Developing Custom Gradle Types

There are several different kinds of "add-ons" to Gradle that you can develop, such as plugins, tasks, project extensions or artifact transforms, that are all implemented as classes and other types that can run on the JVM. This chapter discusses some of the features and concepts that are common to these types. You can use these features to help implement custom Gradle types and provide a consistent DSL for your users.

This chapter applies to the following types:

Plugin types.
Task types.
Artifact transform parameters types.
Extension objects created using ExtensionContainer.create(), for example a project extension registered by a plugin.
Objects created using an ObjectFactory.newInstance().
Elements of a NamedDomainObjectContainer.

Configuration using bean properties

The custom Gradle types that you implement often hold some configuration that you want to make available to build scripts and other plugins. For example, a download task may have configuration that specifies the URL to download from and the file system location to write the result to. This configuration is represented as Java bean properties.

Kotlin and Groovy provide conveniences for declaring Java bean properties, which make them good language choices to use to implement Gradle types. These conveniences are demonstrated in the samples below.

Managed properties

Gradle provides some conveniences for implementing types with bean properties. Gradle can provide an implementation of a property. This is called a managed property, as Gradle takes care of managing the state of the property. A property may be mutable, meaning that it has both a getter method and setter method, or read-only, meaning that it has only a getter method.

✨	Managed properties are currently an incubating feature.

Mutable managed properties

To declare a mutable managed property, add an abstract getter method and an abstract setter method for the property to the type.

Here is an example of a task type with a uri property:

Example 1. Mutable managed property

UrlProcess.java

import org.gradle.api.DefaultTask;
import org.gradle.api.tasks.TaskAction;

import java.net.URI;

public abstract class UrlProcess extends DefaultTask {
    // Use an abstract getter and setter method
    abstract URI getUri();
    abstract void setUri(URI uri);

    @TaskAction
    void run() {
        // Use the `uri` property
        System.out.println("Downloading " + getUri());
    }
}

UrlProcess.kt

import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction
import java.net.URI

abstract class UrlProcess : DefaultTask() {
    // Use an abstract var
    abstract var uri: URI

    @TaskAction
    fun run() {
        // Use the `uri` property
        println("Downloading $uri")
    }
}

UrlProcess.groovy

import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction

abstract class UrlProcess extends DefaultTask {
    // Use an abstract property method
    abstract URI uri

    @TaskAction
    void run() {
        // Use the `uri` property
        println "downloading ${uri}"
    }
}

Note that for a property to be considered a mutable managed property, all of the property’s getter methods and setter methods must be public or protected and abstract.

Read-only managed properties

To declare a read-only managed property, add an abstract getter method for the property to the type. The property should not have any setter methods. This is a useful pattern to use with one of Gradle’s configurable lazy property types.

Here is an example of a task type with a uri property:

Example 2. Read-only managed property

UrlProcess.java

import org.gradle.api.DefaultTask;
import org.gradle.api.provider.Property;
import org.gradle.api.tasks.TaskAction;

import java.net.URI;

public abstract class UrlProcess extends DefaultTask {
    // Use an abstract getter method
    abstract Property<URI> getUri();

    @TaskAction
    void run() {
        // Use the `uri` property
        System.out.println("Downloading " + getUri().get());
    }
}

UrlProcess.kt

import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction
import org.gradle.api.provider.Property
import java.net.URI

abstract class UrlProcess : DefaultTask() {
    // Use an abstract val
    abstract val uri: Property<URI>

    @TaskAction
    fun run() {
        // Use the `uri` property
        println("Downloading ${uri.get()}")
    }
}

UrlProcess.groovy

import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction
import org.gradle.api.provider.Property

abstract class UrlProcess extends DefaultTask {
    // Use an abstract getter method
    abstract Property<URI> getUri()

    @TaskAction
    void run() {
        // Use the `uri` property
        println "downloading ${uri.get()}"
    }
}

Note that for a property to be considered a read only managed property, all of the property’s getter methods must be public or protected and abstract and the property must not have any setter methods. In addition, the property must have one of the following types:

Property
RegularFileProperty
DirectoryProperty
ListProperty
SetProperty
MapProperty
ConfigurableFileCollection

Managed types

A managed type is an abstract class or interface with no fields and whose properties are all managed. That is, it is a type whose state is entirely managed by Gradle.

DSL support and extensibility

When Gradle creates an instance of a custom type, it decorates the instance to mix-in DSL and extensibility support.

Each decorated instance implements ExtensionAware, and so can have extension objects attached to it.

Note that plugins and container elements are currently not decorated, due to backwards compatibility issues.

Service injection

Gradle provides a number of useful services that can be used by custom Gradle types. For example, the WorkerExecutor service can be used by a task to run work in parallel, as seen in the worker API section. The services are made available through service injection.

