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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 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 aria-current="page" href="/thread/ConcurrentHashMap.html" class="router-link-active router-link-exact-active nav-link active sidebar-link sidebar-page active" aria-label="ConcurrentHashMap"><!---->ConcurrentHashMap<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#关键属性及类" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="关键属性及类"><!---->关键属性及类<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#concurrenthashmap的关键属性" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="ConcurrentHashMap的关键属性"><!---->ConcurrentHashMap的关键属性<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#concurrenthashmap中关键内部类" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="ConcurrentHashMap中关键内部类"><!---->ConcurrentHashMap中关键内部类<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#cas关键操作" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="CAS关键操作"><!---->CAS关键操作<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#_1-tabat" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="1. tabAt"><!---->1. tabAt<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#_2-castabat" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="2. casTabAt"><!---->2. casTabAt<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#_3-settabat" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="3. setTabAt"><!---->3. setTabAt<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#重点方法讲解" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="重点方法讲解"><!---->重点方法讲解<!----></a><ul class="sidebar-sub-headers"><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#实例构造器方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="实例构造器方法"><!---->实例构造器方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#inittable方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="initTable方法"><!---->initTable方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#put方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="put方法"><!---->put方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#get方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="get方法"><!---->get方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#transfer方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="transfer方法"><!---->transfer方法<!----></a><ul class="sidebar-sub-headers"></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#与size相关的一些方法" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="与size相关的一些方法"><!---->与size相关的一些方法<!----></a><ul class="sidebar-sub-headers"></ul></li></ul></li><li class="sidebar-sub-header"><a aria-current="page" href="/thread/ConcurrentHashMap.html#总结" class="router-link-active router-link-exact-active nav-link sidebar-link heading" aria-label="总结"><!---->总结<!----></a><ul class="sidebar-sub-headers"></ul></li></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、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">九、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><!---->吊打Java并发面试官之ConcurrentHashMap</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="https://tobebetterjavaer.com" 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 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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 0-40.311-18.049-40.311-40.312V593.79c0-22.263 18.048-40.311 40.31-40.311zM593.927 106.992h282.176c22.263 0 40.31 18.048 40.31 40.31V429.48c0 22.263-18.047 40.31-40.31 40.31H593.927c-22.263 0-40.311-18.047-40.311-40.31V147.302c0-22.263 18.048-40.31 40.31-40.31zM730.22 920.502H623.926c-40.925 0-74.22-33.388-74.22-74.425V623.992c0-41.038 33.387-74.424 74.425-74.424h222.085c41.038 0 74.424 33.226 74.424 74.067v114.233c0 10.244-8.304 18.548-18.547 18.548s-18.548-8.304-18.548-18.548V623.635c0-20.388-16.746-36.974-37.33-36.974H624.13c-20.585 0-37.331 16.747-37.331 37.33v222.086c0 20.585 16.654 37.331 37.126 37.331H730.22c10.243 0 18.547 8.304 18.547 18.547 0 10.244-8.304 18.547-18.547 18.547z"></path></svg><ul class="categories-wrapper"><li class="category category0" role>Java核心</li><li class="category category6" role>并发编程</li><meta property="articleSection" content="Java核心,并发编程"></ul></span><span aria-label="标签🏷" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon tag-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="tag icon"><path d="M939.902 458.563L910.17 144.567c-1.507-16.272-14.465-29.13-30.737-30.737L565.438 84.098h-.402c-3.215 0-5.726 1.005-7.634 2.913l-470.39 470.39a10.004 10.004 0 000 14.164l365.423 365.424c1.909 1.908 4.42 2.913 7.132 2.913s5.223-1.005 7.132-2.913l470.39-470.39c2.01-2.11 3.014-5.023 2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></svg><ul class="tags-wrapper"><li class="tag tag4" role>Java</li></ul><meta property="keywords" content="Java"></span><span class="reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 27 分钟</span><meta property="timeRequired" content="PT27M"></span></div><hr></div><div class="toc-place-holder"><aside 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/ConcurrentHashMap.html#关键属性及类" class="router-link-active router-link-exact-active toc-link level2">关键属性及类</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#concurrenthashmap的关键属性" class="router-link-active router-link-exact-active toc-link level3">ConcurrentHashMap的关键属性</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#concurrenthashmap中关键内部类" class="router-link-active router-link-exact-active toc-link level3">ConcurrentHashMap中关键内部类</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#cas关键操作" class="router-link-active router-link-exact-active toc-link level2">CAS关键操作</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#_1-tabat" class="router-link-active router-link-exact-active toc-link level3">1. tabAt</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#_2-castabat" class="router-link-active router-link-exact-active toc-link level3">2. casTabAt</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#_3-settabat" class="router-link-active router-link-exact-active toc-link level3">3. setTabAt</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#重点方法讲解" class="router-link-active router-link-exact-active toc-link level2">重点方法讲解</a></li><ul class="toc-list"><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#实例构造器方法" class="router-link-active router-link-exact-active toc-link level3">实例构造器方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#inittable方法" class="router-link-active router-link-exact-active toc-link level3">initTable方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#put方法" class="router-link-active router-link-exact-active toc-link level3">put方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#get方法" class="router-link-active router-link-exact-active toc-link level3">get方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#transfer方法" class="router-link-active router-link-exact-active toc-link level3">transfer方法</a></li><!----><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#与size相关的一些方法" class="router-link-active router-link-exact-active toc-link level3">与size相关的一些方法</a></li><!----><!--]--></ul><!--]--><!--[--><li class="toc-item"><a aria-current="page" href="/thread/ConcurrentHashMap.html#总结" class="router-link-active router-link-exact-active toc-link level2">总结</a></li><!----><!--]--></ul></div></aside></div><!----><div class="theme-hope-content"><p>在使用HashMap时，在多线程情况下扩容会出现CPU接近100%的情况，因为hashmap并不是线程安全的，通常我们可以使用在java体系中古老的hashtable类，该类基本上所有的方法都采用synchronized进行线程安全的控制，可想而知，在高并发的情况下，每次只有一个线程能够获取对象监视器锁，这样的并发性能的确不令人满意。</p><p>另外一种方式通过Collections的<code>Map&lt;K,V&gt; synchronizedMap(Map&lt;K,V&gt; m)</code>将hashmap包装成一个线程安全的map。比如SynchronzedMap的put方法源码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code>	<span class="token keyword">public</span> <span class="token class-name">V</span> <span class="token function">put</span><span class="token punctuation">(</span><span class="token class-name">K</span> key<span class="token punctuation">,</span> <span class="token class-name">V</span> value<span class="token punctuation">)</span> <span class="token punctuation">{</span>
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	    <span class="token keyword">synchronized</span> <span class="token punctuation">(</span>mutex<span class="token punctuation">)</span> <span class="token punctuation">{</span><span class="token keyword">return</span> m<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>key<span class="token punctuation">,</span> value<span class="token punctuation">)</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></div><p>实际上SynchronizedMap实现依然是采用synchronized独占式锁进行线程安全的并发控制的。同样，这种方案的性能也是令人不太满意的。针对这种境况，Doug Lea大师不遗余力的为我们创造了一些线程安全的并发容器，让每一个java开发人员倍感幸福。相对于hashmap来说，ConcurrentHashMap就是线程安全的map，其中<strong>利用了锁分段的思想提高了并发度</strong>。</p><p>ConcurrentHashMap在JDK1.6的版本网上资料很多，有兴趣的可以去看看。 JDK 1.6版本关键要素：</p><ol><li>segment继承了ReentrantLock充当锁的角色，为每一个segment提供了线程安全的保障；</li><li>segment维护了哈希散列表的若干个桶，每个桶由HashEntry构成的链表。</li></ol><p>而到了JDK 1.8的ConcurrentHashMap就有了很大的变化，光是代码量就足足增加了很多。1.8版本舍弃了segment，并且大量使用了synchronized，以及CAS无锁操作以保证ConcurrentHashMap操作的线程安全性。</p><p>至于为什么不用ReentrantLock而是Synchronzied呢？实际上，synchronzied做了很多的优化，包括偏向锁，轻量级锁，重量级锁，可以依次向上升级锁状态，但不能降级，因此，使用synchronized相较于ReentrantLock的性能会持平甚至在某些情况更优，具体的性能测试可以去网上查阅一些资料。另外，底层数据结构改变为采用数组+链表+红黑树的数据形式。</p><h2 id="关键属性及类" tabindex="-1"><a class="header-anchor" href="#关键属性及类" aria-hidden="true">#</a> 关键属性及类</h2><p>在了解ConcurrentHashMap的具体方法实现前，我们需要系统的来看一下几个关键的地方。</p><h3 id="concurrenthashmap的关键属性" tabindex="-1"><a class="header-anchor" href="#concurrenthashmap的关键属性" aria-hidden="true">#</a> <strong>ConcurrentHashMap的关键属性</strong></h3><ol><li><strong>table</strong><code>volatile Node&lt;K,V&gt;[] table</code>:</li></ol><p>装载Node的数组，作为ConcurrentHashMap的数据容器，采用懒加载的方式，直到第一次插入数据的时候才会进行初始化操作，数组的大小总是为2的幂次方。</p><ol start="2"><li><strong>nextTable</strong><code>volatile Node&lt;K,V&gt;[] nextTable;</code></li></ol><p>扩容时使用，平时为null，只有在扩容的时候才为非null</p><ol start="3"><li><strong>sizeCtl</strong><code>volatile int sizeCtl;</code></li></ol><p>该属性用来控制table数组的大小，根据是否初始化和是否正在扩容有几种情况：</p><ul><li><strong>当值为负数时：</strong> 如果为-1表示正在初始化，如果为-N则表示当前正有N-1个线程进行扩容操作；</li><li><strong>当值为正数时：</strong> 如果当前数组为null的话表示table在初始化过程中，sizeCtl表示为需要新建数组的长度；</li><li>若已经初始化了，表示当前数据容器（table数组）可用容量也可以理解成临界值（插入节点数超过了该临界值就需要扩容），具体指为数组的长度n 乘以 加载因子loadFactor；</li><li>当值为0时，即数组长度为默认初始值。</li></ul><ol start="4"><li><code>sun.misc.Unsafe U</code></li></ol><p>在ConcurrentHashMapde的实现中可以看到大量的U.compareAndSwapXXXX的方法去修改ConcurrentHashMap的一些属性。</p><p>这些方法实际上是利用了CAS算法保证了线程安全性，这是一种乐观策略，假设每一次操作都不会产生冲突，当且仅当冲突发生的时候再去尝试。</p><p>而CAS操作依赖于现代处理器指令集，通过底层<strong>CMPXCHG</strong>指令实现。CAS(V,O,N)核心思想为：<strong>若当前变量实际值V与期望的旧值O相同，则表明该变量没被其他线程进行修改，因此可以安全的将新值N赋值给变量；若当前变量实际值V与期望的旧值O不相同，则表明该变量已经被其他线程做了处理，此时将新值N赋给变量操作就是不安全的，在进行重试</strong>。</p><p>而在大量的同步组件和并发容器的实现中使用CAS是通过<code>sun.misc.Unsafe</code>类实现的，该类提供了一些可以直接操控内存和线程的底层操作，可以理解为java中的“指针”。该成员变量的获取是在静态代码块中：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token punctuation">{</span>
    <span class="token keyword">try</span> <span class="token punctuation">{</span>
        <span class="token class-name">U</span> <span class="token operator">=</span> <span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>Unsafe</span><span class="token punctuation">.</span><span class="token function">getUnsafe</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
