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。

 
   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 
   } 
  

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