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zhuhjay 22e48d9558 build(www): 添加 Drone CI 流水线配置 
- 新增 .drone.yml 文件用于定义 CI/CD 流程
- 配置了基于 Docker 的部署步骤
- 设置了工作区和卷映射以支持持久化数据
- 添加了构建准备阶段和 Docker 部署阶段
- 定义了环境变量和代理设置
- 配置了 artifacts 目录的处理逻辑
- 添加了 timezone 映射以确保时间同步
- 设置了 docker.sock 映射以支持 Docker in Docker
2025-11-01 13:36:00 +08:00

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<span><svg 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" class="icon outbound"><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="sr-only">(opens new window)</span></span></a></div> <!----></nav> <ul class="sidebar-links"><li><a href="/blogs/java/jvm/JVM-内存结构.html" class="active sidebar-link">JVM之内存结构</a></li><li><a href="/blogs/java/jvm/JVM-垃圾回收.html" class="sidebar-link">JVM之垃圾回收</a></li><li><a href="/blogs/java/jvm/JVM-字节码技术.html" class="sidebar-link">JVM之字节码技术</a></li><li><a href="/blogs/java/jvm/JVM-类加载.html" class="sidebar-link">JVM之类加载</a></li></ul> </aside> <div class="password-shadow password-wrapper-in" style="display:none;" data-v-59e6cb88 data-v-7dd95ae2><h3 class="title" data-v-59e6cb88>JVM之内存结构</h3> <!----> <label id="box" class="inputBox" data-v-59e6cb88><input type="password" value="" data-v-59e6cb88> <span data-v-59e6cb88>Konck! Knock!</span> <button data-v-59e6cb88>OK</button></label> <div class="footer" data-v-59e6cb88><span data-v-59e6cb88><i class="iconfont reco-theme" data-v-59e6cb88></i> <a target="blank" href="https://vuepress-theme-reco.recoluan.com" data-v-59e6cb88>vuePress-theme-reco</a></span> <span data-v-59e6cb88><i class="iconfont reco-copyright" data-v-59e6cb88></i> <a data-v-59e6cb88><span data-v-59e6cb88>ZhuHJay</span>
  
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</a></span></div></div> <div data-v-7dd95ae2><div data-v-7dd95ae2><main class="page"><section style="display:;"><div class="page-title"><h1 class="title">JVM之内存结构</h1> <div data-v-8a445198><i class="iconfont reco-account" data-v-8a445198><span data-v-8a445198>ZhuHJay</span></i> <i class="iconfont reco-date" data-v-8a445198><span data-v-8a445198>2022/11/22</span></i> <!----> <i class="tags iconfont reco-tag" data-v-8a445198><span class="tag-item" data-v-8a445198>JVM</span></i></div></div> <div class="theme-reco-content content__default"><h2 id="_1-程序计数器"><a href="#_1-程序计数器" class="header-anchor">#</a> 1 程序计数器</h2> <p><img src="/jvm/1669015637679.png" alt="1669015637679"></p> <p>Program Counter Register 程序计数器（寄存器）</p> <ul><li>作用：是记住下一条jvm指令的执行地址</li> <li>特点
<ul><li>是线程私有的</li> <li>不会存在内存溢出</li></ul></li></ul> <h2 id="_2-虚拟机栈"><a href="#_2-虚拟机栈" class="header-anchor">#</a> 2 虚拟机栈</h2> <p><img src="/jvm/1669015814320.png" alt="1669015814320"></p> <h3 id="_2-1-定义"><a href="#_2-1-定义" class="header-anchor">#</a> 2.1 定义</h3> <p>Java Virtual Machine Stacks （Java 虚拟机栈）</p> <ul><li>每个线程运行时所需要的内存，称为虚拟机栈</li> <li>每个栈由多个栈帧（Frame）组成，对应着每次方法调用时所占用的内存</li> <li>每个线程只能有一个活动栈帧，对应着当前正在执行的那个方法</li></ul> <p>问题辨析</p> <ol><li>垃圾回收是否涉及栈内存？
<ul><li>每一个栈帧使用完后会自动释放该栈帧所对应的内存，垃圾回收只涉及到堆内存</li></ul></li> <li>栈内存分配越大越好吗？
<ul><li>每个线程都会对应一个栈内存，如果栈内存分配越大则所能使用的线程数则减小，也对程序的执行起不到加快的作用</li></ul></li> <li>方法内的局部变量是否线程安全？
<ul><li>如果方法内局部变量没有逃离方法的作用访问，它是线程安全的</li> <li>如果是局部变量引用了对象，并逃离方法的作用范围，需要考虑线程安全</li></ul></li></ol> <h3 id="_2-2-栈内存溢出"><a href="#_2-2-栈内存溢出" class="header-anchor">#</a> 2.2 栈内存溢出</h3> <ul><li><p>栈帧过多导致栈内存溢出</p> <ul><li>递归没有给到合适的出口</li> <li>第三方组件使用不当导致循环引用（ObjectMapper：对象 -&gt; Json）</li></ul></li> <li><p>栈帧过大导致栈内存溢出（局部变量过多导致，但较难触发）</p></li></ul> <blockquote><p>Tip：</p> <ul><li><p>可以使用 <code>-Xss</code> 来改变栈内存的大小</p></li> <li><p>栈内存溢出的错误信息为：<code>java.lang.StackOverflowError</code></p></li></ul></blockquote> <h3 id="_2-3-线程运行诊断"><a href="#_2-3-线程运行诊断" class="header-anchor">#</a> 2.3 线程运行诊断</h3> <p>eg1：CPU占用过多</p> <p>定位问题步骤（Linux）</p> <ul><li>用 <code>top</code> 命令定位哪个进程对CPU的占用过高</li> <li><code>ps H -eo pid,tid,%cpu | grep 进程id</code> （用<code>ps</code>命令进一步定义是哪个线程引起的cpu占用过高）</li> <li><code>jstack 进程id</code> （可以根据线程id找到有问题的线程，进一步定位到问题代码的源码行号）</li></ul> <p>eg2：程序运行很长时间没有结果（线程死锁）</p> <h2 id="_3-本地方法栈"><a href="#_3-本地方法栈" class="header-anchor">#</a> 3 本地方法栈</h2> <p><img src="/jvm/1669019510517.png" alt="1669019510517"></p> <p>本地方法栈的作用类似于JVM虚拟机栈，就是调用本地方法Native时会开辟一个本地方法的执行内存，也就是本地方法栈，其中有调用库函数的执行栈帧。</p> <h2 id="_4-堆"><a href="#_4-堆" class="header-anchor">#</a> 4 堆</h2> <p><img src="/jvm/1669021362263.png" alt="1669021362263"></p> <h3 id="_4-1-定义"><a href="#_4-1-定义" class="header-anchor">#</a> 4.1 定义</h3> <p>Heap 堆</p> <ul><li>通过 new 关键字，创建对象都会使用堆内存</li></ul> <p>特点</p> <ul><li>它是线程共享的，堆中对象都需要考虑线程安全的问题</li> <li>有垃圾回收机制</li></ul> <h3 id="_4-2-堆内存溢出"><a href="#_4-2-堆内存溢出" class="header-anchor">#</a> 4.2 堆内存溢出</h3> <p>当创建的对象足够多并且这些对象都正在被使用，不会被当成垃圾进行处理时，就会产生堆内存溢出的现象</p> <blockquote><p>Tip：</p> <ul><li>可以使用 <code>-Xmx</code> 来改变堆内存的最大大小</li> <li>堆内存溢出的错误信息为：<code>java.lang.OutOfMemoryError: Java heap space</code></li></ul></blockquote> <h3 id="_4-3-堆内存诊断"><a href="#_4-3-堆内存诊断" class="header-anchor">#</a> 4.3 堆内存诊断</h3> <p>有如下代码进行测试</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_4</span> <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 keyword">throws</span> <span class="token class-name">InterruptedException</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><span class="token string">&quot;1..&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">30000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 分配 10M内存</span>
<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">10</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><span class="token string">&quot;2..&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">30000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
bytes <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token comment">// 进行一次垃圾回收</span>
<span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">gc</span><span class="token punctuation">(</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><span class="token string">&quot;3..&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1000000L</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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><ol><li><p>jps 工具：查看当前系统中有哪些 Java 进程</p> <div class="language-shell line-numbers-mode"><pre class="language-shell"><code>PS JVM<span class="token operator">&gt;</span> jps
<span class="token number">12128</span> Demo1_4
<span class="token number">12116</span> RemoteMavenServer
<span class="token number">14968</span>
<span class="token number">7720</span> Jps
<span class="token number">7800</span> Launcher
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div></li> <li><p>jmap 工具：查看堆内存占用情况 <code>jmap -heap 进程id</code></p> <ul><li><p>在输出1时的堆内存情况</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Heap Usage: # 堆内存占用情况
PS Young Generation
Eden Space: # 新创建对象使用的分区
capacity = 34078720 (32.5MB) # 堆内存容量
used = 4771568 (4.5505218505859375MB) # 堆内存已使用情况
free = 29307152 (27.949478149414062MB) # 可分配
14.001605694110577<span class="token comment">% used</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li> <li><p>在输出2时的堆内存情况，因为创建了一个 10M 的字节数组，所以此时堆内存使用量比上次多了 10M 左右的使用</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Heap Usage: # 堆内存占用情况
PS Young Generation
Eden Space: # 新创建对象使用的分区
capacity = 34078720 (32.5MB) # 堆内存容量
used = 15257344 (14.550537109375MB) # 堆内存已使用情况
free = 18821376 (17.949462890625MB) # 可分配
44.77088341346154<span class="token comment">% used</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li> <li><p>在输出3时的堆内存情况，进行了垃圾回收，将分配的 10M 字节数组进行回收，使得堆内存使用量大大减少</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Heap Usage: # 堆内存占用情况
PS Young Generation
Eden Space: # 新创建对象使用的分区
capacity = 34078720 (32.5MB) # 堆内存容量
used = 681592 (0.6500167846679688MB) # 堆内存已使用情况
