Chapter 19. More about Tasks

Table of Contents

19.1. Defining tasks
19.2. Locating tasks
19.3. Configuring tasks
19.4. Adding dependencies to a task
19.5. Ordering tasks
19.6. Adding a description to a task
19.7. Replacing tasks
19.8. Skipping tasks
19.9. Up-to-date checks (AKA Incremental Build)
19.10. Task rules
19.11. Finalizer tasks
19.12. Summary

In the introductory tutorial (Chapter 16, Build Script Basics) you learned how to create simple tasks. You also learned how to add additional behavior to these tasks later on, and you learned how to create dependencies between tasks. This was all about simple tasks, but Gradle takes the concept of tasks further. Gradle supports enhanced tasks, which are tasks that have their own properties and methods. This is really different from what you are used to with Ant targets. Such enhanced tasks are either provided by you or built into Gradle.

19.1. Defining tasks

We have already seen how to define tasks using a keyword style in Chapter 16, Build Script Basics. There are a few variations on this style, which you may need to use in certain situations. For example, the keyword style does not work in expressions.

Example 19.1. Defining tasks

build.gradle

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

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

You can also use strings for the task names:

Example 19.2. Defining tasks - using strings for task names

build.gradle

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

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

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

Example 19.3. Defining tasks with alternative syntax

build.gradle

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

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

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

19.2. Locating tasks

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

Example 19.4. Accessing tasks as properties

build.gradle

task hello

println hello.name
println project.hello.name

Tasks are also available through the tasks collection.

Example 19.5. Accessing tasks via tasks collection

build.gradle

task hello

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

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

Example 19.6. Accessing tasks by path

build.gradle

project(':projectA') {
    task hello
}

task hello

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

Output of gradle -q hello

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

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

19.3. 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 19.7. Creating a copy task

build.gradle

task myCopy(type: Copy)

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

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

Example 19.8. Configuring a task - various ways

build.gradle

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

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

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

Example 19.9. Configuring a task - with closure

build.gradle

task myCopy(type: Copy)

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

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

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

Example 19.10. Defining a task with closure

build.gradle

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

Don't forget about the build phases

A task has both configuration and actions. When using the <<, 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 Chapter 22, The Build Lifecycle for more details about the build lifecycle.

19.4. Adding dependencies to a task

There are several ways you can define the dependencies of a task. In Section 16.5, “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 19.11. Adding dependency on task from another project

build.gradle

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

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

Output of gradle -q taskX

> gradle -q taskX
taskY
taskX

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

Example 19.12. Adding dependency using task object

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}

task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.dependsOn taskY

Output of gradle -q taskX

> gradle -q taskX
taskY
taskX

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

Example 19.13. Adding dependency using closure

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}

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

task lib1 {
    doLast {
        println 'lib1'
    }
}

task lib2 {
    doLast {
        println 'lib2'
    }
}

task notALib {
    doLast {
        println 'notALib'
    }
}

Output of gradle -q taskX

> gradle -q taskX
lib1
lib2
taskX

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

19.5. Ordering tasks

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

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

Task ordering can be useful in a number of scenarios:

  • Enforce sequential ordering of tasks: e.g. 'build' never runs before 'clean'.
  • Run build validations early in the build: e.g. validate I have the correct credentials before starting the work for a release build.
  • Get feedback faster by running quick verification tasks before long verification tasks: e.g. unit tests should run before integration tests.
  • A task that aggregates the results of all tasks of a particular type: e.g. test report task combines the outputs of all executed test tasks.

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

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

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

Example 19.14. Adding a 'must run after' task ordering

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
taskY.mustRunAfter taskX

Output of gradle -q taskY taskX

> gradle -q taskY taskX
taskX
taskY

Example 19.15. Adding a 'should run after' task ordering

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
taskY.shouldRunAfter taskX

Output of gradle -q taskY taskX

> gradle -q taskY taskX
taskX
taskY

In the examples above, it is still possible to execute taskY without causing taskX to run:

Example 19.16. 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 19.17. A 'should run after' task ordering is ignored if it introduces an ordering cycle

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}
task taskZ {
    doLast {
        println 'taskZ'
    }
}
taskX.dependsOn taskY
taskY.dependsOn taskZ
taskZ.shouldRunAfter taskX

Output of gradle -q taskX

> gradle -q taskX
taskZ
taskY
taskX

19.6. Adding a description to a task

You can add a description to your task. This description is displayed when executing gradle tasks.

