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 JVM 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 (or rootProjectDir/buildSrc/src/main/java or rootProjectDir/buildSrc/src/main/kotlin` depending on which language you prefer). 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 1. 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 2. 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 3. 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 4. 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())
}
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 5. 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 optimize your build so that only out-of-date input files are processed, you can do so with an incremental task.

There is the IncrementalTaskInputs API, which is available in Gradle versions before 5.4. The usage of this API is discouraged, since it does not allow querying individual input properties for changes. The IncrementalTaskInputs API will be deprecated and eventually removed. If you need to use the old API, have a look at the documentation in the user manual for Gradle 5.3.1.

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 has a single InputChanges parameter. That parameter tells Gradle that the action only wants to process the changed inputs. In addition, the task needs to declare at least one incremental file input property by using either @Incremental or @SkipWhenEmpty.

❗️

To query incremental changes for an input file property, that property always needs to return the same instance. The easiest way to accomplish this is to use one of the following types for such properties: RegularFileProperty, DirectoryProperty or ConfigurableFileCollection.

You can learn more about RegularFileProperty and DirectoryProperty in the Lazy Configuration chapter, especially the sections on using read-only and configurable properties and lazy file properties.

The incremental task action can use InputChanges.getFileChanges() to find out what files have changed for a given file-based input property, be it of type RegularFileProperty, DirectoryProperty or ConfigurableFileCollection. The method returns an Iterable of type FileChanges, which in turn can be queried for the following:

The following example demonstrates an incremental task that has a directory input. It assumes that the directory contains a collection of text files and copies them to an output directory, reversing the text within each file. The key things to note are the type of the inputDir property, its annotations, and how the action (execute()) uses getFileChanges() to process the subset of files that have actually changed since the last build. You can also see how the action deletes a target file if the corresponding input file has been removed:

Example 6. Defining an incremental task action
build.gradle
abstract class IncrementalReverseTask extends DefaultTask {
    @Incremental
    @PathSensitive(PathSensitivity.NAME_ONLY)
    @InputDirectory
    abstract DirectoryProperty getInputDir()

    @OutputDirectory
    abstract DirectoryProperty getOutputDir()

    @Input
    abstract Property<String> getInputProperty()

    @TaskAction
    void execute(InputChanges inputChanges) {
        println(inputChanges.incremental
            ? 'Executing incrementally'
            : 'Executing non-incrementally'
        )

        inputChanges.getFileChanges(inputDir).each { change ->
            if (change.fileType == FileType.DIRECTORY) return

            println "${change.changeType}: ${change.normalizedPath}"
            def targetFile = outputDir.file(change.normalizedPath).get().asFile
            if (change.changeType == ChangeType.REMOVED) {
                targetFile.delete()
            } else {
                targetFile.text = change.file.text.reverse()
            }
        }
    }
}
build.gradle.kts
abstract class IncrementalReverseTask : DefaultTask() {
    @get:Incremental
    @get:PathSensitive(PathSensitivity.NAME_ONLY)
    @get:InputDirectory
    abstract val inputDir: DirectoryProperty

    @get:OutputDirectory
    abstract val outputDir: DirectoryProperty

    @get:Input
    abstract val inputProperty: Property<String>

    @TaskAction
    fun execute(inputChanges: InputChanges) {
        println(
            if (inputChanges.isIncremental) "Executing incrementally"
            else "Executing non-incrementally"
        )

        inputChanges.getFileChanges(inputDir).forEach { change ->
            if (change.fileType == FileType.DIRECTORY) return@forEach

            println("${change.changeType}: ${change.normalizedPath}")
            val targetFile = outputDir.file(change.normalizedPath).get().asFile
            if (change.changeType == ChangeType.REMOVED) {
                targetFile.delete()
            } else {
                targetFile.writeText(change.file.readText().reversed())
            }
        }
    }
}
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 executed non-incrementally, for example by running with --rerun-tasks, all files are reported as ADDED, irrespective of the previous state. In this case, Gradle automatically removes the previous outputs, so the incremental task only needs to process the given files.

For a simple transformer task like the above exmaple, 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 there is a previous execution of the task, and the only changes since that execution are to incremental input file properties, then Gradle is able to determine which input files need to be processed (incremental execution). In this case, the InputChanges.getFileChanges() method returns details for all input files for the given property that were added, modified or removed.

However, there are many cases where Gradle is unable to determine which input files need to be processed (non-incremental execution). 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 criterion added to the task returns false.