free = 33397128 (31.84998321533203MB) # 可分配
2.0000516451322117<span class="token comment">% used</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li></ul></li> <li><p>jconsole 工具：图形界面，多功能的监测工具，可以连续监测</p> <p>分为三个阶段的使用情况，阶段一为初始阶段。阶段二时分配了一段内存，阶段三进行了垃圾回收</p> <p><img src="/jvm/1669024208924.png" alt="1669024208924"></p></li> <li><p>jvisualvm 工具：与 jconsole 类似的图形化界面，但是该用具可以 dump堆内存快照，可以查看堆中存活的对象信息</p> <ul><li><p>案例/代码：垃圾回收后，内存占用仍然很高</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_6</span> <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 keyword">throws</span> <span class="token class-name">InterruptedException</span> <span class="token punctuation">{</span>
<span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Student</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">200</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> <span class="token number">200</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
list<span class="token punctuation">.</span><span class="token function">add</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 punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 为了调试而休眠</span>
<span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">sleep</span><span class="token punctuation">(</span><span class="token number">1000000000L</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">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> big <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div></li> <li><p>使用 <code>jps</code> 命令获取到该程序的进程id，使用 <code>jmap -heap 进程id</code> 查看堆内存使用</p> <p>可以发现堆内存总共占用了 (16+187)M，然后使用 <code>jconsole</code> 工具进行该进程的 gc 回收</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Heap Usage:
PS Young Generation
Eden Space:
capacity = 116916224 (111.5MB)
used = 17285928 (16.485145568847656MB)
free = 99630296 (95.01485443115234MB)
14.784883918249019<span class="token comment">% used</span>
PS Old Generation # 老年代堆内存使用情况
capacity = 334495744 (319.0MB)
used = 196736312 (187.62236785888672MB)
free = 137759432 (131.37763214111328MB)
58.815789297456654<span class="token comment">% used</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>可以发现，虽然进行了 gc 垃圾回收，但是堆内存占用还是居高不下，使用 jconsole 工具已经没办法进一步查询到具体问题了</p> <p><img src="/jvm/1669043972054.png" alt="1669043972054"></p></li> <li><p>使用 jvisualvm 进行堆内存 dump</p> <p>使用命令 <code>jvisualvm</code> 打开可视化界面，选择当前执行的进程，查看详细信息，然后进行堆内存 dump</p> <p><img src="/jvm/1669044220427.png" alt="1669044220427"></p> <p>可以发现当前的 ArrayList 对象占用的内存高达约 200M 左右的内存，点击进去查看该对象的详细信息</p> <p><img src="/jvm/1669044331372.png" alt="1669044331372"></p> <p>在 ArrayList 对象中的每一个元素都占用了约 1M 的内存大小，一共有200个元素，则总占用就有了大约 200M 的堆内存空间，由此快速定位到代码中存在的问题</p> <p><img src="/jvm/1669044475571.png" alt="1669044475571"></p></li></ul></li></ol> <h2 id="_5-方法区"><a href="#_5-方法区" class="header-anchor">#</a> 5 方法区</h2> <p><img src="/jvm/1669044952813.png" alt="1669044952813"></p> <h3 id="_5-1-定义"><a href="#_5-1-定义" class="header-anchor">#</a> 5.1 定义</h3> <p><a href="https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-2.html" target="_blank" rel="noopener noreferrer">Java 虚拟机规范<span><svg 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" class="icon outbound"><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="sr-only">(opens new window)</span></span></a></p> <p>以下是官方对方法区的定义</p> <blockquote><p>Java 虚拟机具有 在所有 Java 虚拟机之间共享<em>的方法区域</em> 线程。方法区域类似于已编译的存储区域传统语言的代码或类似于“文本”段的代码 操作系统进程。它存储每个类结构，如运行时常量池、字段和方法数据以及方法和构造函数，包括特殊方法 （<a href="https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-2.html#jvms-2.9" target="_blank" rel="noopener noreferrer">§2.9<span><svg 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" class="icon outbound"><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="sr-only">(opens new window)</span></span></a>） 用于类和实例初始化 和接口初始化。</p> <p>创建方法区域 在虚拟机启动时。虽然方法区域在逻辑上是作为堆的一部分，简单的实现可以选择不垃圾收集或压缩它。这规范不要求方法区域的位置或用于管理已编译代码的策略。方法区域可以是固定大小或可根据计算需要进行扩展，并且可以如果不需要更大的方法区域，则收缩。记忆对于方法区域不需要是连续的。</p> <p>Java 虚拟机实现可以为程序员或用户控制方法区域的初始大小，以及，对于不同尺寸的方法区域，控制最大值和最小方法区域大小。</p> <p>以下特殊条件与方法区域相关联：</p> <ul><li>如果方法区域中的内存无法用于满足分配请求，Java 虚拟机抛出一个。<code>OutOfMemoryError</code></li></ul></blockquote> <h3 id="_5-2-组成"><a href="#_5-2-组成" class="header-anchor">#</a> 5.2 组成</h3> <p>JDK6 和 JDK8 的内存结构发生了变化</p> <blockquote><p><strong>注意</strong>：图中的 常量池 指的是 运行时常量池</p></blockquote> <p><img src="/jvm/1669045525089.png" alt="1669045525089"></p> <h3 id="_5-3-方法区内存溢出"><a href="#_5-3-方法区内存溢出" class="header-anchor">#</a> 5.3 方法区内存溢出</h3> <ul><li>JDK1.8 以前会导致永久代内存溢出：<code>java.lang.OutOfMemoryError: PermGen space</code></li> <li>JDK1.8 以后会导致元空间内存溢出：<code>java.lang.OutOfMemoryError: Metaspace</code></li></ul> <p>演示 JDK1.8 以后元空间内存溢出（JDK1.6 代码几乎相同，永久代使用 <code>-XX:MaxPermSize=8m</code>）</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_7</span> <span class="token keyword">extends</span> <span class="token class-name">ClassLoader</span> <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 keyword">int</span> j <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token class-name">Demo1_7</span> test <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Demo1_7</span><span class="token punctuation">(</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> <span class="token number">10000</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">,</span> j<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// ClassWriter 作用是生成类的二进制字节码</span>
<span class="token class-name">ClassWriter</span> cw <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ClassWriter</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 版本号，public，类名，包名，父类，接口</span>
cw<span class="token punctuation">.</span><span class="token function">visit</span><span class="token punctuation">(</span><span class="token class-name">Opcodes</span><span class="token punctuation">.</span><span class="token constant">V1_8</span><span class="token punctuation">,</span> <span class="token class-name">Opcodes</span><span class="token punctuation">.</span><span class="token constant">ACC_PUBLIC</span><span class="token punctuation">,</span> <span class="token string">&quot;Class&quot;</span> <span class="token operator">+</span> i<span class="token punctuation">,</span> <span class="token keyword">null</span><span class="token punctuation">,</span> <span class="token string">&quot;java/lang/Object&quot;</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 comment">// 返回byte信息</span>
<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> code <span class="token operator">=</span> cw<span class="token punctuation">.</span><span class="token function">toByteArray</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 执行类的加载</span>
test<span class="token punctuation">.</span><span class="token function">defineClass</span><span class="token punctuation">(</span><span class="token string">&quot;Class&quot;</span> <span class="token operator">+</span> i<span class="token punctuation">,</span> code<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> code<span class="token punctuation">.</span>length<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>
<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>j<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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>在 1.8 以后元空间使用的是系统没存，没有限制大小，不会发生溢出的情况，需要设置 <code>-XX:MaxMetaspaceSize=8m</code></p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>5411
Exception in thread &quot;main&quot; java.lang.OutOfMemoryError: Metaspace
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
at com.zhuhjay.Demo1_7.main(Demo1_7.java:23)
