这篇文章主要介绍了Java内存模型可见性问题相关解析,文中通过示例代码介绍的非常详细，对大家的学习或者工作具有一定的参考学习价值,需要的朋友可以参考下

前言

之前的文章中讲到，JMM是内存模型规范在Java语言中的体现。JMM保证了在多核CPU多线程编程环境下，对共享变量读写的原子性、可见性和有序性。

本文就具体来讲讲JMM是如何保证共享变量访问的可见性的。

什么是可见性问题

我们从一段简单的代码来看看到底什么是可见性问题。

public class VolatileDemo {

  boolean started = false;

  public void startSystem(){
    System.out.println(Thread.currentThread().getName()+" begin to start system, time:"+System.currentTimeMillis());
    started = true;
    System.out.println(Thread.currentThread().getName()+" success to start system, time:"+System.currentTimeMillis());
  }

  public void checkStartes(){
    if (started){
      System.out.println("system is running, time:"+System.currentTimeMillis());
    }else {
      System.out.println("system is not running, time:"+System.currentTimeMillis());
    }
  }

  public static void main(String[] args) {
    VolatileDemo demo = new VolatileDemo();
    Thread startThread = new Thread(new Runnable() {
      @Override
      public void run() {
        demo.startSystem();
      }
    });
    startThread.setName("start-Thread");

    Thread checkThread = new Thread(new Runnable() {
      @Override
      public void run() {
        while (true){
          demo.checkStartes();
        }
      }
    });
    checkThread.setName("check-Thread");
    startThread.start();
    checkThread.start();
  }

}

上面的列子中，一个线程来改变started的状态，另外一个线程不停地来检测started的状态，如果是true就输出系统启动，如果是false就输出系统未启动。那么当start-Thread线程将状态改成true后，check-Thread线程在执行时是否能立即“看到”这个变化呢？答案是不一定能立即看到。这边我做了很多测试，大多数情况下是能“感知”到started这个变量的变化的。但是偶尔会存在感知不到的情况。请看下下面日志记录：

start-Thread begin to start system, time:1577079553515
start-Thread success to start system, time:1577079553516 
system is not running, time:1577079553516  ==>此处start-Thread线程已经将状态设置成true,但是check-Thread线程还是没检测到
system is running, time:1577079553516
system is running, time:1577079553516
system is running, time:1577079553516
system is running, time:1577079553516
system is running, time:1577079553516
system is running, time:1577079553516
system is running, time:1577079553517
system is running, time:1577079553517
system is running, time:1577079553517
system is running, time:1577079553517
system is running, time:1577079553517
system is running, time:1577079553517
system is running, time:1577079553517
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519
system is running, time:1577079553519