http://stackoverflow.com/questions/38842780/why-play-2-5-akka-chunk-response-getting-loaded-all-at-once

play-akk-test.java

package controllers;

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

import akka.actor.ActorRef;
import akka.actor.ActorSystem;
import akka.actor.Props;
import akka.actor.UntypedActor;
import com.google.inject.Inject;
import com.google.inject.Singleton;
import akka.stream.javadsl.Source;
import akka.util.ByteString;
import play.cache.CacheApi;
import play.cache.Cached;
import play.filters.csrf.AddCSRFToken;
import play.filters.csrf.CSRF;
import play.libs.Json;
import play.libs.concurrent.HttpExecutionContext;
import play.mvc.Controller;
import play.mvc.Http;
import play.mvc.Http.Cookie;
import play.mvc.Result;

import akka.NotUsed;
import akka.actor.Status;
import akka.stream.OverflowStrategy;
import akka.stream.javadsl.Source;
import akka.util.ByteString;

@Singleton
@AddCSRFToken
public class GetHandler extends Controller {

    @Inject
    private ActorSystem actorSystem;

    @Inject
    private CacheApi cache;

    @Inject
    private HttpExecutionContext httpExecutionContext;


    public static class Worker extends UntypedActor {
        @Override
        public void onReceive(Object msg) {
            if (msg instanceof ActorRef) {
                ActorRef sourceActor = (ActorRef)msg;
                for (int i = 0; i < 20; i++) {
                    sourceActor.tell(ByteString.fromString(String.valueOf(i) + "<br/>\n"), null);
                    try {
                        Thread.sleep(500);//intentional delay
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                sourceActor.tell(new Status.Success(NotUsed.getInstance()), null);
            } else {
                unhandled(msg);
            }
        }
    }

    public CompletionStage<Result> index() {
        return CompletableFuture.supplyAsync(() ->
                Source.<ByteString>actorRef(256, OverflowStrategy.dropNew())
                        .mapMaterializedValue(sourceActor -> {
                            actorSystem
                                    .actorOf(Props.create(Worker.class))
                                    .tell(sourceActor, null);
                            return sourceActor;
                        })
        ).thenApplyAsync(chunks -> ok().chunked(chunks).as("text/html"));
    }
}