Return to main thread as soon as one child thread throws an exception

Question

I'm using Executors.newCachedThreadPool() and invokeAll with a List of Callables to do long-running multithreaded processing. My main thread is blocked until all threads did finish and I can process the Futures returned by invokeAll. I'd like however invokeAll to immediately return if one of the Callables throw an exception and terminate the other threads.

Using execute instead of invokeAll would block on the first future.get() which needs not to be the one that throws the execption.

Using busy waiting to loop through all the futures and checking isDone() seems not to be the best way either.

Answer 1

You can use more complex synchronization mechanisms like latches, barriers or semaphores, but have a look at ExecutorCompletionService. It's a lightweight wrapper around ExecutorService that allows you to listen for the first finished task. Here is a quick example:

final ExecutorService executorService = Executors.newCachedThreadPool();
final ExecutorCompletionService<String> completionService = 
            new ExecutorCompletionService<String>(executorService);
for (int i = 0; i < 10; ++i) {
    completionService.submit(new Task());
}
completionService.take().get();

The code is pretty simple. First you wrap executorService with completionService. Later you use it to submit tasks one after another. The last line is crucial. It takes the first task that finished and tries to retrieve the result. If it thrown an exception, it will be rethrown here, wrapped with ExecutionException:

try {
    completionService.take().get();
} catch (ExecutionException e) {
    e.getCause();       //this was thrown from task!
}

Inside catch block you can somehow handle the exception, e.g. canceling remaining tasks or shutting down the whole thread pool.

Of course you are free to wait for all tasks to finish by calling take() ten times. Each call will block as long as there is at least one task finished.