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class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.11 并发编程</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">2.12 JVM</span><span class="arrow right"></span></button></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">四、数据库</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">五、计算机基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">六、求职面试</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">七、学习资源</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">八、学习建议</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">九、知识库搭建</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">十、联系作者</span><span class="arrow right"></span></button></section></li></ul></aside><main class="page" id="main-content"><nav class="breadcrumb disable"></nav><div class="page-title"><h1>深入理解Java中的hashCode方法</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon calendar-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="calendar icon"><path d="M716.4 110.137c0-18.753-14.72-33.473-33.472-33.473-18.753 0-33.473 14.72-33.473 33.473v33.473h66.993v-33.473zm-334.87 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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 9 分钟</span><meta property="timeRequired" content="PT9M"></span></div><hr></div><div class="theme-hope-content"><h1 id="深入理解java中的hashcode方法" tabindex="-1"><a class="header-anchor" href="#深入理解java中的hashcode方法" aria-hidden="true">#</a> 深入理解Java中的hashCode方法</h1><p>假期结束了，需要快速切换到工作的状态投入到新的一天当中。放假的时候痛快地玩耍，上班的时候积极的工作，这应该是我们大多数“现代人”该有的生活状态。</p><p>我之所以费尽心思铺垫了前面这段话，就是想告诉大家，技术文虽迟但到，来吧，学起来~</p><p>今天我们来谈谈 Java 中的 <code>hashCode()</code> 方法。众所周知，Java 是一门面向对象的编程语言，所有的类都会默认继承自 Object 类，而 Object 的中文意思就是“对象”。</p><p>Object 类中就包含了 <code>hashCode()</code> 方法：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token annotation punctuation">@HotSpotIntrinsicCandidate</span>

<span class="token keyword">public</span> <span class="token keyword">native</span> <span class="token keyword">int</span> <span class="token function">hashCode</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></div><p>意味着所有的类都会有一个 <code>hashCode()</code> 方法，该方法会返回一个 int 类型的值。由于 <code>hashCode()</code> 方法是一个本地方法（<code>native</code> 关键字修饰的方法，用 <code>C/C++</code> 语言实现，由 Java 调用），意味着 Object 类中并没有给出具体的实现。</p><p>具体的实现可以参考 <code>jdk/src/hotspot/share/runtime/synchronizer.cpp</code>（源码可以到 GitHub 上 OpenJDK 的仓库中下载）。<code>get_next_hash()</code> 方法会根据 hashCode 的取值来决定采用哪一种哈希值的生成策略。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/basic-extra-meal/hashcode-1.png" alt=""></p><p>并且 <code>hashCode()</code> 方法被 <code>@HotSpotIntrinsicCandidate</code> 注解修饰，说明它在 HotSpot 虚拟机中有一套高效的实现，基于 CPU 指令。</p><p>那大家有没有想过这样一个问题：为什么 Object 类需要一个 <code>hashCode()</code> 方法呢？</p><p>在 Java 中，<code>hashCode()</code> 方法的主要作用就是为了配合哈希表使用的。</p><p>哈希表（Hash Table），也叫散列表，是一种可以通过关键码值（key-value）直接访问的数据结构，它最大的特点就是可以快速实现查找、插入和删除。其中用到的算法叫做哈希，就是把任意长度的输入，变换成固定长度的输出，该输出就是哈希值。像 MD5、SHA1 都用的是哈希算法。</p><p>像 Java 中的 HashSet、Hashtable（注意是小写的 t）、HashMap 都是基于哈希表的具体实现。其中的 HashMap 就是最典型的代表，不仅面试官经常问，工作中的使用频率也非常的高。</p><p>大家想一下，如果没有哈希表，但又需要这样一个数据结构，它里面存放的数据是不允许重复的，该怎么办呢？</p><p>要不使用 <code>equals()</code> 方法进行逐个比较？这种方案当然是可行的。但如果数据量特别特别大，采用 <code>equals()</code> 方法进行逐个对比的效率肯定很低很低，最好的解决方案就是哈希表。</p><p>拿 HashMap 来说吧。当我们要在它里面添加对象时，先调用这个对象的 <code>hashCode()</code> 方法，得到对应的哈希值，然后将哈希值和对象一起放到 HashMap 中。当我们要再添加一个新的对象时：</p><ul><li>获取对象的哈希值；</li><li>和之前已经存在的哈希值进行比较，如果不相等，直接存进去；</li><li>如果有相等的，再调用 <code>equals()</code> 方法进行对象之间的比较，如果相等，不存了；</li><li>如果不等，说明哈希冲突了，增加一个链表，存放新的对象；</li><li>如果链表的长度大于 8，转为红黑树来处理。</li></ul><p>就这么一套下来，调用 <code>equals()</code> 方法的频率就大大降低了。也就是说，只要哈希算法足够的高效，把发生哈希冲突的频率降到最低，哈希表的效率就特别的高。</p><p>来看一下 HashMap 的哈希算法：</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">hash</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 keyword">return</span> <span class="token punctuation">(</span>key <span class="token operator">==</span> <span class="token keyword">null</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>h <span class="token operator">=</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 operator">^</span> <span class="token punctuation">(</span>h <span class="token operator">>>></span> <span class="token number">16</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>先调用对象的 <code>hashCode()</code> 方法，然后对该值进行右移运算，然后再进行异或运算。</p><p>通常来说，String 会用来作为 HashMap 的键进行哈希运算，因此我们再来看一下 String 的 <code>hashCode()</code> 方法：</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">hashCode</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>h <span class="token operator">==</span> <span class="token number">0</span> <span class="token operator">&&</span> value<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>

hash <span class="token operator">=</span> h <span class="token operator">=</span> <span class="token function">isLatin1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">?</span> <span class="token class-name">StringLatin1</span><span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span>

