如何解决Java Timer / TimerTask-如果未收到消息则触发事件
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。
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