Java多线程并发经常使用类实例之：Countdownlatch

    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如其所写，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，可让主线程等待子线程的结束。下面以一个模拟运动员比赛的例子加以说明。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

publicclass CountDownLatchDemo {
privatestaticfinalint PLAYER_AMOUNT = 5;
public CountDownLatchDemo() {
// TODO Auto-generated constructor stub    
      }
/**
      * @param args
*/
publicstaticvoid main(String[] args) {
// TODO Auto-generated method stub
//对于每位运动员，CountDownLatch减1后即结束比赛
         CountDownLatch begin = new CountDownLatch(1);
//对于整个比赛，全部运动员结束后才算结束
         CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
         Player[] plays = new Player[PLAYER_AMOUNT];

for(int i=0;i<PLAYER_AMOUNT;i++)
             plays[i] = new Player(i+1,begin,end);

//设置特定的线程池，大小为5
         ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
for(Player p:plays)
             exe.execute(p);            //分配线程
         System.out.println("Race begins!");
         begin.countDown();
try{
             end.await();            //等待end状态变为0，即为比赛结束
         }catch (InterruptedException e) {
// TODO: handle exception
             e.printStackTrace();
         }finally{
             System.out.println("Race ends!");
         }
         exe.shutdown();
     }
 }
publicclass Player implements Runnable {

privateint id;
private CountDownLatch begin;
private CountDownLatch end;
public Player(int i, CountDownLatch begin, CountDownLatch end) {
// TODO Auto-generated constructor stub
super();
this.id = i;
this.begin = begin;
this.end = end;
     }

     @Override
publicvoid run() {
// TODO Auto-generated method stub
try{
             begin.await();        //等待begin的状态为0
             Thread.sleep((long)(Math.random()*100));    //随机分配时间，即运动员完成时间
             System.out.println("Play"+id+" arrived.");
         }catch (InterruptedException e) {
// TODO: handle exception
             e.printStackTrace();
         }finally{
             end.countDown();    //使end状态减1，最终减至0
         }
     }
 }