多线程之三

线程的死锁
哲学家进餐的问题
线程1锁住了对象A的监视器，等待对象B的监视器，线程2锁住了对象B的监视器，等待对象A的监视器，就造成了死锁。

 

class TicketsSystem3
...{
     public static void main(String[] args)
     ...{
              SellThread st=new SellThread();
               new Thread(st).start();               
               try
                     ...{
                              Thread.sleep(1000);
                     }
                     catch(Exception e)
                     ...{
                          e.printStackTrace();
                     }
                     st.b=true;
               new Thread(st).start();
               
     }
}
class SellThread implements Runnable
...{
     int tickets=100;
     Object ojb=new Object();//这个可以是任意的对象
     boolean b=false;
     public void run()
     ...{    
              if(b==false)
              ...{
                             while(true)
                             ...{ 
                                     sell();//同步方法利用的是this所代表的对象的锁
                             }
                   }   
                   else
                   ...{
                         while(true)
                             ...{ 
                                     synchronized(obj)
                                     ...{
                                     try
                                         ...{
                                                 Thread.sleep(1000);
                                         }
                                         catch(Exception e)
                                         ...{
                                                  e.printStackTrace();
                                         }
                                   synchronized(this)//能实现方法和同步块同步
                     /** *//**静态方法只属于类本身，它并不属于某一个对象，当我们在调用静态方法的时候
                  并不需要产生类的对象，同步静态方法使用的是方法所在的类所对应的CLASS对象的监视器，
                       每个对象都有自己对应的CLASS对象*/
                                     //synchronized(ojb)//这样的话。两个线程实现的都是ELSE里的语句，
                                     //让线程睡眠一会可以看到结果会出现0这张票
                                     ...{
                                                 if(tickets>0)
                                                 ...{                                                         
                                                               System.out.println("obj:"+Thread.currentThread().getName()+
                                                                                             " sell tickets:"+tickets);
                                                                      tickets--;
                                                 }
                                         }
                             }
                           }
                   }
     }
     
        public synchronized void sell()
        ...{
                                 synchronize(obj)
                                                 ...{
                                                             if(tickets>0)
                                                             /** *//**假设第一个线程进入IF语句中,时间片刚好到期,然后第二个线程运行,进入IF语句中
                                                             时间片刚好到期,然后第三个线程运行,进入IF语句中,时间片刚好到期
                                                             然后第四个线程运行,进入IF语句中,时间片刚好到期.之后,第一个线程获得时间片,
                                                             开始运行语句,开始减减,卖了1这张票,tickets变为0,第二个线程卖了0这张票,
                                                             tickets变为-1.第三个线程卖了-1这张票,tickets变为-2,第四个线程卖了-2这张票,tickets变为-3,
                                                             让线程睡眠一下,可以捕捉结果*/
                                                             ...{
                                                                     try
                                                                   ...{
                                                                           Thread.sleep(1000);
                                                                   }
                                                                   catch(Exception e)
                                                                   ...{
                                                                           e.printStackTrace();
                                                                   }
                                                                           System.out.println("sell:"+Thread.currentThread().getName()+
                                                                                                         " sell tickets:"+tickets);
                                                                                  tickets--;
                                                             }
                                                     }
    }
}