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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 href="/thread/volatile.html" class="nav-link sidebar-link sidebar-page" aria-label="volatile"><!---->volatile<!----></a><ul class="sidebar-sub-headers"></ul><!--]--></li><li><!--[--><a aria-current="page" href="/thread/synchronized.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="synchronized"><!---->synchronized<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#synchronized关键字" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="Synchronized关键字"><!---->Synchronized关键字<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#几种锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="几种锁"><!---->几种锁<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#java对象头" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="Java对象头"><!---->Java对象头<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#偏向锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="偏向锁"><!---->偏向锁<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#轻量级锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="轻量级锁"><!---->轻量级锁<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#重量级锁" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="重量级锁"><!---->重量级锁<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#锁的升级流程" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="锁的升级流程"><!---->锁的升级流程<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/synchronized.html#各种锁的优缺点对比" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="各种锁的优缺点对比"><!---->各种锁的优缺点对比<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li></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" 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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中的synchronized关键字锁的到底是什么？</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/synchronized.html#synchronized关键字" class="router-link-active router-link-exact-active toc-link level2">Synchronized关键字</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#几种锁" class="router-link-active router-link-exact-active toc-link level2">几种锁</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#java对象头" class="router-link-active router-link-exact-active toc-link level3">Java对象头</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#偏向锁" class="router-link-active router-link-exact-active toc-link level3">偏向锁</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#轻量级锁" class="router-link-active router-link-exact-active toc-link level3">轻量级锁</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#重量级锁" class="router-link-active router-link-exact-active toc-link level3">重量级锁</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#锁的升级流程" class="router-link-active router-link-exact-active toc-link level3">锁的升级流程</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/synchronized.html#各种锁的优缺点对比" class="router-link-active router-link-exact-active toc-link level3">各种锁的优缺点对比</a></li><!