Chapter 49. Writing Custom Task Classes

Gradle supports two types of task. One such type is the simple task, where you define the task with an action closure. We have seen these in Chapter 6, 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 Chapter 16, More about Tasks. Most Gradle plugins use enhanced tasks. With enhanced tasks, you don't need to implement the task behaviour as you do with simple tasks. You simply declare the task and configure the task using its properties. In this way, enhanced tasks let you reuse a piece of behaviour in many different places, possibly across different builds.

The behaviour and properties of an enhanced task is defined by the task's class. When you declare an enhanced task, you specify the type, or class of the task.

Implementing your own custom task class in Gradle is easy. You can implement a custom task class in pretty much any language you like, provided it ends up compiled to bytecode. In our examples, we are going to use Groovy as the implementation language, but you could use, for example, Java or Scala. In general, using Groovy is the easiest option, because the Gradle API is designed to work well with Groovy.

49.1. Packaging a task class

There are several places where you can put the source for the task class.

Build script

You can include the task class directly in the build script. This has the benefit that the task class is automatically compiled and included in the classpath of the build script without you having to do anything. However, the task class is not visible outside the build script, and so you cannot reuse the task class outside the build script it is defined in.

buildSrc project

You can put the source for the task class in the rootProjectDir/buildSrc/src/main/groovy directory. Gradle will take care of compiling and testing the task class and making it available on the classpath of the build script. The task class is visible to every build script used by the build. However, it is not visible outside the build, and so you cannot reuse the task class outside the build it is defined in. Using the buildSrc project approach keeps separate the task declaration - that is, what the task should do - from the task implementation - that is, how the task does it.

See Chapter 51, Organizing Build Logic 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.

49.2. Writing a simple task class

To implement a custom task class, you extend DefaultTask.

Example 49.1. Defining a custom task

build.gradle

class GreetingTask extends 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 49.2. A hello world task

build.gradle

task hello(type: GreetingTask)

class GreetingTask extends DefaultTask {
    @TaskAction
    def greet() {
        println 'hello from GreetingTask'
    }
}

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 49.3. A customizable hello world task

build.gradle

// Use the default greeting
task hello(type: GreetingTask)

// Customize the greeting
task greeting(type: GreetingTask) {
    greeting = 'greetings from GreetingTask'
}

class GreetingTask extends DefaultTask {
    def String greeting = 'hello from GreetingTask'

    @TaskAction
    def greet() {
        println greeting
    }
}

Output of gradle -q hello greeting

> gradle -q hello greeting
hello from GreetingTask
greetings from GreetingTask

49.3. 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 49.4. A build for a custom task

build.gradle

apply plugin: 'groovy'

dependencies {
    compile gradleApi()
    groovy localGroovy()
}

Note: The code for this example can be found at samples/customPlugin/plugin which is in both the binary and source distributions of Gradle.

We just follow the convention for where the source for the task class should go.

Example 49.5. 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
    }
}

49.3.1. 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 Section 51.5, “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 49.6. Using a custom task in another project

build.gradle

buildscript {
    repositories {
        maven {
            url uri('../repo')
        }
    }
    dependencies {
        classpath group: 'org.gradle', name: 'customPlugin', version: '1.0-SNAPSHOT'
    }
}

task greeting(type: org.gradle.GreetingTask) {
    greeting = 'howdy!'
}

49.3.2. 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 49.7. 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)
    }
}