[2.1 Java] How to Await on an array of futures?

[Chanan Braunstein]

Hello,

I need to do X amount of work which I will perform inside actors. The actors will respond when they are done. I know how to Await on an actor, but how do you await on an array of actors till they are all done?

[Martin Grotzke]

If you're working with promises you can use Promise.sequence(promises)
which returns a Promise of all results, so that you can finally map on this.

Cheers,
Martin

> --
> You received this message because you are subscribed to the Google
> Groups "play-framework" group.
> To unsubscribe from this group and stop receiving emails from it, send
> an email to play-framewor...@googlegroups.com.
> For more options, visit https://groups.google.com/groups/opt_out.
>
>

[stikkos]

You can use akka.dispatch.Futures.sequence as described on http://doc.akka.io/docs/akka/snapshot/java/futures.html  (under Composing Futures)

Example:

import static akka.dispatch.Futures.sequence;

final ExecutionContext ec = system.dispatcher();

//Some source generating a sequence of Future<Integer>:s

Iterable<Future<Integer>> listOfFutureInts = source;

 

// now we have a Future[Iterable[Integer]]

Future<Iterable<Integer>> futureListOfInts = sequence(listOfFutureInts, ec);

 

// Find the sum of the odd numbers

Future<Long> futureSum = futureListOfInts.map(

  new Mapper<Iterable<Integer>, Long>() {

    public Long apply(Iterable<Integer> ints) {

      long sum = 0;

      for (Integer i : ints)

        sum += i;

      return sum;

    }

  }, ec);

 

futureSum.onSuccess(new PrintResult<Long>(), system.dispatcher());

[Chanan Braunstein]

I am close but don't have it right yet. This is what I have so far:

List<Promise<Object>> list = new ArrayList<Promise<Object>>();

Timeout timeout = new Timeout(Duration.create(5, TimeUnit.SECONDS));

for(int i = 0; i < message.getNumberOfStudents(); i++) {

InsertTestScore msg = new InsertTestScore(message.getCourseId(), message.getTestId(), String.format("U%d", i + 1), 60 + rnd.nextInt(40));

list.add(Akka.asPromise(Patterns.ask(insertScoreActor, message, timeout)));

}

Promise waitAll = Promise.sequence(list);

The problem is that Promise.sequence is expecting a type of java.lang.Iterable<F.Promise<? extends A>

[Skamander]

Maybe this helps (line 39 and later): https://github.com/huntc/FrameworkBenchmarks/blob/0044d06c3ea8d4e823ec2a3d7c89b1143172faab/play-java/app/controllers/Application.java#L39

[Chanan Braunstein]

Thanks! That helped!

I noticed this class: play.libs.F.Promise.PromiseActor - Do you happen to know how to use it?