java基础知识回顾之java Thread类学习（七）--java多线程安全问题（死锁）

死锁：是两个或者两个以上的线程被无限的阻塞，线程之间互相等待所需资源。

线程死锁产生的条件：

• 当两个线程相互调用Join（）方法。
• 当两个线程使用嵌套的同步代码块的时候，一个线程占用了另一个线程的锁，互相等待阻塞，就有可能产生死锁。

下面看代码：

代码1：死锁的案例

package com.lp.ecjtu.Thread;





/*

死锁：常见情景之一：同步的嵌套。



*/

class Ticket implements Runnable

{

    private  int num = 100;

    Object obj = new Object();

    boolean flag = true;

    public void run()

    {

        if(flag)

            while(true)

            {

                synchronized(obj)//同步代码块里面调用同步函数

                {

                    show();

                }

            }

        else

            while(true)

                this.show();

    }



    public synchronized void show()//同步函数里面的调用同步代码块

    {



        synchronized(obj)

        {

            if(num>0)

            {

                try{Thread.sleep(10);}catch (InterruptedException e){}

                

                System.out.println(Thread.currentThread().getName()+".....sale...."+num--);

            }

        }

    }

}





public class DeathLockThread1 

{

    public static void main(String[] args) 

    {

        Ticket t = new Ticket();

        Thread t1 = new Thread(t);

        Thread t2 = new Thread(t);

        t1.start();

        try{Thread.sleep(10);}catch(InterruptedException e){}

        t.flag = false;

        t2.start();

    }

}

代码2：

 

package com.lp.ecjtu.Thread;



 class DeathLockRunable implements Runnable{

    private boolean flag;

    public DeathLockRunable(boolean flag){

        this.flag = flag;

    }



    @Override

    public void run() {

        if(flag){

            synchronized (Lock.mylock1){

                System.out.println("if mylock1"+Thread.currentThread().getName());

                synchronized (Lock.mylock2){

                    System.out.println("if mylock2"+Thread.currentThread().getName());

                }

            }

        }else{

            synchronized (Lock.mylock2){

                System.out.println("else mylock2"+Thread.currentThread().getName());

                synchronized (Lock.mylock1){

                    System.out.println("else mylock1"+Thread.currentThread().getName());

                }

            }

        }

    }



}

 

 class Lock{

    static Object mylock1 = new Object();

    static Object mylock2 = new Object();

 }

 

 public class DeathLockThread{

     public static void main(String[] args){

         Thread t1 = new Thread(new DeathLockRunable(true));

         Thread t2 = new Thread(new DeathLockRunable(false));

         t1.start();

         t2.start();

     }

 }

输出结果：if mylock1Thread-0
else mylock2Thread-1