java基础一.volatile

喜欢ヅ旅行 2022-05-29 10:07 138阅读 0赞

被volatile修改的变量进行写操作的时候，会立即刷新近主存。读的时候，直接去主存中读取。

要说volatile，先简单的说一下java的内存模型。

首先，我先去百度张图片。

![Image 1][]

java中，jvm会给每个线程分配一个独立的内存空间，用于存放本线程的局部变量等，还有一部分空间叫做共享变量副本，这个部分空间存放的是从主存中复制过来的共享变量。目的是为了加快CPU的处理速度。

在这种前提下，并发环境中就很容易出现问题。

假设：

int i=0;

i++;

i++并非是原子操作。先读取到i，然后对i进行+1操作，然后将I+1后的值赋给i。

当线程A刚刚读取到i，线程B横插一足，读取i，i+1，复制给i。这时候(在线程B眼中)i=1,此时，线程A继续执行i+1=1,赋值给i，这时候i仍然是i。而使用了volatile就不会了。这就涉及到了volatile关键字的性质。

### volatile有两个性质： ###

1.可见性：被volatile修改的变量，多个线程修改了其值，其他线程都能立即看到修改后的值。

2.有序性：对于被volatile修饰的变量，JVM会禁止对其指令重排序。

指令重排序是JVM为了加快运行速度，可能对程序的代码进行优化，它不保证程序执行时候的代码顺序和所书写的代码顺序是一致的，但是它保证程序执行完毕的结果和代码顺序执行的结果是一致的(结果一致，过程不同)。

volatile有序性：表现在两个方面：

1.程序执行的时候：被volatile修饰的变量，前面的语句都执行完毕后，才会执行本语句。同样在volatile后面的语句，只有在volatile修改的变量语句之后，才会被执行。

2.指令优化的时候：不能将volatile修饰的变量放到某些语句之后，也不能volatile修饰的变量语句后面的放到volatile之前。

**volatile并不能保证原子性：**

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     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">;j++)</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(255,255,255) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">test.increase();</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(244,244,244) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">};</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(255,255,255) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">}.start();</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(244,244,244) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">}</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(255,255,255) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code>&nbsp; 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(244,244,244) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,255);">while</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">(Thread.activeCount()&gt;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,153,0);">1</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">)&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,130,0);">//保证前面的线程都执行完</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(255,255,255) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">Thread.yield();</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(244,244,244) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">System.out.println(test.inc);</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(255,255,255) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;">&nbsp;&nbsp;&nbsp;&nbsp;</code> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">}</code> 
    </div> 
    <div style="text-align:left;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:12px;color:rgb(0,0,0);margin:0px;padding:0px 1em;background:rgb(244,244,244) none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;min-height:auto;white-space:nowrap;"> 
     <code style="margin:0px;padding:0px;background:none;border:0px;float:none;height:auto;line-height:1.8em;vertical-align:baseline;width:auto;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;min-height:auto;color:rgb(0,0,0);">}</code> 
    </div></td> 
  </tr> 
 </tbody> 
</table>

自增操作不是原子性操作，而且volatile也无法保证对变量的任何操作都是原子性的。

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### volatile的实现原理 ###

##### 1.可见性 #####

处理器为了提高处理速度，不直接和内存进行通讯，而是将系统内存的数据独到内部缓存后再进行操作，但操作完后不知什么时候会写到内存。

如果对声明了volatile变量进行写操作时，JVM会向处理器发送一条Lock前缀的指令，将这个变量所在缓存行的数据写会到系统内存。 这一步确保了如果有其他线程对声明了volatile变量进行修改，则立即更新主内存中数据。

但这时候其他处理器的缓存还是旧的，所以在多处理器环境下，为了保证各个处理器缓存一致，每个处理会通过嗅探在总线上传播的数据来检查 自己的缓存是否过期，当处理器发现自己缓存行对应的内存地址被修改了，就会将当前处理器的缓存行设置成无效状态，当处理器要对这个数据进行修改操作时，会强制重新从系统内存把数据读到处理器缓存里。 这一步确保了其他线程获得的声明了volatile变量都是从主内存中获取最新的。

##### 2.有序性 #####

Lock前缀指令实际上相当于一个内存屏障（也成内存栅栏），它确保指令重排序时不会把其后面的指令排到内存屏障之前的位置，也不会把前面的指令排到内存屏障的后面；即在执行到内存屏障这句指令时，在它前面的操作已经全部完成。

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什么时候会用到volatile关键字呢？

1.对变量的写不依赖于当前值(volatile不能保证原子性的结论下)

2.该变量没有包含在具有其他变量的不变式中。

常用于：状态标记量。单例模式中的double check。

<table> 
 <tbody> 
  <tr> 
   <td>public class Demo2SingleTon { <br><br><br>&nbsp; &nbsp; private volatile static Demo2SingleTon singleTon;<br>&nbsp; &nbsp;&nbsp;<br>&nbsp; &nbsp; private Demo2SingleTon(){ <br>&nbsp; &nbsp;&nbsp;<br>&nbsp; &nbsp; }<br>&nbsp; &nbsp;&nbsp;<br>&nbsp; &nbsp; /**<br>&nbsp; &nbsp; &nbsp;* &nbsp;双重校验锁的方式<br>&nbsp; &nbsp; &nbsp;* &nbsp;这种方式，只是会在创建的对象的时候加锁，也就是第一次创建对象的时候加锁。<br>&nbsp; &nbsp; &nbsp;* @return<br>&nbsp; &nbsp; &nbsp;*/<br>&nbsp; &nbsp; public static Demo2SingleTon getInstance(){ <br>&nbsp; &nbsp; &nbsp; &nbsp; if(null!=singleTon){ <br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; synchronized (singleTon){ <br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if(null!=singleTon) { <br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; singleTon = new Demo2SingleTon();<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }<br>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }<br>&nbsp; &nbsp; &nbsp; &nbsp; }<br>&nbsp; &nbsp; &nbsp; &nbsp; return singleTon;<br>&nbsp; &nbsp; }<br>}</td> 
  </tr> 
 </tbody> 
</table>

instance = new Demo2SingleTon() 这句，实际上做了三件事：

1.给singleTon分配内存。

2.调用SingleTon的构造函数来初始化成员变量。

3.将instance对象指向分配的内存空间。

但是在 JVM 的即时编译器中存在指令重排序的优化。也就是说上面的第二步和第三步的顺序是不能保证的，最终的执行顺序可能是 1-2-3 也可能是 1-3-2。如果是后者，则在 3 执行完毕、2 未执行之前，被线程二抢占了，这时 instance 已经是非 null 了（但却没有初始化），所以线程二会直接返回 instance，然后使用，然后顺理成章地报错。

>  2018-03-21 方小白 畅游大厦

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