		<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span>e<span class="token punctuation">)</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></div><h3 id="concurrenthashmap中关键内部类" tabindex="-1"><a class="header-anchor" href="#concurrenthashmap中关键内部类" aria-hidden="true">#</a> <strong>ConcurrentHashMap中关键内部类</strong></h3><h4 id="_1-node" tabindex="-1"><a class="header-anchor" href="#_1-node" aria-hidden="true">#</a> 1. <strong>Node</strong></h4><p>Node类实现了Map.Entry接口，主要存放key-value对，并且具有next域</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">class</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">implements</span> <span class="token class-name">Map<span class="token punctuation">.</span>Entry</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
        <span class="token keyword">final</span> <span class="token keyword">int</span> hash<span class="token punctuation">;</span>
        <span class="token keyword">final</span> <span class="token class-name">K</span> key<span class="token punctuation">;</span>
        <span class="token keyword">volatile</span> <span class="token class-name">V</span> val<span class="token punctuation">;</span>
        <span class="token keyword">volatile</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> next<span class="token punctuation">;</span>
		<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</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></div><p>另外可以看出很多属性都是用volatile进行修饰的，也就是为了保证内存可见性。</p><h4 id="_2-treenode" tabindex="-1"><a class="header-anchor" href="#_2-treenode" aria-hidden="true">#</a> 2. <strong>TreeNode</strong></h4><p>树节点，继承于承载数据的Node类。而红黑树的操作是针对TreeBin类的，从该类的注释也可以看出，也就是TreeBin会将TreeNode进行再一次封装</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token operator">*</span><span class="token operator">*</span>
 <span class="token operator">*</span> <span class="token class-name">Nodes</span> <span class="token keyword">for</span> use in <span class="token class-name">TreeBins</span>
 <span class="token operator">*</span><span class="token operator">/</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">extends</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> parent<span class="token punctuation">;</span>  <span class="token comment">// red-black tree links</span>
        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> left<span class="token punctuation">;</span>
        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> right<span class="token punctuation">;</span>
        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> prev<span class="token punctuation">;</span>    <span class="token comment">// needed to unlink next upon deletion</span>
        <span class="token keyword">boolean</span> red<span class="token punctuation">;</span>
		<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</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></div><h4 id="_3-treebin" tabindex="-1"><a class="header-anchor" href="#_3-treebin" aria-hidden="true">#</a> 3. <strong>TreeBin</strong></h4><p>这个类并不负责包装用户的key、value信息，而是包装的很多TreeNode节点。实际的ConcurrentHashMap“数组”中，存放的是TreeBin对象，而不是TreeNode对象。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">extends</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> root<span class="token punctuation">;</span>
        <span class="token keyword">volatile</span> <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> first<span class="token punctuation">;</span>
        <span class="token keyword">volatile</span> <span class="token class-name">Thread</span> waiter<span class="token punctuation">;</span>
        <span class="token keyword">volatile</span> <span class="token keyword">int</span> lockState<span class="token punctuation">;</span>
        <span class="token comment">// values for lockState</span>
        <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> WRITER <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// set while holding write lock</span>
        <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> WAITER <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span> <span class="token comment">// set when waiting for write lock</span>
        <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> READER <span class="token operator">=</span> <span class="token number">4</span><span class="token punctuation">;</span> <span class="token comment">// increment value for setting read lock</span>
		<span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</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></div><h4 id="_4-forwardingnode" tabindex="-1"><a class="header-anchor" href="#_4-forwardingnode" aria-hidden="true">#</a> 4. <strong>ForwardingNode</strong></h4><p>在扩容时才会出现的特殊节点，其key,value,hash全部为null。并拥有nextTable指针引用新的table数组。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">ForwardingNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">extends</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
    <span class="token keyword">final</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> nextTable<span class="token punctuation">;</span>
    <span class="token class-name">ForwardingNode</span><span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">super</span><span class="token punctuation">(</span>MOVED<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>nextTable <span class="token operator">=</span> tab<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
   <span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</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></div><h2 id="cas关键操作" tabindex="-1"><a class="header-anchor" href="#cas关键操作" aria-hidden="true">#</a> <strong>CAS关键操作</strong></h2><p>在上面我们提及到在ConcurrentHashMap中会大量使用CAS修改它的属性和一些操作。因此，在理解ConcurrentHashMap的方法前我们需要了解下面几个常用的利用CAS算法来保障线程安全的操作。</p><h3 id="_1-tabat" tabindex="-1"><a class="header-anchor" href="#_1-tabat" aria-hidden="true">#</a> 1. <strong>tabAt</strong></h3><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token function">tabAt</span><span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">)</span><span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">getObjectVolatile</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span>i <span class="token operator">&lt;&lt;</span> ASHIFT<span class="token punctuation">)</span> <span class="token operator">+</span> ABASE<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></div><p>该方法用来获取table数组中索引为i的Node元素。</p><h3 id="_2-castabat" tabindex="-1"><a class="header-anchor" href="#_2-castabat" aria-hidden="true">#</a> 2. <strong>casTabAt</strong></h3><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">boolean</span> <span class="token function">casTabAt</span><span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">,</span>
                                    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> c<span class="token punctuation">,</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> v<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapObject</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span>i <span class="token operator">&lt;&lt;</span> ASHIFT<span class="token punctuation">)</span> <span class="token operator">+</span> ABASE<span class="token punctuation">,</span> c<span class="token punctuation">,</span> v<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></div><p>利用CAS操作设置table数组中索引为i的元素</p><h3 id="_3-settabat" tabindex="-1"><a class="header-anchor" href="#_3-settabat" aria-hidden="true">#</a> 3. <strong>setTabAt</strong></h3><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">void</span> <span class="token function">setTabAt</span><span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">,</span> <span class="token keyword">int</span> i<span class="token punctuation">,</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> v<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">putObjectVolatile</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span>i <span class="token operator">&lt;&lt;</span> ASHIFT<span class="token punctuation">)</span> <span class="token operator">+</span> ABASE<span class="token punctuation">,</span> v<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></div><p>该方法用来设置table数组中索引为i的元素</p><h2 id="重点方法讲解" tabindex="-1"><a class="header-anchor" href="#重点方法讲解" aria-hidden="true">#</a> 重点方法讲解</h2><p>在熟悉上面的这核心信息之后，我们接下来就来依次看看几个常用的方法是怎样实现的。</p><h3 id="实例构造器方法" tabindex="-1"><a class="header-anchor" href="#实例构造器方法" aria-hidden="true">#</a> 实例构造器方法</h3><p>在使用ConcurrentHashMap第一件事自然而然就是new 出来一个ConcurrentHashMap对象，一共提供了如下几个构造器方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 1. 构造一个空的map，即table数组还未初始化，初始化放在第一次插入数据时，默认大小为16</span>
<span class="token class-name">ConcurrentHashMap</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token comment">// 2. 给定map的大小</span>
<span class="token class-name">ConcurrentHashMap</span><span class="token punctuation">(</span><span class="token keyword">int</span> initialCapacity<span class="token punctuation">)</span> 
<span class="token comment">// 3. 给定一个map</span>
<span class="token class-name">ConcurrentHashMap</span><span class="token punctuation">(</span><span class="token class-name">Map</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">K</span><span class="token punctuation">,</span> <span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> m<span class="token punctuation">)</span>
<span class="token comment">// 4. 给定map的大小以及加载因子</span>
<span class="token class-name">ConcurrentHashMap</span><span class="token punctuation">(</span><span class="token keyword">int</span> initialCapacity<span class="token punctuation">,</span> <span class="token keyword">float</span> loadFactor<span class="token punctuation">)</span>
<span class="token comment">// 5. 给定map大小，加载因子以及并发度（预计同时操作数据的线程）</span>
<span class="token class-name">ConcurrentHashMap</span><span class="token punctuation">(</span><span class="token keyword">int</span> initialCapacity<span class="token punctuation">,</span><span class="token keyword">float</span> loadFactor<span class="token punctuation">,</span> <span class="token keyword">int</span> concurrencyLevel<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></div><p>ConcurrentHashMap一共给我们提供了5中构造器方法，具体使用请看注释，我们来看看第2种构造器，传入指定大小时的情况，该构造器源码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">ConcurrentHashMap</span><span class="token punctuation">(</span><span class="token keyword">int</span> initialCapacity<span class="token punctuation">)</span> <span class="token punctuation">{</span>
	<span class="token comment">//1. 小于0直接抛异常</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>initialCapacity <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span>
        <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">//2. 判断是否超过了允许的最大值，超过了话则取最大值，否则再对该值进一步处理</span>
    <span class="token keyword">int</span> cap <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>initialCapacity <span class="token operator">&gt;=</span> <span class="token punctuation">(</span>MAXIMUM_CAPACITY <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">?</span>
               MAXIMUM_CAPACITY <span class="token operator">:</span>
