CountDownLatch浅析

 

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;
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

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