# Shared Build Services

version 8.0

Sometimes, it is useful for several tasks to share some state or resource. For example, tasks might share a cache of pre-computed values in order to do their work faster. Or tasks might do their work using a web service or database instance.

Gradle allows you to declare build services to represent this state. A build service is simply an object that holds the state for tasks to use. Gradle takes care of the service lifecycle, and will create the service instance only when it is required and clean it up once it is no longer required. Gradle can also optionally take care of coordinating access to the build service, so that no more than a specified number of tasks can use the service concurrently.

## Implementing a build service

To implement a build service, create an abstract class that implements BuildService. Define methods on this type that you’d like tasks to use. A build service implementation is treated as a custom Gradle type and can use any of the features available to custom Gradle types.

A build service can optionally take parameters, which Gradle injects into the service instance when creating it. To provide parameters, you define an abstract class (or interface) that holds the parameters. The parameters type must implement (or extend) BuildServiceParameters. The service implementation can access the parameters using `this.getParameters()`. The parameters type is also a custom Gradle type.

When the build service does not require any parameters, you can use BuildServiceParameters.None as the parameters type.

A build service implementation can also optionally implement `AutoCloseable`, in which case Gradle will call the build service instance’s `close()` method when it discards the service instance. This happens some time between completion of the last task that uses the build service and the end of the build.

Here is an example of a service that takes parameters and is closeable:

Example 1. Implementing a build service
WebServer.java

    import org.gradle.api.file.DirectoryProperty;
    import org.gradle.api.provider.Property;
    import org.gradle.api.services.BuildService;
    import org.gradle.api.services.BuildServiceParameters;

    import java.net.URI;
    import java.net.URISyntaxException;

    public abstract class WebServer implements BuildService<WebServer.Params>, AutoCloseable {

        // Some parameters for the web server
        interface Params extends BuildServiceParameters {
            Property<Integer> getPort();

            DirectoryProperty getResources();
        }

        private final URI uri;

        public WebServer() throws URISyntaxException {
            // Use the parameters
            int port = getParameters().getPort().get();
            uri = new URI(String.format("https://localhost:%d/", port));

            // Start the server ...

            System.out.println(String.format("Server is running at %s", uri));
        }

        // A public method for tasks to use
        public URI getUri() {
            return uri;
        }

        @Override
        public void close() {
            // Stop the server ...
        }
    }

Note that you should not implement the BuildService.getParameters() method, as Gradle will provide an implementation of this.

A build service implementation must be thread-safe, as it will potentially be used by multiple tasks concurrently.

## Using a build service from a task

To use a build service from a task, add a property to the task of type `Property<MyServiceType>` and mark the property as `@Internal`. Using a service with any other annotation is currently not supported. For example, it is currently not possible to mark a service as an input to a task. It is also important to declare the association between the task and the service via the Task.usesService method so Gradle can properly honor the build service lifecycle and its usage constraints.

Here is an example of a task that uses the previous service:

Example 2. Using a build service
Download.java

    import org.gradle.api.DefaultTask;
    import org.gradle.api.file.RegularFileProperty;
    import org.gradle.api.provider.Property;
    import org.gradle.api.tasks.Internal;
    import org.gradle.api.tasks.OutputFile;
    import org.gradle.api.tasks.TaskAction;

    import java.net.URI;

    public abstract class Download extends DefaultTask {
        // This property provides access to the service instance
        @Internal
        abstract Property<WebServer> getServer();

        @OutputFile
        abstract RegularFileProperty getOutputFile();

        @TaskAction
        public void download() {
            // Use the server to download a file
            WebServer server = getServer().get();
            URI uri = server.getUri().resolve("somefile.zip");
            System.out.println(String.format("Downloading %s", uri));
        }
    }

## Registering a build service

To create a build service, you register the service instance using the BuildServiceRegistry.registerIfAbsent() method. Registering the service does not create the service instance. This happens on demand when a task first uses the service. If no task uses the service during a build, the service instance will not be created.

Currently, build services are scoped to a build, rather than to a project, and these services are available to be shared by the tasks of all projects. You can access the registry of shared build services via `Project.getGradle().getSharedServices()`.

