iterable<File>*

An iterable of output directories. Using a file tree turns caching off for the task.

@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.

@SkipWhenEmpty

File*

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

Any type

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

File*

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.
Note
*

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…​), ProjectLayout.files(java.lang.Object...), or ProjectLayout.configurableFiles(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.

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 Java 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:

JacocoAgent.java
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 an equivalent for @Nested.

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 85. 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
    }
}
build.gradle.kts
tasks.register("processTemplatesAdHoc") {
    inputs.property("engine", TemplateEngineType.FREEMARKER)
    inputs.files(fileTree("src/templates"))
    inputs.property("templateData.name", "docs")
    inputs.property("templateData.variables", mapOf("year" to "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 files that a task removes can be specified through destroyables.register().

Example 86. Ad-hoc task declaring a destroyable
build.gradle
task removeTempDir {
    destroyables.register("$projectDir/tmpDir")
    doLast {
        delete("$projectDir/tmpDir")
    }
}
build.gradle.kts
tasks.register("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 87. 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)
}
build.gradle.kts
tasks.register<ProcessTemplatesNoAnnotations>("processTemplatesRuntime") {
    templateEngine = TemplateEngineType.FREEMARKER
    sourceFiles = fileTree("src/templates")
    templateData = TemplateData("test", mapOf("year" to "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 (run again)
> 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 88. Using skipWhenEmpty() via the runtime API
build.gradle
task processTemplatesRuntimeConf(type: ProcessTemplatesNoAnnotations) {
    // ...
    sourceFiles = fileTree("src/templates") {
        include "**/*.fm"
    }

    inputs.files(sourceFiles).skipWhenEmpty()
    // ...
}
build.gradle.kts
tasks.register<ProcessTemplatesNoAnnotations>("processTemplatesRuntimeConf") {
    // ...
    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.

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 89. Inferred task dependency via task outputs
build.gradle
task packageFiles(type: Zip) {
    from processTemplates.outputs
}
build.gradle.kts
tasks.register<Zip>("packageFiles") {
    from(processTemplates.get().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 90. Inferred task dependency via a task argument
build.gradle
task packageFiles2(type: Zip) {
    from processTemplates
}
build.gradle.kts
tasks.register<Zip>("packageFiles2") {
    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 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 fingerprint of the inputs. This fingerprint 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 fingerprint of the outputs. This fingerprint contains the set of output files and a hash of the contents of each file. Gradle persists both fingerprints for the next time the task is executed.

Each time after that, before the task is executed, Gradle takes a new fingerprint of the inputs and outputs. If the new fingerprints are the same as the previous fingerprints, 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 fingerprints 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 fingerprint 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 Task output caching.

Note

For tracking the implementation of tasks, task actions and nested inputs, Gradle uses the class name and an identifier for the classpath which contains the implementation. There are some situations when Gradle is not able to track the implementation precisely:

Unknown classloader

When the classloader which loaded the implementation has not been created by Gradle, the classpath cannot be determined.

Java lambda

Java lambda classes are created at runtime with a non-deterministic classname. Therefore, the class name does not identify the implementation of the lambda and changes between different Gradle runs.

When the implementation of a task, task action or a nested input cannot be tracked precisely, Gradle disables any caching for the task. That means that the task will never be up-to-date or loaded from the build cache.
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 91. Declaring a method to add task inputs
build.gradle
task processTemplates(type: ProcessTemplates) {
    templateEngine = TemplateEngineType.FREEMARKER
    templateData = new TemplateData("test", [year: 2012])
    outputDir = file("$buildDir/genOutput")

    sources fileTree("src/templates")
}
build.gradle.kts
tasks.register<ProcessTemplates>("processTemplates") {
    templateEngine = TemplateEngineType.FREEMARKER
    templateData = TemplateData("test", mapOf("year" to "2012"))
    outputDir = file("$buildDir/genOutput")

    sources(fileTree("src/templates"))
}
ProcessTemplates.java
public class ProcessTemplates extends DefaultTask {
    // ...
    private FileCollection sourceFiles = getProject().getLayout().files();

    @SkipWhenEmpty
    @InputFiles
    @PathSensitive(PathSensitivity.NONE)
    public FileCollection getSourceFiles() {
        return this.sourceFiles;
    }

    public void sources(FileCollection sourceFiles) {
        this.sourceFiles = this.sourceFiles.plus(sourceFiles);
    }

    // ...
}
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.layout.files() method like so:

Example 92. Declaring a method to add a task as an input
build.gradle
task copyTemplates(type: Copy) {
    into "$buildDir/tmp"
    from "src/templates"
}

task processTemplates2(type: ProcessTemplates) {
    // ...
    sources copyTemplates
}
build.gradle.kts
val copyTemplates by tasks.registering(Copy::class) {
    into("$buildDir/tmp")
    from("src/templates")
}

tasks.register<ProcessTemplates>("processTemplates2") {
    // ...
    sources(copyTemplates.get())
}
ProcessTemplates.java
    // ...
    public void sources(Task inputTask) {
        this.sourceFiles = this.sourceFiles.plus(getProject().getLayout().files(inputTask));
    }
    // ...
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().getLayout().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.

