Behavior of Java's ScheduledExecutorService.scheduleAtFixedRate()

Question

I have a question regarding the scheduleAtFixedRate() method on ScheduledExecutorService in Java 6.

[ edit : the Javadoc for 1.6 is more complete than that for 1.5. See comment below]

Given that:

• the ScheduledExecutorService is constructed with N = 1 thread in the pool
• the fixed-rate is a period of T seconds
• no initial delay

What happens in this case (times are not meant to be absolute, in the real-time sense):

• at time T, the service kicks off a Runnable task, "task1"
• at time 2T, task1 has not yet completed, and service is scheduled to fire

Is the service guaranteed to do any of the following?

• (a) at 2T, kick off a Runnable task, "task2" (recall N = 1)
• (b) block until task1 is finished
• (c) skip this time and try again at 3T
• (d) behavior is undefined

Or something else? Does the answer change if N > 1 ?

Answer 1

The answer is

(b) block until task1 is finished

and that is regardless of number of threads of the executor (task2 might even be not submitted).

The doc says:

If any execution of this task takes longer than its period, then subsequent executions may start late, . 。

(BTW, since there's no intial delay, "task1" will kickoff right away as doc`ed:

executions will commence after initialDelay

).

Answer 2

From the documentation that you linked...

If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.