Example 19.18. 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')
}

19.7. 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 19.19. Overwriting a task

build.gradle

task copy(type: Copy)

task copy(overwrite: true) {
    doLast {
        println('I am the new one.')
    }
}

Output of gradle -q copy

> gradle -q copy
I am the new one.

This will replace a task of type Copy with the task you've defined, because it uses the same name. When you define the new task, you have to set the overwrite property to true. Otherwise Gradle throws an exception, saying that a task with that name already exists.

19.8. Skipping tasks

Gradle offers multiple ways to skip the execution of a task.

19.8.1. 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 19.20. Skipping a task using a predicate

build.gradle

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

hello.onlyIf { !project.hasProperty('skipHello') }

Output of gradle hello -PskipHello

> gradle hello -PskipHello
:hello SKIPPED

BUILD SUCCESSFUL

Total time: 1 secs

19.8.2. 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 19.21. Skipping tasks with StopExecutionException

build.gradle

task compile {
    doLast {
        println 'We are doing the compile.'
    }
}

compile.doFirst {
    // Here you would put arbitrary conditions in real life.
    // But this is used in an integration test so we want defined behavior.
    if (true) { throw new StopExecutionException() }
}
task myTask(dependsOn: 'compile') {
    doLast {
        println 'I am not affected'
    }
}

Output of gradle -q myTask

> gradle -q myTask
I am not affected

This feature is helpful if you work with tasks provided by Gradle. It allows you to add conditional execution of the built-in actions of such a task. [7]

19.8.3. 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.

Example 19.22. Enabling and disabling tasks

build.gradle

task disableMe {
    doLast {
        println 'This should not be printed if the task is disabled.'
    }
}
disableMe.enabled = false

Output of gradle disableMe

> gradle disableMe
:disableMe SKIPPED

BUILD SUCCESSFUL

Total time: 1 secs

19.9. 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[8] every time the UP-TO-DATE text appears next to the name of a task when you run a build.

How does incremental build work? And what does it take to make use of it in your own tasks? Let’s take a look.

19.9.1. Task inputs and outputs

In the most common case, a task takes some inputs and generates some outputs. If we use the compilation example from earlier, we can see that the source files are the inputs and, in the case of Java, the generated class files are the outputs. Other inputs might include things like whether debug information should be included.

Figure 19.1. Example task inputs and outputs

Example task inputs and outputs

An important characteristic of an input is that it affects one or more outputs, as you can see from the previous figure. Different bytecode is generated depending on the content of the source files and the minimum version of the Java runtime you want to run the code on. That makes them task inputs. But whether the compilation runs in a forked process or not, determined by the fork property, has no impact on what bytecode gets generated. In Gradle terminology, fork 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 fields or properties (via getter methods) for each of your task inputs and outputs
  2. Add the appropriate annotation to each of those fields or getter methods

Gradle supports three main categories of inputs and outputs:

  • Simple values

    Things like strings and numbers. More generally, a simple value can have any type that implements Serializable.

  • Filesystem types

    These consist of the standard File class but also derivatives of Gradle’s FileCollection type and anything else that can be passed to either the Project.file(java.lang.Object) method - for single file/directory properties - or the Project.files(java.lang.Object[]) method.

  • Nested values

    Custom types that don’t conform to the other two categories but have their own properties that are inputs or outputs. In effect, the task inputs or outputs are nested inside these custom types.

As an example, imagine you have a task that processes templates of varying types, such as FreeMarker, Velocity, Moustache, etc. It takes template source files and combines them with some model data to generate populated versions of the template files.

This task will have three inputs and one output:

  • Template source files
  • Model data
  • Template engine
  • Where the output files are written

When you’re writing a custom task class, it’s easy to register properties as inputs or outputs via annotations. To demonstrate, here is a skeleton task implementation with some suitable inputs and outputs, along with their annotations:

Example 19.23. 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
:processTemplates

BUILD SUCCESSFUL

Output of gradle processTemplates

> gradle processTemplates
:processTemplates UP-TO-DATE

BUILD SUCCESSFUL

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 19.1. 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.

To stay compatible with Gradle versions before 3.2, classpath properties should also be annotated with @InputFiles.

