Java set a callback from ExecutorService

Question

I have a fixedThreadPool that I am using to run a bunch of worker threads to achieve parallel execution of a task with many components.

When all threads have finished, I retrieve their results (which are quite large) using a method (getResult) and write them to a file.

Ultimately, to save memory and be able to see intermediate results, I'd like each thread to write its result to the file as soon as it finishes execution and then free its memory.

Ordinarily, I'd add code to that effect to the end of the run() method. However, certain other objects in this class also calls these threads, but DO NOT want them to write their results to file - instead they use their results to perform other calculations, which are eventually written to file.

So, I was wondering if it's possible to attach a callback function to the event of a thread finishing using the ExecutorService. That way, I can immediately retrieve its result and free the memory in that scenario, but not break the code when those threads are used in other scenarios.

Is such a thing possible?

Answer 1

ExecutorService#submit return FutureTask<T> which helps you to retrieve result and the ExecutorService#get method will block execution until the computation is not completed. Example -

ExecutorService executor = Executors.newFixedThreadPool(10);
Future<Long> future = executor.submit(new Callable<Long>(){
       @Override
       public Long call() throws Exception {
           long sum = 0;
           for (long i = 0; i <= 10000000l; i++) {
               sum += i;
           }
           return sum;
       }
});
Long result = future.get();
System.out.println(result);