java - Future.cancel() 没有取消 ScheduledExecutorService 的计划执行

Question

我正在安排一个任务:

ScheduledExecutorService dataService = Executors.newScheduledThreadPool(1);
Future<?> dataTimerHandle = dataService.scheduleAtFixedRate(runnable, 100, freq, TimeUnit.MILLISECONDS);

这很好用，没有缺陷。

但是，当某个标志因用户操作而变为 true 时，任务不再需要定期执行，只需执行一次即可。然后，我尝试取消任务并仅提交一次，如下所示:

if(!dynamicUpdate) {
    dataTimerHandle.cancel(true); 
    dataTimerHandle = dataService.submit(runnable);
}
else { //Reschedule again:
    dataTimerHandle = dataService.scheduleAtFixedRate(runnable, 100, freq, TimeUnit.MILLISECONDS);
}

但似乎 runnable 仍在定期执行，cancel() 未按预期工作。是否有替代策略？

Answer 1

问题可能不在 Future 的 cancel() 方法中。这是一个可运行的小示例，它似乎完全符合您的要求:

import java.util.concurrent.*;

public class CancelPeriodicTask {

    public static void main(String[] args) {

        ScheduledThreadPoolExecutor scheduler = (ScheduledThreadPoolExecutor) Executors.newScheduledThreadPool(1);
        scheduler.setRemoveOnCancelPolicy(true);
        try {
            new CancelPeriodicTask().test(scheduler);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            int openTasks = scheduler.shutdownNow().size();
            println("Finished, open tasks: " + openTasks);
            // openTasks will be 1 when RemoveOnCancelPolicy is false
            // and the executor is closed within the scheduled task-period.
        }
    }

    private static long sleepTime = 25L;

    public void test(ScheduledThreadPoolExecutor scheduler) throws Exception {

        // sleep 5 times at scheduled interval
        SleepTask sleepTask;
        ScheduledFuture<?> scheduledSleep = scheduler.scheduleAtFixedRate(sleepTask = new SleepTask(), 0, 2 * sleepTime, TimeUnit.MILLISECONDS);
        sleepTask.sleepTimes.await();
        println("Cancelling scheduledSleep. Done: " + scheduledSleep.isDone() + ", cancelled: " + scheduledSleep.isCancelled());
        scheduledSleep.cancel(true);
        Thread.sleep(2 * sleepTime);
        println("Running sleepTask once.");
        scheduler.submit(sleepTask);
        Thread.sleep(2 * sleepTime);
        scheduledSleep = scheduler.scheduleAtFixedRate(sleepTask, 0, 2 * sleepTime, TimeUnit.MILLISECONDS);
        println("Re-scheduled scheduledSleep. Done: " + scheduledSleep.isDone() + ", cancelled: " + scheduledSleep.isCancelled());
        Thread.sleep(5 * sleepTime);
        println("Cancelling scheduledSleep. Done: " + scheduledSleep.isDone() + ", cancelled: " + scheduledSleep.isCancelled());
        scheduledSleep.cancel(true);
    }

    class SleepTask implements Runnable {

        public final CountDownLatch sleepTimes = new CountDownLatch(5); 
        public int sleepCount;

        @Override public void run() {
            println("Sleeping " + (++sleepCount));
            try { Thread.sleep(sleepTime); } catch (Exception e) {
                e.printStackTrace();
            }
            sleepTimes.countDown();
        }
    }

    private static final long START_TIME = System.currentTimeMillis(); 

    private static void println(String msg) {
        System.out.println((System.currentTimeMillis() - START_TIME) + "\t " + msg);
    }

}