ExecutorService - create new instances in method vs one per class

Question

Should I create new ExecutorService in every method call or use one per class? Which is preferred option in terms of performance?

public class NotificationService {

    public void sendNotification(User recipient) {

        ExecutorService notificationsPool = Executors.newFixedThreadPool(10);

        // code

        notificationsPool.shutdown();
    }
}

Or

public class NotificationService {

    ExecutorService notificationsPool = Executors.newFixedThreadPool(10);

    public void sendNotification(User recipient) {

        // code
    }
}

Answer 1

In your first code snippet the ExecutorService is local, i.e. a new ExecutorService is created on each method call and the ExecutorService is terminated at the end of the method. As a consequence the threads aren't reused when the method runs the next time. In the second snippet the ExecutorService and its threads are kept as long as the NotificationService instance is alive. As you can see there are not only less instances of ExecutorService to be GC'd but there are less threads to create and they can be reused. As an additional bonus the second method does not incur any no warmup time for thread creation after the ExecutorService has been created.

If you have multiple instances of NotificationService you should declare notificationsPool as static in order to share the pool and its threads between all the instances.

If the amount of threads needed varies depending on the amount of notifications that have to be sent, use a cached thread pool (ExecutorService#newCachedThreadPool()), maybe with an upper limit.