----><!--]--></ul><!--]--></ul></div></aside></div><!----><div class="theme-hope-content"><h1 id="java中的synchronized关键字锁的到底是什么" tabindex="-1"><a class="header-anchor" href="#java中的synchronized关键字锁的到底是什么" aria-hidden="true">#</a> Java中的synchronized关键字锁的到底是什么？</h1><p>这篇文章我们来聊一聊Java多线程里面的“锁”。</p><p>首先需要明确的一点是：<strong>Java多线程的锁都是基于对象的</strong>，Java中的每一个对象都可以作为一个锁。</p><p>还有一点需要注意的是，我们常听到的<strong>类锁</strong>其实也是对象锁。</p><p>Java类只有一个Class对象（可以有多个实例对象，多个实例共享这个Class对象），而Class对象也是特殊的Java对象。所以我们常说的类锁，其实就是Class对象的锁。</p><h2 id="synchronized关键字" tabindex="-1"><a class="header-anchor" href="#synchronized关键字" aria-hidden="true">#</a> Synchronized关键字</h2><p>说到锁，我们通常会谈到<code>synchronized</code>这个关键字。它翻译成中文就是“同步”的意思。</p><p>我们通常使用<code>synchronized</code>关键字来给一段代码或一个方法上锁。它通常有以下三种形式：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 关键字在实例方法上，锁为当前实例</span>
<span class="token keyword">public</span> <span class="token keyword">synchronized</span> <span class="token keyword">void</span> <span class="token function">instanceLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</span>
<span class="token punctuation">}</span>
<span class="token comment">// 关键字在静态方法上，锁为当前Class对象</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">synchronized</span> <span class="token keyword">void</span> <span class="token function">classLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</span>
<span class="token punctuation">}</span>
<span class="token comment">// 关键字在代码块上，锁为括号里面的对象</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">blockLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Object</span> o <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">synchronized</span> <span class="token punctuation">(</span>o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</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 class="line-number"></div></div></div><p>我们这里介绍一下“临界区”的概念。所谓“临界区”，指的是某一块代码区域，它同一时刻只能由一个线程执行。在上面的例子中，如果<code>synchronized</code>关键字在方法上，那临界区就是整个方法内部。而如果是使用synchronized代码块，那临界区就指的是代码块内部的区域。</p><p>通过上面的例子我们可以看到，下面这两个写法其实是等价的作用：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 关键字在实例方法上，锁为当前实例</span>
<span class="token keyword">public</span> <span class="token keyword">synchronized</span> <span class="token keyword">void</span> <span class="token function">instanceLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</span>
<span class="token punctuation">}</span>
<span class="token comment">// 关键字在代码块上，锁为括号里面的对象</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">blockLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</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></div><p>同理，下面这两个方法也应该是等价的：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 关键字在静态方法上，锁为当前Class对象</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">synchronized</span> <span class="token keyword">void</span> <span class="token function">classLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</span>
