java多线程售票程序中synchronized用法反思

通过对以下两个例子,谈谈对synchronized的理解:

代码1:

public class Ticket implements Runnable {

private static int num=100;

public static void main(String[] args) {

Thread t1=new Thread(new Ticket(),"t1");

Thread t2=new Thread(new Ticket(),"t2");

Thread t3=new Thread(new Ticket(),"t3");

Thread t4=new Thread(new Ticket(),"t4");

Thread t5=new Thread(new Ticket(),"t5");

t1.start();

t2.start();

t3.start();

t4.start();

t5.start();

}

synchronized void getTicket(String name){

if(num>0){

System.out.println(name+"-----"+num);

num--;

}

try {

Thread.sleep(500);

} catch (InterruptedException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

public void run() {

while(num>0)

this.getTicket(Thread.currentThread().getName());

}

}
输出结果为什么同一张票被卖出多次，而有的票没卖出，例如：





代码2:

public class Sell implements Runnable{

private int total;

public Sell(int total) {

this.total=total;

}

@Override

public void run() {

// TODO Auto-generated method stub

while (total>0) {

System.out.println("tickets" +total+"is sell");

synchronized (new Integer(total)) {

if (total>0) {

total--;

}

try {

Thread.sleep(2000);

} catch (InterruptedException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

System.out.println("tickets is selled");

}

}

}

public static void main(String[] args) {

int total=10;

Sell sell=new Sell(total);

Thread thread1=new Thread(sell);

Thread thread2=new Thread(sell);

Thread thread3=new Thread(sell);

Thread thread4=new Thread(sell);

thread1.start();

thread2.start();

thread3.start();

thread4.start();

}

}

synchronized 锁的是对象,例子一中,其实也相当于得到了Sell 对象的对象锁,所以同步的只是对象对方法的调用,而售票时最重要的是对票的同步,为了使tickets 同步,可以将票数封装成对象,使synchronized 得到此对象的对象锁,这样就不会得到同一个票被卖多次的情况.