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 93. Failed attempt at setting up an inferred task dependency
build.gradle
plugins {
    id 'java'
}

task badInstrumentClasses(type: Instrument) {
    classFiles = fileTree(compileJava.destinationDir)
    destinationDir = file("$buildDir/instrumented")
}
build.gradle.kts
plugins {
    java
}

tasks.register<Instrument>("badInstrumentClasses") {
    classFiles = fileTree(tasks.compileJava.get().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 94. Setting up an inferred task dependency between output dir and input files
build.gradle
task instrumentClasses(type: Instrument) {
    classFiles = compileJava.outputs.files
    destinationDir = file("$buildDir/instrumented")
}
build.gradle.kts
tasks.register<Instrument>("instrumentClasses") {
    classFiles = tasks.compileJava.get().outputs.files
    destinationDir = file("$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 one of project.files(), project.layout.files() or project.layout.configurableFiles() in place of project.fileTree():

Example 95. Setting up an inferred task dependency with layout.files()
build.gradle
task instrumentClasses2(type: Instrument) {
    classFiles = layout.files(compileJava)
    destinationDir = file("$buildDir/instrumented")
}
build.gradle.kts
tasks.register<Instrument>("instrumentClasses2") {
    classFiles = layout.files(tasks.compileJava.get())
    destinationDir = file("$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(), layout.files() and layout.configurableFiles() 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 96. Setting up an inferred task dependency with builtBy()
build.gradle
task instrumentClassesBuiltBy(type: Instrument) {
    classFiles = fileTree(compileJava.destinationDir) {
        builtBy compileJava
    }
    destinationDir = file("$buildDir/instrumented")
}
build.gradle.kts
tasks.register<Instrument>("instrumentClassesBuiltBy") {
    classFiles = fileTree(tasks.compileJava.get().destinationDir) {
        builtBy(tasks.compileJava.get())
    }
    destinationDir = file("$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 97. Ignoring up-to-date checks
build.gradle
task alwaysInstrumentClasses(type: Instrument) {
    classFiles = layout.files(compileJava)
    destinationDir = file("$buildDir/instrumented")
    outputs.upToDateWhen { false }
}
build.gradle.kts
tasks.register<Instrument>("alwaysInstrumentClasses") {
    classFiles = layout.files(tasks.compileJava.get())
    destinationDir = file("$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 alwaysInstrumentClasses will always be executed, 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 Java 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 98. Runtime classpath normalization
build.gradle
normalization {
    runtimeClasspath {
        ignore 'build-info.properties'
    }
}
build.gradle.kts
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.

Note

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 99. Task rule
build.gradle
tasks.addRule("Pattern: ping<ID>") { String taskName ->
    if (taskName.startsWith("ping")) {
        task(taskName) {
            doLast {
                println "Pinging: " + (taskName - 'ping')
            }
        }
    }
}
build.gradle.kts
tasks.addRule("Pattern: ping<ID>") {
    val taskName = this
    if (startsWith("ping")) {
        task(taskName) {
            doLast {
                println("Pinging: " + (taskName.replace("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 100. 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
}
build.gradle.kts
tasks.addRule("Pattern: ping<ID>") {
    val taskName = this
    if (startsWith("ping")) {
        task(taskName) {
            doLast {
                println("Pinging: " + (taskName.replace("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

Finalizer tasks are automatically added to the task graph when the finalized task is scheduled to run.

Example 101. Adding a task finalizer
build.gradle
task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.finalizedBy taskY
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
    }
}
val taskY by tasks.registering {
    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 102. Task finalizer for a failing task
build.gradle
task taskX {
    doLast {
        println 'taskX'
        throw new RuntimeException()
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.finalizedBy taskY
build.gradle.kts
val taskX by tasks.registering {
    doLast {
        println("taskX")
        throw RuntimeException()
    }
}
val taskY by tasks.registering {
    doLast {
        println("taskY")
    }
}

taskX { finalizedBy(taskY) }
Output of gradle -q taskX
> gradle -q taskX
taskX
taskY

FAILURE: Build failed with an exception.

* Where:
Build file '/home/user/gradle/samples/groovy/build.gradle' line: 4

* What went wrong:
Execution failed for task ':taskX'.
> java.lang.RuntimeException (no error message)

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

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 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.

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 available in Groovy and Kotlin.

A Groovy build script can contain any Groovy language element.[5] A Kotlin build script can contain any Kotlin language element. Gradle assumes that each build script is encoded using UTF-8.

The Project API

In the tutorial 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:

Tip
Getting help writing build scripts

Don’t forget that your build script is simply Groovy or Kotlin 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 103. Accessing property of the Project object
build.gradle
println name
println project.name
build.gradle.kts
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 the top-level reference to the name property of the Project object. 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
Important
Script with other targets

The build scripts described here target Project objects. There are also settings scripts and init scripts that respectively target Settings and Gradle objects.

The script API

When Gradle executes a Groovy build script (.gradle), 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.

When Gradle executes a Kotlin build script (.gradle.kts), it compiles the script into a subclass of KotlinBuildScript. This means that all of the visible properties and functions declared by the KotlinBuildScript type are available in your script. Also see the KotlinSettingsScript and KotlinInitScript types respectively for settings scripts and init scripts.

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.

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

Example 104. Using local variables
build.gradle
def dest = "dest"

task copy(type: Copy) {
    from "source"
    into dest
}
build.gradle.kts
val dest = "dest"

tasks.register<Copy>("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.

Extra properties can be added, read and set via the owning object’s extra property. Alternatively, they can be addressed via Kotlin delegated properties using by extra.

Example 105. Using extra properties
build.gradle
plugins {
    id '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 }
    }
}
build.gradle.kts
plugins {
    java
}

val springVersion by extra("3.1.0.RELEASE")
val emailNotification by extra { "build@master.org" }

sourceSets.all { extra["purpose"] = null }

sourceSets {
    main {
        extra["purpose"] = "production"
    }
    test {
        extra["purpose"] = "test"
    }
    create("plugin") {
        extra["purpose"] = "production"
    }
}

tasks.register("printProperties") {
    doLast {
        println(springVersion)
        println(emailNotification)
        sourceSets.matching { it.extra["purpose"] == "production" }.forEach { 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.

In this example, two extra properties are added to the project object using by extra. Additionally, a property named purpose is added to each source set by setting extra["purpose"] to null (null is a permissible value). Once the properties have been added, they can be read and set on extra.

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 106. Configuring arbitrary objects
build.gradle
import java.text.FieldPosition

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

tasks.register("configure") {
    doLast {
        val pos = FieldPosition(10).apply {
            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.

