Wait for all threads in an Executor to finish?

Question

I'm implementing a parellel quicksort as programming practice, and after I finished, I read the Java tutorial page on Executors, which sound like they could make my code even faster. Unfortunately, I was relying on join()'s to make sure that the program doesn't continue until everything is sorted. Right now I'm using:

public static void quicksort(double[] a, int left, int right) {
    if (right <= left) return;
    int i = partition(a, left, right);

    // threads is an AtomicInteger I'm using to make sure I don't
    // spawn a billion threads.
    if(threads.get() < 5){

        // ThreadSort's run method just calls quicksort()
        Future leftThread = e.submit(new ThreadSort(a, left, i-1));
        Future rightThread = e.submit(new ThreadSort(a, i+1, right));

        threads.getAndAdd(2);
        try {
            leftThread.get();
            rightThread.get();
        }
        catch (InterruptedException ex) {}
        catch (ExecutionException ex) {}
    }
    else{
        quicksort(a, left, i-1);
        quicksort(a, i+1, right);
    }
}

This seems to work ok, but if I run e.shutdown() right after I call my non-recursive quicksort() method, it has a bunch of RejectedExecutionExceptions, so I assume this isn't working as well as I had wanted.

So anyway, I'm basically trying to get the same functionality as leftThread.join() but with an Executor, and my questions is:

Is this the best way to wait until all of the threads are done?

EDIT: Ok, so I figured out why I got a bunch of errors after shutting down my Executor, it was because I was calling this function in a loop (to even out run times) and not creating a new Executor.

Answer 1

What type of executor are you using?

ThreadPoolExecutor.awaitTermination() will do what you are asking about (it's effectively a bulk join operation).

As a total aside, ThreadPoolExecutor will allow you to set limits on the # of threads, etc... (might be better than going recursive like what you are doing if the thread count goes high, not sure).

PS - I doubt that executors will make your code run any faster, but they may make your code easier to read and maintain. Using a Thread pool will make things faster for this sort of algorithm, and the Executor makes it easy to work with thread pools.

Answer 2

Take a look at Executors.newFixedThreadPool which lets you create a pool of at most n threads (gets rid of your "if") and the ExecutorService.shutdown method and the ExecutorsService.awaitTermination method.

Answer 3

You could use a CountDownLatch