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 

} `

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