Available services

The following services are available for injection:

ObjectFactory - Allows model objects to be created. See Creating nested objects for more details.
ProjectLayout - Provides access to key project locations. See lazy configuration for more details.
ProviderFactory - Creates Provider instances. See lazy configuration for more details.
WorkerExecutor - Allows a task to run work in parallel. See the worker API for more details.

Constructor injection

There are 2 ways that an object can receive the services that it needs. The first option is to add the service as a parameter of the class constructor. The constructor must be annotated with the javax.inject.Inject annotation. Gradle uses the declared type of each constructor parameter to determine the services that the object requires. The order of the constructor parameters and their names are not significant and can be whatever you like.

Here is an example that shows a task type that receives an ObjectFactory via its constructor:

Example 3. Constructor service injection

UrlProcess.java

import org.gradle.api.DefaultTask;
import org.gradle.api.file.DirectoryProperty;
import org.gradle.api.model.ObjectFactory;
import org.gradle.api.tasks.OutputDirectory;
import org.gradle.api.tasks.TaskAction;

import javax.inject.Inject;

public class UrlProcess extends DefaultTask {
    private final DirectoryProperty outputDirectory;

    // Inject an ObjectFactory into the constructor
    @Inject
    public UrlProcess(ObjectFactory objectFactory) {
        // Use the factory
        outputDirectory = objectFactory.directoryProperty();
    }

    @OutputDirectory
    public DirectoryProperty getOutputDirectory() {
        return outputDirectory;
    }

    @TaskAction
    void run() {
        // ...
    }
}

UrlProcess.kt

import javax.inject.Inject
import org.gradle.api.model.ObjectFactory
import org.gradle.api.DefaultTask
import org.gradle.api.tasks.TaskAction
import org.gradle.api.file.DirectoryProperty
import org.gradle.api.tasks.OutputDirectory

open class UrlProcess
// Inject an ObjectFactory into the constructor
@Inject constructor(objectFactory: ObjectFactory) : DefaultTask() {
    // Use the factory
    @OutputDirectory
    val outputDirectory = objectFactory.directoryProperty()

    @TaskAction
    fun run() {
        // ...
    }
}

UrlProcess.groovy

import org.gradle.api.DefaultTask
import org.gradle.api.file.DirectoryProperty
import org.gradle.api.model.ObjectFactory
import org.gradle.api.tasks.OutputDirectory
import org.gradle.api.tasks.TaskAction

import javax.inject.Inject

class UrlProcess extends DefaultTask {
    @OutputDirectory
    final DirectoryProperty outputDirectory

    // Inject an ObjectFactory into the constructor
    @Inject
    UrlProcess(ObjectFactory objectFactory) {
        // Use the factory
        outputDirectory = objectFactory.directoryProperty()
    }

    @TaskAction
    void run() {
        // ...
    }
}

Property injection

Alternatively, a service can be injected by adding a property getter method annotated with the javax.inject.Inject annotation to the class. This can be useful, for example, when you cannot change the constructor of the class due to backwards compatibility constraints. This pattern also allows Gradle to defer creation of the service until the getter method is called, rather than when the instance is created. This can help with performance. Gradle uses the declared return type of the getter method to determine the service to make available. The name of the property is not significant and can be whatever you like.

The property getter method must be public or protected. The method can be abstract or, in cases where this isn’t possible, can have a dummy method body. The method body is discarded.

Here is an example that shows a task type that receives a two services via property getter methods:

Example 4. Property service injection

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 abstract class UrlProcess extends DefaultTask {
    // Use an abstract getter method
    @Inject
    protected abstract ObjectFactory getObjectFactory();

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

    @TaskAction
    void run() {
        WorkerExecutor workerExecutor = getWorkerExecutor();
        ObjectFactory objectFactory = getObjectFactory();
        // Use the executor and factory ...
    }
}

UrlProcess.kt

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

abstract class UrlProcess : DefaultTask() {
    // Use an abstract property
    // Note that the @Inject annotation must be attached to the getter
    @get:Inject
    abstract val objectFactory: ObjectFactory

    // Alternatively, use a property getter with a dummy implementation
    // Note that the property must be open and the @Inject annotation must be attached to the getter
    @get:Inject
    open val workerExecutor: WorkerExecutor
        get() {
            // Getter body is ignored
            throw UnsupportedOperationException()
        }

    @TaskAction
    fun run() {
        // Use the executor and factory ...
    }
}

UrlProcess.groovy

import org.gradle.api.DefaultTask
import org.gradle.api.model.ObjectFactory
import org.gradle.api.tasks.TaskAction
import org.gradle.workers.WorkerExecutor

import javax.inject.Inject

abstract class UrlProcess extends DefaultTask {
    // Use an abstract getter method
    @Inject
    protected abstract ObjectFactory getObjectFactory()

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

    @TaskAction
    void run() {
        // Use the executor and factory ...
    }
}

Creating nested objects

A custom Gradle type can use the ObjectFactory service to create instances of Gradle types to use for its property values. These instances can make use of the features discussed in this chapter, allowing you to create 'nested' instances and a nested DSL.

