How to implement CompletableFuture.allOf() that completes exceptionally once any of the futures fail?

Question

I want to implement a hybrid of CompletableFuture.allOf() and CompletableFuture.anyOf() where the returned future completes successfully as soon as

Answer 1

This question is actually very similar to Replace Futures.successfulAsList with Java 8 CompletableFuture?

Although the question is not exactly the same, the same answer (from myself) should satisfy your needs.

You can implement this with a combination of allOf() and chaining each input future with an exceptionally() that would make the future returned by allOf() immediately fail:

CompletableFuture<String> a = …, b = …, c = …;
CompletableFuture<Void> allWithFailFast = CompletableFuture.allOf(a, b, c);
Stream.of(a, b, c)
    .forEach(f -> f.exceptionally(e -> {
        allWithFailFast.completeExceptionally(e);
        return null;
    }));

Answer 2

I believe this will do the job:

/**
 * @param arrayOfFutures an array of futures to wait on
 * @return a {@code CompletableFuture} that completes successfully once all elements have completed successfully, or completes
 * exceptionally after any of the elements has done the same
 * <br>{@code @throws NullPointerException} if {@code arrayOfFutures} is null
 */
public CompletableFuture<Void> waitForAllButAbortOnFirstException(CompletableFuture<?>... arrayOfFutures)
{
    if (arrayOfFutures == null)
      return CompletableFuture.failedFuture(new NullPointerException("arrayOfFutures may not be null"));
    if (arrayOfFutures.length == 0)
        return CompletableFuture.completedFuture(null);
    return CompletableFuture.anyOf(arrayOfFutures).
        thenApply(unused ->
        {
            // Remove any futures that completed normally and try again
            return Arrays.stream(arrayOfFutures).
                filter(element -> !element.isDone() || element.isCompletedExceptionally()).
                toArray(CompletableFuture[]::new);
        }).
        thenCompose(remainingFutures -> waitForAllButAbortOnFirstException(remainingFutures));
}