               <span class="token function">tableSizeFor</span><span class="token punctuation">(</span>initialCapacity <span class="token operator">+</span> <span class="token punctuation">(</span>initialCapacity <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">//3. 赋值给sizeCtl</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>sizeCtl <span class="token operator">=</span> cap<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>这段代码的逻辑请看注释，很容易理解，如果小于0就直接抛出异常，如果指定值大于了所允许的最大值的话就取最大值，否则，在对指定值做进一步处理。最后将cap赋值给sizeCtl,关于sizeCtl的说明请看上面的说明，<strong>当调用构造器方法之后，sizeCtl的大小应该就代表了ConcurrentHashMap的大小，即table数组长度</strong>。tableSizeFor做了哪些事情了？源码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token doc-comment comment">/**
 * Returns a power of two table size for the given desired capacity.
 * See Hackers Delight, sec 3.2
 */</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">tableSizeFor</span><span class="token punctuation">(</span><span class="token keyword">int</span> c<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">int</span> n <span class="token operator">=</span> c <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
    n <span class="token operator">|=</span> n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">1</span><span class="token punctuation">;</span>
    n <span class="token operator">|=</span> n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">2</span><span class="token punctuation">;</span>
    n <span class="token operator">|=</span> n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">4</span><span class="token punctuation">;</span>
    n <span class="token operator">|=</span> n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">8</span><span class="token punctuation">;</span>
    n <span class="token operator">|=</span> n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">16</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span>n <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">1</span> <span class="token operator">:</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;=</span> MAXIMUM_CAPACITY<span class="token punctuation">)</span> <span class="token operator">?</span> MAXIMUM_CAPACITY <span class="token operator">:</span> n <span class="token operator">+</span> <span class="token number">1</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></div><p>通过注释就很清楚了，该方法会将调用构造器方法时指定的大小转换成一个2的幂次方数，也就是说ConcurrentHashMap的大小一定是2的幂次方，比如，当指定大小为18时，为了满足2的幂次方特性，实际上concurrentHashMapd的大小为2的5次方（32）。</p><p>另外，需要注意的是，<strong>调用构造器方法的时候并未构造出table数组（可以理解为ConcurrentHashMap的数据容器），只是算出table数组的长度，当第一次向ConcurrentHashMap插入数据的时候才真正的完成初始化创建table数组的工作</strong>。</p><h3 id="inittable方法" tabindex="-1"><a class="header-anchor" href="#inittable方法" aria-hidden="true">#</a> initTable方法</h3><p>直接上源码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">initTable</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">;</span> <span class="token keyword">int</span> sc<span class="token punctuation">;</span>
    <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>tab <span class="token operator">=</span> table<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> tab<span class="token punctuation">.</span>length <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>sc <span class="token operator">=</span> sizeCtl<span class="token punctuation">)</span> <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span>
			<span class="token comment">// 1. 保证只有一个线程正在进行初始化操作</span>
            <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token keyword">yield</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// lost initialization race; just spin</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapInt</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> SIZECTL<span class="token punctuation">,</span> sc<span class="token punctuation">,</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">try</span> <span class="token punctuation">{</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>tab <span class="token operator">=</span> table<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> tab<span class="token punctuation">.</span>length <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
					<span class="token comment">// 2. 得出数组的大小</span>
                    <span class="token keyword">int</span> n <span class="token operator">=</span> <span class="token punctuation">(</span>sc <span class="token operator">&gt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">?</span> sc <span class="token operator">:</span> DEFAULT_CAPACITY<span class="token punctuation">;</span>
                    <span class="token annotation punctuation">@SuppressWarnings</span><span class="token punctuation">(</span><span class="token string">&quot;unchecked&quot;</span><span class="token punctuation">)</span>
					<span class="token comment">// 3. 这里才真正的初始化数组</span>
                    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> nt <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">,</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span>n<span class="token punctuation">]</span><span class="token punctuation">;</span>
                    table <span class="token operator">=</span> tab <span class="token operator">=</span> nt<span class="token punctuation">;</span>
					<span class="token comment">// 4. 计算数组中可用的大小：实际大小n*0.75（加载因子）</span>
                    sc <span class="token operator">=</span> n <span class="token operator">-</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token punctuation">}</span>
            <span class="token punctuation">}</span> <span class="token keyword">finally</span> <span class="token punctuation">{</span>
                sizeCtl <span class="token operator">=</span> sc<span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> tab<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 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>代码的逻辑请见注释，有可能存在一个情况是多个线程同时走到这个方法中，为了保证能够正确初始化，在第1步中会先通过if进行判断，若当前已经有一个线程正在初始化即sizeCtl值变为-1，这个时候其他线程在If判断为true从而调用Thread.yield()让出CPU时间片。</p><p>正在进行初始化的线程会调用U.compareAndSwapInt方法将sizeCtl改为-1即正在初始化的状态。</p><p>另外还需要注意的事情是，在第四步中会进一步计算数组中可用的大小即为数组实际大小n乘以加载因子0.75.可以看看这里乘以0.75是怎么算的，0.75为四分之三，这里<code>n - (n &gt;&gt;&gt; 2)</code>是不是刚好是<code>n-(1/4)n=(3/4)n</code>，挺有意思的吧:)。</p><p>如果选择是无参的构造器的话，这里在new Node数组的时候会使用默认大小为<code>DEFAULT_CAPACITY</code>（16），然后乘以加载因子0.75为12，也就是说数组的可用大小为12。</p><h3 id="put方法" tabindex="-1"><a class="header-anchor" href="#put方法" aria-hidden="true">#</a> put方法</h3><p>使用ConcurrentHashMap最长用的也应该是put和get方法了吧，我们先来看看put方法是怎样实现的。调用put方法时实际具体实现是putVal方法，源码如下：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token doc-comment comment">/** Implementation for put and putIfAbsent */</span>
<span class="token keyword">final</span> <span class="token class-name">V</span> <span class="token function">putVal</span><span class="token punctuation">(</span><span class="token class-name">K</span> key<span class="token punctuation">,</span> <span class="token class-name">V</span> value<span class="token punctuation">,</span> <span class="token keyword">boolean</span> onlyIfAbsent<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> value <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">NullPointerException</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">//1. 计算key的hash值</span>
    <span class="token keyword">int</span> hash <span class="token operator">=</span> <span class="token function">spread</span><span class="token punctuation">(</span>key<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">int</span> binCount <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab <span class="token operator">=</span> table<span class="token punctuation">;</span><span class="token punctuation">;</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> f<span class="token punctuation">;</span> <span class="token keyword">int</span> n<span class="token punctuation">,</span> i<span class="token punctuation">,</span> fh<span class="token punctuation">;</span>
		<span class="token comment">//2. 如果当前table还没有初始化先调用initTable方法将tab进行初始化</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>tab <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> <span class="token punctuation">(</span>n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span>
            tab <span class="token operator">=</span> <span class="token function">initTable</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
		<span class="token comment">//3. tab中索引为i的位置的元素为null，则直接使用CAS将值插入即可</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>f <span class="token operator">=</span> <span class="token function">tabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i <span class="token operator">=</span> <span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> hash<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">casTabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span>
                         <span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span> value<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token keyword">break</span><span class="token punctuation">;</span>                   <span class="token comment">// no lock when adding to empty bin</span>
        <span class="token punctuation">}</span>
		<span class="token comment">//4. 当前正在扩容</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>fh <span class="token operator">=</span> f<span class="token punctuation">.</span>hash<span class="token punctuation">)</span> <span class="token operator">==</span> MOVED<span class="token punctuation">)</span>
            tab <span class="token operator">=</span> <span class="token function">helpTransfer</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> f<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">else</span> <span class="token punctuation">{</span>
            <span class="token class-name">V</span> oldVal <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
            <span class="token keyword">synchronized</span> <span class="token punctuation">(</span>f<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">tabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">)</span> <span class="token operator">==</span> f<span class="token punctuation">)</span> <span class="token punctuation">{</span>
					<span class="token comment">//5. 当前为链表，在链表中插入新的键值对</span>
                    <span class="token keyword">if</span> <span class="token punctuation">(</span>fh <span class="token operator">&gt;=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                        binCount <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
                        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> e <span class="token operator">=</span> f<span class="token punctuation">;</span><span class="token punctuation">;</span> <span class="token operator">++</span>binCount<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                            <span class="token class-name">K</span> ek<span class="token punctuation">;</span>
                            <span class="token keyword">if</span> <span class="token punctuation">(</span>e<span class="token punctuation">.</span>hash <span class="token operator">==</span> hash <span class="token operator">&amp;&amp;</span>
