Wait for all the results of a CompletableFuture and act on those values?

Question

I'm looking for the best way, in Java, to asynchronously run some tasks calculating some values and when they're all gathered, run some further calculation on them.

I thought I was going down the right path with this

    var f1 = CompletableFuture.supplyAsync(() -> Service.findPlayers(filter, maxCount, LeaderboardType.Unranked));
    var f2 = CompletableFuture.supplyAsync(() -> Service.findPlayers(filter, maxCount, LeaderboardType._1x1_RM));
    var f3 = CompletableFuture.supplyAsync(() -> Service.findPlayers(filter, maxCount, LeaderboardType.TG_RM));

    var x = CompletableFuture.allOf(f1, f2, f3);

until I realized that unfortunately allOf() doesn't return a list with the results of f1, f2 and f3 but void instead? Is there any alternative? Otherwise I'd have to store the results in some sort of "global variable" and then access them from the when() call, which completely defeats the whole purpose of using supplyAync vs runAsync, I guess.

The method `CompletableFuture.allOf()` is not a `void` method ... it returns a `CompletableFuture`! — Seelenvirtuose, May 08 '21 at 08:12

Eugene · Answer 1 · 2021-05-08T11:52:57.803

0

You do CompletableFuture::allOf, just like you do it now, but also join it :

CompletableFuture.allOf(f1, f2, f3).join();

and then you do:

Stream.of(f1, f2, f3)
      .map(CompletableFuture::join)
      .....

There is no joining in the above operation, though, as the features are already completed.

EDIT

If you do not want to block:

 var f1 = ...
 var f2 = ...
 var f3 = ...

 CompletableFuture.allOf(f1, f2, f3)
                  .thenAccept(x -> {
                       f1.join();
                       f2.join();
                       f3.join();
                   })

edited May 08 '21 at 11:52

answered May 08 '21 at 07:58

Eugene

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There _is_ joining because the `allOf` method does not join. That means the tasks could still be running (asynchronously). Alternatively, you could `join` on the returned `CompletableFuture` of the `allOf` method, first. – Seelenvirtuose May 08 '21 at 08:15
@Seelenvirtuose my bad, I missed to mention the _first_ `join` after `allOf`. – Eugene May 08 '21 at 08:33
1

Won't that first join() call block my current thread? – devoured elysium May 08 '21 at 08:46
@devouredelysium yes it will, see edit. Is that what you are looking for? – Eugene May 08 '21 at 11:54

Wait for all the results of a CompletableFuture and act on those values?

1 Answers1