Java中CountDownLatch用法解析

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，可以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;

public class CountDownLatchDemo {
  private static final int PLAYER_AMOUNT = 5;
  public CountDownLatchDemo() {
    // TODO Auto-generated constructor stub
  }
  /**
  * @param args
  */
  public static void 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，所有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();
  }
}

接下来是Player类

import java.util.concurrent.CountDownLatch;


 public class Player implements Runnable {

   private int 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
   public void 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
     }
   }
 }

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