<span class="token punctuation">}</span>
<span class="token comment">// 关键字在代码块上，锁为括号里面的对象</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">blockLock</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// code</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></div><h2 id="几种锁" tabindex="-1"><a class="header-anchor" href="#几种锁" aria-hidden="true">#</a> 几种锁</h2><p>Java 6 为了减少获得锁和释放锁带来的性能消耗，引入了“偏向锁”和“轻量级锁“。在Java 6 以前，所有的锁都是”重量级“锁。所以在Java 6 及其以后，一个对象其实有四种锁状态，它们级别由低到高依次是：</p><ol><li>无锁状态</li><li>偏向锁状态</li><li>轻量级锁状态</li><li>重量级锁状态</li></ol><p>无锁就是没有对资源进行锁定，任何线程都可以尝试去修改它，无锁在这里不再细讲。</p><p>几种锁会随着竞争情况逐渐升级，锁的升级很容易发生，但是锁降级发生的条件会比较苛刻，锁降级发生在Stop The World期间，当JVM进入安全点的时候，会检查是否有闲置的锁，然后进行降级。</p><blockquote><p>关于锁降级有两点说明：</p><p>1.不同于大部分文章说锁不能降级，实际上HotSpot JVM 是支持锁降级的，文末有链接。</p><p>2.上面提到的Stop The World期间，以及安全点，这些知识是属于JVM的知识范畴，本文不做细讲。</p></blockquote><p>下面分别介绍这几种锁以及它们之间的升级。</p><h3 id="java对象头" tabindex="-1"><a class="header-anchor" href="#java对象头" aria-hidden="true">#</a> Java对象头</h3><p>前面我们提到，Java的锁都是基于对象的。首先我们来看看一个对象的“锁”的信息是存放在什么地方的。</p><p>每个Java对象都有对象头。如果是非数组类型，则用2个字宽来存储对象头，如果是数组，则会用3个字宽来存储对象头。在32位处理器中，一个字宽是32位；在64位虚拟机中，一个字宽是64位。对象头的内容如下表：</p><table><thead><tr><th>长度</th><th>内容</th><th>说明</th></tr></thead><tbody><tr><td>32/64bit</td><td>Mark Word</td><td>存储对象的hashCode或锁信息等</td></tr><tr><td>32/64bit</td><td>Class Metadata Address</td><td>存储到对象类型数据的指针</td></tr><tr><td>32/64bit</td><td>Array length</td><td>数组的长度（如果是数组）</td></tr></tbody></table><p>我们主要来看看Mark Word的格式：</p><table><thead><tr><th>锁状态</th><th>29 bit 或 61 bit</th><th>1 bit 是否是偏向锁？</th><th>2 bit 锁标志位</th></tr></thead><tbody><tr><td>无锁</td><td></td><td>0</td><td>01</td></tr><tr><td>偏向锁</td><td>线程ID</td><td>1</td><td>01</td></tr><tr><td>轻量级锁</td><td>指向栈中锁记录的指针</td><td>此时这一位不用于标识偏向锁</td><td>00</td></tr><tr><td>重量级锁</td><td>指向互斥量（重量级锁）的指针</td><td>此时这一位不用于标识偏向锁</td><td>10</td></tr><tr><td>GC标记</td><td></td><td>此时这一位不用于标识偏向锁</td><td>11</td></tr></tbody></table><p>可以看到，当对象状态为偏向锁时，<code>Mark Word</code>存储的是偏向的线程ID；当状态为轻量级锁时，<code>Mark Word</code>存储的是指向线程栈中<code>Lock Record</code>的指针；当状态为重量级锁时，<code>Mark Word</code>为指向堆中的monitor对象的指针。</p><h3 id="偏向锁" tabindex="-1"><a class="header-anchor" href="#偏向锁" aria-hidden="true">#</a> 偏向锁</h3><p>Hotspot的作者经过以往的研究发现大多数情况下<strong>锁不仅不存在多线程竞争，而且总是由同一线程多次获得</strong>，于是引入了偏向锁。</p><p>偏向锁会偏向于第一个访问锁的线程，如果在接下来的运行过程中，该锁没有被其他的线程访问，则持有偏向锁的线程将永远不需要触发同步。也就是说，<strong>偏向锁在资源无竞争情况下消除了同步语句，连CAS操作都不做了，提高了程序的运行性能。</strong></p><blockquote><p>大白话就是对锁置个变量，如果发现为true，代表资源无竞争，则无需再走各种加锁/解锁流程。如果为false，代表存在其他线程竞争资源，那么就会走后面的流程。</p></blockquote><h4 id="实现原理" tabindex="-1"><a class="header-anchor" href="#实现原理" aria-hidden="true">#</a> <strong>实现原理</strong></h4><p>一个线程在第一次进入同步块时，会在对象头和栈帧中的锁记录里存储锁的偏向的线程ID。