Question

I was working on a Java EE app that took in user requests from a UI and then keyed off a lon workflow asynchronously for each of these requests using ExecutorService (SinglethreadExecutor). Now since i was using a SinglethreadExecutor and because there was genuine need for the requests to be served one at a time, i did not feel the need for thread safety.
Is my understanding correct ?

Recently i had asked a question Issue when executing asynchronous tasks using ExecutorService and the solution to this question was that i make my code thread safe.
I’m looking if any shared resources that I’m using in my code is causing the need for this thread safety but would just like to be sure that my understanding of
the scenario is correct.

FYI, I have implemented my ExecutorService in a servlet as mentioned in Running a background Java program in Tomcat

Answer 1

Your requests will be passed to a different thread to be executed. Even if this thread doesn’t access shared data structures, the passing of the request to the thread and the returning of the result need to be properly synchronized.

If you use one of the submit or invoke methods which use a Future object for returning the results, you can assume that the appropriate synchronization is performed. The javadoc for ExecutorService says this:

Memory consistency effects: Actions in a thread prior to the submission of a Runnable or Callable task to an ExecutorService happen-before any actions taken by that task, which in turn happen-before the result is retrieved via Future.get().

In short, if the requests / tasks don’t use shared data structures and you use the interface methods provided, then you should be OK.