java - Future.cancel(false) 为执行中的任务返回 true

Question

在我看来，Future.cancel(false) 应该只在确实有可能阻止任务执行时才返回 true。

但是从下面的代码我们可以看出这是矛盾的。

由于任务被取消，它不应该打印 "Not expecting this statement!!!"

public class Test {
    public static void main(String[] args) throws InterruptedException {
        ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();
        final boolean[] canceled = { false };
        ScheduledFuture<?> future = executor.schedule(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    if (canceled[0])
                        System.out.println("Not expecting this statement!!!");
                } catch (InterruptedException e) {
                }
            }
        }, 0, TimeUnit.SECONDS);
        Thread.sleep(100);
        canceled[0] = future.cancel(false);

        executor.shutdown();
        executor.awaitTermination(2, TimeUnit.SECONDS);
    }
}

不幸的是，这个程序的输出是“没想到这个语句!!!”

有人能解释一下这种行为吗。

Answer 1

因为您使用参数 false 调用了 canceled[0] = future.cancel(false);。来自 Future.cancel 的 javadoc:

If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

@param mayInterruptIfRunning {@code true} if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete

当你调用cancel时，你的任务已经开始并且你允许它完成，所以取消操作实际上是成功的并返回true，如果你调用 canceled[0] = future.cancel(true);，你将看不到输出。