Question

Some questions about Executors best usage for memory and time performance:

1. Is there any cost penalty incurred for the use of

   ExecutorService e = Executors.newSingleThreadExecutor();
   e.execute(callable)
   e.shutdown()
   

   compared to:

   new Thread(runnable).start()
   

2. If a Callable is not a long one, and never won’t be more than one instance running of it it’s ok to use the code from (1)? Or is it best to have ExecutorService as a static instance and reuse between calls?

3. If I have several tasks as described in (2) is there any problem with every task to having its own executor service or is better to have a centralized one? (I’m talking about a client application where normally there won’t be more than one of these tasks running)

4. What resources does an Executor consume if it isn’t shutdown()?

Answer 1

Is there any cost penalty from using

ExecutorService e=Executors.newSingleThreadExecutor();
e.execute(callable)
e.shutdown()

compared to:

new Thread(runnable).start()

Yes, there is a “penalty”: the ExecutorService will most likely be more expensive to create since it also creates a queue for the tasks you’re submitting and if a thread fails prior to shutdown, then the failed thread will be replaced with another one in order to run any subsequent tasks (so there is quite a bit of logic there). However, you probably don’t want to be creating an ExecutiveService each time you want to run a task… that’s probably not the best use of an ExecutorService (more on that in the next question).

If a Callable is not a long one, and never won’t be more than one instance running of it it’s ok to use the code from -1-? Or it’s best to have ExecutorService as static one and reuse between calls?

The suggested use of the ExecutorService is as a thread pool, where you keep the ExecutorService around and you keep submitting tasks to it as long as you have tasks to submit. It can be static or just a regular member, that is irrelevant and highly dependent on your requirements/design. Even if you’re only running one instance at a time (i.e. you only have a single-threaded executor), it’s still more efficient to use the ExecutorService because it reuses the thread, so in the long run it will be less expensive than creating a new thread for each task you submit.

If I have several task… is there any problem for every task to have their executor service or is better to have a centralized one?

There is no problem, but it’s inefficient, so just have a centralized executor service.

What resources consumes an Executor if it isn’t shutdown()?

I don’t think you should worry about that in particular, especially if you’re using the Executor in the correct context it will be minimal.