Example Chatroom with Java API

2 years after

Play Chatroom

This is a simple chatroom using Play and Websockets with the Java API.

This project makes use of dynamic streams from Akka Streams, notably BroadcastHub and MergeHub. By combining MergeHub and BroadcastHub, you can get publish/subscribe functionality.

The good bit

The flow is defined once in the controller, and used everywhere from the chat action:

public class HomeController extends Controller {

    private final Flow userFlow;

    public HomeController(ActorSystem actorSystem,
                          Materializer mat) {
        org.slf4j.Logger logger = org.slf4j.LoggerFactory.getLogger(this.getClass());
        LoggingAdapter logging = Logging.getLogger(actorSystem.eventStream(), logger.getName());

        //noinspection unchecked
        Source<String, Sink<String, NotUsed>> source = MergeHub.of(String.class)
                .log("source", logging)
                .recoverWithRetries(-1, new PFBuilder().match(Throwable.class, e -> Source.empty()).build());
        Sink<String, Source<String, NotUsed>> sink = BroadcastHub.of(String.class);

        Pair<Sink<String, NotUsed>, Source<String, NotUsed>> sinkSourcePair = source.toMat(sink, Keep.both()).run(mat);
        Sink<String, NotUsed> chatSink = sinkSourcePair.first();
        Source<String, NotUsed> chatSource = sinkSourcePair.second();
        this.userFlow = Flow.fromSinkAndSource(chatSink, chatSource).log("userFlow", logging);

    public Result index() {
        Http.Request request = request();
        String url =;
        return Results.ok(views.html.index.render(url));

    public WebSocket chat() {
        return WebSocket.Text.acceptOrResult(request -> {
            if (sameOriginCheck(request)) {
                return CompletableFuture.completedFuture(F.Either.Right(userFlow));
            } else {
                return CompletableFuture.completedFuture(F.Either.Left(forbidden()));


You will need JDK 1.8 and sbt installed.


sbt run

Go to http://localhost:9000 and open it in two different browsers. Typing into one browser will cause it to show up in another browser.


This project is originally taken from Johan Andrén's Akka-HTTP version:

Johan also has a blog post explaining dynamic streams in more detail:

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