When should I use a CompletionService over an ExecutorService?

Question

I just found CompletionService in this blog post. However, this does\'t really showcases the advantages of CompletionService over a standard ExecutorService. The same code c

Answer 1

there is another advantage of using completionservice: Performance

when you call future.get(), you are spin waiting:

from java.util.concurrent.CompletableFuture

  private Object waitingGet(boolean interruptible) {
        Signaller q = null;
        boolean queued = false;
        int spins = -1;
        Object r;
        while ((r = result) == null) {
            if (spins < 0)
                spins = (Runtime.getRuntime().availableProcessors() > 1) ?
                    1 << 8 : 0; // Use brief spin-wait on multiprocessors
            else if (spins > 0) {
                if (ThreadLocalRandom.nextSecondarySeed() >= 0)
                    --spins;
            }

when you have a long-running task, this will be a disaster for performance.

with completionservice, once the task is done, it's result will be enqueued and you can poll the queue with lower performance overhand.

completionservice achieve this by using wrap task with a done hook.

java.util.concurrent.ExecutorCompletionService

    private class QueueingFuture extends FutureTask {
    QueueingFuture(RunnableFuture task) {
        super(task, null);
        this.task = task;
    }
    protected void done() { completionQueue.add(task); }
    private final Future task;
}