java - Play Framework 2.5 JavaAsync throwing CompletionException

Question

I'm using Play 2.5 to build a simple app. For better performance I'm using Akka chunked response with Java 8 CompletionStage strategy. Below is the code by which chunked response is getting generated(it's working fine when not using ComperableFuture):

@Singleton
public class AbstractSource {

    public Source<ByteString, ?> getChunked(String html) {

        return Source.<ByteString>actorRef(256, OverflowStrategy.dropNew())
                .mapMaterializedValue(sourceActor -> {
                    sourceActor.tell(ByteString.fromString(html), null);
                    sourceActor.tell(new Status.Success(NotUsed.getInstance()), null);
                    return null;
                });

    }

}

And here is my controller:

@Singleton
@AddCSRFToken
public class Application extends Controller {

    @Inject
    private AbstractSource abstractSource;

    public CompletionStage<Result> index() {


        CompletionStage<Source<ByteString, ?>> source = CompletableFuture.supplyAsync(() -> 
                                                  abstractSource.getChunked(index.render(CSRF.getToken(request()).map(t -> 
                                                    t.value()).orElse("no token")).body()
                                                   )
                                                );

        return source.thenApply( chunks -> ok().chunked(chunks));

    }

}

Now when I'm running the app it's throwing following exception:

play.api.http.HttpErrorHandlerExceptions$$anon$1: Execution exception[[CompletionException: java.lang.RuntimeException: There is no HTTP Context available from here.]]
    at play.api.http.HttpErrorHandlerExceptions$.throwableToUsefulException(HttpErrorHandler.scala:269)
    at play.api.http.DefaultHttpErrorHandler.onServerError(HttpErrorHandler.scala:195)
    at play.api.GlobalSettings$class.onError(GlobalSettings.scala:160)
    at play.api.DefaultGlobal$.onError(GlobalSettings.scala:188)
    at play.api.http.GlobalSettingsHttpErrorHandler.onServerError(HttpErrorHandler.scala:98)
    at play.core.server.netty.PlayRequestHandler$$anonfun$2$$anonfun$apply$1.applyOrElse(PlayRequestHandler.scala:99)
    at play.core.server.netty.PlayRequestHandler$$anonfun$2$$anonfun$apply$1.applyOrElse(PlayRequestHandler.scala:98)
    at scala.concurrent.Future$$anonfun$recoverWith$1.apply(Future.scala:344)
    at scala.concurrent.Future$$anonfun$recoverWith$1.apply(Future.scala:343)
    at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:32)
Caused by: java.util.concurrent.CompletionException: java.lang.RuntimeException: There is no HTTP Context available from here.
    at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
    at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
    at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1592)
    at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.RuntimeException: There is no HTTP Context available from here.
    at play.mvc.Http$Context.current(Http.java:57)
    at play.mvc.Controller.request(Controller.java:36)
    at com.mabsisa.ui.web.controllers.Application.lambda$index$1(Application.java:31)
    at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
    ... 5 common frames omitted

I'm not using HTTP context anywhere, so why this is not working I'm not getting. Same code is working when returning normal Result with chunked response. Please help with this

Answer 1

You have to supply the HTTP execution context when dealing with CompletableFuture / CompletionStage. In Scala the context information is passed via implicits, these are not available in Java - this is why Play uses ThreadLocal.

However you can lose this information when switching threads and that is why you have the problem. You may think that you don't access the HTTP context but actually you do - you are using request().

So you have to change your code to use supplyAsync with an Executor:

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/CompletableFuture.html#supplyAsync-java.util.function.Supplier-java.util.concurrent.Executor-

From this:

CompletableFuture.supplyAsync(() -> abstractSource.getChunked(index.render(CSRF.getToken(request()).map(t -> 
                                                    t.value()).orElse("no token")).body()
                                                   )
                                                );

to this:

CompletableFuture.supplyAsync(() -> abstractSource.getChunked(index.render(CSRF.getToken(request()).map(t -> 
                                                    t.value()).orElse("no token")).body()
                                                   )
                                                , ec.current());

where ec is your context: @Inject HttpExecutionContext ec;