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right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.8 常用工具类</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.9 Java新特性</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.10 Java重要知识点</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable active"><!----><span class="title">2.11 并发编程</span><span class="arrow down"></span></button><ul class="sidebar-links"><li><!--[--><a href="/thread/wangzhe-thread.html" class="nav-link sidebar-link sidebar-page" aria-label="创建Java线程的3种方式"><!---->创建Java线程的3种方式<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/thread-state-and-method.html" class="nav-link sidebar-link sidebar-page" aria-label="线程的6种状态及切换"><!---->线程的6种状态及切换<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/thread-group-and-thread-priority.html" class="nav-link sidebar-link sidebar-page" aria-label="线程组和线程优先级"><!---->线程组和线程优先级<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/why-need-thread.html" class="nav-link sidebar-link sidebar-page" aria-label="进程与线程的区别"><!---->进程与线程的区别<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/thread-bring-some-problem.html" class="nav-link sidebar-link sidebar-page" aria-label="并发编程带来了哪些问题"><!---->并发编程带来了哪些问题<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/jmm.html" class="nav-link sidebar-link sidebar-page" aria-label="Java内存模型"><!---->Java内存模型<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/thread/volatile.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="volatile"><!---->volatile<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#几个基本概念" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="几个基本概念"><!---->几个基本概念<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#内存可见性" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="内存可见性"><!---->内存可见性<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#重排序" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="重排序"><!---->重排序<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#happens-before规则" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="happens-before规则"><!---->happens-before规则<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#volatile的内存语义" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="volatile的内存语义"><!---->volatile的内存语义<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#内存可见性-1" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="内存可见性"><!---->内存可见性<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#禁止重排序" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="禁止重排序"><!---->禁止重排序<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/volatile.html#volatile的用途" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="volatile的用途"><!---->volatile的用途<!----></a><ul class="sidebar-sub-headers"></ul></li></ul><!--]--></li><li><!--[--><a href="/thread/synchronized.html" class="nav-link sidebar-link sidebar-page" aria-label="synchronized"><!---->synchronized<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/cas.html" class="nav-link sidebar-link sidebar-page" aria-label="CAS的原理"><!---->CAS的原理<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/aqs.html" class="nav-link sidebar-link sidebar-page" aria-label="AQS详解"><!---->AQS详解<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/lock.html" class="nav-link sidebar-link sidebar-page" aria-label="锁"><!---->锁<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/reentrantLock.html" class="nav-link sidebar-link sidebar-page" aria-label="重入锁ReentrantLock"><!---->重入锁ReentrantLock<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ReentrantReadWriteLock.html" class="nav-link sidebar-link sidebar-page" aria-label="读写锁ReentrantReadWriteLock"><!---->读写锁ReentrantReadWriteLock<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/condition.html" class="nav-link sidebar-link sidebar-page" aria-label="线程协作类Condition"><!