ExecutorCompletionService? Why do need one if we have invokeAll?

Question

If we use an ExecutorCompletionService we can submit a series of tasks as Callables and get the result interacting with the CompletionService as a

Answer 1

By using an ExecutorCompletionService, you can get immediately notified when each of your jobs completes. In comparison, ExecutorService.invokeAll(...) waits for all of your jobs to complete before returning the collection of Futures:

// this waits until _all_ of the jobs complete
List> futures = threadPool.invokeAll(...);

Instead, when you use a ExecutorCompletionService, you will be able to get the jobs as soon as each of them completes which allows you to (for example) send them on for processing into another thread pool, log results, etc..

ExecutorService threadPool = Executors.newFixedThreadPool(2);
ExecutorCompletionService compService
      = new ExecutorCompletionService(threadPool);
for (MyJob job : jobs) {
    compService.submit(job);
}
// shutdown the pool but the jobs submitted continue to run
threadPool.shutdown();
while (!threadPool.isTerminated()) {
    // the take() blocks until any of the jobs complete
    // this joins with the jobs in the order they _finish_
    Future future = compService.take();
    // this get() won't block
    Result result = future.get();
    // you can then put the result in some other thread pool or something
    // to immediately start processing it
    someOtherThreadPool.submit(new SomeNewJob(result));
}