如何在我认为完成后停止ScheduledThreadPoolExecutor中的任务

Question

我有一个ScheduledThreadPoolExecutor，我用它来安排任务以固定速率运行。我希望任务以指定的延迟运行最多10次，直到"成功"。在那之后，我将不希望该任务被重试。因此，基本上，当我希望停止计划任务时，我需要停止运行它，但不关闭ScheduledThreadPoolExecutor。知道我该怎么做吗？

这里有一些伪代码-

public class ScheduledThreadPoolExecutorTest
{
  public static ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(15);  // no multiple instances, just one to serve all requests
  class MyTask implements Runnable
  {
    private int MAX_ATTEMPTS = 10;
    public void run()
    {
      if(++attempt <= MAX_ATTEMPTS)
      {
        doX();
        if(doXSucceeded)
        {
          //stop retrying the task anymore
        }
      }
      else
      { 
        //couldn't succeed in MAX attempts, don't bother retrying anymore!
      }
    }
  }
  public void main(String[] args)
  {
    executor.scheduleAtFixedRate(new ScheduledThreadPoolExecutorTest().new MyTask(), 0, 5, TimeUnit.SECONDS);
  }
}

Answer 1

运行此测试，它打印1 2 3 4 5并停止

public class ScheduledThreadPoolExecutorTest {
    static ScheduledThreadPoolExecutor executor = new ScheduledThreadPoolExecutor(15); // no
    static ScheduledFuture<?> t;
    static class MyTask implements Runnable {
        private int attempt = 1;
        public void run() {
            System.out.print(attempt + " ");
            if (++attempt > 5) {
                t.cancel(false);
            }
        }
    }
    public static void main(String[] args) {
        t = executor.scheduleAtFixedRate(new MyTask(), 0, 1, TimeUnit.SECONDS);
    }
}

Answer 2

CountDownLatch是一种替代方法。线程完成后，在锁存器上调用countDown()。调用线程调用latch.await()，直到所有线程完成。这时调用ExecutorService.shutdownNow()，这样主线程就不会变成僵尸。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class ScheduledThreadPoolExecutorTest {
  static int i = 0;
  public static void main(String[] args) throws Exception {
    final ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
    final CountDownLatch latch = new CountDownLatch(1);
    executor.scheduleAtFixedRate(() -> {
        System.out.println(++i);
        if (i > 4) {
          latch.countDown();
        }
    }, 0, 100, TimeUnit.MILLISECONDS);
    latch.await();
    executor.shutdownNow();
  }
}

Answer 3

在线程外很好地取消了：

public class ScheduleTest {
    @Test
    public void testCancel() throws Exception {
        final ScheduledThreadPoolExecutor EXECUTOR = (ScheduledThreadPoolExecutor) Executors.newScheduledThreadPool(2);
        ScheduledFuture f1 = EXECUTOR.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                System.out.println("Im alive 1");
            }
        }, 0, 1, TimeUnit.SECONDS);
        ScheduledFuture f2 = EXECUTOR.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                System.out.println("Im alive 2");
            }
        }, 0, 2, TimeUnit.SECONDS);
        Thread.sleep(10000);
        f1.cancel(true);
        System.out.println("f1 cancel");
        Thread.sleep(10000);
        f2.cancel(false);
        System.out.println("f2 cancel");
        Thread.sleep(10000);
    }
}

有时线程无法取消，这通常通过volatile boolean isCancelled;