Java Timer/TimerTask - 如果未收到消息则触发事件

Question

我有以下代码，其目的是在定期调用停止到达 messageReceived() 时增加普罗米修斯计数器：

...

    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);

    }

  ...

我们的目标是设置一个计时器，该计时器仅在未收到新事件时才会触发操作。每次在十分钟过去之前调用 messageReceived() 时，应该取消计时器，这样它就不会触发。

我所看到的几乎每十分钟就会触发一次操作，即使每分钟调用 messageReceived 不止一次。

MessageReceived 是从服务调用的，因此它不会每次都在同一个线程上调用，但 messageReceived 在单例中。我不确定，但我认为如果多线程是问题所在，我会看到很多次“动作”触发，而不是每 10 分钟一次。

Answer 1

我认为你确实有一个 multi-threading 问题，就像 SnowmanXL 说的那样。这是一个简单的 MCVE 重现问题：

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();
  }
}

控制台日志将如下所示：

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$main[=11=](scratch_3.java:59)
    at java.base/java.lang.Thread.run(Thread.java:832)
09:25:58.147 Timer task running: MiscellaneousMonitor@1ce7fd7d
09:25:58.142 Timer task running: MiscellaneousMonitor@7ba42a49
09:25:58.147 Timer task running: MiscellaneousMonitor@493cb0eb
09:25:58.147 Timer task running: MiscellaneousMonitor@6f9a3afe
09:25:58.148 Timer task running: MiscellaneousMonitor@1d86f308
Exception in thread "Thread-9" java.lang.IllegalStateException: Timer already cancelled.
    at java.base/java.util.Timer.sched(Timer.java:398)
    at java.base/java.util.Timer.schedule(Timer.java:249)
    at MiscellaneousMonitor.messageReceived(scratch_3.java:39)
    at MiscellaneousMonitor.lambda$main[=11=](scratch_3.java:59)
    at java.base/java.lang.Thread.run(Thread.java:832)
09:25:58.445 Timer task running: MiscellaneousMonitor@53c65632
09:25:58.445 Timer task running: MiscellaneousMonitor@6ce24daa
09:25:58.445 Timer task running: MiscellaneousMonitor@784b861f
09:25:58.447 Timer task running: MiscellaneousMonitor@783528c9
09:25:58.447 Timer task running: MiscellaneousMonitor@2cc4944f
09:25:58.597 Timer task running: MiscellaneousMonitor@711e91d9
09:25:58.597 Timer task running: MiscellaneousMonitor@19ddcb88
09:25:58.597 Timer task running: MiscellaneousMonitor@5fbdc1a8
(...)

有时您会看到异常，有时不会，这取决于您运行程序的时机。但是即使你没有看到任何异常，多个定时器任务 - MiscellaneousMonitor 是匿名 TimerTask 实例的内部名称 - 将永远记录并且永远不会被取消，这就是程序继续运行永远，直到你杀死它，尽管你在所有运行ning 任务上调用 join()。但是还是有流氓TimerTask

现在，如果您取消注释我在代码中放置的所有 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@4f963263

程序将终止。

P.S.: 你也许可以在更小的代码部分而不是整个方法上同步，我没有分析过。我刚刚向您展示了线程不安全的基本问题，您的单例被多个其他线程访问，就像您说的那样。

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