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可以看作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。
CountDownLatch的一个非常典型的应用场景是：有一个任务想要往下执行，但必须要等到其他的任务执行完毕后才可以继续往下执行。假如我们这个想要继续往下执行的任务调用一个CountDownLatch对象的await()方法，其他的任务执行完自己的任务后调用同一个CountDownLatch对象上的countDown()方法，这个调用await()方法的任务将一直阻塞等待，直到这个CountDownLatch对象的计数值减到0为止。
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 = ecutors.newFixedThreadPool(PLAYER_AMOUNT);
for(Player p:plays)
exe.execute(p); //分配线程
System.out.println("Race begins!");
begin.countDown();
try{
end.wait(); //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
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 }
下面是张孝祥老师所举例子的程序代码和运行结果：
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CountDownLatchTest {
public static void main(String [] args){
ExecutorService service = Executors.newCachedThreadPool();
final CountDownLatch cdOrder = new CountDownLatch(1);
final CountDownLatch cdAnswer = new CountDownLatch(3);
for(int i=0;i<3;i++){
Runnable runnable=new Runnable(){
public void run(){
try {
System.out.println("线程"+Thread.currentThread().getName()+"正准备接受命令");
cdOrder.await();
System.out.println("线程"+Thread.currentThread().getName()+"已接受命令");
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程"+Thread.currentThread().getName()+"回应命令处理结果");
cdAnswer.countDown();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
};
service.execute(runnable);
}
try {
System.out.println("线程"+Thread.currentThread().getName()+"即将发布命令");
cdOrder.countDown();
System.out.println("线程"+Thread.currentThread().getName()+"已接受命令，正在等待结果");
cdAnswer.await();
Thread.sleep((long)(Math.random()*10000));
System.out.println("线程"+Thread.currentThread().getName()+"已收到所有响应结果");
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
service.shutdown();
}
}
运行结果：
线程pool-1-thread-1正准备接受命令
线程pool-1-thread-2正准备接受命令
线程pool-1-thread-3正准备接受命令
线程main即将发布命令
线程main已接受命令，正在等待结果
线程pool-1-thread-2已接受命令
线程pool-1-thread-1已接受命令
线程pool-1-thread-3已接受命令
线程pool-1-thread-2回应命令处理结果
线程pool-1-thread-1回应命令处理结果
线程pool-1-thread-3回应命令处理结果
线程main已收到所有响应结果