Question

I’ve stumbled upon a problem, that can be summarized as follows:

When I create the thread manually (i.e. by instantiating java.lang.Thread) the UncaughtExceptionHandler is called appropriately. However, when I use an ExecutorService with a ThreadFactory the handler is ommited. What did I miss?

public class ThreadStudy {

private static final int THREAD_POOL_SIZE = 1;

public static void main(String[] args) {

    // create uncaught exception handler

    final UncaughtExceptionHandler exceptionHandler = new UncaughtExceptionHandler() {

        @Override
        public void uncaughtException(Thread t, Throwable e) {
            synchronized (this) {
                System.err.println("Uncaught exception in thread '" + t.getName() + "': " + e.getMessage());
            }
        }
    };

    // create thread factory

    ThreadFactory threadFactory = new ThreadFactory() {

        @Override
        public Thread newThread(Runnable r) {
            // System.out.println("creating pooled thread");
            final Thread thread = new Thread(r);
            thread.setUncaughtExceptionHandler(exceptionHandler);
            return thread;
        }
    };

    // create Threadpool

    ExecutorService threadPool = Executors.newFixedThreadPool(THREAD_POOL_SIZE, threadFactory);

    // create Runnable

    Runnable runnable = new Runnable() {

        @Override
        public void run() {
            // System.out.println("A runnable runs...");
            throw new RuntimeException("Error in Runnable");
        }
    };

    // create Callable

    Callable<Integer> callable = new Callable<Integer>() {

        @Override
        public Integer call() throws Exception {
            // System.out.println("A callable runs...");
            throw new Exception("Error in Callable");
        }
    };

    // a) submitting Runnable to threadpool
    threadPool.submit(runnable);

    // b) submit Callable to threadpool
    threadPool.submit(callable);

    // c) create a thread for runnable manually
    final Thread thread_r = new Thread(runnable, "manually-created-thread");
    thread_r.setUncaughtExceptionHandler(exceptionHandler);
    thread_r.start();

    threadPool.shutdown();
    System.out.println("Done.");
}
}

I expect: Three times the message “Uncaught exception…”

I get: The message once (triggered by the manually created thread).

Reproduced with Java 1.6 on Windows 7 and Mac OS X 10.5.

Answer 1

Because the exception does not go uncaught.

The Thread that your ThreadFactory produces is not given your Runnable or Callable directly. Instead, the Runnable that you get is an internal Worker class, for example see ThreadPoolExecutor$Worker. Try System.out.println() on the Runnable given to newThread in your example.

This Worker catches any RuntimeExceptions from your submitted job.

You can get the exception in the ThreadPoolExecutor#afterExecute method.