Java Timer / TimerTask-如果未收到消息则触发事件

问题描述

我有以下代码，其目的是如果定期调用停止到messageReceived（）时增加一个普罗米修斯计数器：

...

    private static final int tenMinutes = 10 * 60 * 1000;
    private Timer timer = new Timer();
    private boolean newTimer = false;

...
    public void messageReceived() {

        timer.cancel();
        timer = new Timer();
        newTimer = true;

        TimerTask action = new TimerTask() {
            public void run() {
                if (!newTimer)
                    counter.increment();
                else
                    newTimer = false;
            }

        };

        timer.schedule(action,tenMinutes,tenMinutes);

    }

  ...

目标是设置一个计时器，该计时器仅在未接收到新事件的情况下才会触发操作。每次在十分钟之前调用messageReceived（）时，都应取消计时器，以免触发。

我看到的情况几乎是每十分钟触发一次动作，即使messageReceived每分钟调用一次以上。

MessageReceived是从服务调用的，因此并非每次都在同一线程上调用它，而是messageReceived在单例内部。我不确定，但是我认为如果多线程是问题所在，我会看到很多“动作”触发，而不仅仅是每10分钟触发一次。

解决方法

我认为您确实有多线程问题，就像 SnowmanXL 所说。这是一个简单的MCVE再现问题：

import java.text.SimpleDateFormat;
import java.util.*;

class MiscellaneousMonitor {
  private static SimpleDateFormat dateFormat = new SimpleDateFormat("HH:mm:ss.SSS");

  private boolean isRunning;
  private Counter counter;
  private static final int tenMinutes = /*10 * 60 **/ 1000;
  private Timer timer = new Timer();
  private boolean newTimer = false;

  static class Counter {
    private int count = 0;

    public /*synchronized*/ void increment() {
      count++;
    }
  }

  public /*synchronized*/ void start() {
    counter = new Counter();
    isRunning = true;
  }

  public /*synchronized*/ void messageReceived() {
    timer.cancel();
    timer = new Timer();
    newTimer = true;
    TimerTask action = new TimerTask() {
      public void run() {
        System.out.println(dateFormat.format(new Date()) + " Timer task running: " + this);
        if (!newTimer)
          counter.increment();
        else
          newTimer = false;
      }
    };
    timer.schedule(action,tenMinutes,tenMinutes);
  }

  public /*synchronized*/ void stop() {
    timer.cancel();
    isRunning = false;
  }

  public /*synchronized*/ boolean isRunning() {
    return isRunning;
  }

  public static void main(String[] args) throws InterruptedException {
    MiscellaneousMonitor monitor = new MiscellaneousMonitor();
    monitor.start();
    Queue<Thread> threads = new LinkedList<>();
    for (int t = 0; t < 10; t++) {
      Thread thread = new Thread(() -> {
        for (int i = 0; i < 10; i++) {
          try { Thread.sleep(150); } catch (InterruptedException e) { e.printStackTrace(); }
          monitor.messageReceived();
        }
        try { Thread.sleep(1500); } catch (InterruptedException e) { e.printStackTrace(); }
      });
      thread.start();
      threads.add(thread);
    }
    while (!threads.isEmpty()) {
      threads.poll().join();
    }
    monitor.stop();
  }
}

控制台日志如下所示：

Exception in thread "Thread-4" java.lang.IllegalStateException: Timer already cancelled.
    at java.base/java.util.Timer.sched(Timer.java:398)
    at java.base/java.util.Timer.schedule(Timer.java:249)
    at MiscellaneousMonitor.messageReceived(scratch_3.java:39)
    at MiscellaneousMonitor.lambda$main$0(scratch_3.java:59)
    at java.base/java.lang.Thread.run(Thread.java:832)
09:25:58.147 Timer task running: MiscellaneousMonitor$1@1ce7fd7d
09:25:58.142 Timer task running: MiscellaneousMonitor$1@7ba42a49
09:25:58.147 Timer task running: MiscellaneousMonitor$1@493cb0eb
09:25:58.147 Timer task running: MiscellaneousMonitor$1@6f9a3afe
09:25:58.148 Timer task running: MiscellaneousMonitor$1@1d86f308
Exception in thread "Thread-9" java.lang.IllegalStateException: Timer already cancelled.
    at java.base/java.util.Timer.sched(Timer.java:398)
    at java.base/java.util.Timer.schedule(Timer.java:249)
    at MiscellaneousMonitor.messageReceived(scratch_3.java:39)
    at MiscellaneousMonitor.lambda$main$0(scratch_3.java:59)
    at java.base/java.lang.Thread.run(Thread.java:832)
09:25:58.445 Timer task running: MiscellaneousMonitor$1@53c65632
09:25:58.445 Timer task running: MiscellaneousMonitor$1@6ce24daa
09:25:58.445 Timer task running: MiscellaneousMonitor$1@784b861f
09:25:58.447 Timer task running: MiscellaneousMonitor$1@783528c9
09:25:58.447 Timer task running: MiscellaneousMonitor$1@2cc4944f
09:25:58.597 Timer task running: MiscellaneousMonitor$1@711e91d9
09:25:58.597 Timer task running: MiscellaneousMonitor$1@19ddcb88
09:25:58.597 Timer task running: MiscellaneousMonitor$1@5fbdc1a8
(...)

根据运行程序的时间，有时您会看到一些兴奋，有时看不到。但是，即使您没有看到任何异常，多个计时器任务-MiscellaneousMonitor$1是匿名TimerTask实例的内部名称-仍将永远记录并且永远不会被取消，这就是程序永远继续运行的原因直到杀死它为止，尽管您在所有正在运行的任务上都调用了join()。但是仍然有流氓TimerTask。

现在，如果您取消注释我在代码中放置它们的所有synchronized关键字，您的控制台日志将更改为预期的

09:31:44.880 Timer task running: MiscellaneousMonitor$1@4f963263

该程序将终止。

P.S .：您也许可以在较小的代码部分而不是整个方法上进行同步，我没有对此进行分析。我刚刚向您展示了单例线程不安全的基本问题，就像您所说的那样，其他多个线程可以访问该单例。