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>方法区内存溢出场景：</p> <ul><li>Spring</li> <li>Mybatis</li></ul> <p>这些场景都会使用 CGLIB 等代理技术动态的生成字节码，使用不当可能会导致方法区内存溢出的情况。</p> <h3 id="_5-4-运行时常量池"><a href="#_5-4-运行时常量池" class="header-anchor">#</a> 5.4 运行时常量池</h3> <ul><li>常量池，就是一张表，虚拟机指令根据这张常量表找到要执行的类名、方法名、参数类型、字面量等信息</li> <li>运行时常量池，常量池是 *.class 文件中的，当该类被加载，它的常量池信息就会放入运行时常量池，并把里面的符号地址变为真实地址</li></ul> <blockquote><p><strong>Tip</strong>: 常量池的概念解析</p> <p>编译运行以下代码</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_8</span> <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">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;hello world&quot;</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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>将编译后的 class 文件进行反编译 <code>javap -v Demo1_8.class</code></p> <ul><li>源码 1-8 行是文件的 类的基本信息</li> <li>源码 9-42 行是 常量池信息</li> <li>源码 44-72 行是 类方法定义信息</li> <li><strong>当 class 文件被运行之后，所有的 <code>#..</code> 都会被赋予真实的物理地址信息</strong></li></ul> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Classfile /G:/Idea_workspace/JVM/target/classes/com/zhuhjay/Demo1_8.class
Last modified 2022-11-22; size 548 bytes
MD5 checksum 551e83a2ede53badac918978a8846964
Compiled from &quot;Demo1_8.java&quot;
public class com.zhuhjay.Demo1_8
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #6.#20 // java/lang/Object.&quot;&lt;init&gt;&quot;:()V
#2 = Fieldref #21.#22 // java/lang/System.out:Ljava/io/PrintStream;
#3 = String #23 // hello world
#4 = Methodref #24.#25 // java/io/PrintStream.println:(Ljava/lang/String;)V
#5 = Class #26 // com/zhuhjay/Demo1_8
#6 = Class #27 // java/lang/Object
#7 = Utf8 &lt;init&gt;
#8 = Utf8 ()V
#9 = Utf8 Code
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Lcom/zhuhjay/Demo1_8;
#14 = Utf8 main
#15 = Utf8 (<span class="token punctuation">[</span>Ljava/lang/String;)V
#16 = Utf8 args
#17 = Utf8 <span class="token punctuation">[</span>Ljava/lang/String;
#18 = Utf8 SourceFile
#19 = Utf8 Demo1_8.java
#20 = NameAndType #7:#8 // &quot;&lt;init&gt;&quot;:()V
#21 = Class #28 // java/lang/System
#22 = NameAndType #29:#30 // out:Ljava/io/PrintStream;
#23 = Utf8 hello world
#24 = Class #31 // java/io/PrintStream
#25 = NameAndType #32:#33 // println:(Ljava/lang/String;)V
#26 = Utf8 com/zhuhjay/Demo1_8
#27 = Utf8 java/lang/Object
#28 = Utf8 java/lang/System
#29 = Utf8 out
#30 = Utf8 Ljava/io/PrintStream;
#31 = Utf8 java/io/PrintStream
#32 = Utf8 println
#33 = Utf8 (Ljava/lang/String;)V
<span class="token punctuation">{</span>
public com.zhuhjay.Demo1_8();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object.&quot;&lt;init&gt;&quot;:()V
4: return
LineNumberTable:
line 7: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/zhuhjay/Demo1_8;
public static void main(java.lang.String<span class="token punctuation">[</span><span class="token punctuation">]</span>);
descriptor: (<span class="token punctuation">[</span>Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String hello world
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 9: 0
line 10: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 args <span class="token punctuation">[</span>Ljava/lang/String;
<span class="token punctuation">}</span>
SourceFile: &quot;Demo1_8.java&quot;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br><span class="line-number">43</span><br><span class="line-number">44</span><br><span class="line-number">45</span><br><span class="line-number">46</span><br><span class="line-number">47</span><br><span class="line-number">48</span><br><span class="line-number">49</span><br><span class="line-number">50</span><br><span class="line-number">51</span><br><span class="line-number">52</span><br><span class="line-number">53</span><br><span class="line-number">54</span><br><span class="line-number">55</span><br><span class="line-number">56</span><br><span class="line-number">57</span><br><span class="line-number">58</span><br><span class="line-number">59</span><br><span class="line-number">60</span><br><span class="line-number">61</span><br><span class="line-number">62</span><br><span class="line-number">63</span><br><span class="line-number">64</span><br><span class="line-number">65</span><br><span class="line-number">66</span><br><span class="line-number">67</span><br><span class="line-number">68</span><br><span class="line-number">69</span><br><span class="line-number">70</span><br><span class="line-number">71</span><br><span class="line-number">72</span><br><span class="line-number">73</span><br><span class="line-number">74</span><br></div></div><p>进行main方法信息进行分析 62-66 行代码截取，在CPU中是不会读取到 <code>//</code> 后面的注释信息的，这些注释信息是为了让程序员快速明白的注释信息。</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String hello world
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><ul><li><p><code>0: getstatic #2</code></p> <ul><li><p>getstatic：获取类的成员变量</p></li> <li><p>#2：去常量池 <code>Constant pool</code> 中查找对应的信息</p> <ol><li><p><code>#2 = Fieldref #21.#22</code>：需要找到地址 <code>#21.#22</code>，该组成为 成员变量的引用</p></li> <li><p><code>#21 = Class #28</code>：需要找到地址 <code>#28</code>，该地址信息为 Class</p></li> <li><p><code>#28 = Utf8 java/lang/System</code>：找到 <code>System</code> 类名</p></li> <li><p><code>#22 = NameAndType #29:#30</code>：需要找到 方法名和其类型</p></li> <li><p><code>#29 = Utf8 out</code>：找到调用的静态方法</p></li> <li><p><code>#30 = Utf8 Ljava/io/PrintStream;</code>：找到类型</p></li></ol></li> <li><p>即该条命令就是为了找到该结果 <code>Field java/lang/System.out:Ljava/io/PrintStream;</code>，这也就是源码的 <code>System.out</code></p></li></ul></li> <li><p><code>3: ldc #3</code></p> <ul><li>ldc：找到一个地址，并作为下一次指令的参数</li> <li>#3：
<ol><li><code>#3 = String #23</code>：需要找到地址 <code>#23</code></li> <li><code>#23 = Utf8 hello world</code>：找到类型为 <code>Utf8</code> 的字符串</li></ol></li> <li>即该条指令就是为了找到结果 <code>hello world</code></li></ul></li> <li><p><code>5: invokevirtual #4</code></p> <ul><li>invokevirtual：执行一次虚方法调用（方法调用）</li> <li>#4：
<ol><li><code>#4 = Methodref #24.#25</code>：需要找到地址 <code>#24.#25</code>，该组为 方法引用</li> <li><code>#24 = Class #31</code>：需要找到地址 <code>#31</code>，是一个 Class 对象</li> <li><code>#31 = Utf8 java/io/PrintStream</code>：一个输出流的类名</li> <li><code>#25 = NameAndType #32:#33</code>：需要找到地址 <code>#32:#33</code>，方法名及其类型</li> <li><code>#32 = Utf8 println</code>：找到需要调用的方法名称</li> <li><code>#33 = Utf8 (Ljava/lang/String;)V</code>：参数类型为 String，无返回值</li></ol></li> <li>即该条指令就是为了找到结果 <code>Method java/io/PrintStream.println:(Ljava/lang/String;)V</code>，也就是源码中的 <code>println(&quot;hello world&quot;)</code>，需要将上一个指令的数据作为方法参数</li></ul></li></ul></blockquote> <h3 id="_5-5-stringtable"><a href="#_5-5-stringtable" class="header-anchor">#</a> 5.5 StringTable</h3> <p>用来存储字符串的常量池，<strong>使用硬编码的字符串都会被存储到字符串常量池中</strong></p> <p>eg:</p> <ul><li><code>new String(&quot;a&quot;)</code> 会被放入字符串常量池和堆中，返回值是堆中的引用</li> <li><code>&quot;a&quot;</code> 会被直接放到字符串常量池中，返回值是串池中的引用</li></ul> <h5 id="_5-5-1-解析引入"><a href="#_5-5-1-解析引入" class="header-anchor">#</a> 5.5.1 解析引入</h5> <ul><li><p>现有以下代码，将其进行编译后，使用 <code>javap</code> 对其进行反编译，查看常量池的信息</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s1 <span class="token operator">=</span> <span class="token string">&quot;a&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s2 <span class="token operator">=</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s3 <span class="token operator">=</span> <span class="token string">&quot;ab&quot;</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li> <li><p>编译结果如下（只展示部分）</p> <ul><li>常量池中的信息，都会被加载到运行时常量池中，这时 a b ab 都是常量池中的符号，还没有变为 java 字符串对象</li> <li><code>ldc #2</code> 会把 a 符号变为 &quot;a&quot; 字符串对象，然后到 StringTable 中去找是否存在该值，不存在则存入 StringTable 中
<ul><li>StringTable 是 hashtable 结构，不能扩容，一创建就固定了大小</li></ul></li> <li><code>ldc #3</code> <code>ldc #4</code> 同上</li> <li><code>astore_1</code> 即是将上一条指令的内容存入到方法栈桢的变量表的 1 位置上</li></ul> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Constant pool:
