Java ScheduledExecutorService在幕后

Question

How does things like scheduleAtFixedRate work? How does it work behind the scenes and is there a penalty to using it?

More specifically, I have a task that I want to run periodically, say every 12 hours. The period is not strict at all, so my first instinct was to check in every request (tomcat server) if it's been more than >12 hours since the task last executed and if so, execute it and reset the timer. The downside of this is that I have to do a small time check on every request, make sure the task is run only once (using a semaphore or something similar) and the task might not execute in a long time if there's no requests.

scheduleAtFixedRate makes it easier to schedule a recurring task, but since I don't know how it does it, I don't know what the performance impact is. Is there a thread continually checking if the task is due to run? etc.

edit: In Timer.java , there's a mainLoop function which, in my understanding, is something like this (overly simplified):

while(true) {
    currentTime = System.currentTimeMillis();
    if(myTask.nextExecutionTime == currentTime) myTask.run();
}

Won't this loop try to run as fast as possible and use a ton of CPU (I know, obviously not, but why )? There's no Thread.sleep in there to slow things down.

Answer 1

You can read the code if you wish to work out how it works.

There is an overhead using ScheduledExecutorService in terms of CPU and memory, however on the scale of hours, minutes, second even milli-seconds, it probably not work worrying about. If you have a task running in the range of micro-seconds, I would consider something more light weight.

In short, the overhead is probably too small for you to notice. The benefit it gives you is ease of use, and it is likely to be worth it.