  • An input property has changed since the previous execution.

  • A non-incremental input file property has changed since the previous execution.

  • One or more output files have changed since the previous execution.

In all of these cases, Gradle will report all input files as ADDED and the getFileChanges() method will return details for all the files that comprise the given input property.

You can check if the task execution is incremental or not with the InputChanges.isIncremental() method.

An incremental task in action

Given the example incremental task implementation above, let’s walk through some scenarios based on it.

First, consider an instance of IncrementalReverseTask that is executed against a set of inputs for the first time. In this case, all inputs will be considered added, as shown here:

Example 7. Running the incremental task for the first time
build.gradle
task incrementalReverse(type: IncrementalReverseTask) {
    inputDir = file('inputs')
    outputDir = file("$buildDir/outputs")
    inputProperty = project.properties['taskInputProperty'] ?: 'original'
}
build.gradle.kts
tasks.register<IncrementalReverseTask>("incrementalReverse") {
    inputDir.set(file("inputs"))
    outputDir.set(file("$buildDir/outputs"))
    inputProperty.set(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
Executing non-incrementally
ADDED: 1.txt
ADDED: 2.txt
ADDED: 3.txt

Naturally when the task is executed again with no changes, then the entire task is up to date and the task action is not executed:

Example 8. Running the incremental task with unchanged inputs
Output of gradle incrementalReverse
> gradle incrementalReverse
> Task :incrementalReverse UP-TO-DATE

BUILD SUCCESSFUL in 0s
1 actionable task: 1 up-to-date

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 returned by InputChanges.getFileChanges(). The following example modifies the content of one file and adds another before running the incremental task:

Example 9. 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
Executing incrementally
MODIFIED: 1.txt
ADDED: 4.txt
The various mutation tasks (updateInputs, removeInput, etc) are only present to demonstrate the behavior of incremental tasks. They should not be viewed as the kinds of tasks or task implementations you should have in your own build scripts.

When an existing input file is removed, then re-executing the task results in that file being returned by InputChanges.getFileChanges() as REMOVED. The following example removes one of the existing files before executing the incremental task:

Example 10. 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
Executing incrementally
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, details for all the input files for the given property are returned by InputChanges.getFileChanges(). The following example removes just one of the output files from the build directory, but notice how all the input files are considered to be ADDED:

Example 11. 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
Executing non-incrementally
ADDED: 1.txt
ADDED: 2.txt
ADDED: 3.txt

The last scenario we want to cover concerns what happens when a non-file-based input property is modified. In such cases, Gradle is unable to determine how the property impacts the task outputs, so the task is executed non-incrementally. This means that all input files for the given property are returned by InputChanges.getFileChanges() and they are all treated as ADDED. The following example sets the project property taskInputProperty to a new value when running the incrementalReverse task and that project property is used to initialize the task’s inputProperty property, as you can see in the first example of this section. Here’s the output you can expect in this case:

Example 12. Running the incremental task with an input property changed
Output of gradle -q -PtaskInputProperty=changed incrementalReverse
> gradle -q -PtaskInputProperty=changed incrementalReverse
Executing non-incrementally
ADDED: 1.txt
ADDED: 2.txt
ADDED: 3.txt

Storing incremental state for cached tasks

Using Gradle’s InputChanges is not the only way to create tasks that only work 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 some analysis data about the state of the previous execution in some file. If such state files are relocatable, then they can be declared as outputs of the task. This way when the task’s results are loaded from cache, the next execution can already use the analysis data loaded from cache, too.

However, if the state files are non-relocatable, then they can’t be shared via the build cache. Indeed, when the task is loaded from cache, any such state files must be cleaned up to prevent stale state from confusing the tool during the next execution. Gradle can ensure such stale files are removed if they are declared via task.localState.register() or if a property is marked with the @LocalState annotation.

Declaring and Using Command Line Options

The API for exposing command line options is an incubating feature.

Sometimes a user wants to declare the value of an exposed task property on the command line instead of the build script. Being able to pass in property values on the command line is particularly helpful if they change more frequently. The task API supports a mechanism for marking a property to automatically generate a corresponding command line parameter with a specific name at runtime.

Declaring a command-line option

Exposing a new command line option for a task property is straightforward. You just have to annotate the corresponding setter method of a property with Option. An option requires a mandatory identifier. Additionally, you can provide an optional description. A task can expose as many command line options as properties available in the class.