                                <span class="token punctuation">(</span><span class="token punctuation">(</span>ek <span class="token operator">=</span> e<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span>
                                 <span class="token punctuation">(</span>ek <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>ek<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                                oldVal <span class="token operator">=</span> e<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
                                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>onlyIfAbsent<span class="token punctuation">)</span>
                                    e<span class="token punctuation">.</span>val <span class="token operator">=</span> value<span class="token punctuation">;</span>
                                <span class="token keyword">break</span><span class="token punctuation">;</span>
                            <span class="token punctuation">}</span>
                            <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> pred <span class="token operator">=</span> e<span class="token punctuation">;</span>
                            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>e <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                                pred<span class="token punctuation">.</span>next <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span>
                                                          value<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                                <span class="token keyword">break</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">// 6.当前为红黑树，将新的键值对插入到红黑树中</span>
                    <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>f <span class="token keyword">instanceof</span> <span class="token class-name">TreeBin</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> p<span class="token punctuation">;</span>
                        binCount <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
                        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>p <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">)</span>f<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">putTreeVal</span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span>
                                                       value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                            oldVal <span class="token operator">=</span> p<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
                            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>onlyIfAbsent<span class="token punctuation">)</span>
                                p<span class="token punctuation">.</span>val <span class="token operator">=</span> value<span class="token punctuation">;</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">// 7.插入完键值对后再根据实际大小看是否需要转换成红黑树</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>binCount <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span>binCount <span class="token operator">&gt;=</span> TREEIFY_THRESHOLD<span class="token punctuation">)</span>
                    <span class="token function">treeifyBin</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span>oldVal <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                    <span class="token keyword">return</span> oldVal<span class="token punctuation">;</span>
                <span class="token keyword">break</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">//8.对当前容量大小进行检查，如果超过了临界值（实际大小*加载因子）就需要扩容 </span>
    <span class="token function">addCount</span><span class="token punctuation">(</span><span class="token number">1L</span><span class="token punctuation">,</span> binCount<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token keyword">null</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 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 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 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 class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>put方法的代码量有点长，我们按照上面的分解的步骤一步步来看。</p><p><strong>从整体而言，为了解决线程安全的问题，ConcurrentHashMap使用了synchronzied和CAS的方式</strong>。</p><p>在之前了解过HashMap以及1.8版本之前的ConcurrenHashMap都应该知道ConcurrentHashMap结构图，为了方面下面的讲解这里先直接给出，如果对这有疑问的话，可以在网上随便搜搜即可。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/ConcurrentHashMap-01.png" alt="ConcurrentHashMap散列桶数组结构示意图"></p><p>如图（图片摘自网络），ConcurrentHashMap是一个哈希桶数组，如果不出现哈希冲突的时候，每个元素均匀的分布在哈希桶数组中。当出现哈希冲突的时候，是<strong>标准的链地址的解决方式</strong>，将hash值相同的节点构成链表的形式，称为“拉链法”，另外，在1.8版本中为了防止拉链过长，当链表的长度大于8的时候会将链表转换成红黑树。</p><p>table数组中的每个元素实际上是单链表的头结点或者红黑树的根节点。当插入键值对时首先应该定位到要插入的桶，即插入table数组的索引i处。那么，怎样计算得出索引i呢？当然是根据key的hashCode值。</p><h4 id="_1-spread-重哈希-以减小hash冲突" tabindex="-1"><a class="header-anchor" href="#_1-spread-重哈希-以减小hash冲突" aria-hidden="true">#</a> 1. spread()重哈希，以减小Hash冲突</h4><p>我们知道对于一个hash表来说，hash值分散的不够均匀的话会大大增加哈希冲突的概率，从而影响到hash表的性能。因此通过spread方法进行了一次重hash从而大大减小哈希冲突的可能性。spread方法为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> <span class="token function">spread</span><span class="token punctuation">(</span><span class="token keyword">int</span> h<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span>h <span class="token operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">16</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> HASH_BITS<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></div><p>该方法主要是<strong>将key的hashCode的低16位于高16位进行异或运算</strong>，这样不仅能够使得hash值能够分散能够均匀减小hash冲突的概率，另外只用到了异或运算，在性能开销上也能兼顾，做到平衡的trade-off。</p><h4 id="_2-初始化table" tabindex="-1"><a class="header-anchor" href="#_2-初始化table" aria-hidden="true">#</a> 2. 初始化table</h4><p>紧接着到第2步，会判断当前table数组是否初始化了，没有的话就调用initTable进行初始化，该方法在上面已经讲过了。</p><h4 id="_3-能否直接将新值插入到table数组中" tabindex="-1"><a class="header-anchor" href="#_3-能否直接将新值插入到table数组中" aria-hidden="true">#</a> 3. 能否直接将新值插入到table数组中</h4><p>从上面的结构示意图就可以看出存在这样一种情况，如果插入值待插入的位置刚好所在的table数组为null的话就可以直接将值插入即可。那么怎样根据hash确定在table中待插入的索引i呢？很显然可以通过hash值与数组的长度取模操作，从而确定新值插入到数组的哪个位置。而之前我们提过ConcurrentHashMap的大小总是2的幂次方，(n - 1) &amp; hash运算等价于对长度n取模，也就是hash%n，但是位运算比取模运算的效率要高很多，Doug lea大师在设计并发容器的时候也是将性能优化到了极致，令人钦佩。</p><p>确定好数组的索引i后，就可以可以tabAt()方法（该方法在上面已经说明了，有疑问可以回过头去看看）获取该位置上的元素，如果当前Node f为null的话，就可以直接用casTabAt方法将新值插入即可。</p><h4 id="_4-当前是否正在扩容" tabindex="-1"><a class="header-anchor" href="#_4-当前是否正在扩容" aria-hidden="true">#</a> 4.当前是否正在扩容</h4><p>如果当前节点不为null，且该节点为特殊节点（forwardingNode）的话，就说明当前concurrentHashMap正在进行扩容操作，关于扩容操作，下面会作为一个具体的方法进行讲解。</p><p>那么怎样确定当前的这个Node是不是特殊的节点了？是通过判断该节点的hash值是不是等于-1（MOVED）,代码为<code>(fh = f.hash) == MOVED</code>，对MOVED的解释在源码上也写的很清楚了：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> MOVED     <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// hash for forwarding nodes</span>
</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><h4 id="_5-当table-i-为链表的头结点-在链表中插入新值" tabindex="-1"><a class="header-anchor" href="#_5-当table-i-为链表的头结点-在链表中插入新值" aria-hidden="true">#</a> 5. 当table[i]为链表的头结点，在链表中插入新值</h4><p>在table[i]不为null并且不为forwardingNode时，并且当前Node f的hash值大于<code>0（fh &gt;= 0）</code>的话说明当前节点f为当前桶的所有的节点组成的链表的头结点。那么接下来，要想向ConcurrentHashMap插入新值的话就是向这个链表插入新值。通过synchronized (f)的方式进行加锁以实现线程安全性。往链表中插入节点的部分代码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>fh <span class="token operator">&gt;=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    binCount <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> e <span class="token operator">=</span> f<span class="token punctuation">;</span><span class="token punctuation">;</span> <span class="token operator">++</span>binCount<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">K</span> ek<span class="token punctuation">;</span>
		<span class="token comment">// 找到hash值相同的key,覆盖旧值即可</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>e<span class="token punctuation">.</span>hash <span class="token operator">==</span> hash <span class="token operator">&amp;&amp;</span>
            <span class="token punctuation">(</span><span class="token punctuation">(</span>ek <span class="token operator">=</span> e<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span>
             <span class="token punctuation">(</span>ek <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>ek<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            oldVal <span class="token operator">=</span> e<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>onlyIfAbsent<span class="token punctuation">)</span>
                e<span class="token punctuation">.</span>val <span class="token operator">=</span> value<span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> pred <span class="token operator">=</span> e<span class="token punctuation">;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>e <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <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">//如果到链表末尾仍未找到，则直接将新值插入到链表末尾即可</span>
            pred<span class="token punctuation">.</span>next <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span>
                                      value<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">break</span><span class="token punctuation">;</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 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>这部分代码很好理解，就是两种情况：1. 在链表中如果找到了与待插入的键值对的key相同的节点，就直接覆盖即可；2. 如果直到找到了链表的末尾都没有找到的话，就直接将待插入的键值对追加到链表的末尾即可</p><h4 id="_6-当table-i-为红黑树的根节点-在红黑树中插入新值" tabindex="-1"><a class="header-anchor" href="#_6-当table-i-为红黑树的根节点-在红黑树中插入新值" aria-hidden="true">#</a> 6.当table[i]为红黑树的根节点，在红黑树中插入新值</h4><p>按照之前的数组+链表的设计方案，这里存在一个问题，即使负载因子和Hash算法设计的再合理，也免不了会出现拉链过长的情况，一旦出现拉链过长，甚至在极端情况下，查找一个节点会出现时间复杂度为O(n)的情况，则会严重影响ConcurrentHashMap的性能，于是，在JDK1.8版本中，对数据结构做了进一步的优化，引入了红黑树。而当链表长度太长（默认超过8）时，链表就转换为红黑树，利用红黑树快速增删改查的特点提高ConcurrentHashMap的性能，其中会用到红黑树的插入、删除、查找等算法。当table[i]为红黑树的树节点时的操作为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>f <span class="token keyword">instanceof</span> <span class="token class-name">TreeBin</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> p<span class="token punctuation">;</span>
    binCount <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>p <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">)</span>f<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">putTreeVal</span><span class="token punctuation">(</span>hash<span class="token punctuation">,</span> key<span class="token punctuation">,</span>
                                   value<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        oldVal <span class="token operator">=</span> p<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>onlyIfAbsent<span class="token punctuation">)</span>
            p<span class="token punctuation">.</span>val <span class="token operator">=</span> value<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></div><p>首先在if中通过<code>f instanceof TreeBin</code>判断当前table[i]是否是树节点，这下也正好验证了我们在最上面介绍时说的TreeBin会对TreeNode做进一步封装，对红黑树进行操作的时候针对的是TreeBin而不是TreeNode。这段代码很简单，调用putTreeVal方法完成向红黑树插入新节点，同样的逻辑，<strong>如果在红黑树中存在于待插入键值对的Key相同（hash值相等并且equals方法判断为true）的节点的话，就覆盖旧值，否则就向红黑树追加新节点</strong>。</p><h4 id="_7-根据当前节点个数进行调整" tabindex="-1"><a class="header-anchor" href="#_7-根据当前节点个数进行调整" aria-hidden="true">#</a> 7. 根据当前节点个数进行调整</h4><p>当完成数据新节点插入之后，会进一步对当前链表大小进行调整，这部分代码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">if</span> <span class="token punctuation">(</span>binCount <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>binCount <span class="token operator">&gt;=</span> TREEIFY_THRESHOLD<span class="token punctuation">)</span>
