CompletionService

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在java.util.concurrent的包里面，有很多优秀的并发工具，其中CompletionService就是一个接口。
他的实现有多个，ExecutorCompletionService是把executor和CompletionService结合的一种实现。

package com.front.qzq.concurrent;import java.util.Random;import java.util.concurrent.Callable;import java.util.concurrent.CompletionService;import java.util.concurrent.ExecutorCompletionService;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;import java.util.concurrent.TimeUnit;public class ExecutorServiceStudy {    public static void main(String[] args) throws Exception{        ExecutorService executorService = Executors.newCachedThreadPool();        CompletionService<Integer> completionService = new ExecutorCompletionService<Integer>(executorService);        for(int i = 0 ; i < 7; i++){            completionService.submit(new Worker());        }        for(int i = 0; i < 7; i++){            Future<Integer> poll = completionService.poll();            if(poll != null){                System.out.println(poll.get());            }else{                System.out.println("The task is not ready!" + i);            }            TimeUnit.MILLISECONDS.sleep(500);        }    }}class Worker implements Callable<Integer>{    public Integer call() throws Exception {        Random random = new Random();        int result = random.nextInt(4);        System.out.println("sleep " + result);        TimeUnit.SECONDS.sleep(result);        return result;    }}

使用CompletionService的好处是，当任务执行完了，我们只需要调用take（阻塞方法，直到有任务执行完成）或者poll（非阻塞方法）方法就好了，获取future对象，实现了生产者和消费者的解耦。无需我们自己去对结果列表进行调度和轮询。

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