Catching thread exceptions from Java ExecutorService

Question

I\'m working on a software development framework for parallel computing JavaSeis.org. I need a robust mechanism for reporting thread exceptions. During development, knowing

Answer 1

I didn't have a great deal of luck with other answers because I needed the actual exception instance, itself, not just a printed stack trace. For me, the accepted answer involving ThreadPoolExecutor#afterExecute() of the question "Why is UncaughtExceptionHandler not called by ExecutorService?" worked.

See the following sample code:

List tasks = new LinkedList<>();
for (int i = 0; i < numThreads; ++i) {
    Runnable task = new Runnable() {
        @Override
        public void run() {
            throw new RuntimeException();
        }
    };

    tasks.add(task);
}

Optional opEmpty = Optional.empty();
/*
 * Use AtomicReference as a means of capturing the first thrown exception, since a
 * spawned thread can't "throw" an exception to the parent thread.
 */
final AtomicReference> firstThrownException =
        new AtomicReference<>(opEmpty);

/*
 * Use new ThreadPoolExecutor instead of Executors.newFixedThreadPool() so
 * that I can override afterExecute() for the purposes of throwing an
 * exception from the test thread if a child thread fails.
 */
ExecutorService execSvc = new ThreadPoolExecutor(numThreads, numThreads,
            0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue()) {
    @Override
    public void afterExecute(Runnable task, Throwable failureCause) {
        if(failureCause == null) {
            // The Runnable completed successfully.
            return;
        }
        // only sets the first exception because it will only be empty on the first call.
        firstThrownException.compareAndSet(Optional.empty(), Optional.of(failureCause));
    }
};

for (Runnable task : tasks) {
    execSvc.execute(task);
}
execSvc.shutdown();
execSvc.awaitTermination(1, TimeUnit.HOURS);

assertEquals(firstThrownException.get(), Optional.empty());