Let’s have a look at an example to illustrate the functionality. The custom task UrlVerify verifies whether a given URL can be resolved by making a HTTP call and checking the response code. The URL to be verified is configurable through the property url. The setter method for the property is annotated with @Option.

Example: Declaring a command line option

UrlVerify.java
import org.gradle.api.tasks.options.Option;

public class UrlVerify extends DefaultTask {
    private String url;

    @Option(option = "url", description = "Configures the URL to be verified.")
    public void setUrl(String url) {
        this.url = url;
    }

    @Input
    public String getUrl() {
        return url;
    }

    @TaskAction
    public void verify() {
        getLogger().quiet("Verifying URL '{}'", url);

        // verify URL by making a HTTP call
    }
}

All options declared for a task can be rendered as console output by running the help task and the --task option.

Using an option on the command line

Using an option on the command line has to adhere to the following rules:

  • The option uses a double-dash as prefix e.g. --url. A single dash does not qualify as valid syntax for a task option.

  • The option argument follows directly after the task declaration e.g. verifyUrl --url=http://www.google.com/.

  • Multiple options of a task can be declared in any order on the command line following the task name.

Getting back to the previous example, the build script creates a task instance of type UrlVerify and provides a value from the command line through the exposed option.

Example 13. Using a command line option
build.gradle
task verifyUrl(type: UrlVerify)
build.gradle.kts
tasks.register<UrlVerify>("verifyUrl")
Output of gradle -q verifyUrl --url=http://www.google.com/
> gradle -q verifyUrl --url=http://www.google.com/
Verifying URL 'http://www.google.com/'

Supported data types for options

Gradle limits the set of data types that can be used for declaring command line options. The use on the command line differ per type.

boolean, Boolean, Property<Boolean>

Describes an option with the value true or false. Passing the option on the command line treats the value as true. For example --enabled equates to true. The absence of the option uses the default value of the property.

String, Property<String>

Describes an option with an arbitrary String value. Passing the option on the command line also requires a value e.g. --container-id=2x94held or --container-id 2x94held.

enum, Property<enum>

Describes an option as an enumerated type. Passing the option on the command line also requires a value e.g. --log-level=DEBUG or --log-level debug. The value is not case sensitive.

List<String>, List<enum>

Describes an option that can takes multiple values of a given type. The values for the option have to be provided as multiple declarations e.g. --image-id=123 --image-id=456. Other notations such as comma-separated lists or multiple values separated by a space character are currently not supported.

Documenting available values for an option

In theory, an option for a property type String or List<String> can accept any arbitrary value. Expected values for such an option can be documented programmatically with the help of the annotation OptionValues. This annotation may be assigned to any method that returns a List of one of the supported data types. In addition, you have to provide the option identifier to indicate the relationship between option and available values.

Passing a value on the command line that is not supported by the option does not fail the build or throw an exception. You’ll have to implement custom logic for such behavior in the task action.

This example demonstrates the use of multiple options for a single task. The task implementation provides a list of available values for the option output-type.

Example: Declaring available values for an option

UrlProcess.java
import org.gradle.api.tasks.options.Option;
import org.gradle.api.tasks.options.OptionValues;

public class UrlProcess extends DefaultTask {
    private String url;
    private OutputType outputType;

    @Option(option = "url", description = "Configures the URL to be write to the output.")
    public void setUrl(String url) {
        this.url = url;
    }

    @Input
    public String getUrl() {
        return url;
    }

    @Option(option = "output-type", description = "Configures the output type.")
    public void setOutputType(OutputType outputType) {
        this.outputType = outputType;
    }

    @OptionValues("output-type")
    public List<OutputType> getAvailableOutputTypes() {
        return new ArrayList<OutputType>(Arrays.asList(OutputType.values()));
    }

    @Input
    public OutputType getOutputType() {
        return outputType;
    }

    @TaskAction
    public void process() {
        getLogger().quiet("Writing out the URL response from '{}' to '{}'", url, outputType);

        // retrieve content from URL and write to output
    }

    private static enum OutputType {
        CONSOLE, FILE
    }
}

Listing command line options

Command line options using the annotations Option and OptionValues are self-documenting. You will see declared options and their available values reflected in the console output of the help task. The output renders options in alphabetical order.

Example: Listing available values for option

Output of gradle -q help --task processUrl
> gradle -q help --task processUrl
Detailed task information for processUrl

Path
     :processUrl

Type
     UrlProcess (UrlProcess)

Options
     --output-type     Configures the output type.
                       Available values are:
                            CONSOLE
                            FILE

     --url     Configures the URL to be write to the output.