Caution
Only supported from a Groovy script

Configuring arbitrary objects using an external script is not yet supported by the Kotlin DSL. See gradle/kotlin-dsl#659 for more information.
Example 107. 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

Tip
 Looking for some Kotlin basics, the Kotlin reference documentation and Kotlin Koans should be useful to you.

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 108. 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 109. 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 110. 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 111. 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 112. 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 113. 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.*

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 114. How to copy a single file
build.gradle
task copyReport(type: Copy) {
    from file("$buildDir/reports/my-report.pdf")
    into file("$buildDir/toArchive")
}
build.gradle.kts
tasks.register<Copy>("copyReport") {
    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 115. Using implicit string paths
build.gradle
task copyReport2(type: Copy) {
    from "$buildDir/reports/my-report.pdf"
    into "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Copy>("copyReport2") {
    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 116. Prefer task/project properties over hard-coded paths
build.gradle
task copyReport3(type: Copy) {
    from myReportTask.outputFile
    into archiveReportsTask.dirToArchive
}
build.gradle.kts
tasks.register<Copy>("copyReport3") {
    val outputFile: File by myReportTask.get().extra
    val dirToArchive: File by archiveReportsTask.get().extra
    from(outputFile)
    into(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 117. Using multiple arguments with from()
build.gradle
task copyReportsForArchiving(type: Copy) {
    from "$buildDir/reports/my-report.pdf", "src/docs/manual.pdf"
    into "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Copy>("copyReportsForArchiving") {
    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 118. Using a flat filter
build.gradle
task copyPdfReportsForArchiving(type: Copy) {
    from "$buildDir/reports"
    include "*.pdf"
    into "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Copy>("copyPdfReportsForArchiving") {
    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:

copy with flat filter example
Figure 7. 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 119. Using a deep filter
build.gradle
task copyAllPdfReportsForArchiving(type: Copy) {
    from "$buildDir/reports"
    include "**/*.pdf"
    into "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Copy>("copyAllPdfReportsForArchiving") {
    from("$buildDir/reports")
    include("**/*.pdf")
    into("$buildDir/toArchive")
}

This task has the following effect:

copy with deep filter example
Figure 8. 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 120. Copying an entire directory
build.gradle
task copyReportsDirForArchiving(type: Copy) {
    from "$buildDir/reports"
    into "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Copy>("copyReportsDirForArchiving") {
    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 121. Copying an entire directory, including itself
build.gradle
task copyReportsDirForArchiving2(type: Copy) {
    from("$buildDir") {
        include "reports/**"
    }
    into "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Copy>("copyReportsDirForArchiving2") {
    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 122. Archiving a directory as a ZIP
build.gradle
task packageDistribution(type: Zip) {
    archiveName = "my-distribution.zip"
    destinationDir = file("$buildDir/dist")

    from "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Zip>("packageDistribution") {
    archiveName = "my-distribution.zip"
    destinationDir = file("$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 123. 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"
    }
}
build.gradle.kts
plugins {
    base
}

version = "1.0.0"

tasks.register<Zip>("packageDistribution") {
    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 Using child copy 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 124. Unpacking a ZIP file
build.gradle
task unpackFiles(type: Copy) {
    from zipTree("src/resources/thirdPartyResources.zip")
    into "$buildDir/resources"
}
build.gradle.kts
tasks.register<Copy>("unpackFiles") {
    from(zipTree("src/resources/thirdPartyResources.zip"))
    into("$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 125. 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

    dependsOn configurations.runtimeClasspath
    from {
        configurations.runtimeClasspath.findAll { it.name.endsWith('jar') }.collect { zipTree(it) }
    }
}
build.gradle.kts
plugins {
    java
}

version = "1.0.0"

repositories {
    mavenCentral()
}

dependencies {
    implementation("commons-io:commons-io:2.6")
}

tasks.register<Jar>("uberJar") {
    appendix = "uber"

    from(sourceSets.main.get().output)

    dependsOn(configurations.runtimeClasspath)
    from({
        configurations.runtimeClasspath.get().filter { it.name.endsWith("jar") }.map { 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 126. Manually creating a directory
build.gradle
task ensureDirectory {
    doLast {
        mkdir "images"
    }
}
build.gradle.kts
tasks.register("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 127. Moving a directory using the Ant task
build.gradle
task moveReports {
    doLast {
        ant.move file: "${buildDir}/reports",
                 todir: "${buildDir}/toArchive"
    }
}
build.gradle.kts
tasks.register("moveReports") {
    doLast {
        ant.withGroovyBuilder {
            "move"("file" to "${buildDir}/reports", "todir" to "${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 128. Renaming files as they are copied
build.gradle
task copyFromStaging(type: Copy) {
    from "src/main/webapp"
    into "$buildDir/explodedWar"

    rename '(.+)-staging(.+)', '$1$2'
}
build.gradle.kts
tasks.register<Copy>("copyFromStaging") {
    from("src/main/webapp")
    into("$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 129. 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"
}
build.gradle.kts
tasks.register<Copy>("copyWithTruncate") {
    from("$buildDir/reports")
    rename { filename: String ->
        if (filename.length > 10) {
            filename.slice(0..7) + "~" + filename.length
        }
        else filename
    }
    into("$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…​), ProjectLayout.files(java.lang.Object…​), and ProjectLayout.configurableFiles(java.lang.Object…​) methods.

