How can I get the jobs that are in execution in a ScheduledThreadPoolExecutor?

Question

I need to force the release of resources when a task is interrupted. For that, I implemented this solution . But with this, when I call shutdown() all the task in the ScheduledThreadPoolExecutor.getQueue() are forced correctly, but some of my jobs kept the resources. I checked very well the behavior and I figured out this: when a task is in execution, he is not present in the ScheduledThreadPoolExecutor queue (I know it sounds obvious). The problem is that I need to force the release of resources for all the jobs (in the queue or in execution)

So, How can I get the jobs that are in execution? Do you have a better idea?

Answer 1

You can maintain a list of all the Future's you create when you submit the jobs. Use this list to cancel all futures.

Answer 2

Don't you want to call

executor.shutdownNow()

that will attempt to cancel currently running tasks (using Thread.interrupt so you'll need to implement an 'interruption policy' in each task that uses the interrupt flag).

from the javadoc

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt, so any task that fails to respond to interrupts may never terminate.

This will return a list of waiting tasks, so you can always put this back onto a 'wait list' rather than loose them completely. You might also want follow that up with an 'await termination' to avoid run away code. For example, executor.awaitTermination(...) .

tempus-fugit has some handy classes for handling this. You just call

shutdown(executor).waitingForShutdown(millis(400));

see here for details.

Also, the solution you outline in the blog post; I'm not sure if that's quite right. Future.cancel will only stop the task from being scheduled. If you were to update the example in the blog to allow interruption (ie cancel(true) , it'd be equivalent (more or less) with the shutdownNow . That is to say, it will call interrupt on the underlying task which (if you've implemented an interruption policy) will stop it processing. As for cleaning up after interruption, you just need to make sure that you handle that appropriately within the interruption policy implementation. The upshot is that I think you can cancel and cleanup correctly using shutdownNow (or cancel(true))