Removing all queued tasks of an ThreadPoolExecutor
i have this rather simple question about the ThreadPoolExecutor. I have the following situation: I have to consume objects from a queue, create the appropiate worker tasks for them and submit them to the ThreadPoolExecutor. This is quite simple. But within a shutdown scenario many workers may be queued to execution. Since one of those tasks might be running for an hour, and i want a relativly fast graceful shutdown of the application i want to discard all queued tasks from the ThreadPoolExecutor while the already processing tasks should be completed normally.
The ThreadPoolExecutor documentation has a remove() method but only allows specific tasks to be removed. purge() only works for already canceled Future tasks. My idea was to clear the queue holding all queued tasks. The ThreadPoolExecutor provides access to this internal queue but the documentation states:
Method getQueue() allows access to the work queue for purposes of monitoring and debugging. Use of this method for any other purpose is strongly discouraged.
So grabbing this queue and clearing it is not an option. Also, this snippet of the documentation says:
Two supplied methods, remove(java.lang.Runnable) and purge() are available to assist in storage reclamation when large numbers of queued tasks become cancelled.
How? Sure, i can maintain a list of all tasks i submitted to the executor and in a shutdown case i iterate over all entries and remove them from the ThreadPoolExecutor with the remove() method... but... come on, this is a waste of memory and a hassle to maintain this list. (Removing already executed tasks for example)
I appreciate any hints or solutions!
Answer
I used to work on an app with long running threads. We do this at shutdown,
BlockingQueue<Runnable> queue = threadPool.getQueue();
List<Runnable> list = new ArrayList<Runnable>();
int tasks = queue.drainTo(list);
The list is saved to a file. On startup, the list is added back to the pool so we don't lose any jobs.