CountDownLatch类是一个同步计数器,构造时传入int参数,该参数就是计数器的初始值，每调用一次countDown()方法，计数器减1,计数器大于0 时，await()方法会阻塞程序继续执行。

CountDownLatch可以看作是一个倒计数的锁存器，当计数减至0时触发特定的事件。利用这种特性，可以让主线程等待子线程的结束

使用场景：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。

1 public class CountDownLatchDemo { 2     final static SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); 3     public static void main(String[] args) throws InterruptedException { 4         CountDownLatch latch=new CountDownLatch(2); 5         Worker worker1=new Worker("zhang san", 5000, latch); 6         Worker worker2=new Worker("li si", 8000, latch); 7         worker1.start(); 8         worker2.start(); 9         latch.await();10         System.out.println("all work done at "+sdf.format(new Date()));11     }12     13     14     static class Worker extends Thread{15         String workerName; 16         int workTime;17         CountDownLatch latch;18         public Worker(String workerName ,int workTime ,CountDownLatch latch){19              this.workerName=workerName;20              this.workTime=workTime;21              this.latch=latch;22         }23         public void run(){24             System.out.println("Worker "+workerName+" do work begin at "+sdf.format(new Date()));25             doWork();26             System.out.println("Worker "+workerName+" do work complete at "+sdf.format(new Date()));27 28         }29         30         private void doWork(){31             try {32                 Thread.sleep(workTime);33             } catch (InterruptedException e) {34                 e.printStackTrace();35             } finally {36                 latch.countDown();37             }38         }39     }40     42 }43

 

参考：

      http://www.iteye.com/topic/1002652

      http://www.cnblogs.com/whgw/archive/2011/09/29/2195827.html