---->线程协作类Condition<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/LockSupport.html" class="nav-link sidebar-link sidebar-page" aria-label="线程阻塞唤醒类LockSupport"><!---->线程阻塞唤醒类LockSupport<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/map.html" class="nav-link sidebar-link sidebar-page" aria-label="并发集合容器"><!---->并发集合容器<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ConcurrentHashMap.html" class="nav-link sidebar-link sidebar-page" aria-label="ConcurrentHashMap"><!---->ConcurrentHashMap<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ConcurrentLinkedQueue.html" class="nav-link sidebar-link sidebar-page" aria-label="ConcurrentLinkedQueue"><!---->ConcurrentLinkedQueue<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/CopyOnWriteArrayList.html" class="nav-link sidebar-link sidebar-page" aria-label="CopyOnWriteArrayList"><!---->CopyOnWriteArrayList<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ThreadLocal.html" class="nav-link sidebar-link sidebar-page" aria-label="ThreadLocal"><!---->ThreadLocal<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/BlockingQueue.html" class="nav-link sidebar-link sidebar-page" aria-label="BlockingQueue"><!---->BlockingQueue<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/pool.html" class="nav-link sidebar-link sidebar-page" aria-label="线程池"><!---->线程池<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/ScheduledThreadPoolExecutor.html" class="nav-link sidebar-link sidebar-page" aria-label="计划任务"><!---->计划任务<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/atomic.html" class="nav-link sidebar-link sidebar-page" aria-label="原子操作类"><!---->原子操作类<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/CountDownLatch.html" class="nav-link sidebar-link sidebar-page" aria-label="通信工具类CountDownLatch"><!---->通信工具类CountDownLatch<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/fork-join.html" class="nav-link sidebar-link sidebar-page" aria-label="Fork/Join框架"><!---->Fork/Join框架<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a href="/thread/shengchanzhe-xiaofeizhe.html" class="nav-link sidebar-link sidebar-page" aria-label="生产者-消费者模式"><!---->生产者-消费者模式<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li></ul></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">2.12 JVM</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">四、数据库</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">五、计算机基础</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">六、求职面试</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">七、学习资源</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">八、学习建议</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">九、知识库搭建</span><span class="arrow right"></span></button><!----></section><!--]--></li><li><!--[--><section class="sidebar-group"><button class="sidebar-heading clickable"><!----><span class="title">十、联系作者</span><span class="arrow right"></span></button><!----></section><!--]--></li></ul><!--[--><!----><!--]--></aside><!--[--><main class="page" id="main-content"><!--[--><!----><nav class="breadcrumb disable"></nav><div class="page-title"><h1><!---->Java并发编程volatile关键字解析</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><!