For example, the following task deletes the entire contents of a build’s output directory:

Example 130. Deleting a directory
build.gradle
task myClean(type: Delete) {
    delete buildDir
}
build.gradle.kts
tasks.register<Delete>("myClean") {
    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 FileCollection and FileTree. The following example does just that to clear out temporary files from a source directory:

Example 131. Deleting files matching a specific pattern
build.gradle
task cleanTempFiles(type: Delete) {
    delete fileTree("src").matching {
        include "**/*.tmp"
    }
}
build.gradle.kts
tasks.register<Delete>("cleanTempFiles") {
    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 132. 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
}
build.gradle.kts
val archivesDirPath by extra { "$buildDir/archives" }

tasks.register<Zip>("packageClasses") {
    appendix = "classes"
    destinationDir = file(archivesDirPath)

    from(tasks.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 files and directories

Gradle provides the Project.file(java.lang.Object) method for specifying the location of a single file or directory. Relative paths are resolved relative to the project directory, while absolute paths remain unchanged.

Caution

Never use new File(relative path) because this creates a path relative to the current working directory (CWD). Gradle can make no guarantees about the location of the CWD, which means builds that rely on it may break at any time.

Here are some examples of using the file() method with different types of argument:

Example 133. 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'))
build.gradle.kts
// Using a relative path
var configFile = file("src/config.xml")

// Using an absolute path
configFile = file(configFile.absolutePath)

// Using a File object with a relative path
configFile = file(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 134. Creating a path relative to a parent project
build.gradle
File configFile = file("$rootDir/shared/config.xml")
build.gradle.kts
val configFile = file("$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 ProjectLayout.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.

Caution

Although the files() method accepts File instances, never use new File(relative path) with it because this creates a path relative to the current working directory (CWD). Gradle can make no guarantees about the location of the CWD, which means builds that rely on it may break at any time.

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 135. Creating a file collection
build.gradle
FileCollection collection = layout.files('src/file1.txt',
                                  new File('src/file2.txt'),
                                  ['src/file3.csv', 'src/file4.csv'],
                                  Paths.get('src', 'file5.txt'))
build.gradle.kts
val collection: FileCollection = layout.files(
    "src/file1.txt",
    File("src/file2.txt"),
    listOf("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 136. Implementing a file collection
build.gradle
task list {
    doLast {
        File srcDir

        // Create a file collection using a closure
        collection = layout.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 }
    }
}
build.gradle.kts
tasks.register("list") {
    doLast {
        var srcDir: File? = null

        val collection = layout.files({
            srcDir?.listFiles()
        })

        srcDir = file("src")
        println("Contents of ${srcDir.name}")
        collection.map { relativePath(it) }.sorted().forEach { println(it) }

        srcDir = file("src2")
        println("Contents of ${srcDir.name}")
        collection.map { relativePath(it) }.sorted().forEach { 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 (in Groovy) or a Provider (in Kotlin) to the files() method. Your closure/provider 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 (in Groovy) of forEach method (in Kotlin) 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 137. 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

        // Add and subtract collections
        def union = collection + layout.files('src/file2.txt')
        def difference = collection - layout.files('src/file2.txt')
build.gradle.kts
        // Iterate over the files in the collection
        collection.forEach { file: File ->
            println(file.name)
        }

        // Convert the collection to various types
        val set: Set<File> = collection.files
        val list: List<File> = collection.toList()
        val path: String = collection.asPath
        val file: File = collection.singleFile

        // Add and subtract collections
        val union = collection + layout.files("src/file2.txt")
        val difference = collection - layout.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 138. Filtering a file collection
build.gradle
        FileCollection textFiles = collection.filter { File f ->
            f.name.endsWith(".txt")
        }
build.gradle.kts
        val textFiles: FileCollection = collection.filter { f: File ->
            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:

file collection vs file tree
Figure 9. The differences in how file trees and file collections behave when copying files
Note
 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 139. Creating a file tree
build.gradle
// Create a file tree with a base directory
ConfigurableFileTree tree = fileTree(dir: 'src/main')

// Add include and exclude patterns to the tree
tree.include '**/*.java'
tree.exclude '**/Abstract*'

// 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*/**')
build.gradle.kts
// Create a file tree with a base directory
var tree: ConfigurableFileTree = fileTree("src/main")

// Add include and exclude patterns to the tree
tree.include("**/*.java")
tree.exclude("**/Abstract*")

// Create a tree using closure
tree = fileTree("src") {
    include("**/*.java")
}

// Create a tree using a map
tree = fileTree("dir" to "src", "include" to "**/*.java")
tree = fileTree("dir" to "src", "includes" to listOf("**/*.java", "**/*.xml"))
tree = fileTree("dir" to "src", "include" to "**/*.java", "exclude" to "**/*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 140. 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
    }
}
build.gradle.kts
tasks.register<Copy>("forcedCopy") {
    into("$buildDir/inPlaceApp")
    from("src/main/webapp")

    doFirst {
        ant.withGroovyBuilder {
            "defaultexcludes"("remove" to "**/.git")
            "defaultexcludes"("remove" to "**/.git/**")
            "defaultexcludes"("remove" to "**/*~")
        }
    }

    doLast {
        ant.withGroovyBuilder {
            "defaultexcludes"("default" to 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 141. 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"
}
build.gradle.kts
// Iterate over the contents of a tree
tree.forEach{ file: File ->
    println(file)
}

// Filter a tree
val filtered: FileTree = tree.matching {
    include("org/gradle/api/**")
}

// Add trees together
val sum: FileTree = tree + fileTree("src/test")

// Visit the elements of the tree
tree.visit {
    println("${this.relativePath} => ${this.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 142. 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'))
build.gradle.kts
// Create a ZIP file tree using path
val zip: FileTree = zipTree("someFile.zip")

// Create a TAR file tree using path
val tar: FileTree = 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:
val someTar: FileTree = 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 or `Provider.

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 ProjectLayout.files(java.lang.Object...) in your build.