#2 = String #25 // a
#3 = String #26 // b
#4 = String #27 // ab
public static void main(java.lang.String<span class="token punctuation">[</span><span class="token punctuation">]</span>);
Code:
stack=1, locals=4, args_size=1
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: return
LocalVariableTable: # main方法栈帧的变量表
Start Length Slot Name Signature
0 10 0 args <span class="token punctuation">[</span>Ljava/lang/String;
3 7 1 s1 Ljava/lang/String;
6 4 2 s2 Ljava/lang/String;
9 1 3 s3 Ljava/lang/String;
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div></li></ul> <h5 id="_5-5-2-变量拼接"><a href="#_5-5-2-变量拼接" class="header-anchor">#</a> 5.5.2 变量拼接</h5> <ul><li><p>现对源代码进行改变如下，需要了解字符串变量拼接的原理，以及 <code>s3 == s4</code> 是否成立</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s1 <span class="token operator">=</span> <span class="token string">&quot;a&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s2 <span class="token operator">=</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s3 <span class="token operator">=</span> <span class="token string">&quot;ab&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s4 <span class="token operator">=</span> s1 <span class="token operator">+</span> s2<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div></li> <li><p>进行反编译（只展示部分），根据如下执行的指令，可以发现</p> <ul><li><code>String s4 = s1 + s2;</code> 是通过 <code>new StringBuilder().append(&quot;a&quot;).append(&quot;b&quot;).toString()</code> 来进行组成的字符串</li> <li>StringBuilder 中的 toString 方法就是通过 <code>new String(&quot;ab&quot;)</code> 在堆空间中创建一个对象</li> <li><strong>以上即可得出 <code>s3 == s4</code> 不成立，s3 变量指向串池，s4 变量指向堆中的对象</strong></li></ul> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Constant pool:
#2 = String #30 // a
#3 = String #31 // b
#4 = String #32 // ab
#5 = Class #33 // java/lang/StringBuilder
#6 = Methodref #5.#29 // java/lang/StringBuilder.&quot;&lt;init&gt;&quot;:()V
#7 = Methodref #5.#34 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#8 = Methodref #5.#35 // java/lang/StringBuilder.toString:()Ljava/lang/String;
public static void main(java.lang.String<span class="token punctuation">[</span><span class="token punctuation">]</span>);
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder.&quot;&lt;init&gt;&quot;:()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: astore 4
29: return
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br></div></div></li></ul> <h5 id="_5-5-3-常量拼接"><a href="#_5-5-3-常量拼接" class="header-anchor">#</a> 5.5.3 常量拼接</h5> <ul><li><p>现对源代码进行改变如下，需要了解字符串常量拼接的原理，以及 <code>s3 == s5</code> 是否成立</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s1 <span class="token operator">=</span> <span class="token string">&quot;a&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s2 <span class="token operator">=</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s3 <span class="token operator">=</span> <span class="token string">&quot;ab&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s4 <span class="token operator">=</span> s1 <span class="token operator">+</span> s2<span class="token punctuation">;</span>
<span class="token class-name">String</span> s5 <span class="token operator">=</span> <span class="token string">&quot;a&quot;</span> <span class="token operator">+</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div></li> <li><p>进行反编译（只展示部分），根据如下执行的指令，可以发现</p> <ul><li><code>String s5 = &quot;a&quot; + &quot;b&quot;;</code> 直接往 StringTable 串池中查找 &quot;ab&quot;</li> <li>发生以上的原因：javac 在编译期间的优化，结果已经在编译器确定为 &quot;ab&quot;</li> <li><strong>即得出 <code>s3 == s5</code> 成立，s3 和 s5 都是来自串池中的 &quot;ab&quot;，属于同一个对象</strong></li></ul> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>public static void main(java.lang.String<span class="token punctuation">[</span><span class="token punctuation">]</span>);
0: ldc #2 // String a
2: astore_1
3: ldc #3 // String b
5: astore_2
6: ldc #4 // String ab
8: astore_3
9: new #5 // class java/lang/StringBuilder
12: dup
13: invokespecial #6 // Method java/lang/StringBuilder.&quot;&lt;init&gt;&quot;:()V
16: aload_1
17: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: aload_2
21: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
24: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: astore 4
29: ldc #4 // String ab
31: astore 5
33: return
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br></div></div></li></ul> <h5 id="_5-5-4-intern-jdk1-8"><a href="#_5-5-4-intern-jdk1-8" class="header-anchor">#</a> 5.5.4 intern (JDK1.8)</h5> <ul><li><p>现对源代码进行改变如下，需要了解 String.intern() 方法的原理，以及 <code>s6 == s7</code> 是否成立</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s6 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;c&quot;</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;d&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s7 <span class="token operator">=</span> <span class="token string">&quot;cd&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> intern <span class="token operator">=</span> s6<span class="token punctuation">.</span><span class="token function">intern</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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li> <li><p>进行反编译（只展示部分），根据如下执行的指令，可以发现</p> <ul><li>通过 <code>new String(&quot;c&quot;)</code> 方法，会在堆内存和常量池中各创建一份数据，然后通过 变量拼接 的方式在堆内存中生成一份 <code>cd</code>，但此数据没有入串池中</li> <li><code>String s7 = &quot;cd&quot;;</code> 此时串池中没有存在该数据，所以入串池操作</li> <li><code>s6.intern();</code> 将该字符串对象尝试放入串池，如果有则不会放入，如果没有则放入串池，会把串池中的对象返回</li> <li><strong>程序执行完后，会发现 s6 属于堆内存中的对象，而 s7 属于串池中的对象，所以 <code>s6 == s7</code> 不成立</strong></li></ul> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>public static void main(java.lang.String<span class="token punctuation">[</span><span class="token punctuation">]</span>);
0: new #2 // class java/lang/StringBuilder
3: dup
4: invokespecial #3 // Method java/lang/StringBuilder.&quot;&lt;init&gt;&quot;:()V
7: new #4 // class java/lang/String
10: dup
11: ldc #5 // String c
13: invokespecial #6 // Method java/lang/String.&quot;&lt;init&gt;&quot;:(Ljava/lang/String;)V
16: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: new #4 // class java/lang/String
22: dup
23: ldc #8 // String d
25: invokespecial #6 // Method java/lang/String.&quot;&lt;init&gt;&quot;:(Ljava/lang/String;)V
28: invokevirtual #7 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
31: invokevirtual #9 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: astore_1
35: ldc #10 // String cd
37: astore_2
38: aload_1
39: invokevirtual #11 // Method java/lang/String.intern:()Ljava/lang/String;
42: astore_3
43: return
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br></div></div></li></ul> <blockquote><p>思考：</p> <p>​ 将以上代码修改为以下代码，<code>s6 == s7</code> 又会是怎样的结果？</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s6 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;c&quot;</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;d&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> intern <span class="token operator">=</span> s6<span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s7 <span class="token operator">=</span> <span class="token string">&quot;cd&quot;</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>分析：</p> <ol><li>执行完 s6 变量赋值过后，常量池和堆的数据如下
<ul><li>StringTable [&quot;c&quot;, &quot;d&quot;]</li> <li>Heap ['new String(&quot;c&quot;)', 'new String(&quot;d&quot;)', 'new String(&quot;cd&quot;)']</li></ul></li> <li>执行完 s6.intern 后，会判断常量池中是否存在 &quot;cd&quot;
