ForkJoinPool重置线程中断状态

Question

我刚刚注意到，当取消ForkJoinPool返回的未来时，会出现以下现象。给出以下示例代码：

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);

这个程序从不打印interrupted。ForkJoinTask#cancel(布尔型)的博士们说：

mayInterruptIfRunning -此值在默认实现中不起作用，因为中断不用于控制取消。

如果ForkJoinTasks忽略中断，您还应该如何检查提交给ForkJoinPool的调用中是否取消？

Answer 1

这是因为Future<?>是一个扩展ForkJoinTask的ForkJoinTask.AdaptedCallable，其cancel方法是：

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;
        }
    }
}

它不做任何中断，它只是设置状态。我认为这种情况的发生是因为ForkJoinPools的Future可能有一个非常复杂的树结构，并且不清楚取消它们的顺序。

Answer 2

在@Mkhail的回答上分享更多的光线：

使用ForkJoinPool execute()而不是submit()将强制一个失败的Runnable抛出一个工作异常，而这个异常将被线程UncaughtExceptionHandler捕获。

取自Java 8代码：

submit使用AdaptedRunnableAction()。

execute使用RunnableExecuteAction() (参见重新抛出(Ex))。

 /**
 * Adaptor for Runnables without results
 */
static final class AdaptedRunnableAction extends ForkJoinTask<Void>
    implements RunnableFuture<Void> {
    final Runnable runnable;
    AdaptedRunnableAction(Runnable runnable) {
        if (runnable == null) throw new NullPointerException();
        this.runnable = runnable;
    }
    public final Void getRawResult() { return null; }
    public final void setRawResult(Void v) { }
    public final boolean exec() { runnable.run(); return true; }
    public final void run() { invoke(); }
    private static final long serialVersionUID = 5232453952276885070L;
}

/**
 * Adaptor for Runnables in which failure forces worker exception
 */
static final class RunnableExecuteAction extends ForkJoinTask<Void> {
    final Runnable runnable;
    RunnableExecuteAction(Runnable runnable) {
        if (runnable == null) throw new NullPointerException();
        this.runnable = runnable;
    }
    public final Void getRawResult() { return null; }
    public final void setRawResult(Void v) { }
    public final boolean exec() { runnable.run(); return true; }
    void internalPropagateException(Throwable ex) {
        rethrow(ex); // rethrow outside exec() catches.
    }
    private static final long serialVersionUID = 5232453952276885070L;
}