ExecutorCompletionService?如果我们有 invokeAll 为什么需要一个?

问题描述

如果我们使用 ExecutorCompletionService，我们可以提交一个一系列任务作为 Callable 并获得与 CompletionService 交互的结果作为 queue.

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

但是还有ExecutorService的invokeAll，它接受一个Collection的任务，我们得到一个Future检索结果.

But there is also the invokeAll of ExecutorService that accepts a Collection of tasks and we get a list of Future to retrieve the results.

据我所知，使用一个或另一个没有任何好处(除了我们使用 invokeAll 避免了 for 循环，我们将有submit 任务到 CompletionService)，本质上它们是相同的想法，但略有不同.

As far as I can tell, there is no benefit in using one or over the other (except that we avoid a for loop using an invokeAll that we would have to submit the tasks to the CompletionService) and essentially they are the same idea with a slight difference.

那么为什么有两种不同的方式来提交一系列任务呢?我是否纠正了性能方面它们是等效的?是否存在一种比另一种更合适的情况?我想不出一个.

So why are there 2 different ways to submit a series of tasks? Am I correct that performance wise they are equivalent? Is there a case that one is more suitable than the other? I can't think of one.

推荐答案

使用 ExecutorCompletionService.poll/take，您将在 Future 完成时收到它们，在完成顺序(或多或少).使用ExecutorService.invokeAll，你没有这个权力；您要么阻塞直到全部完成，要么指定一个超时时间，在此之后取消未完成的部分.

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.

static class SleepingCallable implements Callable<String> {

  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;
  }
}

现在，下面我将演示 invokeAll 是如何工作的:

Now, below I will demonstrate how invokeAll works:

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

这会产生以下输出:

C:devscrap>java CompletionExample
... after 5 s ...
quick
slow

使用 CompletionService，我们看到不同的输出:

---

Using CompletionService, we see a different output:

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

这会产生以下输出:

C:devscrap>java CompletionExample
... after 500 ms ...
quick
... after 5 s ...
slow

注意时间是相对于程序开始的，而不是之前的消息.

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

您可以在此处上找到完整代码.

You can find full code on both here.

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