<ul><li>StringTable [&quot;c&quot;, &quot;d&quot;, &quot;cd&quot;] （<em>实际上这里的 &quot;cd&quot; 是堆中 'new String(&quot;cd&quot;)' 的一个引用，是用一个对象</em>）</li> <li>Heap ['new String(&quot;c&quot;)', 'new String(&quot;d&quot;)', 'new String(&quot;cd&quot;)']</li></ul></li> <li>执行到 s7 时，发现常量池中存在了 &quot;cd&quot;，所以直接引用</li></ol> <p><strong>综上所述，当前情况下的 <code>s6 == s7</code> 是成立的</strong></p></blockquote> <p>​</p> <h5 id="_5-5-5-intern-jdk1-6"><a href="#_5-5-5-intern-jdk1-6" class="header-anchor">#</a> 5.5.5 intern (JDK1.6)</h5> <p>在 JDK1.6 中，以下执行的结果与 JDK1.8 的相同： <code>s6 == s7</code> 不成立</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s6 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;c&quot;</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;d&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s7 <span class="token operator">=</span> <span class="token string">&quot;cd&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> intern <span class="token operator">=</span> s6<span class="token punctuation">.</span><span class="token function">intern</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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>但是当改为以下代码时，结果却不尽相同</p> <ol><li>执行完 s6 变量赋值过后，常量池和堆的数据如下
<ul><li>StringTable [&quot;c&quot;, &quot;d&quot;]</li> <li>Heap ['new String(&quot;c&quot;)', 'new String(&quot;d&quot;)', 'new String(&quot;cd&quot;)']</li></ul></li> <li>执行完 s6.intern 后，会判断常量池中是否存在 &quot;cd&quot;，如果不存在，那么将 <strong>s6 拷贝一份放入常量池中，并返回</strong> <ul><li>StringTable [&quot;c&quot;, &quot;d&quot;, &quot;cd&quot;] （<em>在此时，&quot;cd&quot; 和 'new String(&quot;cd&quot;)' 不是同一个对象，因为是通过拷贝的方式</em>）</li> <li>Heap ['new String(&quot;c&quot;)', 'new String(&quot;d&quot;)', 'new String(&quot;cd&quot;)']</li></ul></li> <li>执行到 s7 时，发现常量池中存在了 &quot;cd&quot;，所以直接引用</li></ol> <p><strong>综上所述，当前情况下的 <code>s6 == s7</code> 是不成立的</strong></p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_9</span> <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">String</span> s6 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;c&quot;</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;d&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> intern <span class="token operator">=</span> s6<span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s7 <span class="token operator">=</span> <span class="token string">&quot;cd&quot;</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h5 id="_5-5-6-特性总结"><a href="#_5-5-6-特性总结" class="header-anchor">#</a> 5.5.6 特性总结</h5> <ul><li><p>常量池中的字符串仅是符号，第一次用到时才变为对象</p></li> <li><p>利用串池的机制，来避免重复创建字符串对象</p></li> <li><p>字符串变量拼接的原理是 StringBuilder（1.8）</p></li> <li><p>字符串常量拼接的原理是编译期优化</p></li> <li><p>可以使用 intern 方法，主动将串池中还没有的字符串对象放入串池</p> <ul><li>1.8 将这个字符串对象尝试放入串池，如果有则并不会放入，如果没有则放入串池， 会把串池中的对象返回</li> <li>1.6 将这个字符串对象尝试放入串池，如果有则并不会放入，如果没有会把此对象<strong>复制</strong>一份，放入串池，会把串池中的对象返回</li></ul></li></ul> <h4 id="_5-6-stringtable-面试题"><a href="#_5-6-stringtable-面试题" class="header-anchor">#</a> 5.6 StringTable 面试题</h4> <p>学习完 StringTable 字符串常量池后，能正确的理解回答下面的题目，那么恭喜你掌握了一个新的知识！</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">String</span> s1 <span class="token operator">=</span> <span class="token string">&quot;a&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s2 <span class="token operator">=</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s3 <span class="token operator">=</span> <span class="token string">&quot;a&quot;</span> <span class="token operator">+</span> <span class="token string">&quot;b&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s4 <span class="token operator">=</span> s1 <span class="token operator">+</span> s2<span class="token punctuation">;</span>
<span class="token class-name">String</span> s5 <span class="token operator">=</span> <span class="token string">&quot;ab&quot;</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> s6 <span class="token operator">=</span> s4<span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 问</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>s3 <span class="token operator">==</span> s4<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>s3 <span class="token operator">==</span> s5<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>s3 <span class="token operator">==</span> s6<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> x2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;c&quot;</span><span class="token punctuation">)</span> <span class="token operator">+</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">(</span><span class="token string">&quot;d&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> x1 <span class="token operator">=</span> <span class="token string">&quot;cd&quot;</span><span class="token punctuation">;</span>
x2<span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 问，如果调换了【最后两行代码】的位置呢？如果是jdk1.6呢？</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>x1 <span class="token operator">==</span> x2<span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>答案：</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>false true true
jdk1.8: false true
jdk1.6: false false
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br></div></div><h4 id="_5-7-stringtable-位置"><a href="#_5-7-stringtable-位置" class="header-anchor">#</a> 5.7 StringTable 位置</h4> <p>在 JDK1.6 时，StringTable 是常量池的一部分，常量池存储在永久代中（在永久代中需要进行 Full GC 才会进行垃圾回收，回收效率低）</p> <p>在 JDK1.8 时，StringTable 是堆中的一部分，垃圾回收跟随堆进行，回收效率高</p> <p>怎么证明以上两个版本存储的位置不相同？</p> <ul><li>在 JDK1.6 中，如果常量池中发生了内存溢出，那么就会报错 <code>java.lang.OutOfMemoryError: PermGen space</code>，永久代内存溢出</li> <li>在 JDK1.8 中，如果常量池中发生了内存溢出，那么就会报错 <code>java.lang.OutOfMemoryError: Java heap space</code>，堆内存溢出</li></ul> <p>通过以上思路，来证明 JDK1.6 和 JDK1.8 StringTable 常量池的位置不同</p> <p>案例代码</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_10</span> <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">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</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">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> j <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> j <span class="token operator">&lt;</span> <span class="token number">260000</span><span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span>j<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
i<span class="token operator">++</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">Throwable</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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>
<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>i<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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p><strong>JDK1.8 验证</strong>：需要设置堆内存的大小 <code>-Xmx10m</code>，会发现有以下结果，报错信息和预想的结果不一样？</p> <ul><li>原来，在 JVM规范中 当GC垃圾回收花了 98% 的时间只释放了 2% 的堆空间时，JVM 就会认为该程序没救了，就直接抛出该异常</li> <li>那么要跳过这个限制，需要再多配置一个参数 <code>-XX:-UseGCOverheadLimit</code></li></ul> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>java.lang.OutOfMemoryError: GC overhead limit exceeded
at java.lang.Integer.toString(Integer.java:403)
at java.lang.String.valueOf(String.java:3099)
at com.zhuhjay.Demo1_10.main(Demo1_10.java:16)
145367
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>最终结果就是预想的结果了！</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>*** java.lang.instrument ASSERTION FAILED ***: &quot;!errorOutstanding&quot; with message can't create byte arrau at JPLISAgent.c line: 813
java.lang.OutOfMemoryError: Java heap space
at java.lang.Integer.toString(Integer.java:401)
at java.lang.String.valueOf(String.java:3099)
at com.zhuhjay.Demo1_10.main(Demo1_10.java:16)
146837