Description
     -

Group
     -

Limitations

Support for declaring command line options currently comes with a few limitations.

  • Command line options can only be declared for custom tasks via annotation. There’s no programmatic equivalent for defining options.

  • Options cannot be declared globally e.g. on a project-level or as part of a plugin.

  • When assigning an option on the command line then the task exposing the option needs to be spelled out explicitly e.g. gradle check --tests abc does not work even though the check task depends on the test task.

Service injection

Gradle provides a number of services that can be used in a task implementation. For example, the WorkerExecutor service can be used by a task to run work in parallel, as we’ll see in the following section.

There are 2 ways that a task can receive the services that it needs. The first option is to add the service as a parameter of the task class constructor. The constructor must be annotated with the javax.inject.Inject annotation. Gradle uses the declared type of each parameter to determine the services that the task requires. The order of the constructor parameters and their names are ignored.

Alternatively, a service can be injected by adding a property getter method annotated with the javax.inject.Inject annotation to the task class. This can be useful, for example, when you cannot change the constructor of the task class due to backwards compatibility constraints. This pattern also allows Gradle to defer creation of the service until it is required, rather than when the task instance is created, which helps with performance. Gradle uses the declared return type of the getter methods to determine the services that the task requires. The name of the property is ignored.

Here is an example that shows these two approaches:

UrlProcess.java
import javax.inject.Inject;
import org.gradle.api.model.ObjectFactory;
import org.gradle.api.DefaultTask ;
import org.gradle.api.tasks.TaskAction;
import org.gradle.workers.WorkerExecutor;

public class UrlProcess extends DefaultTask {
    // Inject an ObjectFactory into the constructor
    @Inject
    public UrlProcess(ObjectFactory objectFactory) {
        // Use the factory ...
    }

    // Alternatively, use a getter method with a dummy implementation
    @Inject
    public WorkerExecutor getWorkerExecutor() {
        // Method body is ignored
        throw new UnsupportedOperationException();
    }

    @TaskAction
    void run() {
        WorkerExecutor workerExecutor = getWorkerExecutor();
        // Use the executor ....
    }
}

Available services

The following services are available for injection:

The Worker API

The Worker API is an incubating feature.

As can be seen from the discussion of incremental tasks, the work that a task performs can be viewed as discrete units (i.e. a subset of inputs that are transformed to a certain subset of outputs). Many times, these units of work are highly independent of each other, meaning they can be performed in any order and simply aggregated together to form the overall action of the task. In a single threaded execution, these units of work would execute in sequence, however if we have multiple processors, it would be desirable to perform independent units of work concurrently. By doing so, we can fully utilize the available resources at build time and complete the activity of the task faster.

The Worker API provides a mechanism for doing exactly this. It allows for safe, concurrent execution of multiple items of work during a task action. But the benefits of the Worker API are not confined to parallelizing the work of a task. You can also configure a desired level of isolation such that work can be executed in an isolated classloader or even in an isolated process. Furthermore, the benefits extend beyond even the execution of a single task. Using the Worker API, Gradle can begin to execute tasks in parallel by default. In other words, once a task has submitted its work to be executed asynchronously, and has exited the task action, Gradle can then begin the execution of other independent tasks in parallel, even if those tasks are in the same project.

Using the Worker API

In order to submit work to the Worker API, two things must be provided: an implementation of the unit of work, and a configuration for the unit of work. The implementation is simply a class that extends java.lang.Runnable. This class should have a constructor that is annotated with javax.inject.Inject and accepts parameters that configure the class for a single unit of work. When a unit of work is submitted to the WorkerExecutor, an instance of this class will be created and the parameters configured for the unit of work will be passed to the constructor.

Example 14. Creating a unit of work implementation
build.gradle
import org.gradle.workers.WorkerExecutor

import javax.inject.Inject

// The implementation of a single unit of work
class ReverseFile implements Runnable {
    File fileToReverse
    File destinationFile

    @Inject
    public ReverseFile(File fileToReverse, File destinationFile) {
        this.fileToReverse = fileToReverse
        this.destinationFile = destinationFile
    }

    @Override
    public void run() {
        destinationFile.text = fileToReverse.text.reverse()
    }
}
build.gradle.kts
import org.gradle.workers.WorkerExecutor

import javax.inject.Inject

// The implementation of a single unit of work
open class ReverseFile @Inject constructor(val fileToReverse: File, val destinationFile: File) : Runnable {

    override fun run() {
        destinationFile.writeText(fileToReverse.readText().reversed())
    }
}

The configuration of the worker is represented by a WorkerConfiguration and is set by configuring an instance of this object at the time of submission. However, in order to submit the unit of work, it is necessary to first acquire the WorkerExecutor. To do this, a constructor should be provided that is annotated with javax.inject.Inject and accepts a WorkerExecutor parameter. Gradle will inject the instance of WorkerExecutor at runtime when the task is created.