        <span class="token function">treeifyBin</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>oldVal <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
        <span class="token keyword">return</span> oldVal<span class="token punctuation">;</span>
    <span class="token keyword">break</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></div><p>很容易理解，如果当前链表节点个数大于等于8（TREEIFY_THRESHOLD）的时候，就会调用treeifyBin方法将tabel[i]（第i个散列桶）拉链转换成红黑树。</p><p>至此，关于Put方法的逻辑就基本说的差不多了，现在来做一些总结：</p><p>整体流程：</p><ol><li>首先对于每一个放入的值，首先利用spread方法对key的hashcode进行一次hash计算，由此来确定这个值在 table中的位置；</li><li>如果当前table数组还未初始化，先将table数组进行初始化操作；</li><li>如果这个位置是null的，那么使用CAS操作直接放入；</li><li>如果这个位置存在结点，说明发生了hash碰撞，首先判断这个节点的类型。如果该节点fh==MOVED(代表forwardingNode,数组正在进行扩容)的话，说明正在进行扩容；</li><li>如果是链表节点（fh&gt;0）,则得到的结点就是hash值相同的节点组成的链表的头节点。需要依次向后遍历确定这个新加入的值所在位置。如果遇到key相同的节点，则只需要覆盖该结点的value值即可。否则依次向后遍历，直到链表尾插入这个结点；</li><li>如果这个节点的类型是TreeBin的话，直接调用红黑树的插入方法进行插入新的节点；</li><li>插入完节点之后再次检查链表长度，如果长度大于8，就把这个链表转换成红黑树；</li><li>对当前容量大小进行检查，如果超过了临界值（实际大小*加载因子）就需要扩容。</li></ol><h3 id="get方法" tabindex="-1"><a class="header-anchor" href="#get方法" aria-hidden="true">#</a> get方法</h3><p>看完了put方法再来看get方法就很容易了，用逆向思维去看就好，这样存的话我反过来这么取就好了。get方法源码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">V</span> <span class="token function">get</span><span class="token punctuation">(</span><span class="token class-name">Object</span> key<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">;</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> e<span class="token punctuation">,</span> p<span class="token punctuation">;</span> <span class="token keyword">int</span> n<span class="token punctuation">,</span> eh<span class="token punctuation">;</span> <span class="token class-name">K</span> ek<span class="token punctuation">;</span>
	<span class="token comment">// 1. 重hash</span>
    <span class="token keyword">int</span> h <span class="token operator">=</span> <span class="token function">spread</span><span class="token punctuation">(</span>key<span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</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><span class="token punctuation">(</span>tab <span class="token operator">=</span> table<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span>n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">&gt;</span> <span class="token number">0</span> <span class="token operator">&amp;&amp;</span>
        <span class="token punctuation">(</span>e <span class="token operator">=</span> <span class="token function">tabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> <span class="token punctuation">(</span>n <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> h<span class="token punctuation">)</span><span class="token punctuation">)</span> <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">// 2. table[i]桶节点的key与查找的key相同，则直接返回</span>
		<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>eh <span class="token operator">=</span> e<span class="token punctuation">.</span>hash<span class="token punctuation">)</span> <span class="token operator">==</span> h<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>ek <span class="token operator">=</span> e<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span> <span class="token punctuation">(</span>ek <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>ek<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> e<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
		<span class="token comment">// 3. 当前节点hash小于0说明为树节点，在红黑树中查找即可</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>eh <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span>
            <span class="token keyword">return</span> <span class="token punctuation">(</span>p <span class="token operator">=</span> e<span class="token punctuation">.</span><span class="token function">find</span><span class="token punctuation">(</span>h<span class="token punctuation">,</span> key<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">?</span> p<span class="token punctuation">.</span>val <span class="token operator">:</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>e <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <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">//4. 从链表中查找，查找到则返回该节点的value，否则就返回null即可</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>e<span class="token punctuation">.</span>hash <span class="token operator">==</span> h <span class="token operator">&amp;&amp;</span>
                <span class="token punctuation">(</span><span class="token punctuation">(</span>ek <span class="token operator">=</span> e<span class="token punctuation">.</span>key<span class="token punctuation">)</span> <span class="token operator">==</span> key <span class="token operator">||</span> <span class="token punctuation">(</span>ek <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span> key<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>ek<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token keyword">return</span> e<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> <span class="token keyword">null</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 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>代码的逻辑请看注释，首先先看当前的hash桶数组节点即table[i]是否为查找的节点，若是则直接返回；若不是，则继续再看当前是不是树节点？通过看节点的hash值是否为小于0，如果小于0则为树节点。如果是树节点在红黑树中查找节点；如果不是树节点，那就只剩下为链表的形式的一种可能性了，就向后遍历查找节点，若查找到则返回节点的value即可，若没有找到就返回null。</p><h3 id="transfer方法" tabindex="-1"><a class="header-anchor" href="#transfer方法" aria-hidden="true">#</a> transfer方法</h3><p>当ConcurrentHashMap容量不足的时候，需要对table进行扩容。这个方法的基本思想跟HashMap是很像的，但是由于它是支持并发扩容的，所以要复杂的多。原因是它支持多线程进行扩容操作，而并没有加锁。我想这样做的目的不仅仅是为了满足concurrent的要求，而是希望利用并发处理去减少扩容带来的时间影响。transfer方法源码为：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">transfer</span><span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">,</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> nextTab<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">int</span> n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">,</span> stride<span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>stride <span class="token operator">=</span> <span class="token punctuation">(</span>NCPU <span class="token operator">&gt;</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">3</span><span class="token punctuation">)</span> <span class="token operator">/</span> NCPU <span class="token operator">:</span> n<span class="token punctuation">)</span> <span class="token operator">&lt;</span> MIN_TRANSFER_STRIDE<span class="token punctuation">)</span>
        stride <span class="token operator">=</span> MIN_TRANSFER_STRIDE<span class="token punctuation">;</span> <span class="token comment">// subdivide range</span>
	<span class="token comment">//1. 新建Node数组，容量为之前的两倍</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>nextTab <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">// initiating</span>
        <span class="token keyword">try</span> <span class="token punctuation">{</span>
            <span class="token annotation punctuation">@SuppressWarnings</span><span class="token punctuation">(</span><span class="token string">&quot;unchecked&quot;</span><span class="token punctuation">)</span>
            <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> nt <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">,</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span>n <span class="token operator">&lt;&lt;</span> <span class="token number">1</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
            nextTab <span class="token operator">=</span> nt<span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Throwable</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>      <span class="token comment">// try to cope with OOME</span>
            sizeCtl <span class="token operator">=</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE<span class="token punctuation">;</span>
            <span class="token keyword">return</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        nextTable <span class="token operator">=</span> nextTab<span class="token punctuation">;</span>
        transferIndex <span class="token operator">=</span> n<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">int</span> nextn <span class="token operator">=</span> nextTab<span class="token punctuation">.</span>length<span class="token punctuation">;</span>
	<span class="token comment">//2. 新建forwardingNode引用，在之后会用到</span>
    <span class="token class-name">ForwardingNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> fwd <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ForwardingNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>nextTab<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">boolean</span> advance <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    <span class="token keyword">boolean</span> finishing <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span> <span class="token comment">// to ensure sweep before committing nextTab</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span> bound <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span><span class="token punctuation">;</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> f<span class="token punctuation">;</span> <span class="token keyword">int</span> fh<span class="token punctuation">;</span>
        <span class="token comment">// 3. 确定遍历中的索引i</span>
		<span class="token keyword">while</span> <span class="token punctuation">(</span>advance<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">int</span> nextIndex<span class="token punctuation">,</span> nextBound<span class="token punctuation">;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">--</span>i <span class="token operator">&gt;=</span> bound <span class="token operator">||</span> finishing<span class="token punctuation">)</span>
                advance <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
            <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>nextIndex <span class="token operator">=</span> transferIndex<span class="token punctuation">)</span> <span class="token operator">&lt;=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                i <span class="token operator">=</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
                advance <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">U</span><span class="token punctuation">.</span>compareAndSwapInt
                     <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> TRANSFERINDEX<span class="token punctuation">,</span> nextIndex<span class="token punctuation">,</span>
                      nextBound <span class="token operator">=</span> <span class="token punctuation">(</span>nextIndex <span class="token operator">&gt;</span> stride <span class="token operator">?</span>