Here is an example of a plugin that registers the previous service:

Example 3. Build service registration
DownloadPlugin.java

    import org.gradle.api.Plugin;
    import org.gradle.api.Project;
    import org.gradle.api.provider.Provider;

    public class DownloadPlugin implements Plugin<Project> {
        public void apply(Project project) {
            // Register the service
            Provider<WebServer> serviceProvider = project.getGradle().getSharedServices().registerIfAbsent("web", WebServer.class, spec -> {
                // Provide some parameters
                spec.getParameters().getPort().set(5005);
            });

            project.getTasks().register("download", Download.class, task -> {
                // Connect the service provider to the task
                task.getServer().set(serviceProvider);
                // Declare the association between the task and the service
                task.usesService(serviceProvider);
                task.getOutputFile().set(project.getLayout().getBuildDirectory().file("result.zip"));
            });
        }
    }

The plugin registers the service and receives a `Provider<WebService>` back. This provider can be connected to task properties to pass the service to the task.

Generally, build services are intended to be used by tasks, as they usually represent some state that is potentially expensive to create, and you should avoid using them at configuration time. However, sometimes it can make sense to use the service at configuration time. This is possible, simply call `get()` on the provider.

## Other ways of using a build service

In addition to using a build service from a task, you can use a build service from a worker API action, an artifact transform or another build service. To do this, pass the build service `Provider` as a parameter of the consuming action or service, in the same way you pass other parameters to the action or service. For example, to pass a `MyServiceType` service to worker API action, you might add a property of type `Property<MyServiceType>` to the action’s parameters object and then connect the `Provider<MyServiceType>` that you receive when registering the service to this property.

Currently, it is not possible to use a build service with a worker API action that uses ClassLoader or process isolation modes.

## Concurrent access to the service

You can constrain concurrent execution when you register the service, by using the `Property` object returned from BuildServiceSpec.getMaxParallelUsages(). When this property has no value, which is the default, Gradle does not constrain access to the service. When this property has a value > 0, Gradle will allow no more than the specified number of tasks to use the service concurrently. IMPORTANT: For the constraint to take effect the build service must be registered with the using tasks via Task.usesService(Provider<? extends BuildService<?>>)

## Receiving information about task execution

A build service can be used to receive events as tasks are executed. To do this, create and register a build service that implements OperationCompletionListener:

Example 4. Build service implementing `OperationCompletionListener`
TaskEventsService.java

    import org.gradle.api.services.BuildService;
    import org.gradle.api.services.BuildServiceParameters;
    import org.gradle.tooling.events.FinishEvent;
    import org.gradle.tooling.events.OperationCompletionListener;
    import org.gradle.tooling.events.task.TaskFinishEvent;

    public abstract class TaskEventsService implements BuildService<BuildServiceParameters.None>,
        OperationCompletionListener { (1)

        @Override
        public void onFinish(FinishEvent finishEvent) {
            if (finishEvent instanceof TaskFinishEvent) { (2)
                // Handle task finish event...
            }
        }
    }

1 | Implement the `OperationCompletionListener` interface in addition to the `BuildService` interface.
2 | Check if the finish event is a TaskFinishEvent.

Then, in the plugin you can use the methods on the BuildEventsListenerRegistry service to start receiving events:

Example 5. Registering BuildService in the `BuildEventsListenerRegistry`
TaskEventsPlugin.java

    import org.gradle.api.Plugin;
    import org.gradle.api.Project;
    import org.gradle.api.provider.Provider;
    import org.gradle.build.event.BuildEventsListenerRegistry;

    import javax.inject.Inject;

    public abstract class TaskEventsPlugin implements Plugin<Project> {
        @Inject
        public abstract BuildEventsListenerRegistry getEventsListenerRegistry(); (1)

        @Override
        public void apply(Project project) {
            Provider<TaskEventsService> serviceProvider =
                project.getGradle().getSharedServices().registerIfAbsent(
                    "taskEvents", TaskEventsService.class, spec -> {}); (2)

            getEventsListenerRegistry().onTaskCompletion(serviceProvider); (3)
        }
    }

1 | Use service injection to obtain an instance of the `BuildEventsListenerRegistry`.
2 | Register the build service as usual.
3 | Use the service `Provider` to subscribe the build service to build events.