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