In the following example, a project extension receives an ObjectFactory instance through its constructor. The constructor uses this to create a nested Server object (also a custom Gradle type) and makes this object available through the server property.

Example 5. Nested object creation

DownloadExtension.java

import org.gradle.api.model.ObjectFactory;

import javax.inject.Inject;

public class DownloadExtension {
    // A nested instance
    private final Server server;

    @Inject
    public DownloadExtension(ObjectFactory objectFactory) {
        // Use an injected ObjectFactory to create a Server object
        server = objectFactory.newInstance(Server.class);
    }

    public Server getServer() {
        return server;
    }
}

DownloadExtension.kt

import javax.inject.Inject
import org.gradle.api.model.ObjectFactory

open class DownloadExtension @Inject constructor(objectFactory: ObjectFactory) {
    // Use an injected ObjectFactory to create a nested Server object
    val server: Server = objectFactory.newInstance(Server::class.java)
}

DownloadExtension.groovy

import org.gradle.api.model.ObjectFactory

import javax.inject.Inject

class DownloadExtension {
    // A nested instance
    final Server server

    @Inject
    DownloadExtension(ObjectFactory objectFactory) {
        // Use an injected ObjectFactory to create a Server object
        server = objectFactory.newInstance(Server)
    }
}

Collection types

Gradle provides types for maintaining collections of objects, intended to work well with the Gradle DSL and provide useful features such as lazy configuration.

NamedDomainObjectContainer

A NamedDomainObjectContainer manages a set of objects, where each elements has a name associated with it. The container takes care of creating and configuring the elements, and provides a DSL that build scripts can use to define and configure elements. It is intended to hold objects which are themselves configurable, for example a set of custom Gradle objects.

Gradle uses NamedDomainObjectContainer type extensively throughout the API. For example, the project.tasks object used to manage the tasks of a project is a NamedDomainObjectContainer<Task>.

You can create a container instance using the ObjectFactory service, which provides the ObjectFactory.domainObjectContainer() method. This is also available using the Project.container() method, however in a custom Gradle type it’s generally better to use the injected ObjectFactory service instead of passing around a Project instance.

In order to use a type with any of the domainObjectContainer() methods, it must expose a property named “name” as the unique, and constant, name for the object. The domainObjectContainer(Class) variant of the method creates new instances by calling the constructor of the class that takes a string argument, which is the desired name of the object. Objects created this way are treated as custom Gradle types, and so can make use of the features discussed in this chapter, for example service injection or managed properties.

See the above link for domainObjectContainer() method variants that allow custom instantiation strategies.

Example 6. Managing a collection of objects

DownloadExtension.java

import org.gradle.api.NamedDomainObjectContainer;
import org.gradle.api.model.ObjectFactory;

import javax.inject.Inject;

public class DownloadExtension {
    // A container of `Server` objects
    private final NamedDomainObjectContainer<Server> servers;

    @Inject
    public DownloadExtension(ObjectFactory objectFactory) {
        // Use an injected ObjectFactory to create a container
        servers = objectFactory.domainObjectContainer(Server.class);
    }

    public NamedDomainObjectContainer<Server> getServers() {
        return servers;
    }
}

DownloadExtension.kt

import javax.inject.Inject
import org.gradle.api.model.ObjectFactory
import org.gradle.api.NamedDomainObjectContainer

open class DownloadExtension @Inject constructor(objectFactory: ObjectFactory) {
    // Use an injected ObjectFactory to create a container of `Server` objects
    val servers = objectFactory.domainObjectContainer(Server::class.java)
}

DownloadExtension.groovy

import org.gradle.api.NamedDomainObjectContainer
import org.gradle.api.model.ObjectFactory

import javax.inject.Inject

class DownloadExtension {
    // A container of `Server` instances
    final NamedDomainObjectContainer<Server> servers

    @Inject
    DownloadExtension(ObjectFactory objectFactory) {
        // Use an injected ObjectFactory to create a Server object
        servers = objectFactory.domainObjectContainer(Server)
    }
}

DomainObjectSet

A DomainObjectSet simply holds a set of objects. Compared to the NamedDomainObjectContainer, the DomainObjectSet doesn’t manage the objects in the collection. They need to be created and added manually.

You can create an instance using the ObjectFactory.domainObjectSet() method.