最近遇到一个需求，其中一个操作比较耗时。为了提高效率，我选择新开线程异步去做。由于需要对方法的返回值进行操作，我选择使用FutureTask，可以循环获取task，并调用get方法去获取task执行结果，但是如果task还未完成，获取结果的线程将阻塞直到task完成。后来发现了CompletionService类，它整合了Executor和BlockingQueue的功能。可以将Callable任务提交给它去执行，然后使用类似于队列中的 take 方法获取线程的返回值。

demo 如下：

package com.aliyun.callback;import org.junit.Test;import java.util.Random;import java.util.concurrent.*;/** * Created by Demon, on 2018/4/25 */public class CompletionServiceTest {    private static final int DEFAULT_CORE_POOL_SIZE = Runtime.getRuntime().availableProcessors() <= 1 ? 2 :            Runtime.getRuntime().availableProcessors() * 2;    private static final int DEFAULT_MAXNUM_POOL_SIZE = 32;    private static final int DEFAULT_KEEP_ALIVE_TIME = 30;    private  ThreadPoolExecutor threadPool = new ThreadPoolExecutor(DEFAULT_CORE_POOL_SIZE, DEFAULT_MAXNUM_POOL_SIZE            , DEFAULT_KEEP_ALIVE_TIME, TimeUnit.SECONDS, new ArrayBlockingQueue
    
     (20));    @Test    public void test() {        CompletionService
     
       completionService = new ExecutorCompletionService<>(threadPool);        try {            for (int i = 0; i < 5; i++) {                Task task = new Task("threadName" + i);                // 使用submit 主线程可以捕捉到异步线程中的异常                completionService.submit(task);            }        } catch (Exception e) {            e.printStackTrace();        }        for (int i = 0; i < 5; i++) {            try {                Future
      
        future = completionService.take();                System.out.println(future.get());            } catch (InterruptedException e) {                e.printStackTrace();            } catch (ExecutionException e) {                e.printStackTrace();            }        }    }    private class Task implements Callable
       
         {        private String name;        private Task(String name) {            this.name = name;        }        @Override        public String call() throws Exception {            int sleepTime = new Random().nextInt(1000);            try {                Thread.sleep(sleepTime);            } catch (InterruptedException e) {                e.printStackTrace();            }            // 返回给调用者的值            String str = name + " sleep time:" + sleepTime;            System.out.println(name + " finished...");            // throw new RuntimeException("test catch error");            return str;        }    }}运行结果：threadName0 finished...threadName0 sleep time:140threadName4 finished...threadName4 sleep time:250threadName3 finished...threadName3 sleep time:258threadName1 finished...threadName1 sleep time:416threadName2 finished...threadName2 sleep time:712
       
      
     
    

使用CompletionService来维护处理线程的返回结果时，主线程总是能够拿到最先完成的任务的返回值，而不管它们加入线程池的顺序, 这样就可以减少主线程阻塞时间，优于原生的 future.get方法。