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

Using a ExecutorCompletionService.poll/take, you are receiving the Futures as they finish, in completion order (more or less). Using ExecutorService.invokeAll, you do not have this power; you either block until are all completed, or you specify a timeout after which the incomplete are cancelled.

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static class SleepingCallable implements Callable {

  final String name;
  final long period;

  SleepingCallable(final String name, final long period) {
    this.name = name;
    this.period = period;
  }

  public String call() {
    try {
      Thread.sleep(period);
    } catch (InterruptedException ex) { }
    return name;
  }
}

Now, below I will demonstrate how invokeAll works:

final ExecutorService pool = Executors.newFixedThreadPool(2);
final List> callables = Arrays.asList(
    new SleepingCallable("quick", 500),
    new SleepingCallable("slow", 5000));
try {
  for (final Future future : pool.invokeAll(callables)) {
    System.out.println(future.get());
  }
} catch (ExecutionException | InterruptedException ex) { }
pool.shutdown();

This produces the following output:

C:\dev\scrap>java CompletionExample
... after 5 s ...
quick
slow

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Using CompletionService, we see a different output:

final ExecutorService pool = Executors.newFixedThreadPool(2);
final CompletionService service = new ExecutorCompletionService(pool);
final List> callables = Arrays.asList(
    new SleepingCallable("slow", 5000),
    new SleepingCallable("quick", 500));
for (final Callable callable : callables) {
  service.submit(callable);
}
pool.shutdown();
try {
  while (!pool.isTerminated()) {
    final Future future = service.take();
    System.out.println(future.get());
  }
} catch (ExecutionException | InterruptedException ex) { }

This produces the following output:

C:\dev\scrap>java CompletionExample
... after 500 ms ...
quick
... after 5 s ...
slow

Note the times are relative to program start, not the previous message.

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You can find full code on both here.