What should Timertask.scheduleAtFixedRate do if the clock changes?

Question

We want to run a task every 1000 seconds (say).

So we have

timer.scheduleAtFixedRate(task, delay, interval);

Mostly, this works fin

Answer 1

It is reported here http://bugs.sun.com/view_bug.do?bug_id=4290274

Similarly, when the system clock is set to a later time, the task may be run multiple times without any delay to "catch up" the missed executions. Exactly this happens when the computer is set to standby/hibernate and the application is resumed (this is how I found out).

This behavior can also be seen in a Java debugger by suspending the timer thread and resuming it.

Answer 2

Looking at the source of Timer for Java 1.7, it appears that is uses System.currentTimeMillis() to determine the next execution of a task.

However, looking at the source of ScheduledThreadPoolExecutor, it uses System.nanoTime().

Which means you won't see that behaviour if you use one in place of a Timer. To create one, use, for instance, Executors.newScheduledThreadPool().

Why you wouldn't see this behaviour is because of what the doc for System.nanoTime() says:

This method can only be used to measure elapsed time and is not related to any other notion of system or wall-clock time. The value returned represents nanoseconds since some fixed but arbitrary origin time [emphasis mine].

As to whether this is a bug in Timer, maybe...

Note that unlike a ScheduledExecutorService, a Timer supports absolute time, and maybe this explains its use of System.currentTimeMillis(); also, Timer has been there since Java 1.3 while System.nanoTime() only appears in 1.5.

But a consequence of using System.currentTimeMillis() is that Timer is sensitive to the system date/time... And that is not documented in the javadoc.