What happens if a TimerTask takes longer to execut

问题:

When using

Timer.schedule(TimerTask task, long delay, long period)

(i.e. with fixed-delay execution), what happens if the specified TimerTask's run() method takes longer than period to complete? Is it possible that two concurrent TimerTask threads will be running because of this? And if so, is there a way to avoid it?

回答1:

Timer's documentation says the following:

Timer tasks should complete quickly. If a timer task takes excessive time to complete, it "hogs" the timer's task execution thread. This can, in turn, delay the execution of subsequent tasks, which may "bunch up" and execute in rapid succession when (and if) the offending task finally completes.

That is, concurrent TimerTask threads will not be running. The tasks will accumulate into a queue. This may or may not be appropriate (more likely, not).

回答2:

Timer and TimerTask don't handle this sort of situation well. If you want to handle it better, then don't use those classes.

java.util.concurrent.ScheduledExecutorService provides two scheduling methods, scheduleAtFixedRate and scheduledWithFixedDelay, which govern what happens when tasks "bunch up".

scheduleAtFixedRate:

Creates and executes a periodic action that becomes enabled first after the given initial delay, and subsequently with the given period; that is executions will commence after initialDelay then initialDelay+period, then initialDelay + 2 * period, and so on. If any execution of the task encounters an exception, subsequent executions are suppressed. Otherwise, the task will only terminate via cancellation or termination of the executor. If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.

scheduleWithFixedDelay:

Creates and executes a periodic action that becomes enabled first after the given initial delay, and subsequently with the given delay between the termination of one execution and the commencement of the next. If any execution of the task encounters an exception, subsequent executions are suppressed. Otherwise, the task will only terminate via cancellation or termination of the executor.

You can create ScheduledExecutorService instances using the Executors factory class.