Java中CountDownLatch用法详解编程语言

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

“`java 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 
     } 
 } 

} “`