----><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="calendar icon"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 0c0-18.753-14.72-33.473-33.473-33.473s-33.52 14.72-33.52 33.473v33.473h66.993v-33.473zm468.81 33.52H716.4v100.465c0 18.753-14.72 33.473-33.472 33.473a33.145 33.145 0 01-33.473-33.473V143.657H381.53v100.465c0 18.753-14.72 33.473-33.473 33.473a33.145 33.145 0 01-33.473-33.473V143.657H180.6A134.314 134.314 0 0046.66 277.595v535.756A134.314 134.314 0 00180.6 947.289h669.74a134.36 134.36 0 00133.94-133.938V277.595a134.314 134.314 0 00-133.94-133.938zm33.473 267.877H147.126a33.145 33.145 0 01-33.473-33.473c0-18.752 14.72-33.473 33.473-33.473h736.687c18.752 0 33.472 14.72 33.472 33.473a33.145 33.145 0 01-33.472 33.473z"></path></svg><span>2022年3月23日</span><meta property="datePublished" content="2022-03-23T12:08:58.000Z"></span><span class="category-info" aria-label="分类🌈" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon category-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="category icon"><path d="M148.41 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H148.41c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.311-40.31zM147.556 553.478H429.73c22.263 0 40.311 18.048 40.311 40.31v282.176c0 22.263-18.048 40.312-40.31 40.312H147.555c-22.263 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id="toc"><div class="toc-header">此页内容</div><div class="toc-wrapper"><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#几个基本概念" class="router-link-active router-link-exact-active toc-link level2">几个基本概念</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#内存可见性" class="router-link-active router-link-exact-active toc-link level3">内存可见性</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#重排序" class="router-link-active router-link-exact-active toc-link level3">重排序</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#happens-before规则" class="router-link-active router-link-exact-active toc-link level3">happens-before规则</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#volatile的内存语义" class="router-link-active router-link-exact-active toc-link level2">volatile的内存语义</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#内存可见性-1" class="router-link-active router-link-exact-active toc-link level3">内存可见性</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#禁止重排序" class="router-link-active router-link-exact-active toc-link level3">禁止重排序</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/volatile.html#volatile的用途" class="router-link-active router-link-exact-active toc-link level2">volatile的用途</a></li><!----><!--]--></ul></div></aside></div><!----><div class="theme-hope-content"><h1 id="java并发编程volatile关键字解析" tabindex="-1"><a class="header-anchor" href="#java并发编程volatile关键字解析" aria-hidden="true">#</a> Java并发编程volatile关键字解析</h1><h2 id="几个基本概念" tabindex="-1"><a class="header-anchor" href="#几个基本概念" aria-hidden="true">#</a> 几个基本概念</h2><p>在介绍volatile之前，我们先回顾及介绍几个基本的概念。</p><h3 id="内存可见性" tabindex="-1"><a class="header-anchor" href="#内存可见性" aria-hidden="true">#</a> 内存可见性</h3><p>在Java内存模型那一章我们介绍了JMM有一个主内存，每个线程有自己私有的工作内存，工作内存中保存了一些变量在主内存的拷贝。</p><p><strong>内存可见性，指的是线程之间的可见性，当一个线程修改了共享变量时，另一个线程可以读取到这个修改后的值</strong>。</p><h3 id="重排序" tabindex="-1"><a class="header-anchor" href="#重排序" aria-hidden="true">#</a> 重排序</h3><p>为优化程序性能，对原有的指令执行顺序进行优化重新排序。重排序可能发生在多个阶段，比如编译重排序、CPU重排序等。</p><h3 id="happens-before规则" tabindex="-1"><a class="header-anchor" href="#happens-before规则" aria-hidden="true">#</a> happens-before规则</h3><p>是一个给程序员使用的规则，只要程序员在写代码的时候遵循happens-before规则，JVM就能保证指令在多线程之间的顺序性符合程序员的预期。</p><h2 id="volatile的内存语义" tabindex="-1"><a class="header-anchor" href="#volatile的内存语义" aria-hidden="true">#</a> volatile的内存语义</h2><p>在Java中，volatile关键字有特殊的内存语义。volatile主要有以下两个功能：</p><ul><li>保证变量的<strong>内存可见性</strong></li><li>禁止volatile变量与普通变量<strong>重排序</strong>（JSR133提出，Java 5 开始才有这个“增强的volatile内存语义”）</li></ul><h3 id="内存可见性-1" tabindex="-1"><a class="header-anchor" href="#内存可见性-1" aria-hidden="true">#</a> 内存可见性</h3><p>以一段示例代码开始：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">VolatileExample</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> a <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">volatile</span> <span class="token keyword">boolean</span> flag <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">writer</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
