如何从 CompletableFuture 抛出自定义异常？

Question

问题：如何直接从 抛出自定义异常.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());
}

问题：我总是得到一个CompletionExceptionwho ex.getCause()is instanceof BadRequestException。

这可能吗？

Answer 1

正如Didier L 所说，函数抛出的异常（或者通常是完成a 的异常CompletableFuture）总是包含在 a 中CompletionException（除非它们已经是 aCompletionException或CancellationException）。

但请注意，即使不尝试通过exceptionally以下方式翻译异常，您的代码也会变得更加简单：

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;
}

或者

… 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;
}

由于exceptionally的主要目的是将异常转换为非异常结果值，因此使用它来将异常转换为另一个抛出的异常并不是最合适的，它也需要一个instanceof. 因此，在catch子句中执行此翻译可以避免另一个翻译步骤。