MinimalWebSocketServer.java

import akka.NotUsed;
import akka.actor.ActorSystem;
import akka.http.javadsl.ConnectHttp;
import akka.http.javadsl.Http;
import akka.http.javadsl.model.HttpRequest;
import akka.http.javadsl.model.HttpResponse;
import akka.http.javadsl.model.ws.Message;
import akka.http.javadsl.model.ws.TextMessage;
import akka.http.javadsl.server.Route;
import akka.stream.ActorMaterializer;
import akka.stream.Materializer;
import akka.stream.javadsl.Flow;
import akka.stream.javadsl.Source;

import java.util.concurrent.CompletionStage;

import static akka.http.javadsl.server.Directives.*;

public class MinimalWebSocketServer {

  public static void main(String[] args) throws Exception {
    ActorSystem actorSystem = ActorSystem.create();
    Materializer materializer = ActorMaterializer.create(actorSystem);
    Http http = Http.get(actorSystem);

    final Flow<Message, Message, NotUsed> webSocketFlow =
      Flow.of(Message.class)
        .mapAsync(1, (message) -> {
          if (message instanceof TextMessage) {
            // Note: it is always safe to call getStreamedText - both on strict and on streamed - strict is just
            // a single element stream in this case
            final Source<String, ?> textStream = ((TextMessage) message).getStreamedText();

            // note that you would likely want to make sure you don't collect any number of elements
            // and fill up memory, but fail if you reach some limit to protect against DoS
            final CompletionStage<String> entireTextInMemory =
                    textStream.runFold("", (textSoFar, textChunk) -> textSoFar + textChunk, materializer);

            return entireTextInMemory;
          } else throw new RuntimeException("I don't do binary, pal");
        }).map(entireText -> {
        System.out.println("Got a text: " + entireText);
        return TextMessage.create("Why, thank you for that text message, client!");
      });


    final Route route = route(
      path("test", () ->
        get(() ->
          handleWebSocketMessages(webSocketFlow)
        )
      )
    );


    final Flow<HttpRequest, HttpResponse, NotUsed> routeFlow = route.flow(actorSystem, materializer);
    http.bindAndHandle(routeFlow, ConnectHttp.toHost("127.0.0.1", 8082), materializer);

    System.out.println("Server started at http://127.0.0.1:8082, press the famous any key to kill server");
    System.in.read();
    actorSystem.terminate();
  }

}