Java计时器/计时任务 – 如果未收到消息,则触发事件。

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英文:

Java Timer/TimerTask - fire event if message not received

问题

...

    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);

    }

  ...

这段代码的目的是,如果周期性调用messageReceived()停止发生,则递增一个Prometheus计数器。

目标是设置一个计时器,仅当未收到新事件时才触发动作。每次在十分钟内调用messageReceived(),计时器都应该被取消,以便它不会触发。

我看到的现象是,几乎每隔十分钟动作都会触发,即使在一分钟内调用了多次messageReceived

messageReceived从一个服务中调用,因此不是在每次相同的线程上调用,但messageReceived位于单例中。我不确定,但我认为如果多线程是问题,我应该会看到多次触发“action”,而不仅仅是每隔10分钟一次。

英文:

I have the following code whose purpose is to increment a prometheus counter if periodic calls stop coming to messageReceived():

...

    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);

    }

  ...

The goal is to set a timer that will only fire an action if a new event is not received. Every time messageReceived() is called before ten minutes have passed, the timer should be cancelled so it will not fire.

What I am seeing happen is pretty much exactly every ten minutes the action fires, even though messageReceived is called more than once per minute.

MessageReceived is called from a service so its not called on the same thread every time, but messageReceived is inside a singleton. I am not sure, but I would think that if multithreading was the problem, I would see many firings of "action" and not just one every 10 minutes.

答案1

得分: 0

我认为你确实存在多线程问题,就像_SnowmanXL_所说的那样。以下是一个简单的 MCVE 用来重现这个问题:

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();
  }
}

控制台日志将会类似于:

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
(...)

有时你会看到异常,有时不会,这取决于运行程序时的时机。但即使你没有看到任何异常,多个定时任务 - MiscellaneousMonitor$1 是匿名的 TimerTask 实例的内部名称 - 会一直记录下去,永远不会被取消,这就是为什么程序会一直运行,直到你终止它,尽管你对所有运行中的任务调用了 join()。但仍然存在问题的 TimerTask

现在,如果你取消所有我在代码中标记的地方的 synchronized 关键字,你的控制台日志将变成预期的:

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

并且程序将终止。

附注:你可能可以在代码的较小部分上同步,而不是整个方法上同步,我没有分析这一点。我只是向你展示了你的单例存在多个其他线程访问的线程不安全的基本问题,就像你所说的那样。

英文:

I think you do have a multi-threading problem, just like SnowmanXL said. Here is a simple MCVE reproducing the problem:

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

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

  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()) + &quot; Timer task running: &quot; + 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&lt;Thread&gt; threads = new LinkedList&lt;&gt;();
    for (int t = 0; t &lt; 10; t++) {
      Thread thread = new Thread(() -&gt; {
        for (int i = 0; i &lt; 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();
  }
}

The console log will look something like this:

Exception in thread &quot;Thread-4&quot; 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$1@1d86f308
Exception in thread &quot;Thread-9&quot; java.lang.IllegalStateException: Timer already cancelled.
at java.base/java.util.Timer.sched(Timer.java:398)
at java.base/java.util.Timer.schedule(Timer.java:249)
at MiscellaneousMonitor.messageReceived(scratch_3.java:39)
at MiscellaneousMonitor.lambda$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
(...)

Sometimes you see the execeptions, sometimes not, depending on the timing when you run the program. But even if you do not see any exceptions, multiple timer tasks - MiscellaneousMonitor$1 is the internal name of the anonymous TimerTask instance - will log forever and never be cancelled, which is why the program continues to run forever until you kill it, despite you calling join() on all running tasks. But there are still rogue TimerTasks.

Now if you uncomment all synchronized keywords where I put them in the code, your console log will change to the expected

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

and the program will terminate.

P.S.: You maybe could synchronise on smaller sections of code instead of on whole methods, I did not analyse that. I just showed you the basic problem of thread unsafety with your singleton which is accessed by multiple other threads, like you said.

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  • 本文由 发表于 2020年10月2日 22:51:05
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