java CountDownLatch类的使用

CountDownLatch 是一个同步辅助类，犹如倒计时计数器，创建对象时通过构造方法设置初始值，调用CountDownLatch对象的await()方法则处于等待状态，调用countDown()方法就将计数器减1，当计数到达0时，则所有等待者或单个等待者开始执行。

主要方法

 public CountDownLatch(int count);

 public void countDown();

 public void await() throws InterruptedException

以下举例子说明用法：

1. 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()));      latch.countDown();//工人完成工作，计数器减一          }           private void doWork(){      try {  Thread.sleep(workTime);  } catch (InterruptedException e) {  e.printStackTrace();  }      }      }  }

输出：

Worker zhang san do work begin at 2011-04-14 11:05:11
Worker li si do work begin at 2011-04-14 11:05:11
Worker zhang san do work complete at 2011-04-14 11:05:16
Worker li si do work complete at 2011-04-14 11:05:19
all work done at 2011-04-14 11:05:19

2. 

package com.thread;  import java.util.concurrent.CountDownLatch;  import java.util.concurrent.CyclicBarrier;  import java.util.concurrent.ExecutorService;  import java.util.concurrent.Executors;  /**   *    * @author Administrator   *该程序用来模拟发送命令与执行命令，主线程代表指挥官，新建3个线程代表战士，战士一直等待着指挥官下达命令，   *若指挥官没有下达命令，则战士们都必须等待。一旦命令下达，战士们都去执行自己的任务，指挥官处于等待状态，战士们任务执行完毕则报告给   *指挥官，指挥官则结束等待。   */  public class CountdownLatchTest {          public static void main(String[] args) {          ExecutorService service = Executors.newCachedThreadPool(); //创建一个线程池          final CountDownLatch cdOrder = new CountDownLatch(1);//指挥官的命令，设置为1，指挥官一下达命令，则cutDown,变为0，战士们执行任务          final CountDownLatch cdAnswer = new CountDownLatch(3);//因为有三个战士，所以初始值为3，每一个战士执行任务完毕则cutDown一次，当三个都执行完毕，变为0，则 指挥官停止等待。                  for(int i=0;i<3;i++){              Runnable runnable = new Runnable(){                      public void run(){                      try {                          System.out.println("线程" + Thread.currentThread().getName() +                                   "正准备接受命令");                                                  cdOrder.await(); //战士们都处于等待命令状态                          System.out.println("线程" + Thread.currentThread().getName() +                           "已接受命令");                                                          Thread.sleep((long)(Math.random()*10000));                              System.out.println("线程" + Thread.currentThread().getName() +                                   "回应命令处理结果");                                                  cdAnswer.countDown(); //任务执行完毕，返回给指挥官，cdAnswer减1。                                          } catch (Exception e) {                          e.printStackTrace();                      }                                  }              };              service.execute(runnable);//为线程池添加任务          }                  try {              Thread.sleep((long)(Math.random()*10000));                        System.out.println("线程" + Thread.currentThread().getName() +                       "即将发布命令");                                      cdOrder.countDown(); //发送命令，cdOrder减1，处于等待的战士们停止等待转去执行任务。              System.out.println("线程" + Thread.currentThread().getName() +               "已发送命令，正在等待结果");                  cdAnswer.await(); //命令发送后指挥官处于等待状态，一旦cdAnswer为0时停止等待继续往下执行              System.out.println("线程" + Thread.currentThread().getName() +               "已收到所有响应结果");              } catch (Exception e) {              e.printStackTrace();          }                          service.shutdown(); //任务结束，停止线程池的所有线程      }  }