                                   nextIndex <span class="token operator">-</span> stride <span class="token operator">:</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                bound <span class="token operator">=</span> nextBound<span class="token punctuation">;</span>
                i <span class="token operator">=</span> nextIndex <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">;</span>
                advance <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
		<span class="token comment">//4.将原数组中的元素复制到新数组中去</span>
		<span class="token comment">//4.5 for循环退出，扩容结束修改sizeCtl属性</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>i <span class="token operator">&lt;</span> <span class="token number">0</span> <span class="token operator">||</span> i <span class="token operator">&gt;=</span> n <span class="token operator">||</span> i <span class="token operator">+</span> n <span class="token operator">&gt;=</span> nextn<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">int</span> sc<span class="token punctuation">;</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>finishing<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                nextTable <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
                table <span class="token operator">=</span> nextTab<span class="token punctuation">;</span>
                sizeCtl <span class="token operator">=</span> <span class="token punctuation">(</span>n <span class="token operator">&lt;&lt;</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">-</span> <span class="token punctuation">(</span>n <span class="token operator">&gt;&gt;&gt;</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                <span class="token keyword">return</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapInt</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> SIZECTL<span class="token punctuation">,</span> sc <span class="token operator">=</span> sizeCtl<span class="token punctuation">,</span> sc <span class="token operator">-</span> <span class="token number">1</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><span class="token punctuation">(</span>sc <span class="token operator">-</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token function">resizeStamp</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span> <span class="token operator">&lt;&lt;</span> RESIZE_STAMP_SHIFT<span class="token punctuation">)</span>
                    <span class="token keyword">return</span><span class="token punctuation">;</span>
                finishing <span class="token operator">=</span> advance <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
                i <span class="token operator">=</span> n<span class="token punctuation">;</span> <span class="token comment">// recheck before commit</span>
            <span class="token punctuation">}</span>
        <span class="token punctuation">}</span>
		<span class="token comment">//4.1 当前数组中第i个元素为null，用CAS设置成特殊节点forwardingNode(可以理解成占位符)</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>f <span class="token operator">=</span> <span class="token function">tabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
            advance <span class="token operator">=</span> <span class="token function">casTabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> fwd<span class="token punctuation">)</span><span class="token punctuation">;</span>
		<span class="token comment">//4.2 如果遍历到ForwardingNode节点  说明这个点已经被处理过了 直接跳过  这里是控制并发扩容的核心</span>
        <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>fh <span class="token operator">=</span> f<span class="token punctuation">.</span>hash<span class="token punctuation">)</span> <span class="token operator">==</span> MOVED<span class="token punctuation">)</span>
            advance <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span> <span class="token comment">// already processed</span>
        <span class="token keyword">else</span> <span class="token punctuation">{</span>
            <span class="token keyword">synchronized</span> <span class="token punctuation">(</span>f<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">tabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">)</span> <span class="token operator">==</span> f<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                    <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> ln<span class="token punctuation">,</span> hn<span class="token punctuation">;</span>
                    <span class="token keyword">if</span> <span class="token punctuation">(</span>fh <span class="token operator">&gt;=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
						<span class="token comment">//4.3 处理当前节点为链表的头结点的情况，构造两个链表，一个是原链表  另一个是原链表的反序排列</span>
                        <span class="token keyword">int</span> runBit <span class="token operator">=</span> fh <span class="token operator">&amp;</span> n<span class="token punctuation">;</span>
                        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> lastRun <span class="token operator">=</span> f<span class="token punctuation">;</span>
                        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> p <span class="token operator">=</span> f<span class="token punctuation">.</span>next<span class="token punctuation">;</span> p <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> p <span class="token operator">=</span> p<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                            <span class="token keyword">int</span> b <span class="token operator">=</span> p<span class="token punctuation">.</span>hash <span class="token operator">&amp;</span> n<span class="token punctuation">;</span>
                            <span class="token keyword">if</span> <span class="token punctuation">(</span>b <span class="token operator">!=</span> runBit<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                                runBit <span class="token operator">=</span> b<span class="token punctuation">;</span>
                                lastRun <span class="token operator">=</span> p<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>runBit <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                            ln <span class="token operator">=</span> lastRun<span class="token punctuation">;</span>
                            hn <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
                        <span class="token punctuation">}</span>
                        <span class="token keyword">else</span> <span class="token punctuation">{</span>
                            hn <span class="token operator">=</span> lastRun<span class="token punctuation">;</span>
                            ln <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
                        <span class="token punctuation">}</span>
                        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> p <span class="token operator">=</span> f<span class="token punctuation">;</span> p <span class="token operator">!=</span> lastRun<span class="token punctuation">;</span> p <span class="token operator">=</span> p<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                            <span class="token keyword">int</span> ph <span class="token operator">=</span> p<span class="token punctuation">.</span>hash<span class="token punctuation">;</span> <span class="token class-name">K</span> pk <span class="token operator">=</span> p<span class="token punctuation">.</span>key<span class="token punctuation">;</span> <span class="token class-name">V</span> pv <span class="token operator">=</span> p<span class="token punctuation">.</span>val<span class="token punctuation">;</span>
                            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>ph <span class="token operator">&amp;</span> n<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span>
                                ln <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>ph<span class="token punctuation">,</span> pk<span class="token punctuation">,</span> pv<span class="token punctuation">,</span> ln<span class="token punctuation">)</span><span class="token punctuation">;</span>
                            <span class="token keyword">else</span>
                                hn <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>ph<span class="token punctuation">,</span> pk<span class="token punctuation">,</span> pv<span class="token punctuation">,</span> hn<span class="token punctuation">)</span><span class="token punctuation">;</span>
                        <span class="token punctuation">}</span>
                       <span class="token comment">//在nextTable的i位置上插入一个链表</span>
                         <span class="token function">setTabAt</span><span class="token punctuation">(</span>nextTab<span class="token punctuation">,</span> i<span class="token punctuation">,</span> ln<span class="token punctuation">)</span><span class="token punctuation">;</span>
                         <span class="token comment">//在nextTable的i+n的位置上插入另一个链表</span>
                         <span class="token function">setTabAt</span><span class="token punctuation">(</span>nextTab<span class="token punctuation">,</span> i <span class="token operator">+</span> n<span class="token punctuation">,</span> hn<span class="token punctuation">)</span><span class="token punctuation">;</span>
                         <span class="token comment">//在table的i位置上插入forwardNode节点  表示已经处理过该节点</span>
                         <span class="token function">setTabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">,</span> fwd<span class="token punctuation">)</span><span class="token punctuation">;</span>
                         <span class="token comment">//设置advance为true 返回到上面的while循环中 就可以执行i--操作</span>
                         advance <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
                    <span class="token punctuation">}</span>
					<span class="token comment">//4.4 处理当前节点是TreeBin时的情况，操作和上面的类似</span>
                    <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>f <span class="token keyword">instanceof</span> <span class="token class-name">TreeBin</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                        <span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> t <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">)</span>f<span class="token punctuation">;</span>
                        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> lo <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">,</span> loTail <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
                        <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> hi <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">,</span> hiTail <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
                        <span class="token keyword">int</span> lc <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">,</span> hc <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
                        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> e <span class="token operator">=</span> t<span class="token punctuation">.</span>first<span class="token punctuation">;</span> e <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">;</span> e <span class="token operator">=</span> e<span class="token punctuation">.</span>next<span class="token punctuation">)</span> <span class="token punctuation">{</span>
