如何确定取消的ScheduledFuture是否实际上未取消？

Question

我正在使用ScheduledExecutorService并提交这样的任务：

future = scheduledExecutorService.schedule(myRunnableTask, delay, timeunit)

但是，一定时间后可能会发生某个事件，这表明不再需要此任务。所以我需要取消此任务，而我正在使用

boolean cancelled = future.cancel(false) 线。

取消后，我必须根据提交的可运行对象是否实际运行来采取不同的操作。在这里，让我们首先进入Oracle文档并阅读cancelled标志的含义：

https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Future.html#cancel(boolean）

返回：如果无法取消任务（通常是因为它已经正常完成），则返回false；否则返回false。否则为真

这就是关于返回值的全部内容。似乎写此文本行的人false在这里不确定返回值，但我认为我可以接受。

现在，让我们集中讨论返回的情况true。这里有两种可能性：

1. 该任务实际上已取消，并且无法运行。
2. 可运行对象正在运行中，因此无法取消。（除非我执行一些线程中断逻辑，而这并不是我真正想做的）

我对这两种情况都可以，但是我想知道实际发生的一种并采取相应的措施。如果可运行对象正在处理中，那么我可以完成它的工作，我想等待它完成，然后做一件事。但是，如果它被取消并且根本不会运行，我想做另一件事。

您能推荐一种解决方法吗？我想念什么吗？

Answer 1

我最终为这个问题写了这样的东西。源代码和一些单元测试可以在https://github.com/nuzayats/cancellabletaskexecutor找到

public class CancellableTaskExecutor {

    private final ScheduledExecutorService es;
    private final Logger log;

    /**
     * For a unit test to replicate a particular timing
     */
    private final Runnable hookBetweenCancels;

    public CancellableTaskExecutor(ScheduledExecutorService es, Logger log) {
        this(es, log, () -> {
            // nop
        });
    }

    // For unit tests
    CancellableTaskExecutor(ScheduledExecutorService es, Logger log, Runnable hookBetweenCancels) {
        this.es = es;
        this.log = log;
        this.hookBetweenCancels = hookBetweenCancels;
    }

    public Execution schedule(Runnable task, long delay, TimeUnit unit) {
        CancellableRunnable runnable = new CancellableRunnable(task);
        ScheduledFuture<?> future = es.schedule(runnable, delay, unit);
        return new Execution(future, runnable);
    }

    public class Execution {

        private final ScheduledFuture<?> future;
        private final CancellableRunnable runnable;

        private Execution(ScheduledFuture<?> future, CancellableRunnable runnable) {
            this.future = future;
            this.runnable = runnable;
        }

        /**
         * @return true when the task has been successfully cancelled and it's guaranteed that
         * the task won't get executed. otherwise false
         */
        public boolean cancel() {
            boolean cancelled = runnable.cancel();
            hookBetweenCancels.run();

            // the return value of this call is unreliable; see /sf/ask/3914601211/
            future.cancel(false);

            return cancelled;
        }
    }

    private class CancellableRunnable implements Runnable {

        private final AtomicBoolean cancelledOrStarted = new AtomicBoolean();
        private final Runnable task;

        private CancellableRunnable(Runnable task) {
            this.task = task;
        }

        @Override
        public void run() {
            if (!cancelledOrStarted.compareAndSet(false, true)) {
                return; // cancelled, forget about the task
            }
            try {
                task.run();
            } catch (Throwable e) {
                log.log(Level.WARNING, "Uncaught Exception", e);
            }
        }

        boolean cancel() {
            return cancelledOrStarted.compareAndSet(false, true);
        }
    }
}