当下次该线程进入这个同步块时，会去检查锁的Mark Word里面是不是放的自己的线程ID。</p><p>如果是，表明该线程已经获得了锁，以后该线程在进入和退出同步块时不需要花费CAS操作来加锁和解锁 ；如果不是，就代表有另一个线程来竞争这个偏向锁。这个时候会尝试使用CAS来替换Mark Word里面的线程ID为新线程的ID，这个时候要分两种情况：</p><ul><li>成功，表示之前的线程不存在了， Mark Word里面的线程ID为新线程的ID，锁不会升级，仍然为偏向锁；</li><li>失败，表示之前的线程仍然存在，那么暂停之前的线程，设置偏向锁标识为0，并设置锁标志位为00，升级为轻量级锁，会按照轻量级锁的方式进行竞争锁。</li></ul><blockquote><p>CAS: Compare and Swap</p><p>比较并设置。用于在硬件层面上提供原子性操作。在 Intel 处理器中，比较并交换通过指令cmpxchg实现。 比较是否和给定的数值一致，如果一致则修改，不一致则不修改。</p></blockquote><p>线程竞争偏向锁的过程如下：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/synchronized-c223913c-6c10-4dd0-849c-5e8f981cba48.jpg" alt=""></p><p>图中涉及到了lock record指针指向当前堆栈中的最近一个lock record，是轻量级锁按照先来先服务的模式进行了轻量级锁的加锁。</p><h4 id="撤销偏向锁" tabindex="-1"><a class="header-anchor" href="#撤销偏向锁" aria-hidden="true">#</a> 撤销偏向锁</h4><p>偏向锁使用了一种<strong>等到竞争出现才释放锁的机制</strong>，所以当其他线程尝试竞争偏向锁时， 持有偏向锁的线程才会释放锁。</p><p>偏向锁升级成轻量级锁时，会暂停拥有偏向锁的线程，重置偏向锁标识，这个过程看起来容易，实则开销还是很大的，大概的过程如下：</p><ol><li>在一个安全点（在这个时间点上没有字节码正在执行）停止拥有锁的线程。</li><li>遍历线程栈，如果存在锁记录的话，需要修复锁记录和Mark Word，使其变成无锁状态。</li><li>唤醒被停止的线程，将当前锁升级成轻量级锁。</li></ol><p>所以，如果应用程序里所有的锁通常处于竞争状态，那么偏向锁就会是一种累赘，对于这种情况，我们可以一开始就把偏向锁这个默认功能给关闭：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token operator">-</span>XX<span class="token operator">:</span><span class="token class-name">UseBiasedLocking</span><span class="token operator">=</span><span class="token boolean">false</span>。
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><p>下面这个经典的图总结了偏向锁的获得和撤销：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/synchronized-23a5729f-71d5-44ce-af19-72ee20ae753e.png" alt=""></p><h3 id="轻量级锁" tabindex="-1"><a class="header-anchor" href="#轻量级锁" aria-hidden="true">#</a> 轻量级锁</h3><p>多个线程在不同时段获取同一把锁，即不存在锁竞争的情况，也就没有线程阻塞。针对这种情况，JVM采用轻量级锁来避免线程的阻塞与唤醒。</p><h4 id="轻量级锁的加锁" tabindex="-1"><a class="header-anchor" href="#轻量级锁的加锁" aria-hidden="true">#</a> 轻量级锁的加锁</h4><p>JVM会为每个线程在当前线程的栈帧中创建用于存储锁记录的空间，我们称为Displaced Mark Word。如果一个线程获得锁的时候发现是轻量级锁，会把锁的Mark Word复制到自己的Displaced Mark Word里面。</p><p>然后线程尝试用CAS将锁的Mark Word替换为指向锁记录的指针。如果成功，当前线程获得锁，如果失败，表示Mark Word已经被替换成了其他线程的锁记录，说明在与其它线程竞争锁，当前线程就尝试使用自旋来获取锁。</p><blockquote><p>自旋：不断尝试去获取锁，一般用循环来实现。</p></blockquote><p>自旋是需要消耗CPU的，如果一直获取不到锁的话，那该线程就一直处在自旋状态，白白浪费CPU资源。解决这个问题最简单的办法就是指定自旋的次数，例如让其循环10次，如果还没获取到锁就进入阻塞状态。</p><p>但是JDK采用了更聪明的方式——适应性自旋，简单来说就是线程如果自旋成功了，则下次自旋的次数会更多，如果自旋失败了，则自旋的次数就会减少。</p><p>自旋也不是一直进行下去的，如果自旋到一定程度（和JVM、操作系统相关），依然没有获取到锁，称为自旋失败，那么这个线程会阻塞。同时这个锁就会<strong>升级成重量级锁</strong>。</p><p><strong>轻量级锁的释放：</strong></p><p>在释放锁时，当前线程会使用CAS操作将Displaced Mark Word的内容复制回锁的Mark Word里面。如果没有发生竞争，那么这个复制的操作会成功。如果有其他线程因为自旋多次导致轻量级锁升级成了重量级锁，那么CAS操作会失败，此时会释放锁并唤醒被阻塞的线程。</p><p>一张图说明加锁和释放锁的过程：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/synchronized-1bb5f956-d3da-4e7b-b426-1d5d9314fe5b.png" alt=""></p><h3 id="重量级锁" tabindex="-1"><a class="header-anchor" href="#重量级锁" aria-hidden="true">#</a> 重量级锁</h3><p>重量级锁依赖于操作系统的互斥量（mutex） 实现的，而操作系统中线程间状态的转换需要相对比较长的时间，所以重量级锁效率很低，但被阻塞的线程不会消耗CPU。</p><p>前面说到，每一个对象都可以当做一个锁，当多个线程同时请求某个对象锁时，对象锁会设置几种状态用来区分请求的线程：</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>Contention List：所有请求锁的线程将被首先放置到该竞争队列