                            <span class="token keyword">int</span> h <span class="token operator">=</span> e<span class="token punctuation">.</span>hash<span class="token punctuation">;</span>
                            <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span> p <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">TreeNode</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span>
                                <span class="token punctuation">(</span>h<span class="token punctuation">,</span> e<span class="token punctuation">.</span>key<span class="token punctuation">,</span> e<span class="token punctuation">.</span>val<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
                            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>h <span class="token operator">&amp;</span> n<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>p<span class="token punctuation">.</span>prev <span class="token operator">=</span> loTail<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                                    lo <span class="token operator">=</span> p<span class="token punctuation">;</span>
                                <span class="token keyword">else</span>
                                    loTail<span class="token punctuation">.</span>next <span class="token operator">=</span> p<span class="token punctuation">;</span>
                                loTail <span class="token operator">=</span> p<span class="token punctuation">;</span>
                                <span class="token operator">++</span>lc<span class="token punctuation">;</span>
                            <span class="token punctuation">}</span>
                            <span class="token keyword">else</span> <span class="token punctuation">{</span>
                                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>p<span class="token punctuation">.</span>prev <span class="token operator">=</span> hiTail<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                                    hi <span class="token operator">=</span> p<span class="token punctuation">;</span>
                                <span class="token keyword">else</span>
                                    hiTail<span class="token punctuation">.</span>next <span class="token operator">=</span> p<span class="token punctuation">;</span>
                                hiTail <span class="token operator">=</span> p<span class="token punctuation">;</span>
                                <span class="token operator">++</span>hc<span class="token punctuation">;</span>
                            <span class="token punctuation">}</span>
                        <span class="token punctuation">}</span>
                        ln <span class="token operator">=</span> <span class="token punctuation">(</span>lc <span class="token operator">&lt;=</span> UNTREEIFY_THRESHOLD<span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token function">untreeify</span><span class="token punctuation">(</span>lo<span class="token punctuation">)</span> <span class="token operator">:</span>
                            <span class="token punctuation">(</span>hc <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token keyword">new</span> <span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>lo<span class="token punctuation">)</span> <span class="token operator">:</span> t<span class="token punctuation">;</span>
                        hn <span class="token operator">=</span> <span class="token punctuation">(</span>hc <span class="token operator">&lt;=</span> UNTREEIFY_THRESHOLD<span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token function">untreeify</span><span class="token punctuation">(</span>hi<span class="token punctuation">)</span> <span class="token operator">:</span>
                            <span class="token punctuation">(</span>lc <span class="token operator">!=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token keyword">new</span> <span class="token class-name">TreeBin</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span>hi<span class="token punctuation">)</span> <span class="token operator">:</span> t<span class="token punctuation">;</span>
                        <span class="token function">setTabAt</span><span class="token punctuation">(</span>nextTab<span class="token punctuation">,</span> i<span class="token punctuation">,</span> ln<span class="token punctuation">)</span><span class="token punctuation">;</span>
                        <span class="token function">setTabAt</span><span class="token punctuation">(</span>nextTab<span class="token punctuation">,</span> i <span class="token operator">+</span> n<span class="token punctuation">,</span> hn<span class="token punctuation">)</span><span class="token punctuation">;</span>
                        <span class="token function">setTabAt</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> i<span class="token punctuation">,</span> fwd<span class="token punctuation">)</span><span class="token punctuation">;</span>
                        advance <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
                    <span class="token punctuation">}</span>
                <span class="token punctuation">}</span>
            <span class="token punctuation">}</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 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 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 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 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 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 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 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 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>代码逻辑请看注释,整个扩容操作分为<strong>两个部分</strong>：</p><p><strong>第一部分</strong>是构建一个nextTable,它的容量是原来的两倍，这个操作是单线程完成的。新建table数组的代码为:<code>Node&lt;K,V&gt;[] nt = (Node&lt;K,V&gt;[])new Node&lt;?,?&gt;[n &lt;&lt; 1]</code>,在原容量大小的基础上右移一位。</p><p><strong>第二个部分</strong>就是将原来table中的元素复制到nextTable中，主要是遍历复制的过程。 根据运算得到当前遍历的数组的位置i，然后利用tabAt方法获得i位置的元素再进行判断：</p><ol><li>如果这个位置为空，就在原table中的i位置放入forwardNode节点，这个也是触发并发扩容的关键点；</li><li>如果这个位置是Node节点（fh&gt;=0），如果它是一个链表的头节点，就构造一个反序链表，把他们分别放在nextTable的i和i+n的位置上</li><li>如果这个位置是TreeBin节点（fh&lt;0），也做一个反序处理，并且判断是否需要untreefi，把处理的结果分别放在nextTable的i和i+n的位置上</li><li>遍历过所有的节点以后就完成了复制工作，这时让nextTable作为新的table，并且更新sizeCtl为新容量的0.75倍 ，完成扩容。设置为新容量的0.75倍代码为 <code>sizeCtl = (n &lt;&lt; 1) - (n &gt;&gt;&gt; 1)</code>，仔细体会下是不是很巧妙，n&lt;&lt;1相当于n右移一位表示n的两倍即2n,n&gt;&gt;&gt;1左右一位相当于n除以2即0.5n,然后两者相减为2n-0.5n=1.5n,是不是刚好等于新容量的0.75倍即2n*0.75=1.5n。最后用一个示意图来进行总结（图片摘自网络）：</li></ol><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/thread/ConcurrentHashMap-02.png" alt="ConcurrentHashMap扩容示意图"></p><h3 id="与size相关的一些方法" tabindex="-1"><a class="header-anchor" href="#与size相关的一些方法" aria-hidden="true">#</a> 与size相关的一些方法</h3><p>对于ConcurrentHashMap来说，这个table里到底装了多少东西其实是个不确定的数量，因为<strong>不可能在调用size()方法的时候像GC的“stop the world”一样让其他线程都停下来让你去统计，因此只能说这个数量是个估计值。对于这个估计值</strong>，ConcurrentHashMap也是大费周章才计算出来的。</p><p>为了统计元素个数，ConcurrentHashMap定义了一些变量和一个内部类</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token doc-comment comment">/**
 * A padded cell for distributing counts.  Adapted from LongAdder
 * and Striped64.  See their internal docs for explanation.
 */</span>
<span class="token annotation punctuation">@sun.misc.Contended</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">CounterCell</span> <span class="token punctuation">{</span>
    <span class="token keyword">volatile</span> <span class="token keyword">long</span> value<span class="token punctuation">;</span>
    <span class="token class-name">CounterCell</span><span class="token punctuation">(</span><span class="token keyword">long</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span> value <span class="token operator">=</span> x<span class="token punctuation">;</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>

<span class="token doc-comment comment">/******************************************/</span> 

<span class="token doc-comment comment">/**
 * 实际上保存的是hashmap中的元素个数  利用CAS锁进行更新
 但它并不用返回当前hashmap的元素个数 

 */</span>
<span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token keyword">volatile</span> <span class="token keyword">long</span> baseCount<span class="token punctuation">;</span>
<span class="token doc-comment comment">/**
 * Spinlock (locked via CAS) used when resizing and/or creating CounterCells.
 */</span>
<span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token keyword">volatile</span> <span class="token keyword">int</span> cellsBusy<span class="token punctuation">;</span>

<span class="token doc-comment comment">/**
 * Table of counter cells. When non-null, size is a power of 2.
 */</span>
<span class="token keyword">private</span> <span class="token keyword">transient</span> <span class="token keyword">volatile</span> <span class="token class-name">CounterCell</span><span class="token punctuation">[</span><span class="token punctuation">]</span> counterCells<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 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><h4 id="mappingcount与size方法" tabindex="-1"><a class="header-anchor" href="#mappingcount与size方法" aria-hidden="true">#</a> <strong>mappingCount与size方法</strong></h4><p><strong>mappingCount</strong>与<strong>size</strong>方法的类似 从给出的注释来看，应该使用mappingCount代替size方法 两个方法都没有直接返回basecount 而是统计一次这个值，而这个值其实也是一个大概的数值，因此可能在统计的时候有其他线程正在执行插入或删除操作。</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">int</span> <span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">long</span> n <span class="token operator">=</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>n <span class="token operator">&lt;</span> <span class="token number">0L</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">0</span> <span class="token operator">:</span>
            <span class="token punctuation">(</span>n <span class="token operator">&gt;</span> <span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span><span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE<span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token class-name">Integer</span><span class="token punctuation">.</span>MAX_VALUE <span class="token operator">:</span>
            <span class="token punctuation">(</span><span class="token keyword">int</span><span class="token punctuation">)</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
 <span class="token doc-comment comment">/**
 * Returns the number of mappings. This method should be used
 * instead of <span class="token punctuation">{</span><span class="token keyword">@link</span> <span class="token reference"><span class="token punctuation">#</span><span class="token field">size</span></span><span class="token punctuation">}</span> because a ConcurrentHashMap may
 * contain more mappings than can be represented as an int. The
 * value returned is an estimate; the actual count may differ if
 * there are concurrent insertions or removals.
