How to throw a custom exception from CompletableFuture?

Question

Question: how can I directly throw a custom exception from .exceptionally() ?

List<CompletableFuture<Object>> futures =
    tasks.stream()
        .map(task -> CompletableFuture.supplyAsync(() -> businessLogic(task))
        .exceptionally(ex -> {
                if (ex instanceof BusinessException) return null;

                //TODO how to throw a custom exception here??
                throw new BadRequestException("at least one async task had an exception");
        }))
        .collect(Collectors.toList());

try {
    List<Object> results = futures.stream()
        .map(CompletableFuture::join)
        .collect(Collectors.toList());
} catch (CompletionException e) {
        if (e.getCause() instanceof RuntimeException) {
              throw (RuntimeException) e.getCause();
        }
        throw new RuntimeException(e.getCause());
}

Problem: I just always get a CompletionException whose ex.getCause() is instanceof BadRequestException .

Is that possible at all?

Answer 1

As said by Didier L , exceptions thrown by the functions (or generally exceptions that completed a CompletableFuture ) are always wrapped in a CompletionException (unless they are already a CompletionException or CancellationException ).

But note that your code becomes much simpler when not even trying to translate the exception via exceptionally :

List<CompletableFuture<Object>> futures =
    tasks.stream()
        .map(task -> CompletableFuture.supplyAsync(() -> businessLogic(task)))
        .collect(Collectors.toList());
try {
    List<Object> results = futures.stream()
        .map(CompletableFuture::join)
        .collect(Collectors.toList());
} catch (CompletionException e) {
    throw e.getCause() instanceof BusinessException?
        new BadRequestException("at least one async task had an exception"): e;
}

or

… catch (CompletionException e) {
    throw e.getCause() instanceof BusinessException?
        new BadRequestException("at least one async task had an exception"):
        e.getCause() instanceof BusinessException? (RuntimeException)e.getCause(): e;
}

Since exceptionally 's primary purpose is translating an exception to a non-exceptional result value, using it for translating the exception to another thrown exception was not the best fit anyway and it also needed an instanceof . So performing this translation in the catch clause saves you from another translation step.

Answer 2

This is not possible. The Javadoc of join() clearly states:

Returns the result value when complete, or throws an (unchecked) exception if completed exceptionally. To better conform with the use of common functional forms, if a computation involved in the completion of this CompletableFuture threw an exception, this method throws an (unchecked) CompletionException with the underlying exception as its cause.

(emphasis is mine)