Chapter 41. Writing Custom Plugins

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

In our examples, we will start with the plugin in the build script, to keep things simple. Then we will look at creating a standalone project.

To create a Gradle plugin, you need to write a class that implements the Plugin interface. When the plugin is applied to a project, Gradle creates an instance of the plugin class and calls the instance’s Plugin.apply(T) method. The project object is passed as a parameter, which the plugin can use to configure the project however it needs to. The following sample contains a greeting plugin, which adds a hello task to the project.

class GreetingPlugin implements Plugin<Project> {
    void apply(Project project) {
        project.task('hello') {
            doLast {
                println 'Hello from the GreetingPlugin'
            }
        }
    }
}

// Apply the plugin
apply plugin: GreetingPlugin

> gradle -q hello
Hello from the GreetingPlugin

One thing to note is that a new instance of a plugin is created for each project it is applied to. Also note that the Plugin class is a generic type. This example has it receiving the Project type as a type parameter. A plugin can instead receive a parameter of type Settings, in which case the plugin can be applied in a settings script, or a parameter of type Gradle, in which case the plugin can be applied in an initialization script.

Most plugins need to obtain some configuration from the build script. One method for doing this is to use extension objects. The Gradle Project has an associated ExtensionContainer object that contains all the settings and properties for the plugins that have been applied to the project. You can provide configuration for your plugin by adding an extension object to this container. An extension object is simply a Java Bean compliant class. Groovy is a good language choice to implement an extension object because plain old Groovy objects contain all the getter and setter methods that a Java Bean requires. Java and Kotlin are other good choices.

Let’s add a simple extension object to the project. Here we add a greeting extension object to the project, which allows you to configure the greeting.

class GreetingPluginExtension {
    String message = 'Hello from GreetingPlugin'
}

class GreetingPlugin implements Plugin<Project> {
    void apply(Project project) {
        // Add the 'greeting' extension object
        def extension = project.extensions.create('greeting', GreetingPluginExtension)
        // Add a task that uses configuration from the extension object
        project.task('hello') {
            doLast {
                println extension.message
            }
        }
    }
}

apply plugin: GreetingPlugin

// Configure the extension
greeting.message = 'Hi from Gradle'

> gradle -q hello
Hi from Gradle

In this example, GreetingPluginExtension is a plain old Groovy object with a property called message. The extension object is added to the plugin list with the name greeting. This object then becomes available as a project property with the same name as the extension object.

Oftentimes, you have several related properties you need to specify on a single plugin. Gradle adds a configuration closure block for each extension object, so you can group settings together. The following example shows you how this works.

class GreetingPluginExtension {
    String message
    String greeter
}

class GreetingPlugin implements Plugin<Project> {
    void apply(Project project) {
        def extension = project.extensions.create('greeting', GreetingPluginExtension)
        project.task('hello') {
            doLast {
                println "${extension.message} from ${extension.greeter}"
            }
        }
    }
}

apply plugin: GreetingPlugin

// Configure the extension using a DSL block
greeting {
    message = 'Hi'
    greeter = 'Gradle'
}

> gradle -q hello
Hi from Gradle

In this example, several settings can be grouped together within the greeting closure. The name of the closure block in the build script (greeting) needs to match the extension object name. Then, when the closure is executed, the fields on the extension object will be mapped to the variables within the closure based on the standard Groovy closure delegate feature.

When developing custom tasks and plugins, it’s a good idea to be very flexible when accepting input configuration for file locations. To do this, you can leverage the Project.file(java.lang.Object) method to resolve values to files as late as possible.

class GreetingToFileTask extends DefaultTask {

    def destination

    File getDestination() {
        project.file(destination)
    }

    @TaskAction
    def greet() {
        def file = getDestination()
        file.parentFile.mkdirs()
        file.write 'Hello!'
    }
}

task greet(type: GreetingToFileTask) {
    destination = { project.greetingFile }
}

task sayGreeting(dependsOn: greet) {
    doLast {
        println file(greetingFile).text
    }
}

ext.greetingFile = "$buildDir/hello.txt"

> gradle -q sayGreeting
Hello!

In this example, we configure the greet task destination property as a closure, which is evaluated with the Project.file(java.lang.Object) method to turn the return value of the closure into a File object at the last minute. You will notice that in the example above we specify the greetingFile property value after we have configured to use it for the task. This kind of lazy evaluation is a key benefit of accepting any value when setting a file property, then resolving that value when reading the property.

Capturing user input from the build script through an extension and mapping it to input/output properties of a custom task is considered a best practice. The end user only interacts with the exposed DSL defined by the extension. The imperative logic is hidden in the plugin implementation.

The extension declaration in the build script as well as the mapping between extension properties and custom task properties occurs during Gradle’s configuration phase of the build lifecycle. To avoid evaluation order issues, the actual value of a mapped property has to be resolved during the execution phase. For more information please see Section 22.1, “Build phases”. Gradle’s API offers types for representing a property that should be lazily evaluated e.g. during execution time. Refer to <<lazyConfiguration> for more information.