 *
 * <span class="token keyword">@return</span> the number of mappings
 * <span class="token keyword">@since</span> 1.8
 */</span>
<span class="token keyword">public</span> <span class="token keyword">long</span> <span class="token function">mappingCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">long</span> n <span class="token operator">=</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token keyword">return</span> <span class="token punctuation">(</span>n <span class="token operator">&lt;</span> <span class="token number">0L</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token number">0L</span> <span class="token operator">:</span> n<span class="token punctuation">;</span> <span class="token comment">// ignore transient negative values</span>
<span class="token punctuation">}</span>

 <span class="token keyword">final</span> <span class="token keyword">long</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">CounterCell</span><span class="token punctuation">[</span><span class="token punctuation">]</span> as <span class="token operator">=</span> counterCells<span class="token punctuation">;</span> <span class="token class-name">CounterCell</span> a<span class="token punctuation">;</span>
    <span class="token keyword">long</span> sum <span class="token operator">=</span> baseCount<span class="token punctuation">;</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>as <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> as<span class="token punctuation">.</span>length<span class="token punctuation">;</span> <span class="token operator">++</span>i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>a <span class="token operator">=</span> as<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span>
                sum <span class="token operator">+=</span> a<span class="token punctuation">.</span>value<span class="token punctuation">;</span><span class="token comment">//所有counter的值求和</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> sum<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 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><h4 id="addcount方法" tabindex="-1"><a class="header-anchor" href="#addcount方法" aria-hidden="true">#</a> <strong>addCount方法</strong></h4><p>在put方法结尾处调用了addCount方法，把当前ConcurrentHashMap的元素个数+1这个方法一共做了两件事,更新baseCount的值，检测是否进行扩容。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">addCount</span><span class="token punctuation">(</span><span class="token keyword">long</span> x<span class="token punctuation">,</span> <span class="token keyword">int</span> check<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">CounterCell</span><span class="token punctuation">[</span><span class="token punctuation">]</span> as<span class="token punctuation">;</span> <span class="token keyword">long</span> b<span class="token punctuation">,</span> s<span class="token punctuation">;</span>
    <span class="token comment">//利用CAS方法更新baseCount的值 </span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>as <span class="token operator">=</span> counterCells<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">||</span>
        <span class="token operator">!</span><span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapLong</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> BASECOUNT<span class="token punctuation">,</span> b <span class="token operator">=</span> baseCount<span class="token punctuation">,</span> s <span class="token operator">=</span> b <span class="token operator">+</span> x<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">CounterCell</span> a<span class="token punctuation">;</span> <span class="token keyword">long</span> v<span class="token punctuation">;</span> <span class="token keyword">int</span> m<span class="token punctuation">;</span>
        <span class="token keyword">boolean</span> uncontended <span class="token operator">=</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>as <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> <span class="token punctuation">(</span>m <span class="token operator">=</span> as<span class="token punctuation">.</span>length <span class="token operator">-</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token operator">&lt;</span> <span class="token number">0</span> <span class="token operator">||</span>
            <span class="token punctuation">(</span>a <span class="token operator">=</span> as<span class="token punctuation">[</span><span class="token class-name">ThreadLocalRandom</span><span class="token punctuation">.</span><span class="token function">getProbe</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;</span> m<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span>
            <span class="token operator">!</span><span class="token punctuation">(</span>uncontended <span class="token operator">=</span>
              <span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapLong</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span> CELLVALUE<span class="token punctuation">,</span> v <span class="token operator">=</span> a<span class="token punctuation">.</span>value<span class="token punctuation">,</span> v <span class="token operator">+</span> x<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token function">fullAddCount</span><span class="token punctuation">(</span>x<span class="token punctuation">,</span> uncontended<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token keyword">return</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>check <span class="token operator">&lt;=</span> <span class="token number">1</span><span class="token punctuation">)</span>
            <span class="token keyword">return</span><span class="token punctuation">;</span>
        s <span class="token operator">=</span> <span class="token function">sumCount</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">//如果check值大于等于0 则需要检验是否需要进行扩容操作</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>check <span class="token operator">&gt;=</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">Node</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> tab<span class="token punctuation">,</span> nt<span class="token punctuation">;</span> <span class="token keyword">int</span> n<span class="token punctuation">,</span> sc<span class="token punctuation">;</span>
        <span class="token keyword">while</span> <span class="token punctuation">(</span>s <span class="token operator">&gt;=</span> <span class="token punctuation">(</span><span class="token keyword">long</span><span class="token punctuation">)</span><span class="token punctuation">(</span>sc <span class="token operator">=</span> sizeCtl<span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span>tab <span class="token operator">=</span> table<span class="token punctuation">)</span> <span class="token operator">!=</span> <span class="token keyword">null</span> <span class="token operator">&amp;&amp;</span>
               <span class="token punctuation">(</span>n <span class="token operator">=</span> tab<span class="token punctuation">.</span>length<span class="token punctuation">)</span> <span class="token operator">&lt;</span> MAXIMUM_CAPACITY<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">int</span> rs <span class="token operator">=</span> <span class="token function">resizeStamp</span><span class="token punctuation">(</span>n<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token comment">//</span>
            <span class="token keyword">if</span> <span class="token punctuation">(</span>sc <span class="token operator">&lt;</span> <span class="token number">0</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token punctuation">(</span>sc <span class="token operator">&gt;&gt;&gt;</span> RESIZE_STAMP_SHIFT<span class="token punctuation">)</span> <span class="token operator">!=</span> rs <span class="token operator">||</span> sc <span class="token operator">==</span> rs <span class="token operator">+</span> <span class="token number">1</span> <span class="token operator">||</span>
                    sc <span class="token operator">==</span> rs <span class="token operator">+</span> MAX_RESIZERS <span class="token operator">||</span> <span class="token punctuation">(</span>nt <span class="token operator">=</span> nextTable<span class="token punctuation">)</span> <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span>
                    transferIndex <span class="token operator">&lt;=</span> <span class="token number">0</span><span class="token punctuation">)</span>
                    <span class="token keyword">break</span><span class="token punctuation">;</span>
                 <span class="token comment">//如果已经有其他线程在执行扩容操作</span>
                <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapInt</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> SIZECTL<span class="token punctuation">,</span> sc<span class="token punctuation">,</span> sc <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                    <span class="token function">transfer</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> nt<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token punctuation">}</span>
            <span class="token comment">//当前线程是唯一的或是第一个发起扩容的线程  此时nextTable=null</span>
            <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token class-name">U</span><span class="token punctuation">.</span><span class="token function">compareAndSwapInt</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">,</span> SIZECTL<span class="token punctuation">,</span> sc<span class="token punctuation">,</span>
                                         <span class="token punctuation">(</span>rs <span class="token operator">&lt;&lt;</span> RESIZE_STAMP_SHIFT<span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
                <span class="token function">transfer</span><span class="token punctuation">(</span>tab<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            s <span class="token operator">=</span> <span class="token function">sumCount</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
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</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 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 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="总结" tabindex="-1"><a class="header-anchor" href="#总结" aria-hidden="true">#</a> 总结</h2><p>JDK6,7中的ConcurrentHashmap主要使用Segment来实现减小锁粒度，分割成若干个Segment，在put的时候需要锁住Segment，get时候不加锁，使用volatile来保证可见性，当要统计全局时（比如size），首先会尝试多次计算modcount来确定，这几次尝试中，是否有其他线程进行了修改操作，如果没有，则直接返回size。如果有，则需要依次锁住所有的Segment来计算。</p><p>1.8之前put定位节点时要先定位到具体的segment，然后再在segment中定位到具体的桶。而在1.8的时候摒弃了segment臃肿的设计，直接针对的是Node[] tale数组中的每一个桶，进一步减小了锁粒度。并且防止拉链过长导致性能下降，当链表长度大于8的时候采用红黑树的设计。</p><p>主要设计上的变化有以下几点:</p><ol><li>不采用segment而采用node，锁住node来实现减小锁粒度。</li><li>设计了MOVED状态 当resize的中过程中 线程2还在put数据，线程2会帮助resize。</li><li>使用3个CAS操作来确保node的一些操作的原子性，这种方式代替了锁。</li><li>sizeCtl的不同值来代表不同含义，起到了控制的作用。</li><li>采用synchronized而不是ReentrantLock</li></ol><p>更多关于1.7版本与1.8版本的ConcurrentHashMap的实现对比，可以参考<a href="http://www.jianshu.com/p/e694f1e868ec" target="_blank" rel="noopener noreferrer">这篇文章<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>。</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" 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" 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></ul></blockquote><img 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