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

Question

如果我们使用ExecutorCompletionService,我们可以提交一系列任务作为Callables,并将结果与CompletionServiceas进行交互queue.

但也有在invokeAll的ExecutorService,它接受一个Collection任务,我们得到的名单Future,以检索结果.

据我所知,有在使用一个或比其他任何好处(除了我们避免for使用循环invokeAll,我们将不得不submit对任务的CompletionService),基本上他们是有轻微的差别同样的想法.

那么为什么有两种不同的方式来提交一系列任务呢？我是否正确表现他们是相同的？是否有一个比另一个更合适的情况？我想不出一个.

Answer 1

使用a ExecutorCompletionService.poll/take,您Future将按完成顺序(或多或少)收到s.使用ExecutorService.invokeAll,你没有这种力量; 你要么阻止直到全部完成,要么你指定一个超时,之后取消不完整.

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,下面我将演示如何invokeAll工作:

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

使用CompletionService,我们看到不同的输出:

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

请注意,时间与程序启动有关,而不是之前的消息.

你可以在这里找到完整的代码.

Answer 2

通过使用ExecutorCompletionService,您可以在每个作业完成时立即得到通知.相比之下,在返回s 集合之前ExecutorService.invokeAll(...)等待所有作业完成Future:

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

相反,当您使用a时ExecutorCompletionService,您将能够在每个作业完成后立即获取作业,这允许您(例如)将它们发送到另一个线程池,记录结果等.

ExecutorService threadPool = Executors.newFixedThreadPool(2);
ExecutorCompletionService<Result> compService
      = new ExecutorCompletionService<Result>(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<Result> 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));
}