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

First of all, if we do not want to waste processor time, we will not use

while (!future.isDone()) {
        // Do some work...
}

We must use

service.shutdown();
service.awaitTermination(14, TimeUnit.DAYS);

The bad thing about this code is that it will shut down ExecutorService. If we want to continue work with it (i.e. we have some recursicve task creation), we have two alternatives: invokeAll or ExecutorService.

invokeAll will wait untill all tasks will be complete. ExecutorService grants us ability to take or poll results one by one.

And, finily, recursive example:

ExecutorService executorService = Executors.newFixedThreadPool(THREAD_NUMBER);
ExecutorCompletionService completionService = new ExecutorCompletionService(executorService);

while (Tasks.size() > 0) {
    for (final Task task : Tasks) {
        completionService.submit(new Callable() {   
            @Override
            public String call() throws Exception {
                return DoTask(task);
            }
        });
    } 

    try {                   
        int taskNum = Tasks.size();
        Tasks.clear();
        for (int i = 0; i < taskNum; ++i) {
            Result result = completionService.take().get();
            if (result != null)
                Tasks.add(result.toTask());
        }           
    } catch (InterruptedException e) {
    //  error :(
    } catch (ExecutionException e) {
    //  error :(
    }
}