学习CountDownLatch

原文链接：http://www.iteye.com/topic/1002652

CountDownLatch，一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。

CountDownLatch的一个非常典型的应用场景是：有一个任务想要往下执行，但必须要等到其他的任务执行完毕后才可以继续往下执行。假如我们这个想要继续往下执行的任务调用一个CountDownLatch对象的await()方法，其他的任务执行完自己的任务后调用同一个CountDownLatch对象上的countDown()方法，这个调用await()方法的任务将一直阻塞等待，直到这个CountDownLatch对象的计数值减到0为止。

主要方法

public CountDownLatch(int count)构造方法参数指定了计数的次数public void countDown()当前线程调用此方法，则计数减一public void await() throws InterruptedException调用此方法会一直阻塞当前线程，直到计时器的值为0

import java.text.SimpleDateFormat;import java.util.Date;import java.util.concurrent.CountDownLatch;public class CountDownLatchDemo {      final static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");      public static void main(String[] args) throws InterruptedException {          CountDownLatch latch=new CountDownLatch(2);//两个工人的协作          Worker worker1=new Worker("zhang san", 5000, latch);          Worker worker2=new Worker("li si", 8000, latch);          worker1.start();          worker2.start();        latch.await();//等待所有工人完成工作          System.out.println("all work done at " + sdf.format(new Date()));      }      static class Worker extends Thread {          String workerName;           int workTime;          CountDownLatch latch;          public Worker(String workerName, int workTime, CountDownLatch latch) {               this.workerName = workerName;               this.workTime = workTime;               this.latch = latch;          }          public void run() {              System.out.println("Worker " + workerName + " do work begin at " + sdf.format(new Date()));              doWork();//工作了              System.out.println("Worker " + workerName + " do work complete at " + sdf.format(new Date()));          }          private void doWork() {              try {                  Thread.sleep(workTime);              } catch (InterruptedException e) {                  e.printStackTrace();              } finally {                latch.countDown();//工人完成工作，计数器减一              }         }      }  }

输出：

Worker li si do work begin at 2017-06-22 14:19:37Worker zhang san do work begin at 2017-06-22 14:19:37Worker zhang san do work complete at 2017-06-22 14:19:42Worker li si do work complete at 2017-06-22 14:19:45all work done at 2017-06-22 14:19:45