java线程中锁存器CountDownLatch的使用

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。

1importjava.util.concurrent.CountDownLatch;
2importjava.util.concurrent.Executor;
3importjava.util.concurrent.ExecutorService;
4importjava.util.concurrent.Executors;
5
6publicclassCountDownLatchDemo {
7privatestaticfinalintPLAYER_AMOUNT=5;
8publicCountDownLatchDemo() {
9//TODO Auto-generated constructor stub
10}
11/**
12*@paramargs
13*/
14publicstaticvoidmain(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
17CountDownLatch begin=newCountDownLatch(1);
18//对于整个比赛，所有运动员结束后才算结束
19CountDownLatch end=newCountDownLatch(PLAYER_AMOUNT);
20Player[] plays=newPlayer[PLAYER_AMOUNT];
21
22for(inti=0;i<PLAYER_AMOUNT;i++)
23plays[i]=newPlayer(i+1,begin,end);
24
25//设置特定的线程池，大小为5
26ExecutorService exe=Executors.newFixedThreadPool(PLAYER_AMOUNT);
27for(Player p:plays)
28exe.execute(p);//分配线程
29System.out.println("Race begins!");
30begin.countDown();
31try{
32end.wait();//等待end状态变为0，即为比赛结束
33}catch(InterruptedException e) {
34//TODO: handle exception
35e.printStackTrace();
36}finally{
37System.out.println("Race ends!");
38}
39exe.shutdown();
40}
41}

接下来是Player类

注：countDown最好是在finally里面调用

1importjava.util.concurrent.CountDownLatch;
2
3
4publicclassPlayerimplementsRunnable {
5
6privateintid;
7privateCountDownLatch begin;
8privateCountDownLatch end;
9publicPlayer(inti, CountDownLatch begin, CountDownLatch end) {
10//TODO Auto-generated constructor stub
11super();
12this.id=i;
13this.begin=begin;
14this.end=end;
15}
16
17@Override
18publicvoidrun() {
19//TODO Auto-generated method stub
20try{
21begin.await();//等待begin的状态为0
22Thread.sleep((long)(Math.random()*100));//随机分配时间，即运动员完成时间
23System.out.println("Play"+id+"arrived.");
24}catch(InterruptedException e) {
25//TODO: handle exception
26e.printStackTrace();
27}finally{
28end.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