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p><strong>JDK1.6 验证</strong>：需要设置永久代内存的大小 <code>-XX:MaxPermSize=10m</code></p> <h4 id="_5-8-stringtable-垃圾回收"><a href="#_5-8-stringtable-垃圾回收" class="header-anchor">#</a> 5.8 StringTable 垃圾回收</h4> <p>查看字符串常量池 StringTable 的垃圾回收是否会在空间不足时进行字符串常量池的垃圾回收</p> <p>示例代码，为了更进一步查看堆内存和字符串常量池的空间，需要添加以下参数</p> <ul><li><code>-Xmx10m</code>：设置堆的大小，为了方便测试GC垃圾回收</li> <li><code>-XX:+PrintStringTableStatistics</code>：打印字符串表的信息</li> <li><code>-XX:+PrintGCDetails -verbose:gc</code>：统计垃圾回收的信息</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_11</span> <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 keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token comment">// TODO 待插入代码</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Throwable</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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>
<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>i<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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>查看输出结果，认识一下下面所代表的含义</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>Heap # 堆内存的占用情况
PSYoungGen total 2560K, used 1676K <span class="token punctuation">[</span>0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 81<span class="token comment">% used [0x00000000ffd00000,0x00000000ffea3198,0x00000000fff00000)</span>
from space 512K, 0<span class="token comment">% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)</span>
to space 512K, 0<span class="token comment">% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)</span>
ParOldGen total 7168K, used 0K <span class="token punctuation">[</span>0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
object space 7168K, 0<span class="token comment">% used [0x00000000ff600000,0x00000000ff600000,0x00000000ffd00000)</span>
Metaspace used 3167K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 345K, capacity 388K, committed 512K, reserved 1048576K
SymbolTable statistics: # 符号表的详细信息（也是常量池的一部分，但是并不关心这部分内容）
Number of buckets : 20011 = 160088 bytes, avg 8.000
Number of entries : 13260 = 318240 bytes, avg 24.000
Number of literals : 13260 = 568232 bytes, avg 42.853
Total footprint : = 1046560 bytes
Average bucket size : 0.663
Variance of bucket size : 0.666
Std. dev. of bucket size: 0.816
Maximum bucket size : 6
StringTable statistics: # StringTable 统计信息，使用 hashtable 结构
Number of buckets : 60013 = 480104 bytes, avg 8.000 # 桶个数 = 字节数
Number of entries : 1739 = 41736 bytes, avg 24.000 # 键值对个数 = 字节数
Number of literals : 1739 = 156592 bytes, avg 90.047 # 字符串常量个数 = 字节数
Total footprint : = 678432 bytes # 总的空间
Average bucket size : 0.029
Variance of bucket size : 0.029
Std. dev. of bucket size: 0.171
Maximum bucket size : 3
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br></div></div><p>现在在以上代码中插入以下代码：将10000个字符串添加到字符串常量池中，此时字符串常量池中理应会在原来的 1739 基础上多 10000</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">int</span> j <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> j <span class="token operator">&lt;</span> <span class="token number">10000</span><span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span><span class="token punctuation">.</span><span class="token function">valueOf</span><span class="token punctuation">(</span>j<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">intern</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
i<span class="token operator">++</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p>可以看到此时运行就有了 GC垃圾回收 的信息，此时字符串常量池没有达到预期的效果，说明当常量池中有不被使用的数据会被垃圾回收</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code><span class="token punctuation">[</span>GC (Allocation Failure) <span class="token punctuation">[</span>PSYoungGen: 2048K-&gt;488K(2560K)<span class="token punctuation">]</span> 2048K-&gt;704K(9728K), 0.0031150 secs<span class="token punctuation">]</span> <span class="token punctuation">[</span>Times: user=0.00 sys=0.00, real=0.00 secs<span class="token punctuation">]</span>
10000
Heap
...省略展示
SymbolTable statistics:
...省略展示
StringTable statistics:
Number of buckets : 60013 = 480104 bytes, avg 8.000
Number of entries : 4026 = 96624 bytes, avg 24.000
Number of literals : 4026 = 266888 bytes, avg 66.291
Total footprint : = 843616 bytes
Average bucket size : 0.067
Variance of bucket size : 0.065
Std. dev. of bucket size: 0.256
Maximum bucket size : 3
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><h4 id="_5-9-stringtable-性能调优"><a href="#_5-9-stringtable-性能调优" class="header-anchor">#</a> 5.9 StringTable 性能调优</h4> <ul><li><p>字符串常量池的性能随着分配桶的个数越多，效率越高，参数配置 <code>-XX:StringTableSize=1009</code>，配置最小值为 1009</p></li> <li><p>String.intern 调优了一大部分性能，防止滋生很多的重复数据</p></li></ul> <h2 id="_6-直接内存"><a href="#_6-直接内存" class="header-anchor">#</a> 6 直接内存</h2> <p>直接内存是系统内存</p> <h3 id="_6-1-定义"><a href="#_6-1-定义" class="header-anchor">#</a> 6.1 定义</h3> <p>Direct Memory</p> <ul><li>常见于 NIO 操作时，用于数据缓冲区</li> <li>分配回收成本较高，但读写性能高</li> <li>不受 JVM 内存回收管理</li></ul> <h3 id="_6-2-使用直接内存"><a href="#_6-2-使用直接内存" class="header-anchor">#</a> 6.2 使用直接内存</h3> <h4 id="_6-2-1-测试io与direct-memory读写效率"><a href="#_6-2-1-测试io与direct-memory读写效率" class="header-anchor">#</a> 6.2.1 测试IO与Direct Memory读写效率</h4> <p>使用差不多 1G 的文件来进行读写对比</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_12</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> <span class="token constant">FROM</span> <span class="token operator">=</span> <span class="token string">&quot;G:\\maven\\maven_repository_0.zip&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> <span class="token constant">TO</span> <span class="token operator">=</span> <span class="token string">&quot;G:\\IDM_DownLoad\\Video\\Temp\\maven.zip&quot;</span><span class="token punctuation">;</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> _1MB <span class="token operator">=</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span><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 function">io</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">directBuffer</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">private</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">directBuffer</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> start <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</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 class-name">FileChannel</span> from <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span><span class="token constant">FROM</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getChannel</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">FileChannel</span> <span class="token keyword">to</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span><span class="token constant">TO</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getChannel</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 class-name">ByteBuffer</span> bb <span class="token operator">=</span> <span class="token class-name">ByteBuffer</span><span class="token punctuation">.</span><span class="token function">allocateDirect</span><span class="token punctuation">(</span>_1MB<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> length <span class="token operator">=</span> from<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>bb<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>length <span class="token operator">==</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 keyword">break</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
bb<span class="token punctuation">.</span><span class="token function">flip</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">to</span><span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>bb<span class="token punctuation">)</span><span class="token punctuation">;</span>
