Is there a converter from List to CompletionStage in Java?

问题

Like in this hypothetical example of using of:

List<CompletionStage<Long>> listOfFutureLongs = getFutureLongs(...)
CompletionStage<List<Long>> futureListOfLongs = CompletionStage.of(listOfFutureLongs)

回答1:

Strangely no.

There's CompletableFuture.allOf for CompletableFuture, which is kind of like what you want, but no similar function for CompletionStage.

You can either use CompletionStage.toCompletableFuture to get futures, or you can write your own.

Unfortunately, the inability to check a CompletionStage to see if it's done already means that you can't do this quite as efficiently.

This code is in a not-yet-released (also not yet tested) open source project of mine. Feel free to use it:

public static <T> CompletionStage<List<T>> list(List<CompletionStage<T>> list)
{
    if (list.size() <= 0)
    {
        return CompletableFuture.completedFuture(Collections.emptyList());
    }
    if (list.size() == 1)
    {
        return list.get(0).thenApply(Collections::singletonList);
    }
    final AtomicInteger waiters = new AtomicInteger(1);
    final List<T> ret = new ArrayList<>(list.size());
    final AtomicReference<Throwable> retErr = new AtomicReference<>();
    final CompletableFuture<List<T>> retFuture = new CompletableFuture<>();
    for (int i = 0; i < list.size(); ++i)
    {
        ret.add(null);
        final int pos = i;
        final CompletionStage<T> cs = list.get(i);
        if (cs == null)
        {
            continue;
        }
        waiters.incrementAndGet();
        cs.whenComplete((val, err) -> {
            if (err != null)
            {
                retErr.compareAndSet(null, err);
            }
            ret.set(pos, val);
            _doneListItem(waiters, retFuture, ret, retErr.get());
        });
    }
    _doneListItem(waiters, retFuture, ret, retErr.get());
    return retFuture;
}

private static <T> void _doneListItem(AtomicInteger waitCount, CompletableFuture<List<T>> ret, List<T> val, Throwable err)
{
    if (waitCount.decrementAndGet() == 0)
    {
        if (err != null)
        {
            ret.completeExceptionally(err);
        }
        else
        {
            ret.complete(val);
        }
    }
}

回答2:

Internet says, use completable future:

List<CompletionStage<Long>> futureLongs = getFutureLongs();
var arr = futureLongs.toArray(new CompletableFuture[futureLongs.size()]);
CompletionStage<List<Long>> result = CompletableFuture.allOf(arr)
  .thenApply(unused -> futureLongs.stream().map(f -> f.toCompletableFuture().join()).collect(Collectors.toList()));

来源：https://stackoverflow.com/questions/56135121/is-there-a-converter-from-listcompletionstage-to-completionstagelist-in-java

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