Entry List：Contention List中那些有资格成为候选人的线程被移到Entry List
Wait Set：那些调用wait方法被阻塞的线程被放置到Wait Set
OnDeck：任何时刻最多只能有一个线程正在竞争锁，该线程称为OnDeck
Owner：获得锁的线程称为Owner
!Owner：释放锁的线程
</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></div><p>当一个线程尝试获得锁时，如果该锁已经被占用，则会将该线程封装成一个<code>ObjectWaiter</code>对象插入到Contention List的队列的队首，然后调用<code>park</code>函数挂起当前线程。</p><p>当线程释放锁时，会从Contention List或EntryList中挑选一个线程唤醒，被选中的线程叫做<code>Heir presumptive</code>即假定继承人，假定继承人被唤醒后会尝试获得锁，但<code>synchronized</code>是非公平的，所以假定继承人不一定能获得锁。这是因为对于重量级锁，线程先自旋尝试获得锁，这样做的目的是为了减少执行操作系统同步操作带来的开销。如果自旋不成功再进入等待队列。这对那些已经在等待队列中的线程来说，稍微显得不公平，还有一个不公平的地方是自旋线程可能会抢占了Ready线程的锁。</p><p>如果线程获得锁后调用<code>Object.wait</code>方法，则会将线程加入到WaitSet中，当被<code>Object.notify</code>唤醒后，会将线程从WaitSet移动到Contention List或EntryList中去。需要注意的是，当调用一个锁对象的<code>wait</code>或<code>notify</code>方法时，<strong>如当前锁的状态是偏向锁或轻量级锁则会先膨胀成重量级锁</strong>。</p><h3 id="锁的升级流程" tabindex="-1"><a class="header-anchor" href="#锁的升级流程" aria-hidden="true">#</a> 锁的升级流程</h3><p>每一个线程在准备获取共享资源时： 第一步，检查MarkWord里面是不是放的自己的ThreadId ,如果是，表示当前线程是处于 “偏向锁” 。</p><p>第二步，如果MarkWord不是自己的ThreadId，锁升级，这时候，用CAS来执行切换，新的线程根据MarkWord里面现有的ThreadId，通知之前线程暂停，之前线程将Markword的内容置为空。</p><p>第三步，两个线程都把锁对象的HashCode复制到自己新建的用于存储锁的记录空间，接着开始通过CAS操作， 把锁对象的MarKword的内容修改为自己新建的记录空间的地址的方式竞争MarkWord。</p><p>第四步，第三步中成功执行CAS的获得资源，失败的则进入自旋 。</p><p>第五步，自旋的线程在自旋过程中，成功获得资源(即之前获的资源的线程执行完成并释放了共享资源)，则整个状态依然处于 轻量级锁的状态，如果自旋失败 。</p><p>第六步，进入重量级锁的状态，这个时候，自旋的线程进行阻塞，等待之前线程执行完成并唤醒自己。</p><h3 id="各种锁的优缺点对比" tabindex="-1"><a class="header-anchor" href="#各种锁的优缺点对比" aria-hidden="true">#</a> 各种锁的优缺点对比</h3><p>下表来自《Java并发编程的艺术》：</p><table><thead><tr><th>锁</th><th>优点</th><th>缺点</th><th>适用场景</th></tr></thead><tbody><tr><td>偏向锁</td><td>加锁和解锁不需要额外的消耗，和执行非同步方法比仅存在纳秒级的差距。</td><td>如果线程间存在锁竞争，会带来额外的锁撤销的消耗。</td><td>适用于只有一个线程访问同步块场景。</td></tr><tr><td>轻量级锁</td><td>竞争的线程不会阻塞，提高了程序的响应速度。</td><td>如果始终得不到锁竞争的线程使用自旋会消耗CPU。</td><td>追求响应时间。同步块执行速度非常快。</td></tr><tr><td>重量级锁</td><td>线程竞争不使用自旋，不会消耗CPU。</td><td>线程阻塞，响应时间缓慢。</td><td>追求吞吐量。同步块执行时间较长。</td></tr></tbody></table><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" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a></li><li><a href="https://github.com/CL0610/Java-concurrency" target="_blank" rel="noopener noreferrer">并发编程知识总结<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" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a></li><li><a href="https://github.com/CoderLeixiaoshuai/java-eight-part" target="_blank" rel="noopener noreferrer">Java八股文<span><svg class="external-link-icon" 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