Example 15. Submitting a unit of work for execution
build.gradle
class ReverseFiles extends SourceTask {
    final WorkerExecutor workerExecutor

    @OutputDirectory
    File outputDir

    // The WorkerExecutor will be injected by Gradle at runtime
    @Inject
    public ReverseFiles(WorkerExecutor workerExecutor) {
        this.workerExecutor = workerExecutor
    }

    @TaskAction
    void reverseFiles() {
        // Create and submit a unit of work for each file
        source.each { file ->
            workerExecutor.submit(ReverseFile.class) { WorkerConfiguration config ->
                // Use the minimum level of isolation
                config.isolationMode = IsolationMode.NONE

                // Constructor parameters for the unit of work implementation
                config.params file, project.file("$outputDir/${file.name}")
            }
        }
    }
}
build.gradle.kts
// The WorkerExecutor will be injected by Gradle at runtime
open class ReverseFiles @Inject constructor(val workerExecutor: WorkerExecutor) : SourceTask() {
    @OutputDirectory
    lateinit var outputDir: File

    @TaskAction
    fun reverseFiles() {
        // Create and submit a unit of work for each file
        source.forEach { file ->
            workerExecutor.submit(ReverseFile::class) {
                // Use the minimum level of isolation
                isolationMode = IsolationMode.NONE

                // Constructor parameters for the unit of work implementation
                params(file, project.file("$outputDir/${file.name}"))
            }
        }
    }
}

Note that one element of the WorkerConfiguration is the params property. These are the parameters passed to the constructor of the unit of work implementation for each item of work submitted. Any parameters provided to the unit of work must be java.io.Serializable.

Once all of the work for a task action has been submitted, it is safe to exit the task action. The work will be executed asynchronously and in parallel (up to the setting of max-workers). Of course, any tasks that are dependent on this task (and any subsequent task actions of this task) will not begin executing until all of the asynchronous work completes. However, other independent tasks that have no relationship to this task can begin executing immediately.

If any failures occur while executing the asynchronous work, the task will fail and a WorkerExecutionException will be thrown detailing the failure for each failed work item. This will be treated like any failure during task execution and will prevent any dependent tasks from executing.

In some cases, however, it might be desirable to wait for work to complete before exiting the task action. This is possible using the WorkerExecutor.await() method. As in the case of allowing the work to complete asynchronously, any failures that occur while executing an item of work will be surfaced as a WorkerExecutionException thrown from the WorkerExecutor.await() method.

Note that Gradle will only begin running other independent tasks in parallel when a task has exited a task action and returned control of execution to Gradle. When WorkerExecutor.await() is used, execution does not leave the task action. This means that Gradle will not allow other tasks to begin executing and will wait for the task action to complete before doing so.

Example 16. Waiting for asynchronous work to complete
build.gradle
        // Create and submit a unit of work for each file
        source.each { file ->
            workerExecutor.submit(ReverseFile.class) { config ->
                config.isolationMode = IsolationMode.NONE
                // Constructor parameters for the unit of work implementation
                config.params file, project.file("${outputDir}/${file.name}")
            }
        }

        // Wait for all asynchronous work to complete before continuing
        workerExecutor.await()
        logger.lifecycle("Created ${outputDir.listFiles().size()} reversed files in ${project.relativePath(outputDir)}")
build.gradle.kts
        // Create and submit a unit of work for each file
        source.forEach { file ->
            workerExecutor.submit(ReverseFile::class) {
                isolationMode = IsolationMode.NONE
                // Constructor parameters for the unit of work implementation
                params(file, project.file("$outputDir/${file.name}"))
            }
        }

        // Wait for all asynchronous work to complete before continuing
        workerExecutor.await()
        logger.lifecycle("Created ${outputDir.listFiles().size} reversed files in ${project.relativePath(outputDir)}")

Isolation Modes

Gradle provides three isolation modes that can be configured on a unit of work and are specified using the IsolationMode enum:

IsolationMode.NONE

This states that the work should be run in a thread with a minimum of isolation. For instance, it will share the same classloader that the task is loaded from. This is the fastest level of isolation.

