ForkJoinPool resets thread interrupted state

Question

I just noticed the following phenomena when cancelling a Future returned by ForkJoinPool. Given the following example code:

ForkJoinPool pool = new ForkJoinPool();
Future<?> fut = pool.submit(new Callable<Void>() {

  @Override
  public Void call() throws Exception {
    while (true) {
      if (Thread.currentThread().isInterrupted()) { // <-- never true
        System.out.println("interrupted");
        throw new InterruptedException();
      }
    }
  }
});

Thread.sleep(1000);
System.out.println("cancel");
fut.cancel(true);

The program never prints interrupted. The docs of ForkJoinTask#cancel(boolean) say:

mayInterruptIfRunning - this value has no effect in the default implementation because interrupts are not used to control cancellation.

If ForkJoinTasks ignore interrupts, how else are you supposed to check for cancellation inside Callables submitted to a ForkJoinPool?

Answer 1

This happens because Future<?> is a ForkJoinTask.AdaptedCallable which extends ForkJoinTask, whose cancel method is:

public boolean cancel(boolean mayInterruptIfRunning) {
    return setCompletion(CANCELLED) == CANCELLED;
}

private int setCompletion(int completion) {
    for (int s;;) {
        if ((s = status) < 0)
            return s;
        if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
            if (s != 0)
                synchronized (this) { notifyAll(); }
            return completion;
        }
    }
}

It does not do any interruptions, it just sets status. I suppose this happens becouse ForkJoinPools's Futures might have a very complicated tree structure, and it is unclear in which order to cancel them.