1.看图自己体会

2.体会不了就给你个小程序

package cs.util;
 
public class VolatileDemo {
    
    private volatile int count =0;
     
     
    public int getCount() {
        return this.count;
    }
 
 
    public void setCount() {
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        this.count++;
    }
 
 
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        VolatileDemo demo=new VolatileDemo();
        for (int i = 0; i < 1000; i++) {
            new Thread(new Runnable() {
                 
                @Override
                public void run() {
                    // TODO Auto-generated method stub
                    demo.setCount();
                }
            }).start();
        }
        while(Thread.activeCount()>1)
        {
            Thread.yield();
        }
        System.out.println(demo.getCount());
    }
 
}

　　输出的结果是

不等于1000其实也不怪，这是由于count++其实是有三个操作组成 1.从主存拿共享变量count 2.进行count++ 3.把count写进内存

本该为7的，最后却为6，少了1，道理知道了吧

3.怎么解决没有出现1000的情况呢很简单有几种做法

一.可以加入 synchronized

public void setCount() {
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }<br>　　　　　　　　　//对count进行原子性操作
        synchronized (this)
        {
             
            this.count++;
        }
    }

　二、使用jdk1.5推出的方法具体修改如下

//定义一个Lock         private Lock lock=new ReentrantLock();
private volatile int count =0;
 
public int getCount() {
    return this.count;
}
 
 
public void setCount() {
    try {
        Thread.sleep(100);
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
            //加锁
    lock.lock();
    try {
        this.count++;
    } finally {
            //解锁
        lock.unlock();
    }
     
     
     
}        