如下场景：我们在一个线程池中提交了多个任务，多个任务的执行时长不一样，而且我们想按执行时间的长短（先拿到执行时间短的任务的结果，再拿执行时间长的任务的结果）来依次获取任务的返回结果。
那可以应用CompletionService来方便的处理这个场景。
示例代码如下：
package completionServiceDemo; import java.util.Date; import java.util.concurrent.*; public class Demo { public static void main(String[] args) throws InterruptedException, ExecutionException { ExecutorService executorService = Executors.newFixedThreadPool(5); CompletionService<Integer> completionService = new ExecutorCompletionService<>(executorService); completionService.submit(new MyCallable(10)); completionService.submit(new MyCallable(20)); completionService.submit(new MyCallable(5)); for (int i = 0; i < 3; i++) { Integer integer = completionService.take().get(); System.out.println(new Date() + ":" + integer); } } }
package completionServiceDemo; import java.util.concurrent.Callable; public class MyCallable implements Callable<Integer> { Integer seconds; public MyCallable(int seconds) { this.seconds = seconds; } @Override public Integer call() throws Exception { Thread.sleep(seconds * 1000); return seconds; } }
打印结果如下：
Thu Jul 29 23:26:55 CST 2021:5 Thu Jul 29 23:27:00 CST 2021:10 Thu Jul 29 23:27:10 CST 2021:20
原理：
CompletionService将各任务执行的结果放到一个BlockingQueue中（先执行完的先往里放），再从队列往外取的时候（队列先进先出）就自然达到了按执行快慢获取结果的效果。