Shutdown ExecutorService gracefully in webapp?

Question

In my webapp, I created a service that is using an ExecutorService with fixed size ThreadPool. I reuse the same ExecutorService during the whole application lifetime.

private static ExecutorService pool = Executors.newFixedThreadPool(8); 

All is running in Tomcat which gives me the following error while shuting down:

appears to have started a thread named [pool-1-thread-1] but has failed to stop it. This is very likely to create a memory leak. 

I do realize I need to shutdown the ExecutorService before shuting tomcat down. Soms SO thread already speak about this but I could not find a clean way to handle this.

Should I use a ShutdownHook as suggested @Tim-bender in Graceful shutdown of threads and executor ? Or should I use a CachedThreadPool instead?

Answer 1

Shutdown hook is not a good approach in Tomcat because:

• it will close the pool too late (on shutdown), Tomcat will already warn you about not closed resources

• you actually want to shutdown that pool when application is undeployed so that redeployment works (otherwise each application will create new pool and they will all be closed only on complete shutdown)

• shutting down the thread pool might take some time (see below), shutdown hook should be as fast as possible

Much better place is ServletContextListener.contextDestroyed(). Remember you have to both shutdownNow() the pool (to cancel running and reject new tasks) and awaitTermination() to wait for already running tasks to finish and all threads to stop.

Answer 2

In addition to what Tomasz suggested you can also use CachedThreadPool

Threads that have not been used for sixty seconds are terminated and removed from the cache. Thus, a pool that remains idle for long enough will not consume any resources

So a very good solution would be use CachedThreadPool and shutdown it in ServletContextListener.contextDestroyed().