scheduleAtFixedRate vs scheduleWithFixedDelay

Try adding a Thread.sleep(1000); call within your run() method... Basically it's the difference between scheduling something based on when the previous execution ends and when it (logically) starts.

For example, suppose I schedule an alarm to go off with a fixed rate of once an hour, and every time it goes off, I have a cup of coffee, which takes 10 minutes. Suppose that starts at midnight, I'd have:

00:00: Start making coffee
00:10: Finish making coffee
01:00: Start making coffee
01:10: Finish making coffee
02:00: Start making coffee
02:10: Finish making coffee

If I schedule with a fixed delay of one hour, I'd have:

00:00: Start making coffee
00:10: Finish making coffee
01:10: Start making coffee
01:20: Finish making coffee
02:20: Start making coffee
02:30: Finish making coffee

Which one you want depends on your task.

Visualize time series of invocation scheduleAtFixedRate method. Next executions will start immediately if the last one takes longer than period. Otherwise, it will start after period time.

Time series of invocation scheduleWithFixedDelay method. Next execution will start after delay time between termination of one execution and the commencement of the next, regardless of its execution time

Hope can help you

The scheduleAtFixedRate() method creates a new task and submits it to the executor every period, regardless of whether or not the previous task finished.

On the other hand, the scheduleWithFixedDelay() method creates a new task after the previous task has finished.

If you read the Java Doc it will be clearer

ScheduledFuture scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) 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.

ScheduledFuture scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) 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.