Here are some examples of the different types of arguments that the source property can take:

Example 143. Specifying a set of files
build.gradle
task compile(type: JavaCompile) {

    // Use a File object to specify the source directory
    source = file('src/main/java')

    // Use a String path to specify the source directory
    source = 'src/main/java'

    // Use a collection to specify multiple source directories
    source = ['src/main/java', '../shared/java']

    // Use a FileCollection (or FileTree in this case) to specify the source files
    source = fileTree(dir: 'src/main/java').matching { include 'org/gradle/api/**' }

    // Using a closure to specify the source files.
    source = {
        // Use the contents of each zip file in the src dir
        file('src').listFiles().findAll {it.name.endsWith('.zip')}.collect { zipTree(it) }
    }
}
build.gradle.kts
tasks.register<JavaCompile>("compile") {
    // Use a File object to specify the source directory
    source = fileTree(file("src/main/java"))

    // Use a String path to specify the source directory
    source = fileTree("src/main/java")

    // Use a collection to specify multiple source directories
    source = fileTree(listOf("src/main/java", "../shared/java"))

    // Use a FileCollection (or FileTree in this case) to specify the source files
    source = fileTree("src/main/java").matching { include("org/gradle/api/**") }

    // Using a closure to specify the source files.
    setSource({
        // Use the contents of each zip file in the src dir
        file("src").listFiles().filter { it.name.endsWith(".zip") }.map { 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 144. 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() }
}
build.gradle.kts
tasks.named<JavaCompile>("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
    setSource({ 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…​), ProjectLayout.files(java.lang.Object…​), and ProjectLayout.configurableFiles(java.lang.Object…​), so see those methods 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 (in Groovy) or a Provider (in Kotlin) — a technique that also works with from():

Example 145. 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() }
}
build.gradle.kts
tasks.register<Copy>("anotherCopyTask") {
    // 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(tasks["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 146. Selecting the files to copy
build.gradle
task copyTaskWithPatterns (type: Copy) {
    from 'src/main/webapp'
    into "$buildDir/explodedWar"
    include '**/*.html'
    include '**/*.jsp'
    exclude { FileTreeElement details ->
        details.file.name.endsWith('.html') &&
            details.file.text.contains('DRAFT')
    }
}
build.gradle.kts
tasks.register<Copy>("copyTaskWithPatterns") {
    from("src/main/webapp")
    into("$buildDir/explodedWar")
    include("**/*.html")
    include("**/*.jsp")
    exclude { details: FileTreeElement ->
        details.file.name.endsWith(".html") &&
            details.file.readText().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 147. 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$2')
}
build.gradle.kts
tasks.register<Copy>("rename") {
    from("src/main/webapp")
    into("$buildDir/explodedWar")
    // Use a closure to convert all file names to upper case
    rename { fileName: String ->
        fileName.toUpperCase()
    }
    // Use a regular expression to map the file name
    rename("(.+)-staging-(.+)", "$1$2")
    rename("(.+)-staging-(.+)".toRegex().pattern, "$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.

Note
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 148. 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'
}
build.gradle.kts
import org.apache.tools.ant.filters.FixCrLfFilter
import org.apache.tools.ant.filters.ReplaceTokens
tasks.register<Copy>("filter") {
    from("src/main/webapp")
    into("$buildDir/explodedWar")
    // Substitute property tokens in files
    expand("copyright" to "2009", "version" to "2.3.1")
    expand(project.properties)
    // Use some of the filters provided by Ant
    filter(FixCrLfFilter::class)
    filter(ReplaceTokens::class, "tokens" to mapOf("copyright" to "2009", "version" to "2.3.1"))
    // Use a closure to filter each line
    filter { line: String ->
        "[$line]"
    }
    // Use a closure to remove lines
    filter { line: String ->
        if (line.startsWith('-')) null else 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.

Note
 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 149. 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
}
build.gradle.kts
val webAssetsSpec: CopySpec = copySpec {
    from("src/main/webapp")
    include("**/*.html", "**/*.png", "**/*.jpg")
    rename("(.+)-staging(.+)", "$1$2")
}

tasks.register<Copy>("copyAssets") {
    into("$buildDir/inPlaceApp")
    with(webAssetsSpec)
}

tasks.register<Zip>("distApp") {
    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.

Note

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 150. 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
}
build.gradle.kts
val webAssetPatterns = Action<CopySpec> {
    include("**/*.html", "**/*.png", "**/*.jpg")
}

tasks.register<Copy>("copyAppAssets") {
    into("$buildDir/inPlaceApp")
    from("src/main/webapp", webAssetPatterns)
}

tasks.register<Zip>("archiveDistAssets") {
    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:

exploded war child copy spec example
Figure 10. 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 151. Nested copy specs
build.gradle
task nestedSpecs(type: Copy) {
    into "$buildDir/explodedWar"
    exclude '**/*staging*'
    from('src/dist') {
        include '**/*.html', '**/*.png', '**/*.jpg'
    }
    from(sourceSets.main.output) {
        into 'WEB-INF/classes'
    }
    into('WEB-INF/lib') {
        from configurations.runtimeClasspath
    }
}
build.gradle.kts
tasks.register<Copy>("nestedSpecs") {
    into("$buildDir/explodedWar")
    exclude("**/*staging*")
    from("src/dist") {
        include("**/*.html", "**/*.png", "**/*.jpg")
    }
    from(sourceSets.main.get().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.