@OutputFile File* A single output file (not directory)
@OutputDirectory File* A single output directory (not file)
@OutputFiles Map<String, File>** or Iterable<File>* An iterable of output files (no directories)
@OutputDirectories Map<String, File>** or Iterable<File>* An iterable of output directories (no files)
@Nested Any custom type A custom type that may not implement Serializable but does have at least one field or property marked with one of the annotations in this table. It could even be another @Nested.
@Console Any type Indicates that the property is neither an input nor an output. It simply affects the console output of the task in some way, such as increasing or decreasing the verbosity of the task.
@Internal Any type Indicates that the property is used internally but is neither an input nor an output.
* In fact, File can be any type accepted by Project.file(java.lang.Object) and Iterable<File> can be any type accepted by Project.files(java.lang.Object[]). This includes instances of Callable, such as closures, allowing for lazy evaluation of the property values. Be aware that the types FileCollection and FileTree are Iterable<File>s.
** Similar to the above, File can be any type accepted by Project.file(java.lang.Object). The Map itself can be wrapped in Callables, such as closures.

Table 19.2. Additional annotations used to further qualifying property type annotations

Annotation Description
@SkipWhenEmpty

Used with @InputFiles or @InputDirectory to tell Gradle to skip the task if the corresponding files or directory are empty, along with all other input files declared with this annotation. Tasks that have been skipped due to all of their input files that were declared with this annotation being empty will result in a distinct “no source” outcome. For example, NO-SOURCE will be emitted in the console output.

@Optional Used with any of the property type annotations listed in the Optional API documentation. This annotation disables validation checks on the corresponding property. See the section on validation for more details.
@OrderSensitive

Used with @InputFiles or @InputDirectory to tell Gradle that a change in the order of the files should mark the task out-of-date.

This functionality is deprecated and will be removed in Gradle 4.0. For classpath properties, use @Classpath instead.

@PathSensitive Used with any input file property to tell Gradle to only consider the given part of the file paths as important. For example, if a property is annotated with @PathSensitive(PathSensitivity.NAME_ONLY), then moving the files around without changing their contents will not make the task out-of-date.

Annotations are inherited from all parent types including implemented interfaces. Property type annotations override any other property type annotation declared in a parent type. This way an @InputFile property can be turned into an @InputDirectory property in a child task type.

Annotations on a property declared in a type override similar annotations declared by the superclass and in any implemented interfaces. Superclass annotations take precedence over annotations declared in implemented interfaces.

The Console and Internal annotations in the table are special cases as they don’t declare either task inputs or task outputs. So why use them? It's so that you can take advantage of the Java Gradle Plugin Development plugin to help you develop and publish your own plugins. This plugin checks whether any properties of your custom task classes lack an incremental build annotation. This protects you from forgetting to add an appropriate annotation during development.

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.

Runtime API

When you don’t have access to the source for a custom task class, there is no way to add any of the annotations we covered in the previous section. Fortunately, Gradle provides a runtime API for scenarios just like that. It can also be used for ad-hoc tasks, as you'll see next.

Using it for ad-hoc tasks

This runtime API is provided through a couple of aptly named properties that are available on every Gradle task:

These objects have methods that allow you to specify files, directories and values which constitute the task’s inputs and outputs. In fact, the runtime API has almost feature parity with the annotations. All it lacks is validation of whether declared files are actually files and declared directories are directories. Nor will it create output directories if they don't exist. But that's it.

Let’s take the template processing example from before and see how it would look as an ad-hoc task that uses the runtime API:

Example 19.24. Ad-hoc task

build.gradle

task processTemplatesAdHoc {
    inputs.property("engine", TemplateEngineType.FREEMARKER)
    inputs.files(fileTree("src/templates"))
    inputs.property("templateData.name", "docs")
    inputs.property("templateData.variables", [year: 2013])
    outputs.dir("$buildDir/genOutput2")

    doLast {
        // Process the templates here
    }
}

Output of gradle processTemplatesAdHoc

> gradle processTemplatesAdHoc
:processTemplatesAdHoc

BUILD SUCCESSFUL

As before, there’s much to talk about. To begin with, you should really write a custom task class for this as it’s a non-trivial implementation that has several configuration options. In this case, there are no task properties to store the root source folder, the location of the output directory or any of the other settings. That’s deliberate to highlight the fact that the runtime API doesn’t require the task to have any state. In terms of incremental build, the above ad-hoc task will behave the same as the custom task class.