<span class="token operator">:</span> <span class="token class-name">StringUTF16</span><span class="token punctuation">.</span><span class="token function">hashCode</span><span class="token punctuation">(</span>value<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> h<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> value<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token keyword">int</span> length <span class="token operator">=</span> value<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 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"><</span> length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

h <span class="token operator">=</span> <span class="token number">31</span> <span class="token operator">*</span> h <span class="token operator">+</span> <span class="token function">getChar</span><span class="token punctuation">(</span>value<span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> h<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token class-name">Student</span> s1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token number">18</span><span class="token punctuation">,</span> <span class="token string">"张三"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">Map</span><span class="token generics"><span class="token punctuation"><</span><span class="token class-name">Student</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span></span> scores <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation"><</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

scores<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>s1<span class="token punctuation">,</span> <span class="token number">98</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>scores<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token number">18</span><span class="token punctuation">,</span> <span class="token string">"张三"</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">class</span> <span class="token class-name">Student</span> <span class="token punctuation">{</span>

<span class="token keyword">private</span> <span class="token keyword">int</span> age<span class="token punctuation">;</span>

<span class="token keyword">private</span> <span class="token class-name">String</span> name<span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token keyword">int</span> age<span class="token punctuation">,</span> <span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token annotation punctuation">@Override</span>

<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">equals</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token class-name">Student</span> student <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">Student</span><span class="token punctuation">)</span> o<span class="token punctuation">;</span>

<span class="token keyword">return</span> age <span class="token operator">==</span> student<span class="token punctuation">.</span>age <span class="token operator">&&</span>

<span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span>name<span class="token punctuation">,</span> student<span class="token punctuation">.</span>name<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></div><p>很不巧，结果为 null，而不是预期当中的 98。这是为什么呢？</p><p>原因就在于重写 <code>equals()</code> 方法的时候没有重写 <code>hashCode()</code> 方法。默认情况下，<code>hashCode()</code> 方法是一个本地方法，会返回对象的存储地址，显然 <code>put()</code> 中的 s1 和 <code>get()</code> 中的 <code>new Student(18, "张三")</code> 是两个对象，它们的存储地址肯定是不同的。</p><p>HashMap 的 <code>get()</code> 方法会调用 <code>hash(key.hashCode())</code> 计算对象的哈希值，虽然两个不同的 <code>hashCode()</code> 结果经过 <code>hash()</code> 方法计算后有可能得到相同的结果，但这种概率微乎其微，所以就导致 <code>scores.get(new Student(18, "张三"))</code> 无法得到预期的值 18。</p><p>怎么解决这个问题呢？很简单，重写 <code>hashCode()</code> 方法。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code> <span class="token annotation punctuation">@Override</span>

<span class="token keyword">public</span> <span class="token keyword">int</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 keyword">return</span> <span class="token class-name">Objects</span><span class="token punctuation">.</span><span class="token function">hash</span><span class="token punctuation">(</span>age<span class="token punctuation">,</span> name<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>Objects 类的 <code>hash()</code> 方法可以针对不同数量的参数生成新的 <code>hashCode()</code> 值。</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">static</span> <span class="token keyword">int</span> <span class="token function">hashCode</span><span class="token punctuation">(</span><span class="token class-name">Object</span> a<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> <span class="token number">0</span><span class="token punctuation">;</span>

<span class="token keyword">int</span> result <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">Object</span> element <span class="token operator">:</span> a<span class="token punctuation">)</span>

result <span class="token operator">=</span> <span class="token number">31</span> <span class="token operator">*</span> result <span class="token operator">+</span> <span class="token punctuation">(</span>element <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">?</span> <span class="token number">0</span> <span class="token operator">:</span> element<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">return</span> result<span class="token punctuation">;</span>

intptr_t value <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <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">// This form uses global Park-Miller RNG.</span>

<span class="token comment">// On MP system we'll have lots of RW access to a global, so the</span>

<span class="token comment">// mechanism induces lots of coherency traffic.</span>

value <span class="token operator">=</span> os<span class="token operator">::</span><span class="token function">random</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">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">1</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

<span class="token comment">// This variation has the property of being stable (idempotent)</span>

<span class="token comment">// between STW operations. This can be useful in some of the 1-0</span>

