Lesson Learned - Java Timer

Issue

When you use Java Timer (java.util.Timer) to set a task for future execution, you should be really careful. Java Timer use currentTimeMillis, which will be affected by system clock.

https://github.com/openjdk-mirror/jdk7u-jdk/blob/master/src/share/classes/java/util/Timer.java

If you want to start a one time timer, you can call schedule(TimerTask task, long delay), here is the sample code.

import java.util.Timer;
import java.util.TimerTask;

public class TimerTest {
	static Timer timer;
	public static void main(String[] args) {
		timer = new Timer();
		timer.schedule(new TimerTask(){

			@Override
			public void run() {
				System.out.println(Thread.currentThread().getId() + " Time's up!");
				timer.cancel();
			}
		}, 3000);

		System.out.println(Thread.currentThread().getId() + " Timer scheduled.");
	}
}

When you run the code, if should print out something like this

1 Timer scheduled.
12 Time's up!

It takes about 3 seconds to finish since the delay is set to 3000 milliseconds.

But what if you change the system clock immediately after running the code? The result is that if you change the system back to 10 minutes ago, you have to wait 10 minutes + 3 seconds to get the timeout message.

The even worse case is for repeated timer. like scheduleAtFixedRate(TimerTask task, long delay, long period). When you change the system clock to a future time, the timer will try to catch up with the time, and then the timer task will be executed continuously, which may take up all your CPU cycles.

https://bugs.openjdk.java.net/browse/JDK-4290274