a <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// step 1</span>
flag <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span> <span class="token comment">// step 2</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">reader</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>flag<span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// step 3</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// step 4</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>在这段代码里，我们使用<code>volatile</code>关键字修饰了一个<code>boolean</code>类型的变量<code>flag</code>。</p><p>所谓内存可见性，指的是当一个线程对<code>volatile</code>修饰的变量进行<strong>写操作</strong>（比如step 2）时，JMM会立即把该线程对应的本地内存中的共享变量的值刷新到主内存；当一个线程对<code>volatile</code>修饰的变量进行<strong>读操作</strong>（比如step 3）时，JMM会把立即该线程对应的本地内存置为无效，从主内存中读取共享变量的值。</p><blockquote><p>在这一点上，volatile与锁具有相同的内存效果，volatile变量的写和锁的释放具有相同的内存语义，volatile变量的读和锁的获取具有相同的内存语义。</p></blockquote><p>假设在时间线上，线程A先执行方法<code>writer</code>方法，线程B后执行<code>reader</code>方法。那必然会有下图：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/volatile-1f5e263e-dd3e-4fb9-a21f-67e160b3dbf2.jpg" alt="volatile内存示意图"></p><p>而如果<code>flag</code>变量<strong>没有</strong>用<code>volatile</code>修饰，在step 2，线程A的本地内存里面的变量就不会立即更新到主内存，那随后线程B也同样不会去主内存拿最新的值，仍然使用线程B本地内存缓存的变量的值<code>a = 0，flag = false</code>。</p><h3 id="禁止重排序" tabindex="-1"><a class="header-anchor" href="#禁止重排序" aria-hidden="true">#</a> 禁止重排序</h3><p>在JSR-133之前的旧的Java内存模型中，是允许volatile变量与普通变量重排序的。那上面的案例中，可能就会被重排序成下列时序来执行：</p><ol><li>线程A写volatile变量，step 2，设置flag为true；</li><li>线程B读同一个volatile，step 3，读取到flag为true；</li><li>线程B读普通变量，step 4，读取到 a = 0；</li><li>线程A修改普通变量，step 1，设置 a = 1；</li></ol><p>可见，如果volatile变量与普通变量发生了重排序，虽然volatile变量能保证内存可见性，也可能导致普通变量读取错误。</p><p>所以在旧的内存模型中，volatile的写-读就不能与锁的释放-获取具有相同的内存语义了。为了提供一种比锁更轻量级的<strong>线程间的通信机制</strong>，<strong>JSR-133</strong>专家组决定增强volatile的内存语义：严格限制编译器和处理器对volatile变量与普通变量的重排序。</p><p>编译器还好说，JVM是怎么还能限制处理器的重排序的呢？它是通过<strong>内存屏障</strong>来实现的。</p><p>什么是内存屏障？硬件层面，内存屏障分两种：读屏障（Load Barrier）和写屏障（Store Barrier）。内存屏障有两个作用：</p><ol><li>阻止屏障两侧的指令重排序；</li><li>强制把写缓冲区/高速缓存中的脏数据等写回主内存，或者让缓存中相应的数据失效。</li></ol><blockquote><p>注意这里的缓存主要指的是CPU缓存，如L1，L2等</p></blockquote><p>编译器在<strong>生成字节码时</strong>，会在指令序列中插入内存屏障来禁止特定类型的处理器重排序。编译器选择了一个<strong>比较保守的JMM内存屏障插入策略</strong>，这样可以保证在任何处理器平台，任何程序中都能得到正确的volatile内存语义。这个策略是：</p><ul><li>在每个volatile写操作前插入一个StoreStore屏障；</li><li>在每个volatile写操作后插入一个StoreLoad屏障；</li><li>在每个volatile读操作后插入一个LoadLoad屏障；</li><li>在每个volatile读操作后再插入一个LoadStore屏障。</li></ul><p>大概示意图是这个样子：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/volatile-aaa0fa44-341f-401f-bfb7-03d0c03dc2b1.png" alt="内存屏障"></p><blockquote><p>再逐个解释一下这几个屏障。注：下述Load代表读操作，Store代表写操作</p><p><strong>LoadLoad屏障</strong>：对于这样的语句Load1; LoadLoad; Load2，在Load2及后续读取操作要读取的数据被访问前，保证Load1要读取的数据被读取完毕。<br><strong>StoreStore屏障</strong>：对于这样的语句Store1; StoreStore; Store2，在Store2及后续写入操作执行前，这个屏障会把Store1强制刷新到内存，保证Store1的写入操作对其它处理器可见。<br><strong>LoadStore屏障</strong>：对于这样的语句Load1; LoadStore; Store2，在Store2及后续写入操作被刷出前，保证Load1要读取的数据被读取完毕。<br><strong>StoreLoad屏障</strong>：对于这样的语句Store1; StoreLoad; Load2，在Load2及后续所有读取操作执行前，保证Store1的写入对所有处理器可见。它的开销是四种屏障中最大的（冲刷写缓冲器，清空无效化队列）。在大多数处理器的实现中，这个屏障是个万能屏障，兼具其它三种内存屏障的功能</p></blockquote><p>对于连续多个volatile变量读或者连续多个volatile变量写，编译器做了一定的优化来提高性能，比如：</p><blockquote><p>第一个volatile读;</p><p>LoadLoad屏障；</p><p>第二个volatile读；</p><p>LoadStore屏障</p></blockquote><p>再介绍一下volatile与普通变量的重排序规则:</p><ol><li><p>如果第一个操作是volatile读，那无论第二个操作是什么，都不能重排序；</p></li><li><p>如果第二个操作是volatile写，那无论第一个操作是什么，都不能重排序；</p></li><li><p>如果第一个操作是volatile写，第二个操作是volatile读，那不能重排序。</p></li></ol><p>举个例子，我们在案例中step 1，是普通变量的写，step 2是volatile变量的写，那符合第2个规则，这两个steps不能重排序。而step 3是volatile变量读，step 4是普通变量读，符合第1个规则，同样不能重排序。</p><p>但如果是下列情况：第一个操作是普通变量读，第二个操作是volatile变量读，那是可以重排序的：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 声明变量</span>