The following demonstrates the usage of the type for mapping an extension property to a task property:

class GreetingPlugin implements Plugin<Project> {
    void apply(Project project) {
        def extension = project.extensions.create('greeting', GreetingPluginExtension, project)
        project.tasks.create('hello', Greeting) {
            message = extension.message
            outputFiles = extension.outputFiles
        }
    }
}

class GreetingPluginExtension {
    final Property<String> message
    final ConfigurableFileCollection outputFiles

    GreetingPluginExtension(Project project) {
        message = project.objects.property(String)
        message.set('Hello from GreetingPlugin')
        outputFiles = project.files()
    }

    void setOutputFiles(FileCollection outputFiles) {
        this.outputFiles.setFrom(outputFiles)
    }
}

class Greeting extends DefaultTask {
    final Property<String> message = project.objects.property(String)
    final ConfigurableFileCollection outputFiles = project.files()

    void setOutputFiles(FileCollection outputFiles) {
        this.outputFiles.setFrom(outputFiles)
    }

    @TaskAction
    void printMessage() {
        outputFiles.each {
            logger.quiet "Writing message 'Hi from Gradle' to file"
            it.text = message.get()
        }
    }
}

apply plugin: GreetingPlugin

greeting {
    message = 'Hi from Gradle'
    outputFiles = files('a.txt', 'b.txt')
}

> gradle -q hello
Writing message 'Hi from Gradle' to file
Writing message 'Hi from Gradle' to file

Now we will move our plugin 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 plugin classes. Here is a simple build script for the project. It applies the Groovy plugin, and adds the Gradle API as a compile-time dependency.

apply plugin: 'groovy'

dependencies {
    compile gradleApi()
    compile localGroovy()
}

So how does Gradle find the Plugin implementation? The answer is you need to provide a properties file in the jar’s META-INF/gradle-plugins directory that matches the id of your plugin.

implementation-class=org.gradle.GreetingPlugin

Notice that the properties filename matches the plugin id and is placed in the resources folder, and that the implementation-class property identifies the Plugin implementation class.

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

plugins {
    id 'com.jfrog.bintray' version '0.4.1'
}

class GreetingPluginTest {
    @Test
    public void greeterPluginAddsGreetingTaskToProject() {
        Project project = ProjectBuilder.builder().build()
        project.pluginManager.apply 'org.samples.greeting'

        assertTrue(project.tasks.hello instanceof GreetingTask)
    }
}

plugins {
    id 'java-gradle-plugin'
}

As we saw above, you can use an extension object to provide configuration for your plugin. Using an extension object also extends the Gradle DSL to add a project property and DSL block for the plugin. An extension object is simply a regular object, and so you can provide DSL elements nested inside this block by adding properties and methods to the extension object.

Gradle provides several conveniences to help create a well-behaved DSL for your plugin.

class Person {
    String name
}

class GreetingPluginExtension {
    String message
    final Person greeter

    @javax.inject.Inject
    GreetingPluginExtension(ObjectFactory objectFactory) {
        // Create a Person instance
        greeter = objectFactory.newInstance(Person)
    }

    void greeter(Action<? super Person> action) {
        action.execute(greeter)
    }
}

class GreetingPlugin implements Plugin<Project> {
    void apply(Project project) {
        // Create the extension, passing in an ObjectFactory for it to use
        def extension = project.extensions.create('greeting', GreetingPluginExtension, project.objects)
        project.task('hello') {
            doLast {
                println "${extension.message} from ${extension.greeter.name}"
            }
        }
    }
}

apply plugin: GreetingPlugin

greeting {
    message = 'Hi'
    greeter {
        name = 'Gradle'
    }
}

> gradle -q hello
Hi from Gradle

class Book {
    final String name
    File sourceFile

    Book(String name) {
        this.name = name
    }
}

class DocumentationPlugin implements Plugin<Project> {
    void apply(Project project) {
        // Create a container of Book instances
        def books = project.container(Book)
        books.all {
            sourceFile = project.file("src/docs/$name")
        }
        // Add the container as an extension object
        project.extensions.books = books
    }
}

apply plugin: DocumentationPlugin

// Configure the container
books {
    quickStart {
        sourceFile = file('src/docs/quick-start')
    }
    userGuide {

    }
    developerGuide {

    }
}

task books {
    doLast {
        books.each { book ->
            println "$book.name -> $book.sourceFile"
        }
    }
}

> gradle -q books
developerGuide -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPluginWithDomainObjectContainer/src/docs/developerGuide
quickStart -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPluginWithDomainObjectContainer/src/docs/quick-start
userGuide -> /home/user/gradle/samples/userguide/organizeBuildLogic/customPluginWithDomainObjectContainer/src/docs/userGuide