为了防止无良网站的爬虫抓取文章，特此标识，转载请注明文章出处。LaplaceDemon/ShiJiaqi。
https://www.cnblogs.com/shijiaqi1066/p/10454237.html
CompletionService是Java8的新增接口，JDK为其提供了一个实现类ExecutorCompletionService。这个类是为线程池中Task的执行结果服务的，即为Executor中Task返回Future而服务的。CompletionService的实现目标是任务先完成可优先获取到，即结果按照完成先后顺序排序。
CompletionService的使用非常简单。从源码查看ExecutorCompletionService类，该类只有三个成员变量：
public class ExecutorCompletionService implements CompletionService {
private final Executor executor;
private final AbstractExecutorService aes;
private final BlockingQueue> completionQueue;
...
} 可以看到ExecutorCompletionService主要是增强executor线程池的。Task包装后被塞入completionQueue，当Task结束，其Future就可以从completionQueue中获取到。
其基本原理可以参看下图：
CompletionService接口源码：
public interface CompletionService {
// 提交
Future submit(Callable task);
Future submit(Runnable task, V result);
// 获取
Future take() throws InterruptedException;
Future poll();
Future poll(long timeout, TimeUnit unit) throws InterruptedException;
}
例：向CompletionService中提交10个Task，当Task有任务返回则会优先从CompletionService内部的队列中获取到Task的Future。
package test;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
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;
public class TestCompletionService {
public static void main(String[] args) {
Long start = System.currentTimeMillis();
//开启3个线程
ExecutorService exs = Executors.newFixedThreadPool(5);
try {
int taskCount = 10;
// 结果集
List list = new ArrayList();
List> futureList = new ArrayList>();
// 1.定义CompletionService
CompletionService completionService = new ExecutorCompletionService(exs);
// 2.添加任务
for(int i=0;i future = completionService.submit(new Task(i+1));
futureList.add(future);
}
// 3.获取结果
for(int i=0;i