IsolationMode.CLASSLOADER

This states that the work should be run in a thread with an isolated classloader. The classloader will have the classpath from the classloader that the unit of work implementation class was loaded from as well as any additional classpath entries added through WorkerConfiguration.classpath(java.lang.Iterable).

IsolationMode.PROCESS

This states that the work should be run with a maximum level of isolation by executing the work in a separate process. The classloader of the process will use the classpath from the classloader that the unit of work was loaded from as well as any additional classpath entries added through WorkerConfiguration.classpath(java.lang.Iterable). Furthermore, the process will be a Worker Daemon which will stay alive and can be reused for future work items that may have the same requirements. This process can be configured with different settings than the Gradle JVM using WorkerConfiguration.forkOptions(org.gradle.api.Action).

Worker Daemons

When using IsolationMode.PROCESS, gradle will start a long-lived Worker Daemon process that can be reused for future work items.

Example 17. Submitting an item of work to run in a worker daemon
build.gradle
            workerExecutor.submit(ReverseFile.class) { WorkerConfiguration config ->
                // Run this work in an isolated process
                config.isolationMode = IsolationMode.PROCESS

                // Configure the options for the forked process
                config.forkOptions { JavaForkOptions options ->
                    options.maxHeapSize = "512m"
                    options.systemProperty "org.gradle.sample.showFileSize", "true"
                }

                // Constructor parameters for the unit of work implementation
                config.params file, project.file("${outputDir}/${file.name}")
            }
build.gradle.kts
            workerExecutor.submit(ReverseFile::class) {
                // Run this work in an isolated process
                isolationMode = IsolationMode.PROCESS

                // Configure the options for the forked process
                forkOptions {
                    maxHeapSize = "512m"
                    systemProperty("org.gradle.sample.showFileSize", "true")
                }

                // Constructor parameters for the unit of work implementation
                params(file, project.file("$outputDir/${file.name}"))
            }

When a unit of work for a Worker Daemon is submitted, Gradle will first look to see if a compatible, idle daemon already exists. If so, it will send the unit of work to the idle daemon, marking it as busy. If not, it will start a new daemon. When evaluating compatibility, Gradle looks at a number of criteria, all of which can be controlled through WorkerConfiguration.forkOptions(org.gradle.api.Action).

executable

A daemon is considered compatible only if it uses the same java executable.

classpath

A daemon is considered compatible if its classpath contains all of the classpath entries requested. Note that a daemon is considered compatible if it has more classpath entries in addition to those requested.

heap settings

A daemon is considered compatible if it has at least the same heap size settings as requested. In other words, a daemon that has higher heap settings than requested would be considered compatible.

jvm arguments

A daemon is considered compatible if it has set all of the jvm arguments requested. Note that a daemon is considered compatible if it has additional jvm arguments beyond those requested (except for arguments treated specially such as heap settings, assertions, debug, etc).

system properties

A daemon is considered compatible if it has set all of the system properties requested with the same values. Note that a daemon is considered compatible if it has additional system properties beyond those requested.

environment variables

A daemon is considered compatible if it has set all of the environment variables requested with the same values. Note that a daemon is considered compatible if it has more environment variables in addition to those requested.

bootstrap classpath

A daemon is considered compatible if it contains all of the bootstrap classpath entries requested. Note that a daemon is considered compatible if it has more bootstrap classpath entries in addition to those requested.

debug

A daemon is considered compatible only if debug is set to the same value as requested (true or false).

enable assertions

A daemon is considered compatible only if enable assertions is set to the same value as requested (true or false).

default character encoding

A daemon is considered compatible only if the default character encoding is set to the same value as requested.

Worker daemons will remain running until either the build daemon that started them is stopped, or system memory becomes scarce. When available system memory is low, Gradle will begin stopping worker daemons in an attempt to minimize memory consumption.

Re-using logic between task classes

There are different ways to re-use logic between task classes. The easiest case is when you can extract the logic you want to share in a separate method or class and then use the extracted piece of code in your tasks. For example, the Copy task re-uses the logic of the Project.copy(org.gradle.api.Action) method. Another option is to add a task dependency on the task which outputs you want to re-use. Other options include using task rules or the worker API.