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
}
}
}