Note
 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 152. Copying files using the copy() method without up-to-date check
build.gradle
task copyMethod {
    doLast {
        copy {
            from 'src/main/webapp'
            into "$buildDir/explodedWar"
            include '**/*.html'
            include '**/*.jsp'
        }
    }
}
build.gradle.kts
tasks.register("copyMethod") {
    doLast {
        copy {
            from("src/main/webapp")
            into("$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 153. 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'
        }
    }
}
build.gradle.kts
tasks.register("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 154. Using the Sync task to copy dependencies
build.gradle
task libs(type: Sync) {
    from configurations.runtime
    into "$buildDir/libs"
}
build.gradle.kts
tasks.register<Sync>("libs") {
    from(configurations["runtime"])
    into("$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 155. Archiving a directory as a ZIP
build.gradle
task packageDistribution(type: Zip) {
    archiveName = "my-distribution.zip"
    destinationDir = file("$buildDir/dist")

    from "$buildDir/toArchive"
}
build.gradle.kts
tasks.register<Zip>("packageDistribution") {
    archiveName = "my-distribution.zip"
    destinationDir = file("$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 156. 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)
    }
}
build.gradle.kts
plugins {
    base
}

version = "1.0"

tasks.register<Zip>("myZip") {
    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 157. Configuration of archive task - custom archive name
build.gradle
task myCustomZip(type: Zip) {
    baseName = 'customName'
    from 'somedir'

    doLast {
        println archiveName
    }
}
build.gradle.kts
tasks.register<Zip>("myCustomZip") {
    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 158. 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
    }
}
build.gradle.kts
plugins {
    base
}

version = "1.0"
base.archivesBaseName = "gradle"

val myZip by tasks.registering(Zip::class) {
    from("somedir")
}

val myOtherZip by tasks.registering(Zip::class) {
    appendix = "wrapper"
    classifier = "src"
    from("somedir")
}

tasks.register("echoNames") {
    doLast {
        println("Project name: ${project.name}")
        println(myZip.get().archiveName)
        println(myOtherZip.get().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:

archiveName — String, 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.

archivePath — File, read-only, default: destinationDir/archiveName

The absolute file path of the generated archive.

destinationDir — File, 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.

baseName — String, 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.

appendix — String, 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.

version — String, default: project.version

The version portion of the archive file name, typically in the form of a normal project or product version.

classifier — String, default: null

The classifier portion of the archive file name. Often used to distinguish between archives that target different platforms.

extension — String, 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 support for 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 159. Activating reproducible archives
build.gradle
tasks.withType(AbstractArchiveTask) {
    preserveFileTimestamps = false
    reproducibleFileOrder = true
}
build.gradle.kts
tasks.withType<AbstractArchiveTask>().configureEach {
    isPreserveFileTimestamps = false
    isReproducibleFileOrder = true
}

Often you will want to publish an archive, so that it is usable from another project. This process is described in Legacy Publishing.

Writing Custom Task Classes

Gradle supports two types of task. One such type is the simple task, where you define the task with an action closure. We have seen these in Build Script Basics. For this type of task, the action closure determines the behaviour of the task. This type of task is good for implementing one-off tasks in your build script.

The other type of task is the enhanced task, where the behaviour is built into the task, and the task provides some properties which you can use to configure the behaviour. We have seen these in Authoring Tasks. Most Gradle plugins use enhanced tasks. With enhanced tasks, you don’t need to implement the task behaviour as you do with simple tasks. You simply declare the task and configure the task using its properties. In this way, enhanced tasks let you reuse a piece of behaviour in many different places, possibly across different builds.

The behaviour and properties of an enhanced task is defined by the task’s class. When you declare an enhanced task, you specify the type, or class of the task.

Implementing your own custom task class in Gradle is easy. You can implement a custom task class in pretty much any language you like, provided it ends up compiled to bytecode. In our examples, we are going to use Groovy as the implementation language. Groovy, Java or Kotlin are all good choices as the language to use to implement a task class, as the Gradle API has been designed to work well with these languages. In general, a task implemented using Java or Kotlin, which are statically typed, will perform better than the same task implemented using Groovy.

Packaging a task class

There are several places where you can put the source for the task class.

Build script

You can include the task class directly in the build script. This has the benefit that the task class is automatically compiled and included in the classpath of the build script without you having to do anything. However, the task class is not visible outside the build script, and so you cannot reuse the task class outside the build script it is defined in.

buildSrc project

You can put the source for the task class in the rootProjectDir/buildSrc/src/main/groovy directory. Gradle will take care of compiling and testing the task class and making it available on the classpath of the build script. The task class is visible to every build script used by the build. However, it is not visible outside the build, and so you cannot reuse the task class outside the build it is defined in. Using the buildSrc project approach separates the task declaration - that is, what the task should do - from the task implementation - that is, how the task does it.

See Organizing Gradle Projects for more details about the buildSrc project.

Standalone project

You can create a separate project for your task class. This project produces and publishes a JAR which you can then use in multiple builds and share with others. Generally, this JAR might include some custom plugins, or bundle several related task classes into a single library. Or some combination of the two.

In our examples, we will start with the task class in the build script, to keep things simple. Then we will look at creating a standalone project.

Writing a simple task class

To implement a custom task class, you extend DefaultTask.

Example 160. Defining a custom task
build.gradle
class GreetingTask extends DefaultTask {
}
build.gradle.kts
open class GreetingTask : DefaultTask() {
}

This task doesn’t do anything useful, so let’s add some behaviour. To do so, we add a method to the task and mark it with the TaskAction annotation. Gradle will call the method when the task executes. You don’t have to use a method to define the behaviour for the task. You could, for instance, call doFirst() or doLast() with a closure in the task constructor to add behaviour.

Example 161. A hello world task
build.gradle
class GreetingTask extends DefaultTask {
    @TaskAction
    def greet() {
        println 'hello from GreetingTask'
    }
}

// Create a task using the task type
task hello(type: GreetingTask)
build.gradle.kts
open class GreetingTask : DefaultTask() {
    @TaskAction
    fun greet() {
        println("hello from GreetingTask")
    }
}

// Create a task using the task type
tasks.register<GreetingTask>("hello")
Output of gradle -q hello
> gradle -q hello
hello from GreetingTask

Let’s add a property to the task, so we can customize it. Tasks are simply POGOs, and when you declare a task, you can set the properties or call methods on the task object. Here we add a greeting property, and set the value when we declare the greeting task.

