java中Executor和ExecutorCompletionservice之间的区别 [英] Difference between Executor and ExecutorCompletionservice in java

问题描述

正如问题标题本身所说的那样，Executors和ExecutorCompletionService类之间的区别是什么？

As the question title itself says what is the difference between Executors and ExecutorCompletionService classes in java?

我是Threading的新手，所以如果任何人可以用一段代码，这将有很大的帮助。

I am new to the Threading,so if any one can explain with a piece of code, that would help a lot.

推荐答案

假设你有一组任务 A ，B，C，D，E 并且您希望在 Executor 中异步执行它们，并在完成时逐个处理结果。

Suppose you had a set of tasks A, B, C, D, E and you want to execute each of them asynchronously in an Executor and process the results 1 by 1 as they complete.

使用执行者，您可以这样做：

List<Future<?>> futures = new ArrayList<Future<?>>();
futures.add(executorService.submit(A));
futures.add(executorService.submit(B));
futures.add(executorService.submit(C));
futures.add(executorService.submit(D));
futures.add(executorService.submit(E));

//This loop must process the tasks in the order they were submitted: A, B, C, D, E
for (Future<?> future:futures) {
    ? result = future.get();
    // Some processing here
}

这种方法的问题在于无法保证任务 A 将首先完成。因此，主线程可能会阻塞空闲等待任务 A 完成，当它可以处理另一个任务的结果时（比如任务乙）。使用 ExecutorCompletionService 可以减少结果处理延迟。

The problem with this method is that there is no guarantee that task A will complete first. Thus, it is possible that the main thread will be blocking idly waiting for task A to complete when it could be processing the result of another task (say task B). Result processing latency could be reduced by using an ExecutorCompletionService.

List<Future<?>> futures = new ArrayList<Future<?>>();
futures.add(executorCompletionService.submit(A));
futures.add(executorCompletionService.submit(B));
futures.add(executorCompletionService.submit(C));
futures.add(executorCompletionService.submit(D));
futures.add(executorCompletionService.submit(E));

//This for loop will process the tasks in the order they are completed,  
//regardless of submission order
for (int i=0; i<futures.size(); i++) {
    ? result = executorCompletionService.take().get();
    // Some processing here
}

所以，实质上，<$当处理任务结果的顺序无关紧要时，可以使用c $ c> ExecutorCompletionService 来提高效率。

So, in essence, ExecutorCompletionService could be used to squeeze out a little more efficiency when the order of processing task results does not matter.

一个重要的但要注意的事情。 ExecutorCompletionService的实现包含结果队列。如果没有调用 take 或 poll 来排空该队列，则会发生内存泄漏。有些人使用 submit 返回的 Future 来处理结果，这是不正确的用法。

One important thing to note though. The implementation of ExecutorCompletionService contains a queue of results. If take or poll are not called to drain that queue, a memory leak will occur. Some people use the Future returned by submit to process results and this is NOT correct usage.

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