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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