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§Action composition

This chapter introduces several ways to define generic action functionality.

§Reminder about actions

Previously, we said that an action is a Java method that returns a play.mvc.Result value. Actually, Play manages internally actions as functions. An action provided by the Java API is an instance of play.mvc.Action. Play builds a root action for you that just calls the proper action method. This allows for more complicated action composition.

§Composing actions

Here is the definition of the VerboseAction:

public class VerboseAction extends play.mvc.Action.Simple {
  public CompletionStage<Result> call(Http.Request req) {
    log.info("Calling action for {}", req);
    return delegate.call(req);
  }
}

You can compose the code provided by the action method with another play.mvc.Action, using the @With annotation:

@With(VerboseAction.class)
public Result verboseIndex() {
  return ok("It works!");
}

At one point you need to delegate to the wrapped action using delegate.call(...).

You also mix several actions by using custom action annotations:

@Security.Authenticated
@Cached(key = "index.result")
public Result authenticatedCachedIndex() {
  return ok("It works!");
}

Note: Every request must be served by a distinct instance of your play.mvc.Action. If a singleton pattern is used, requests will be routed incorrectly during multiple request scenarios. For example, if you are using Spring as a DI container for Play, you need to make sure that your action beans are prototype scoped.

Note: play.mvc.Security.Authenticated and play.cache.Cached annotations and the corresponding predefined Actions are shipped with Play. See the relevant API documentation for more information.

§Action annotations and WebSocket action methods

By default, action composition is not applied when handling WebSockets. A guide how to enable action composition, including an example, can be found in the WebSockets documentation.

§Defining custom action annotations

You can also mark action composition with your own annotation, which must itself be annotated using @With:

@With(VerboseAnnotationAction.class)
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface VerboseAnnotation {
  boolean value() default true;
}

Your Action definition retrieves the annotation as configuration:

public class VerboseAnnotationAction extends Action<VerboseAnnotation> {
  public CompletionStage<Result> call(Http.Request req) {
    if (configuration.value()) {
      log.info("Calling action for {}", req);
    }
    return delegate.call(req);
  }
}

You can then use your new annotation with an action method:

@VerboseAnnotation(false)
public Result verboseAnnotationIndex() {
  return ok("It works!");
}

§Annotating controllers

You can also put any action composition annotation directly on the Controller class. In this case it will be applied to all action methods defined by this controller.

@Security.Authenticated
public class Admin extends Controller {
...

}

Note: If you want the action composition annotation(s) put on a Controller class to be executed before the one(s) put on action methods set play.http.actionComposition.controllerAnnotationsFirst = true in application.conf. However, be aware that if you use a third party module in your project it may rely on a certain execution order of its annotations.

§Passing objects from action to controller

You can pass an object from an action to a controller by utilizing request attributes.

public class Attrs {
  public static final TypedKey<User> USER = TypedKey.<User>create("user");
}

public class PassArgAction extends play.mvc.Action.Simple {
  public CompletionStage<Result> call(Http.Request req) {
    return delegate.call(req.addAttr(Attrs.USER, User.findById(1234)));
  }
}

Then in an action you can get the request attribute like this:

@With(PassArgAction.class)
public static Result passArgIndex(Http.Request request) {
  User user = request.attrs().get(Attrs.USER);
  return ok(Json.toJson(user));
}

§Debugging the action composition order

To see in which order the actions of the action composition chain will be executed, please add the following to logback.xml:

<logger name="play.mvc.Action" level="DEBUG" />

You will now see the whole action composition chain with the according annotations (and their associated method/controller) in the logs:

[debug] play.mvc.Action - ### Start of action order
[debug] play.mvc.Action - 1. ...
[debug] play.mvc.Action - 2. ...
[debug] play.mvc.Action - ### End of action order

§Action composition in interaction with body parsing

By default body parsing takes place before action composition happens, meaning you are able to access the already parsed request body inside every action’s call(...) method via request.body(). However, there are use cases where it makes sense to defer body parsing after action composition took place. For example:

When you want to pass request specific information to the body parser via request attributes. E.g. user dependent maximum file upload size or user dependent credentials for a webservice or object storage where the body parser should redirect an upload to.
When using action composition for (granular) authorization you may not want to even parse the request body and cancel the request early if permission checks fail.

Of course, when deferring body parsing, the request body won’t be parsed yet inside a call(...) method and therefore request.body() will return null.

You can enable deferred body parsing globally in conf/application.conf:

play.server.deferBodyParsing = true

Just be aware that, like all play.server.* config keys, this config won’t be picked up by Play when running in DEV mode, but only in PROD mode. To set this config in DEV mode you have to set it in build.sbt:

PlayKeys.devSettings += "play.server.deferBodyParsing" -> "true"

Instead of enabling deferred body parsing globally, you can enable it just for specific routes by using the routes modifier deferBodyParsing:

+ deferBodyParsing
POST    /      controllers.HomeController.uploadFileToS3(request: Request)

The opposite is true as well. If you globally enable deferred body parsing you can disable it for specific routes by using the routes modifier dontDeferBodyParsing:

+ dontDeferBodyParsing
POST    /      controllers.HomeController.processUpload(request: Request)

§Using Dependency Injection

You can use runtime Dependency Injection or compile-time Dependency Injectiontogether with action composition.

§Runtime Dependency Injection

For example, if you want to define your own result cache solution, first define the annotation:

@With(MyOwnCachedAction.class)
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface WithCache {
  String key();
}

And then you can define your action with the dependencies injected:

public class MyOwnCachedAction extends Action<WithCache> {

  private final AsyncCacheApi cacheApi;

  @Inject
  public MyOwnCachedAction(AsyncCacheApi cacheApi) {
    this.cacheApi = cacheApi;
  }

  @Override
  public CompletionStage<Result> call(Http.Request req) {
    return cacheApi.getOrElseUpdate(configuration.key(), () -> delegate.call(req));
  }
}

Note: As stated above, every request must be served by a distinct instance of your play.mvc.Action and you must not annotate your action as a @Singleton.

§Compile-time Dependency Injection

When using compile-time Dependency Injection, you need to manually add your Action supplier to JavaHandlerComponents. You do that by overriding method javaHandlerComponents in BuiltInComponents:

public class MyComponents extends BuiltInComponentsFromContext
    implements NoHttpFiltersComponents, CaffeineCacheComponents {

  public MyComponents(ApplicationLoader.Context context) {
    super(context);
  }

  @Override
  public Router router() {
    return Router.empty();
  }

  @Override
  public MappedJavaHandlerComponents javaHandlerComponents() {
    return super.javaHandlerComponents()
        // Add action that does not depends on any other component
        .addAction(VerboseAction.class, VerboseAction::new)
        // Add action that depends on the cache api
        .addAction(MyOwnCachedAction.class, () -> new MyOwnCachedAction(defaultCacheApi()));
  }
}

Note: As stated above, every request must be served by a distinct instance of your play.mvc.Action and that is why you add a java.util.function.Supplier<Action> instead of the instance itself. Of course, you can have a Supplier returning the same instance every time, but this is not encouraged.

Next: Content negotiation

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