一道多线程通信实例分析

程序如下：

  public static void main(String[] args) throws  Exception{ 
     final List list = new ArrayList(); 
     final Object lock = new Object(); 
     Thread t1 = new Thread(new Runnable() { 
         @Override 
         public void run() { 
             synchronized (lock){ 
                 for(int i = 0 ; i < 10 ; i++){ 
                     list.add(i); 
                     if(list.size() == 5){ 
                         lock.notify(); 
                         System.out.println(Thread.currentThread().getName() + "发出通知!"); 
                     } 
                 } 
             } 
             System.out.println(Thread.currentThread().getName() + "execute over!"); 
         } 
     }); 
     Thread t2 = new Thread(new Runnable() { 
         @Override 
         public void run() { 
             synchronized (lock){ 
                 if(list.size() != 5){ 
                     try { 
                         lock.wait(); 
                     } catch (InterruptedException e) { 
                         e.printStackTrace(); 
                     } 
                 } 
                 System.out.println(Thread.currentThread().getName() + " 收到通知!"); 
             } 
             System.out.println(Thread.currentThread().getName() + "execute over!"); 
         } 
     }); 
     t2.start(); 
     Thread.sleep(1000); 
     t1.start(); 
}

分析：

程序的意图本是利用多线程之间的通信，利用wait/notify实现，可是运行的结果是虽然线程T1发出了通知，但是线程T2并没有立即收到通知进行执行，这是为什么呢？因为只有线程T1执行完毕释放了锁，T2才能执行，那么也就是说wait/notify并不是实时的（wait释放了锁，而notify没有释放锁导致的），那么线程之间实时的通信该怎么做呢？可以利用CountDownLatch来实现。

对程序的改进：

  public static void main(String[] args) throws  Exception{ 
     final List list = new ArrayList(); 
     final Object lock = new Object(); 
     final CountDownLatch countDownLatch = new CountDownLatch(1); 
     Thread t1 = new Thread(new Runnable() { 
         @Override 
         public void run() { 
                 for(int i = 0 ; i < 10 ; i++){ 
                     list.add(i); 
                     if(list.size() == 5){ 
                         countDownLatch.countDown(); 
                         System.out.println(Thread.currentThread().getName() + "发出通知!"); 
                     } 
                 } 
             System.out.println(Thread.currentThread().getName() + "execute over!"); 
         } 
     }); 
     Thread t2 = new Thread(new Runnable() { 
         @Override 
         public void run() { 
                 if(list.size() != 5){ 
                     try { 
                         countDownLatch.await(); 
                     } catch (InterruptedException e) { 
                         e.printStackTrace(); 
                     } 
                 System.out.println(Thread.currentThread().getName() + " 收到通知!"); 
             } 
             System.out.println(Thread.currentThread().getName() + "execute over!"); 
         } 
     }); 
     t2.start(); 
     Thread.sleep(1000); 
     t1.start(); 
}