bb<span class="token punctuation">.</span><span class="token function">clear</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>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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">long</span> end <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</span><span class="token punctuation">(</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><span class="token string">&quot;directBuffer 用时:&quot;</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end <span class="token operator">-</span> start<span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">1000_000.0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">io</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> start <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</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 class-name">FileInputStream</span> from <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span><span class="token constant">FROM</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">FileOutputStream</span> <span class="token keyword">to</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">FileOutputStream</span><span class="token punctuation">(</span><span class="token constant">TO</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">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> buf <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">byte</span><span class="token punctuation">[</span>_1MB<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span> length <span class="token operator">=</span> from<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span>buf<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>length <span class="token operator">==</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 keyword">break</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">to</span><span class="token punctuation">.</span><span class="token function">write</span><span class="token punctuation">(</span>buf<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> length<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>
e<span class="token punctuation">.</span><span class="token function">printStackTrace</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">long</span> end <span class="token operator">=</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">nanoTime</span><span class="token punctuation">(</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><span class="token string">&quot;io 用时:&quot;</span> <span class="token operator">+</span> <span class="token punctuation">(</span>end <span class="token operator">-</span> start<span class="token punctuation">)</span> <span class="token operator">/</span> <span class="token number">1000_000.0</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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br><span class="line-number">22</span><br><span class="line-number">23</span><br><span class="line-number">24</span><br><span class="line-number">25</span><br><span class="line-number">26</span><br><span class="line-number">27</span><br><span class="line-number">28</span><br><span class="line-number">29</span><br><span class="line-number">30</span><br><span class="line-number">31</span><br><span class="line-number">32</span><br><span class="line-number">33</span><br><span class="line-number">34</span><br><span class="line-number">35</span><br><span class="line-number">36</span><br><span class="line-number">37</span><br><span class="line-number">38</span><br><span class="line-number">39</span><br><span class="line-number">40</span><br><span class="line-number">41</span><br><span class="line-number">42</span><br><span class="line-number">43</span><br><span class="line-number">44</span><br><span class="line-number">45</span><br><span class="line-number">46</span><br><span class="line-number">47</span><br><span class="line-number">48</span><br><span class="line-number">49</span><br><span class="line-number">50</span><br><span class="line-number">51</span><br><span class="line-number">52</span><br></div></div><p>对比结果如下，单位ms，可以发现使用直接内存读写效率更高</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>io 用时:19611.177
directBuffer 用时:5490.16
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><p><img src="/jvm/1669128719697.png" alt="1669128719697"></p> <ul><li>Java的 io 操作涉及到 CPU 的用户态和内核态
<ul><li>io 操作需要操作系统从磁盘中进行磁盘文件的读取，将磁盘文件读取到系统内存中的系统缓存区</li> <li>此时 Java 不能直接操作该缓存区，需要在 Java 堆内存中开辟一块 java缓冲区byte[]</li> <li>然后将 系统缓冲区 的数据写到 Java缓冲区 中才能进一步操作</li></ul></li></ul> <p><img src="/jvm/1669128690245.png" alt="1669128690245"></p> <ul><li>调用 <code>ByteBuffer bb = ByteBuffer.allocateDirect(_1MB);</code> 时，会在 系统内存 和 Java堆内存 中开辟一块 <code>direct Memory</code> 内存来进行使用，这就是直接内存，使得 Java 可以直接操作</li></ul> <h4 id="_6-2-2-直接内存溢出"><a href="#_6-2-2-直接内存溢出" class="header-anchor">#</a> 6.2.2 直接内存溢出</h4> <p>每次分配 100M 内存，直到 Direct buffer memory 溢出</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_13</span> <span class="token punctuation">{</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> _100MB <span class="token operator">=</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">100</span><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">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">ByteBuffer</span><span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</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">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token keyword">while</span> <span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">ByteBuffer</span> byteBuffer <span class="token operator">=</span> <span class="token class-name">ByteBuffer</span><span class="token punctuation">.</span><span class="token function">allocateDirect</span><span class="token punctuation">(</span>_100MB<span class="token punctuation">)</span><span class="token punctuation">;</span>
list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>byteBuffer<span class="token punctuation">)</span><span class="token punctuation">;</span>
i<span class="token operator">++</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>
<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>i<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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br></div></div><p>可以看到直接内存分配到大约 1.7G 时就溢出了</p> <div class="language-tex line-numbers-mode"><pre class="language-tex"><code>17
Exception in thread &quot;main&quot; java.lang.OutOfMemoryError: Direct buffer memory
at java.nio.Bits.reserveMemory(Bits.java:695)
at java.nio.DirectByteBuffer.&lt;init&gt;(DirectByteBuffer.java:123)
at java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:311)
at com.zhuhjay.Demo1_13.main(Demo1_13.java:19)
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><h4 id="_6-2-3-直接内存释放"><a href="#_6-2-3-直接内存释放" class="header-anchor">#</a> 6.2.3 直接内存释放</h4> <p>查看以下案例，分配 1GB 的内存，然后使用 GC 进行回收</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_14</span> <span class="token punctuation">{</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> _1GB <span class="token operator">=</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span><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 keyword">throws</span> <span class="token class-name">IOException</span> <span class="token punctuation">{</span>
<span class="token class-name">ByteBuffer</span> byteBuffer <span class="token operator">=</span> <span class="token class-name">ByteBuffer</span><span class="token punctuation">.</span><span class="token function">allocateDirect</span><span class="token punctuation">(</span>_1GB<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><span class="token string">&quot;分配完毕...&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>in<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</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><span class="token string">&quot;开始释放...&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