<span class="token keyword">int</span> a <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> <span class="token comment">// 声明普通变量</span>
<span class="token keyword">volatile</span> <span class="token keyword">boolean</span> flag <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span> <span class="token comment">// 声明volatile变量</span>
<span class="token comment">// 以下两个变量的读操作是可以重排序的</span>
<span class="token keyword">int</span> i <span class="token operator">=</span> a<span class="token punctuation">;</span> <span class="token comment">// 普通变量读</span>
<span class="token keyword">boolean</span> j <span class="token operator">=</span> flag<span class="token punctuation">;</span> <span class="token comment">// volatile变量读</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h2 id="volatile的用途" tabindex="-1"><a class="header-anchor" href="#volatile的用途" aria-hidden="true">#</a> volatile的用途</h2><p>从volatile的内存语义上来看，volatile可以保证内存可见性且禁止重排序。</p><p>在保证内存可见性这一点上，volatile有着与锁相同的内存语义，所以可以作为一个“轻量级”的锁来使用。但由于volatile仅仅保证对单个volatile变量的读/写具有原子性，而锁可以保证整个<strong>临界区代码</strong>的执行具有原子性。所以<strong>在功能上，锁比volatile更强大；在性能上，volatile更有优势</strong>。</p><p>在禁止重排序这一点上，volatile也是非常有用的。比如我们熟悉的单例模式，其中有一种实现方式是“双重锁检查”，比如这样的代码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> instance<span class="token punctuation">;</span> <span class="token comment">// 不使用volatile关键字</span>
<span class="token comment">// 双重锁检验</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 第7行</span>
<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token class-name">Singleton</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>instance <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 第10行</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> instance<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>如果这里的变量声明不使用volatile关键字，是可能会发生错误的。它可能会被重排序：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>instance <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 第10行</span>
<span class="token comment">// 可以分解为以下三个步骤</span>
<span class="token number">1</span> memory<span class="token operator">=</span><span class="token function">allocate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// 分配内存 相当于c的malloc</span>
<span class="token number">2</span> <span class="token function">ctorInstanc</span><span class="token punctuation">(</span>memory<span class="token punctuation">)</span> <span class="token comment">//初始化对象</span>
<span class="token number">3</span> s<span class="token operator">=</span>memory <span class="token comment">//设置s指向刚分配的地址</span>
<span class="token comment">// 上述三个步骤可能会被重排序为 1-3-2，也就是：</span>
<span class="token number">1</span> memory<span class="token operator">=</span><span class="token function">allocate</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">// 分配内存 相当于c的malloc</span>
<span class="token number">3</span> s<span class="token operator">=</span>memory <span class="token comment">//设置s指向刚分配的地址</span>
<span class="token number">2</span> <span class="token function">ctorInstanc</span><span class="token punctuation">(</span>memory<span class="token punctuation">)</span> <span class="token comment">//初始化对象</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>而一旦假设发生了这样的重排序，比如线程A在第10行执行了步骤1和步骤3，但是步骤2还没有执行完。这个时候另一个线程B执行到了第7行，它会判定instance不为空，然后直接返回了一个未初始化完成的instance！</p><p>所以JSR-133对volatile做了增强后，volatile的禁止重排序功能还是非常有用的。</p><hr><blockquote><p>编辑：沉默王二，内容大部分来源以下三个开源仓库：</p><ul><li><a href="http://concurrent.redspider.group/" target="_blank" rel="noopener noreferrer">深入浅出 Java 多线程<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" 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