一个解释volatile关键字作用的最好的例子

Posted 2021-03-20 龙宇在天

闲话少叙，上代码

package com.dwz.concurrency2.chapter3;

public class VolatileTest {
    private static volatile int INIT_VALUE = 0; 
    private static final int MAX_LIMIT = 5;
    
    public static void main(String[] args) {
        new Thread(() -> {
            int localValue = INIT_VALUE;
            while (localValue < MAX_LIMIT) {
                if(localValue != INIT_VALUE) {
                    System.out.printf("The value updated to [%d]\n", INIT_VALUE);
                    localValue = INIT_VALUE;
                }
            }
        }, "READER").start();
        
        new Thread(() -> {
            int localValue = INIT_VALUE;
            while (localValue < MAX_LIMIT) {
                System.out.printf("update the value to [%d]\n", ++localValue);
                INIT_VALUE = localValue;
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "UPDATER").start();
    }
}

测试结果，出现两种情况：

1.INIT_VALUE被volatile关键字修饰时，READER线程是可以感知到UPDATER的变化

update the value to [1]
The value updated to [1]
update the value to [2]
The value updated to [2]
update the value to [3]
The value updated to [3]
update the value to [4]
The value updated to [4]
update the value to [5]
The value updated to [5]

2.INIT_VALUE缺少volatile关键字修饰时，READER线程感知不到UPDATER的变化

update the value to [1]
update the value to [2]
update the value to [3]
update the value to [4]
update the value to [5]

 volatile可以保证内存可见性，有序性，不让jvm自作主张去优化，可以保证读之前的写操作完成，但是不能保证原子性