Wait for cancel() on FutureTask

Question

I want to cancel a FutureTask that I get from a ThreadPoolExecutor but I want to be sure the that Callable on the thread pool has stopped it's work.

If I call FutureTask#cancel(false) and then get() (to block until completion) I get a CancelledException. Is this exception thrown immediately or after the task has stopped executing?

Answer 1

Yes, CancellationException is thrown immediately. You may extend FutureTask to add get() method's version which waits until Callable's thread is finished.

public class ThreadWaitingFutureTask<T> extends FutureTask<T> {

    private final Semaphore semaphore;

    public ThreadWaitingFutureTask(Callable<T> callable) {
        this(callable, new Semaphore(1));
    }

    public T getWithJoin() throws InterruptedException, ExecutionException {
        try {
            return super.get();
        }
        catch (CancellationException e) {
            semaphore.acquire();
            semaphore.release();
            throw e;
        }
    }

    private ThreadWaitingFutureTask(final Callable<T> callable, 
                final Semaphore semaphore) {
        super(new Callable<T>() {
            public T call() throws Exception {
                semaphore.acquire();
                try {
                    return callable.call();
                }
                finally {
                    semaphore.release();
                }
            }
        });
        this.semaphore = semaphore;
    }
}