All the input and output definitions are done through the methods on inputs and outputs, such as property(), files(), and dir(). Gradle performs up-to-date checks on the argument values to determine whether the task needs to run again or not. Each method corresponds to one of the incremental build annotations, for example inputs.property() maps to @Input and outputs.dir() maps to @OutputDirectory. The only difference is that the file(), files(), dir() and dirs() methods don’t validate the type of file object at the given path (file or directory), unlike the annotations.

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.

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 19.25. Using runtime API with custom task type

build.gradle

task processTemplatesRuntime(type: ProcessTemplatesNoAnnotations) {
    templateEngine = TemplateEngineType.FREEMARKER
    sourceFiles = fileTree("src/templates")
    templateData = new TemplateData("test", [year: 2014])
    outputDir = file("$buildDir/genOutput3")

    inputs.property("engine",templateEngine)
    inputs.files(sourceFiles)
    inputs.property("templateData.name", templateData.name)
    inputs.property("templateData.variables", templateData.variables)
    outputs.dir(outputDir)
}

Output of gradle processTemplatesRuntime

> gradle processTemplatesRuntime
:processTemplatesRuntime

BUILD SUCCESSFUL

Output of gradle processTemplatesRuntime

> gradle processTemplatesRuntime
:processTemplatesRuntime UP-TO-DATE

BUILD SUCCESSFUL

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 let’s 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 19.26. Using skipWhenEmpty() via the runtime API

build.gradle

task processTemplatesRuntimeConf(type: ProcessTemplatesNoAnnotations) {
    // ...
    sourceFiles = fileTree("src/templates") {
        include "**/*.fm"
    }

    inputs.files(sourceFiles).skipWhenEmpty()
    // ...
}

Output of gradle clean processTemplatesRuntimeConf

> gradle clean processTemplatesRuntimeConf
:processTemplatesRuntimeConf NO-SOURCE

BUILD SUCCESSFUL

The TaskInputs.files() method returns a builder that has a skipWhenEmpty() method. Invoking this method is equivalent to annotating to the property with @SkipWhenEmpty.

Prior to Gradle 3.0, you had to use the TaskInputs.source() and TaskInputs.sourceDir() methods to get the same behavior as with skipWhenEmpty(). These methods are now deprecated and should not be used with Gradle 3.0 and above.

Now that you have seen both the annotations and the runtime API, you may be wondering which API you should be using. Our recommendation is to use the annotations wherever possible, and it’s sometimes worth creating a custom task class just so that you can make use of them. The runtime API is more for situations in which you can’t use the annotations.

Important beneficial side effects

Once you declare a task’s formal inputs and outputs, Gradle can then infer things about those properties. For example, if an input of one task is set to the output of another, that means the first task depends on the second, right? Gradle knows this and can act upon it.

We’ll look at this feature next and also some other features that come from Gradle knowing things about inputs and outputs.

Inferred task dependencies

Consider an archive task that packages the output of the processTemplates task. A build author will see that the archive task obviously requires processTemplates to run first and so may add an explicit dependsOn. However, if you define the archive task like so:

Example 19.27. Inferred task dependency via task outputs

build.gradle

task packageFiles(type: Zip) {
    from processTemplates.outputs
}

Output of gradle clean packageFiles

> gradle clean packageFiles
:processTemplates
:packageFiles

BUILD SUCCESSFUL

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 19.28. Inferred task dependency via a task argument

build.gradle

task packageFiles2(type: Zip) {
    from processTemplates
}

Output of gradle clean packageFiles2

> gradle clean packageFiles2
:processTemplates
:packageFiles2

BUILD SUCCESSFUL

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

One last 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 Chapter 9, Continuous build.

19.9.2. 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 19.29. Declaring a method to add task inputs

buildSrc/src/main/java/org/example/ProcessTemplates.java

public class ProcessTemplates extends DefaultTask {
    // ...
    private FileCollection sourceFiles = getProject().files();

    @SkipWhenEmpty
    @InputFiles
    @PathSensitive(PathSensitivity.NONE)
    public FileCollection getSourceFiles() {
        return this.sourceFiles;
    }

    public void sources(FileCollection sourceFiles) {
        this.sourceFiles = this.sourceFiles.plus(sourceFiles);
    }

    // ...
}

build.gradle

task processTemplates(type: ProcessTemplates) {
    templateEngine = TemplateEngineType.FREEMARKER
    templateData = new TemplateData("test", [year: 2012])
    outputDir = file("$buildDir/genOutput")

    sources fileTree("src/templates")
}

Output of gradle processTemplates

> gradle processTemplates
:processTemplates

BUILD SUCCESSFUL

In other words, as long as you add values and files to formal task inputs and outputs during the configuration phase, they will be treated as such regardless from where in the build you add them.

