浅析Java中CountDownLatch用法
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。
1 import java.util.concurrent.CountDownLatch;
2 import java.util.concurrent.Executor;
3 import java.util.concurrent.ExecutorService;
4 import java.util.concurrent.Executors;
5
6 public class CountDownLatchDemo {
7 private static final int PLAYER_AMOUNT = 5;
8 public CountDownLatchDemo() {
9 // TODO Auto-generated constructor stub
10 }
11 /**
12 * @param args
13 */
14 public static void main(String[] args) {
15 // TODO Auto-generated method stub
16 //对于每位运动员，CountDownLatch减1后即结束比赛
17 CountDownLatch begin = new CountDownLatch(1);
18 //对于整个比赛，所有运动员结束后才算结束
19 CountDownLatch end = new CountDownLatch(PLAYER_AMOUNT);
20 Player[] plays = new Player[PLAYER_AMOUNT];
21
22 for(int i=0;i<PLAYER_AMOUNT;i++)
23 plays[i] = new Player(i+1,begin,end);
24
25 //设置特定的线程池，大小为5
26 ExecutorService exe = Executors.newFixedThreadPool(PLAYER_AMOUNT);
27 for(Player p:plays)
28 exe.execute(p); //分配线程
29 System.out.println("Race begins!");
30 begin.countDown();
31 try{
32 end.wait(); //等待end状态变为0，即为比赛结束
33 }catch (InterruptedException e) {
34 // TODO: handle exception
35 e.printStackTrace();
36 }finally{
37 System.out.println("Race ends!");
38 }
39 exe.shutdown();
40 }
41 }
接下来是Player类
1 import java.util.concurrent.CountDownLatch;
2
3
4 public class Player implements Runnable {
5
6 private int id;
7 private CountDownLatch begin;
8 private CountDownLatch end;
9 public Player(int i, CountDownLatch begin, CountDownLatch end) {
10 // TODO Auto-generated constructor stub
11 super();
12 this.id = i;
13 this.begin = begin;
14 this.end = end;
15 }
16
17 @Override
18 public void run() {
19 // TODO Auto-generated method stub
20 try{
21 begin.await(); //等待begin的状态为0
22 Thread.sleep((long)(Math.random()*100)); //随机分配时间，即运动员完成时间
23 System.out.println("Play"+id+" arrived.");
24 }catch (InterruptedException e) {
25 // TODO: handle exception
26 e.printStackTrace();
27 }finally{
28 end.countDown(); //使end状态减1，最终减至0
29 }
30 }
31 }