Example 162. A customizable hello world task
build.gradle
class GreetingTask extends DefaultTask {
    String greeting = 'hello from GreetingTask'

    @TaskAction
    def greet() {
        println greeting
    }
}

// Use the default greeting
task hello(type: GreetingTask)

// Customize the greeting
task greeting(type: GreetingTask) {
    greeting = 'greetings from GreetingTask'
}
build.gradle.kts
open class GreetingTask : DefaultTask() {
    var greeting = "hello from GreetingTask"

    @TaskAction
    fun greet() {
        println(greeting)
    }
}

// Use the default greeting
tasks.register<GreetingTask>("hello")

// Customize the greeting
tasks.register<GreetingTask>("greeting") {
    greeting = "greetings from GreetingTask"
}
Output of gradle -q hello greeting
> gradle -q hello greeting
hello from GreetingTask
greetings from GreetingTask

A standalone project

Now we will move our task to a standalone project, so we can publish it and share it with others. This project is simply a Groovy project that produces a JAR containing the task class. Here is a simple build script for the project. It applies the Groovy plugin, and adds the Gradle API as a compile-time dependency.

Example 163. A build for a custom task
build.gradle
plugins {
    id 'groovy'
}

dependencies {
    implementation gradleApi()
    implementation localGroovy()
}
build.gradle.kts
plugins {
    groovy
}

dependencies {
    implementation(gradleApi())
    implementation(localGroovy())
}
Note
 The code for this example can be found at samples/customPlugin in the ‘-all’ distribution of Gradle.

We just follow the convention for where the source for the task class should go.

Example: A custom task
src/main/groovy/org/gradle/GreetingTask.groovy
package org.gradle

import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction

class GreetingTask extends DefaultTask {
    String greeting = 'hello from GreetingTask'

    @TaskAction
    def greet() {
        println greeting
    }
}
Using your task class in another project

To use a task class in a build script, you need to add the class to the build script’s classpath. To do this, you use a buildscript { } block, as described in External dependencies for the build script. The following example shows how you might do this when the JAR containing the task class has been published to a local repository:

Example 164. Using a custom task in another project
build.gradle
buildscript {
    repositories {
        maven {
            url = uri(repoLocation)
        }
    }
    dependencies {
        classpath 'org.gradle:customPlugin:1.0-SNAPSHOT'
    }
}

task greeting(type: org.gradle.GreetingTask) {
    greeting = 'howdy!'
}
build.gradle.kts
buildscript {
    repositories {
        maven {
            url = uri(repoLocation)
        }
    }
    dependencies {
        classpath("org.gradle:customPlugin:1.0-SNAPSHOT")
    }
}

tasks.register<org.gradle.GreetingTask>("greeting") {
    greeting = "howdy!"
}
Writing tests for your task class

You can use the ProjectBuilder class to create Project instances to use when you test your task class.

Example: Testing a custom task
src/test/groovy/org/gradle/GreetingTaskTest.groovy
class GreetingTaskTest {
    @Test
    public void canAddTaskToProject() {
        Project project = ProjectBuilder.builder().build()
        def task = project.task('greeting', type: GreetingTask)
        assertTrue(task instanceof GreetingTask)
    }
}

Incremental tasks

With Gradle, it’s very simple to implement a task that is skipped when all of its inputs and outputs are up to date (see Incremental Builds). However, there are times when only a few input files have changed since the last execution, and you’d like to avoid reprocessing all of the unchanged inputs. This can be particularly useful for a transformer task, that converts input files to output files on a 1:1 basis.

If you’d like to optimise your build so that only out-of-date inputs are processed, you can do so with an incremental task.

Implementing an incremental task

For a task to process inputs incrementally, that task must contain an incremental task action. This is a task action method that contains a single IncrementalTaskInputs parameter, which indicates to Gradle that the action will process the changed inputs only.

The incremental task action may supply an IncrementalTaskInputs.outOfDate(org.gradle.api.Action) action for processing any input file that is out-of-date, and a IncrementalTaskInputs.removed(org.gradle.api.Action) action that executes for any input file that has been removed since the previous execution.

Example 165. Defining an incremental task action
build.gradle
class IncrementalReverseTask extends DefaultTask {
    @InputDirectory
    def File inputDir

    @OutputDirectory
    def File outputDir

    @Input
    def inputProperty

    @TaskAction
    void execute(IncrementalTaskInputs inputs) {
        println inputs.incremental ? 'CHANGED inputs considered out of date'
                                   : 'ALL inputs considered out of date'
        if (!inputs.incremental)
            project.delete(outputDir.listFiles())

        inputs.outOfDate { change ->
            if (change.file.directory) return

            println "out of date: ${change.file.name}"
            def targetFile = new File(outputDir, change.file.name)
            targetFile.text = change.file.text.reverse()
        }

        inputs.removed { change ->
            if (change.file.directory) return

            println "removed: ${change.file.name}"
            def targetFile = new File(outputDir, change.file.name)
            targetFile.delete()
        }
    }
}
build.gradle.kts
open class IncrementalReverseTask : DefaultTask() {
    @InputDirectory
    lateinit var inputDir: File

    @OutputDirectory
    lateinit var outputDir: File

    @Input
    lateinit var inputProperty: String

    @TaskAction
    fun execute(inputs: IncrementalTaskInputs) {
        println(
            if (inputs.isIncremental) "CHANGED inputs considered out of date"
            else "ALL inputs considered out of date"
        )
        if (!inputs.isIncremental) {
            project.delete(outputDir.listFiles())
        }

        inputs.outOfDate {
            if (file.isDirectory) return@outOfDate

            println("out of date: ${file.name}")
            val targetFile = File(outputDir, file.name)
            targetFile.writeText(file.readText().reversed())
        }

        inputs.removed {
            if (file.isDirectory) return@removed

            println("removed: ${file.name}")
            val targetFile = File(outputDir, file.name)
            targetFile.delete()
        }
    }
}
Note
 The code for this example can be found at samples/userguide/tasks/incrementalTask in the ‘-all’ distribution of Gradle.