If we want to support tasks as arguments as well and treat their outputs as the inputs, we can use the project.files() method like so:

Example 19.30. Declaring a method to add a task as an input

buildSrc/src/main/java/org/example/ProcessTemplates.java

// ...
public void sources(Task inputTask) {
    this.sourceFiles = this.sourceFiles.plus(getProject().files(inputTask));
}
// ...

build.gradle

task copyTemplates(type: Copy) {
    into "$buildDir/tmp"
    from "src/templates"
}

task processTemplates2(type: ProcessTemplates) {
    // ...
    sources copyTemplates
}

Output of gradle processTemplates2

> gradle processTemplates2
:copyTemplates
:processTemplates2

BUILD SUCCESSFUL

This technique can make your custom task easier to use and result in cleaner build files. As an added benefit, our use of getProject().files() means that our custom method can set up an inferred task dependency.

One last thing to note: if you are developing a task that takes collections of source files as inputs, like this example, consider using the built-in SourceTask. It will save you having to implement some of the plumbing that we put into ProcessTemplates.

Linking an @OutputDirectory to an @InputFiles

When you want to link the output of one task to the input of another, the types often match and a simple property assignment will provide that link. For example, a File output property can be assigned to a File input.

Unfortunately, this approach breaks down when you want the files in a task’s @OutputDirectory (of type File) to become the source for another task’s @InputFiles property (of type FileCollection). Since the two have different types, property assignment won’t work.

As an example, imagine you want to use the output of a Java compilation task - via the destinationDir property - as the input of a custom task that instruments a set of files containing Java bytecode. This custom task, which we’ll call Instrument, has a classFiles property annotated with @InputFiles. You might initially try to configure the task like so:

Example 19.31. Failed attempt at setting up an inferred task dependency

build.gradle

apply plugin: "java"

task badInstrumentClasses(type: Instrument) {
    classFiles = fileTree(compileJava.destinationDir)
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean badInstrumentClasses

> gradle clean badInstrumentClasses
:clean UP-TO-DATE
:badInstrumentClasses NO-SOURCE

BUILD SUCCESSFUL

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 19.32. Setting up an inferred task dependency between output dir and input files

build.gradle

task instrumentClasses(type: Instrument) {
    classFiles = compileJava.outputs.files
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean instrumentClasses

> gradle clean instrumentClasses
:clean UP-TO-DATE
:compileJava
:instrumentClasses

BUILD SUCCESSFUL

Alternatively, you can get Gradle to access the appropriate property itself by using the project.files() method in place of project.fileTree():

Example 19.33. Setting up an inferred task dependency with files()

build.gradle

task instrumentClasses2(type: Instrument) {
    classFiles = files(compileJava)
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean instrumentClasses2

> gradle clean instrumentClasses2
:clean UP-TO-DATE
:compileJava
:instrumentClasses2

BUILD SUCCESSFUL

Remember that files() can take tasks as arguments, whereas fileTree() cannot.

The downside of this approach is that all file outputs of the source task become the input files of the target - instrumentClasses in this case. That’s fine as long as the source task only has a single file-based output, like the JavaCompile task. But if you have to link just one output property among several, then you need to explicitly tell Gradle which task generates the input files using the builtBy method:

Example 19.34. Setting up an inferred task dependency with builtBy()

build.gradle

task instrumentClassesBuiltBy(type: Instrument) {
    classFiles = fileTree(compileJava.destinationDir) {
        builtBy compileJava
    }
    destinationDir = file("$buildDir/instrumented")
}

Output of gradle clean instrumentClassesBuiltBy

> gradle clean instrumentClassesBuiltBy
:clean UP-TO-DATE
:compileJava
:instrumentClassesBuiltBy

BUILD SUCCESSFUL

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 19.35. Ignoring up-to-date checks

build.gradle

task alwaysInstrumentClasses(type: Instrument) {
    classFiles = files(compileJava)
    destinationDir = file("$buildDir/instrumented")
    outputs.upToDateWhen { false }
}

Output of gradle clean alwaysInstrumentClasses

> gradle clean alwaysInstrumentClasses
:compileJava
:alwaysInstrumentClasses

BUILD SUCCESSFUL

Output of gradle alwaysInstrumentClasses

> gradle alwaysInstrumentClasses
:compileJava UP-TO-DATE
:alwaysInstrumentClasses

BUILD SUCCESSFUL

The { false } closure ensures that copyResources will always perform the copy, irrespective of whether there is no change in the inputs or outputs.