<span class="token comment">// synchronization schemes.</span>

intptr_t addr_bits <span class="token operator">=</span> cast_from_oop<span class="token generics"><span class="token punctuation"><</span>intptr_t<span class="token punctuation">></span></span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span> <span class="token operator">>></span> <span class="token number">3</span><span class="token punctuation">;</span>

value <span class="token operator">=</span> addr_bits <span class="token operator">^</span> <span class="token punctuation">(</span>addr_bits <span class="token operator">>></span> <span class="token number">5</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token class-name">GVars</span><span class="token punctuation">.</span>stw_random<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>hashCode <span class="token operator">==</span> <span class="token number">2</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

value <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">;</span> <span class="token comment">// for sensitivity testing</span>

<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token keyword">if</span> <span class="token punctuation">(</span>hashCode <span class="token operator">==</span> <span class="token number">3</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

value <span class="token operator">=</span> <span class="token operator">++</span><span class="token class-name">GVars</span><span class="token punctuation">.</span>hc_sequence<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>hashCode <span class="token operator">==</span> <span class="token number">4</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

value <span class="token operator">=</span> cast_from_oop<span class="token generics"><span class="token punctuation"><</span>intptr_t<span class="token punctuation">></span></span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">// Marsaglia's xor-shift scheme with thread-specific state</span>

<span class="token comment">// This is probably the best overall implementation -- we'll</span>

<span class="token comment">// likely make this the default in future releases.</span>

unsigned t <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateX<span class="token punctuation">;</span>

t <span class="token operator">^=</span> <span class="token punctuation">(</span>t <span class="token operator"><<</span> <span class="token number">11</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

current<span class="token operator">-></span>_hashStateX <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateY<span class="token punctuation">;</span>

current<span class="token operator">-></span>_hashStateY <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateZ<span class="token punctuation">;</span>

current<span class="token operator">-></span>_hashStateZ <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateW<span class="token punctuation">;</span>

unsigned v <span class="token operator">=</span> current<span class="token operator">-></span>_hashStateW<span class="token punctuation">;</span>

v <span class="token operator">=</span> <span class="token punctuation">(</span>v <span class="token operator">^</span> <span class="token punctuation">(</span>v <span class="token operator">>></span> <span class="token number">19</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">^</span> <span class="token punctuation">(</span>t <span class="token operator">^</span> <span class="token punctuation">(</span>t <span class="token operator">>></span> <span class="token number">8</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

current<span class="token operator">-></span>_hashStateW <span class="token operator">=</span> v<span class="token punctuation">;</span>

value <span class="token operator">=</span> v<span class="token punctuation">;</span>

value <span class="token operator">&=</span> markWord<span class="token operator">::</span><span class="token function">hash_mask</span><span class="token punctuation">;</span>

<span class="token keyword">if</span> <span class="token punctuation">(</span>value <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span> value <span class="token operator">=</span> <span class="token number">0xBAD</span><span class="token punctuation">;</span>

<span class="token keyword">assert</span><span class="token punctuation">(</span>value <span class="token operator">!=</span> markWord<span class="token operator">::</span><span class="token function">no_hash</span><span class="token punctuation">,</span> <span class="token string">"invariant"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">return</span> value<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></div><p>如果没有 C++ 基础的话，不用细致去看每一行代码，我们只通过表面去了解一下 <code>get_next_hash()</code> 这个方法就行。其中的 <code>hashCode</code> 变量是 JVM 启动时的一个全局参数，可以通过它来切换哈希值的生成策略。</p><ul><li><code>hashCode==0</code>，调用操作系统 OS 的 <code>random()</code> 方法返回随机数。</li><li><code>hashCode == 1</code>，在 STW（stop-the-world）操作中，这种策略通常用于同步方案中。利用对象地址进行计算，使用不经常更新的随机数（<code>GVars.stw_random</code>）参与其中。</li><li><code>hashCode == 2</code>，使用返回 1，用于某些情况下的测试。</li><li><code>hashCode == 3</code>，从 0 开始计算哈希值，不是线程安全的，多个线程可能会得到相同的哈希值。</li><li><code>hashCode == 4</code>，与创建对象的内存位置有关，原样输出。</li><li><code>hashCode == 5</code>，默认值，支持多线程，使用了 Marsaglia 的 xor-shift 算法产生伪随机数。所谓的 xor-shift 算法，简单来说，看起来就是一个移位寄存器，每次移入的位由寄存器中若干位取异或生成。所谓的伪随机数，不是完全随机的，但是真随机生成比较困难，所以只要能通过一定的随机数统计检测，就可以当作真随机数来使用。</li></ul><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/xingbiaogongzhonghao.png" alt=""></p></div><footer class="page-meta"><div class="meta-item edit-link"><a href="https://github.com/itwanger/toBeBetterJavaer/edit/master/docs/basic-extra-meal/hashcode.md" rel="noopener noreferrer" target="_blank" aria-label="在 GitHub 上编辑此页" class="nav-link label"><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="edit icon"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path d="M728.802 962H252.891A190.883 190.883 0 0 1 62.008 771.98V296.934a190.883 190.883 0 0 1 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