If for some reason the task is not run incremental, e.g. by running with --rerun-tasks, only the outOfDate action is executed, even if there were deleted input files. You should consider handling this case at the beginning, as is done in the example above.

For a simple transformer task like this, the task action simply needs to generate output files for any out-of-date inputs, and delete output files for any removed inputs.

A task may only contain a single incremental task action.

Which inputs are considered out of date?

When Gradle has history of a previous task execution, and the only changes to the task execution context since that execution are to input files, then Gradle is able to determine which input files need to be reprocessed by the task. In this case, the IncrementalTaskInputs.outOfDate(org.gradle.api.Action) action will be executed for any input file that was added or modified, and the IncrementalTaskInputs.removed(org.gradle.api.Action) action will be executed for any removed input file.

However, there are many cases where Gradle is unable to determine which input files need to be reprocessed. Examples include:

  • There is no history available from a previous execution.

  • You are building with a different version of Gradle. Currently, Gradle does not use task history from a different version.

  • An upToDateWhen criteria added to the task returns false.

  • An input property has changed since the previous execution.

  • One or more output files have changed since the previous execution.

In any of these cases, Gradle will consider all of the input files to be outOfDate. The IncrementalTaskInputs.outOfDate(org.gradle.api.Action) action will be executed for every input file, and the IncrementalTaskInputs.removed(org.gradle.api.Action) action will not be executed at all.

You can check if Gradle was able to determine the incremental changes to input files with IncrementalTaskInputs.isIncremental().

An incremental task in action

Given the incremental task implementation above, we can explore the various change scenarios by example. Note that the various mutation tasks ('updateInputs', 'removeInput', etc) are only present for demonstration purposes: these would not normally be part of your build script.

First, consider the IncrementalReverseTask executed against a set of inputs for the first time. In this case, all inputs will be considered “out of date”:

Example 166. Running the incremental task for the first time
build.gradle
task incrementalReverse(type: IncrementalReverseTask) {
    inputDir = file('inputs')
    outputDir = file("$buildDir/outputs")
    inputProperty = project.properties['taskInputProperty'] ?: 'original'
}
build.gradle.kts
tasks.register<IncrementalReverseTask>("incrementalReverse") {
    inputDir = file("inputs")
    outputDir = file("$buildDir/outputs")
    inputProperty = project.properties["taskInputProperty"] as String? ?: "original"
}
Build layout
.
├── build.gradle
└── inputs
    ├── 1.txt
    ├── 2.txt
    └── 3.txt
Output of gradle -q incrementalReverse
> gradle -q incrementalReverse
ALL inputs considered out of date
out of date: 1.txt
out of date: 2.txt
out of date: 3.txt

Naturally when the task is executed again with no changes, then the entire task is up to date and no files are reported to the task action:

Example: Running the incremental task with unchanged inputs
Output of gradle -q incrementalReverse
> gradle -q incrementalReverse

When an input file is modified in some way or a new input file is added, then re-executing the task results in those files being reported to IncrementalTaskInputs.outOfDate(org.gradle.api.Action):

Example 167. Running the incremental task with updated input files
build.gradle
task updateInputs() {
    doLast {
        file('inputs/1.txt').text = 'Changed content for existing file 1.'
        file('inputs/4.txt').text = 'Content for new file 4.'
    }
}
build.gradle.kts
tasks.register("updateInputs") {
    doLast {
        file("inputs/1.txt").writeText("Changed content for existing file 1.")
        file("inputs/4.txt").writeText("Content for new file 4.")
    }
}
Output of gradle -q updateInputs incrementalReverse
> gradle -q updateInputs incrementalReverse
CHANGED inputs considered out of date
out of date: 1.txt
out of date: 4.txt

When an existing input file is removed, then re-executing the task results in that file being reported to IncrementalTaskInputs.removed(org.gradle.api.Action):

Example 168. Running the incremental task with an input file removed
build.gradle
task removeInput() {
    doLast {
        file('inputs/3.txt').delete()
    }
}
build.gradle.kts
tasks.register("removeInput") {
    doLast {
        file("inputs/3.txt").delete()
    }
}
Output of gradle -q removeInput incrementalReverse
> gradle -q removeInput incrementalReverse
CHANGED inputs considered out of date
removed: 3.txt

When an output file is deleted (or modified), then Gradle is unable to determine which input files are out of date. In this case, all input files are reported to the IncrementalTaskInputs.outOfDate(org.gradle.api.Action) action, and no input files are reported to the IncrementalTaskInputs.removed(org.gradle.api.Action) action:

Example 169. Running the incremental task with an output file removed
build.gradle
task removeOutput() {
    doLast {
        file("$buildDir/outputs/1.txt").delete()
    }
}
build.gradle.kts
tasks.register("removeOutput") {
    doLast {
        file("$buildDir/outputs/1.txt").delete()
    }
}
Output of gradle -q removeOutput incrementalReverse
> gradle -q removeOutput incrementalReverse
ALL inputs considered out of date
out of date: 1.txt
out of date: 2.txt
out of date: 3.txt

When a task input property is modified, Gradle is unable to determine how this property impacted the task outputs, so all input files are assumed to be out of date. So similar to the changed output file example, all input files are reported to the IncrementalTaskInputs.outOfDate(org.gradle.api.Action) action, and no input files are reported to the IncrementalTaskInputs.removed(org.gradle.api.Action) action:

Example: Running the incremental task with an input property changed
Output of gradle -q -PtaskInputProperty=changed incrementalReverse
> gradle -q -PtaskInputProperty=changed incrementalReverse
ALL inputs considered out of date
out of date: 1.txt
out of date: 2.txt
out of date: 3.txt
Storing incremental state for cached tasks

Using Gradle’s IncrementalTaskInputs is not the only way to create tasks that only works on changes since the last execution. Tools like the Kotlin compiler provide incrementality as a built-in feature. The way this is typically implemented is that the tool stores som