You can of course put more complex logic into the closure. You could check whether a particular record in a database table exists or has changed for example. Just be aware that up-to-date checks should _save_ you time. Don’t add checks that cost as much or more time than the standard execution of the task. In fact, if a task ends up running frequently anyway, because it’s rarely up to date, then it may not be worth having an up-to-date check at all. Remember that your checks will always run if the task is in the execution task graph.

One common mistake is to use upToDateWhen() instead of Task.onlyIf(). If you want to skip a task on the basis of some condition unrelated to the task inputs and outputs, then you should use onlyIf(). For example, in cases where you want to skip a task when a particular property is set or not set.

19.9.3. How does it work?

Before a task is executed for the first time, Gradle takes a snapshot of the inputs. This snapshot contains the paths of input files and a hash of the contents of each file. Gradle then executes the task. If the task completes successfully, Gradle takes a snapshot of the outputs. This snapshot contains the set of output files and a hash of the contents of each file. Gradle persists both snapshots for the next time the task is executed.

Each time after that, before the task is executed, Gradle takes a new snapshot of the inputs and outputs. If the new snapshots are the same as the previous snapshots, Gradle assumes that the outputs are up to date and skips the task. If they are not the same, Gradle executes the task. Gradle persists both snapshots for the next time the task is executed.

Gradle also considers the code of the task as part of the inputs to the task. When a task, its actions, or its dependencies change between executions, Gradle considers the task as out-of-date.

Gradle understands if a file property (e.g. one holding a Java classpath) is order-sensitive. When comparing the snapshot of such a property, even a change in the order of the files will result in the task becoming out-of-date.

Note that if a task has an output directory specified, any files added to that directory since the last time it was executed are ignored and will NOT cause the task to be out of date. This is so unrelated tasks may share an output directory without interfering with each other. If this is not the behaviour you want for some reason, consider using TaskOutputs.upToDateWhen(groovy.lang.Closure)

19.10. 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 19.36. Task rule

build.gradle

tasks.addRule("Pattern: ping<ID>") { String taskName ->
    if (taskName.startsWith("ping")) {
        task(taskName) {
            doLast {
                println "Pinging: " + (taskName - 'ping')
            }
        }
    }
}

Output of gradle -q pingServer1

> gradle -q pingServer1
Pinging: Server1

The String parameter is used as a description for the rule, which is shown with gradle tasks.

Rules are not only used when calling tasks from the command line. You can also create dependsOn relations on rule based tasks:

Example 19.37. Dependency on rule based tasks

build.gradle

tasks.addRule("Pattern: ping<ID>") { String taskName ->
    if (taskName.startsWith("ping")) {
        task(taskName) {
            doLast {
                println "Pinging: " + (taskName - 'ping')
            }
        }
    }
}

task groupPing {
    dependsOn pingServer1, pingServer2
}

Output of gradle -q groupPing

> gradle -q groupPing
Pinging: Server1
Pinging: Server2

If you run “gradle -q tasks” you won't find a task named “pingServer1” or “pingServer2”, but this script is executing logic based on the request to run those tasks.

19.11. Finalizer tasks

Finalizers tasks are an incubating feature (see Section C.1.2, “Incubating”).

Finalizer tasks are automatically added to the task graph when the finalized task is scheduled to run.

Example 19.38. Adding a task finalizer

build.gradle

task taskX {
    doLast {
        println 'taskX'
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.finalizedBy taskY

Output of gradle -q taskX

> gradle -q taskX
taskX
taskY

Finalizer tasks will be executed even if the finalized task fails.

Example 19.39. Task finalizer for a failing task

build.gradle

task taskX {
    doLast {
        println 'taskX'
        throw new RuntimeException()
    }
}
task taskY {
    doLast {
        println 'taskY'
    }
}

taskX.finalizedBy taskY

Output of gradle -q taskX

> gradle -q taskX
taskX
taskY

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[]).

19.12. 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.



[7] You might be wondering why there is neither an import for the StopExecutionException nor do we access it via its fully qualified name. The reason is, that Gradle adds a set of default imports to your script (see Section 18.8, “Default imports”).

[8] You will also see UP-TO-DATE next to tasks that have no actions, even though that’s nothing to do with incremental build.