How to wait for all tasks in an ThreadPoolExecutor to finish without shutting down the Executor?

Question

I can't use shutdown() and awaitTermination() because it is possible new tasks will be added to the ThreadPoolExecutor while it is waiting.

So I'm looking for a way to wait until the ThreadPoolExecutor has emptied it's queue and finished all of it's tasks without stopping new tasks from being added before that point.

If it makes any difference, this is for Android.

Thanks

Answer 1

If you are interested in knowing when a certain task completes, or a certain batch of tasks, you may use ExecutorService.submit(Runnable). Invoking this method returns a Future object which may be placed into a Collection which your main thread will then iterate over calling Future.get() for each one. This will cause your main thread to halt execution until the ExecutorService has processed all of the Runnable tasks.

Collection<Future<?>> futures = new LinkedList<Future<?>>();
futures.add(executorService.submit(myRunnable));
for (Future<?> future:futures) {
    future.get();
}

Answer 2

Maybe you are looking for a CompletionService to manage batches of task, see also this answer.

Answer 3

I think this is a related question: http://stackoverflow.com/questions/3402895/java-threadpool-usage. You might want to check out the answers listed there...

Answer 4

(This is an attempt to reproduce Thilo's earlier, deleted answer with my own adjustments.)

I think you may need to clarify your question since there is an implicit infinite condition... at some point you have to decide to shut down your executor, and at that point it won't accept any more tasks. Your question seems to imply that you want to wait until you know that no further tasks will be submitted, which you can only know in your own application code.

The following answer will allow you to smoothly transition to a new TPE (for whatever reason), completing all the currently-submitted tasks, and not rejecting new tasks to the new TPE. It might answer your question. @Thilo's might also.

Assuming you have defined somewhere a visible TPE in use as such:

AtomicReference<ThreadPoolExecutor> publiclyAvailableTPE = ...;

You can then write the TPE swap routine as such. It could also be written using a synchronized method, but I think this is simpler:

void rotateTPE()
{
   ThreadPoolExecutor newTPE = createNewTPE();
   // atomic swap with publicly-visible TPE
   ThreadPoolExecutor oldTPE = publiclyAvailableTPE.getAndSet(newTPE);
   oldTPE.shutdown();

   // and if you want this method to block awaiting completion of old tasks in  
   // the previously visible TPE
   oldTPE.awaitTermination();
} 

Alternatively, if you really no kidding want to kill the thread pool, then your submitter side will need to cope with rejected tasks at some point, and you could use null for the new TPE:

void killTPE()
{
   ThreadPoolExecutor oldTPE = publiclyAvailableTPE.getAndSet(null);
   oldTPE.shutdown();

   // and if you want this method to block awaiting completion of old tasks in  
   // the previously visible TPE
   oldTPE.awaitTermination();
} 

Which could cause upstream problems, the caller would need to know what to do with a null.

You could also swap out with a dummy TPE that simply rejected every new execution, but that's equivalent to what happens if you call shutdown() on the TPE.