byteBuffer <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">gc</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>in<span class="token punctuation">.</span><span class="token function">read</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-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>结果如图所示，这样一来感觉好像可以被 GC垃圾回收 进行 直接内存 的回收，但其实并不然。</p> <p><img src="/jvm/1669130149269.png" alt="1669130149269"></p> <p>接下来 Unsafe 脱离 JVM 的内存管理来进行 直接内存 分配</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Demo1_15</span> <span class="token punctuation">{</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> _1GB <span class="token operator">=</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span> <span class="token operator">*</span> <span class="token number">1024</span><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 keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">{</span>
<span class="token class-name">Unsafe</span> unsafe <span class="token operator">=</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 comment">// 分配内存，并返回分配到的内存首地址</span>
<span class="token keyword">long</span> base <span class="token operator">=</span> unsafe<span class="token punctuation">.</span><span class="token function">allocateMemory</span><span class="token punctuation">(</span>_1GB<span class="token punctuation">)</span><span class="token punctuation">;</span>
unsafe<span class="token punctuation">.</span><span class="token function">setMemory</span><span class="token punctuation">(</span>base<span class="token punctuation">,</span> _1GB<span class="token punctuation">,</span> <span class="token punctuation">(</span><span class="token keyword">byte</span><span class="token punctuation">)</span> <span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>in<span class="token punctuation">.</span><span class="token function">read</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 手动释放内存</span>
unsafe<span class="token punctuation">.</span><span class="token function">freeMemory</span><span class="token punctuation">(</span>base<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">System</span><span class="token punctuation">.</span>in<span class="token punctuation">.</span><span class="token function">read</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">private</span> <span class="token keyword">static</span> <span class="token class-name">Unsafe</span> <span class="token function">getUnsafe</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">Exception</span> <span class="token punctuation">{</span>
<span class="token class-name">Field</span> field <span class="token operator">=</span> <span class="token class-name">Unsafe</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getDeclaredField</span><span class="token punctuation">(</span><span class="token string">&quot;theUnsafe&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
field<span class="token punctuation">.</span><span class="token function">setAccessible</span><span class="token punctuation">(</span><span class="token boolean">true</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 class-name">Unsafe</span><span class="token punctuation">)</span> field<span class="token punctuation">.</span><span class="token function">get</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 punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br></div></div><p>可以发现使用 直接内存 是通过 Unsafe 进行管理的，GC垃圾回收 只能对 JVM内存有效</p> <p><img src="/jvm/1669130788224.png" alt="1669130788224"></p> <p>查看 <code>ByteBuffer.allocateDirect()</code> 方法的源码</p> <ul><li>使用了 Unsafe 进行 直接内存的分配</li> <li>使用了 <code>Cleaner</code> 特殊对象，虚引用 <code>PhantomReference&lt;Object&gt;</code> 对象</li> <li>虚引用对象 <code>Cleaner</code> 所关联的实际对象 <code>ByteBuffer</code> 被 GC垃圾回收之后，就会主动调用回调对象 <code>Deallocator</code> 主动的释放 直接内存</li></ul> <p><img src="/jvm/1669131731576.png" alt="1669131731576"></p> <h3 id="_6-3-分配和回收原理"><a href="#_6-3-分配和回收原理" class="header-anchor">#</a> 6.3 分配和回收原理</h3> <ul><li>使用了 Unsafe 对象完成直接内存的分配回收，并且回收需要主动调用 freeMemory 方法</li> <li>ByteBuffer 的实现类内部，使用了 Cleaner（虚引用）来监测 ByteBuffer 对象，一旦 ByteBuffer 对象被垃圾回收，那么就会由 ReferenceHandler 线程通过 Cleaner 的 clean 方法调 用 freeMemory 来释放直接内存</li></ul> <p>禁用显示回收对直接内存的影响（就是禁用代码层面的 <code>System.gc()</code>，使其无效）</p> <ul><li><code>System.gc()</code> 触发的是 <code>Full GC</code>，比较影响性能的，不光回收新生代，还会回收老年代，会使得程序暂停时间较长</li> <li>使用虚拟机参数 <code>-XX:+DisableExplicitGC</code></li> <li>代码层面的 GC 失效后，<code>ByteBuffer</code> 得不到内存的释放，使得 直接内存 也无法被释放，需要等到下一次 GC 时才会触发</li> <li><strong>问题解决</strong>：手动调用 Unsafe 释放直接内存</li></ul> <p><img src="/jvm/1669132766803.png" alt="1669132766803"></p></div></section> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><!----> <span class="next"><a href="/blogs/java/jvm/JVM-垃圾回收.html">
JVM之垃圾回收
</a></span></p></div> <div class="comments-wrapper"><!----></div></main></div> <!----></div> <ul class="sub-sidebar sub-sidebar-wrapper" style="width:12rem;" data-v-b57cc07c data-v-7dd95ae2><li class="level-2" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_1-程序计数器" class="sidebar-link reco-side-_1-程序计数器" data-v-b57cc07c>1 程序计数器</a></li><li class="level-2" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_2-虚拟机栈" class="sidebar-link reco-side-_2-虚拟机栈" data-v-b57cc07c>2 虚拟机栈</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_2-1-定义" class="sidebar-link reco-side-_2-1-定义" data-v-b57cc07c>2.1 定义</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_2-2-栈内存溢出" class="sidebar-link reco-side-_2-2-栈内存溢出" data-v-b57cc07c>2.2 栈内存溢出</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_2-3-线程运行诊断" class="sidebar-link reco-side-_2-3-线程运行诊断" data-v-b57cc07c>2.3 线程运行诊断</a></li><li class="level-2" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_3-本地方法栈" class="sidebar-link reco-side-_3-本地方法栈" data-v-b57cc07c>3 本地方法栈</a></li><li class="level-2" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_4-堆" class="sidebar-link reco-side-_4-堆" data-v-b57cc07c>4 堆</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_4-1-定义" class="sidebar-link reco-side-_4-1-定义" data-v-b57cc07c>4.1 定义</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_4-2-堆内存溢出" class="sidebar-link reco-side-_4-2-堆内存溢出" data-v-b57cc07c>4.2 堆内存溢出</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_4-3-堆内存诊断" class="sidebar-link reco-side-_4-3-堆内存诊断" data-v-b57cc07c>4.3 堆内存诊断</a></li><li class="level-2" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_5-方法区" class="sidebar-link reco-side-_5-方法区" data-v-b57cc07c>5 方法区</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_5-1-定义" class="sidebar-link reco-side-_5-1-定义" data-v-b57cc07c>5.1 定义</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_5-2-组成" class="sidebar-link reco-side-_5-2-组成" data-v-b57cc07c>5.2 组成</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_5-3-方法区内存溢出" class="sidebar-link reco-side-_5-3-方法区内存溢出" data-v-b57cc07c>5.3 方法区内存溢出</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_5-4-运行时常量池" class="sidebar-link reco-side-_5-4-运行时常量池" data-v-b57cc07c>5.4 运行时常量池</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_5-5-stringtable" class="sidebar-link reco-side-_5-5-stringtable" data-v-b57cc07c>5.5 StringTable</a></li><li class="level-2" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_6-直接内存" class="sidebar-link reco-side-_6-直接内存" data-v-b57cc07c>6 直接内存</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_6-1-定义" class="sidebar-link reco-side-_6-1-定义" data-v-b57cc07c>6.1 定义</a></li><li class="level-3" data-v-b57cc07c><a href="/blogs/java/jvm/JVM-%E5%86%85%E5%AD%98%E7%BB%93%E6%9E%84.html#_6-2-使用直接内存" class="sidebar-link reco-side-_6-2-使用直接内存" data-v-b57cc07c>6.2 使用直接内存</a></li><li class="level-3" data-v-b57cc07c><a 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