ForkJoinPool重置线程中断状态

问题描述

当取消 ForkJoinPool .给出以下示例代码:

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);

程序从不打印interrupted. ForkJoinTask#的文档cancel(boolean)说:

如果ForkJoinTasks忽略了中断，您还应该如何检查提交给ForkJoinPool的Callable中的取消?

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

推荐答案

之所以会发生这种情况，是因为Future<?>是扩展了ForkJoinTask的ForkJoinTask.AdaptedCallable，其取消方法是:

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;
        }
    }
}

它不做任何打扰，它只是设置状态.我想发生这种情况是因为ForkJoinPools的Future可能具有非常复杂的树结构，并且尚不清楚取消它们的顺序.

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.

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