java-tutorials/play-framework/introduction/app/controllers/AsyncController.java

package controllers;

import akka.actor.ActorSystem;

import javax.inject.*;

import play.*;
import play.mvc.*;

import java.util.concurrent.Executor;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.TimeUnit;

import scala.concurrent.duration.Duration;
import scala.concurrent.ExecutionContextExecutor;

/**
 * This controller contains an action that demonstrates how to write
 * simple asynchronous code in a controller. It uses a timer to
 * asynchronously delay sending a response for 1 second.
 *
 * @param actorSystem We need the {@link ActorSystem}'s
 *                    {@link Scheduler} to run code after a delay.
 * @param exec        We need a Java {@link Executor} to apply the result
 *                    of the {@link CompletableFuture} and a Scala
 *                    {@link ExecutionContext} so we can use the Akka {@link Scheduler}.
 *                    An {@link ExecutionContextExecutor} implements both interfaces.
 */
@Singleton
public class AsyncController extends Controller {

    private final ActorSystem actorSystem;
    private final ExecutionContextExecutor exec;

    @Inject
    public AsyncController(ActorSystem actorSystem, ExecutionContextExecutor exec) {
        this.actorSystem = actorSystem;
        this.exec = exec;
    }

    /**
     * An action that returns a plain text message after a delay
     * of 1 second.
     * <p>
     * The configuration in the <code>routes</code> file means that this method
     * will be called when the application receives a <code>GET</code> request with
     * a path of <code>/message</code>.
     */
    public CompletionStage<Result> message() {
        return getFutureMessage(1, TimeUnit.SECONDS).thenApplyAsync(Results::ok, exec);
    }

    private CompletionStage<String> getFutureMessage(long time, TimeUnit timeUnit) {
        CompletableFuture<String> future = new CompletableFuture<>();
        actorSystem.scheduler().scheduleOnce(
          Duration.create(time, timeUnit),
          () -> future.complete("Hi!"),
          exec
        );
        return future;
    }

}