Removing all queued tasks of an ThreadPoolExecutor

Question

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 1

You could create your own task queue and pass it to ThreadPoolExecutor constructor:

int poolSize = 1; // number of threads
BlockingQueue<Runnable> queue = new ArrayBlockingQueue<>();
Executor executor = new ThreadPoolExecutor(poolSize, poolSize, 0L, TimeUnit.MILLISECONDS, queue);

When you clear the queue somewhere in your code then the remaining tasks will not be executed:

queue.clear();

Answer 2

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.

Answer 3

Doesn't awaitTermination(long timeout, TimeUnit unit) work after shutdown?

executor.shutdown(); executor.awaitTermination(60, TimeUnit.SECONDS)

Answer 4

This is an old question, but in case this helps somebody else: you could set a volatile boolean when you call shutdown(), and have each submitted task terminate if that boolean is set before really starting. This will allow tasks which have genuinely started to complete, but will prevent queued tasks from starting their actual activity.

Answer 5

Bombe's answer is exactly what you want. shutdownNow() stops everything using the nuke and pave approach. This is the best thing you can do, short of subclassing the implementation of ThreadPoolExecutor that you're using.

Answer 6

Have you considered wrapping the ExecutorService? Create a

CleanShutdownExecutorService implements Executor 

that delegates all calls to another Executor, but keeps the Futures in a list of its own. CleanShutdownExecutorService can then have a cancelRemainingTasks() method that calls shutdown(), then calls cancel(false) on all the Futures in its list.