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sidebar-page" aria-label="自己编译JDK">自己编译JDK</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/neicun-jiegou.html" class="nav-link sidebar-link sidebar-page" aria-label="深入理解JVM的内存结构">深入理解JVM的内存结构</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/whereis-the-object.html" class="nav-link sidebar-link sidebar-page" aria-label="Java 创建的对象到底放在哪">Java 创建的对象到底放在哪</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/gc.html" class="nav-link sidebar-link sidebar-page" aria-label="从头到尾说一次Java垃圾回收">从头到尾说一次Java垃圾回收</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/tujie-gc.html" class="nav-link sidebar-link sidebar-page" aria-label="图解Java的垃圾回收机制">图解Java的垃圾回收机制</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/problem-tools.html" class="nav-link sidebar-link sidebar-page" aria-label="Java问题诊断和排查工具">Java问题诊断和排查工具</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/jit.html" class="nav-link sidebar-link sidebar-page" aria-label="JIT原理解析及实践">JIT原理解析及实践</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/oom.html" class="nav-link sidebar-link sidebar-page" aria-label="内存溢出排查优化实战">内存溢出排查优化实战</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/cpu-percent-100.html" class="nav-link sidebar-link sidebar-page" aria-label="CPU 100% 排查优化实践">CPU 100% 排查优化实践</a><ul class="sidebar-sub-headers"></ul></li><li><a href="/jvm/zongjie.html" class="nav-link sidebar-link sidebar-page" aria-label="JVM 核心知识点总结">JVM 核心知识点总结</a><ul class="sidebar-sub-headers"></ul></li></ul></section></li></ul></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">三、Java企业级开发</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">四、数据库</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">五、计算机基础</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">六、求职面试</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">七、学习资源</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">八、学习建议</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">九、知识库搭建</span><span class="arrow right"></span></button></section></li><li><section class="sidebar-group"><button class="sidebar-heading clickable"><span class="title">十、联系作者</span><span class="arrow right"></span></button></section></li></ul></aside><main class="page" id="main-content"><nav class="breadcrumb disable"></nav><div class="page-title"><h1>JVM到底是如何运行Java代码的？</h1><div class="page-info"><span class="author-info" aria-label="作者🖊" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon author-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="author icon"><path d="M649.6 633.6c86.4-48 147.2-144 147.2-249.6 0-160-128-288-288-288s-288 128-288 288c0 108.8 57.6 201.6 147.2 249.6-121.6 48-214.4 153.6-240 288-3.2 9.6 0 19.2 6.4 25.6 3.2 9.6 12.8 12.8 22.4 12.8h704c9.6 0 19.2-3.2 25.6-12.8 6.4-6.4 9.6-16 6.4-25.6-25.6-134.4-121.6-240-243.2-288z"></path></svg><span><a class="author-item" href="/about-the-author/" target="_blank" rel="noopener noreferrer">沉默王二</a></span><span property="author" content="沉默王二"></span></span><span class="date-info" aria-label="写作日期📅" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon 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2.813-8.036zm-240.067-72.121c-35.458 0-64.286-28.828-64.286-64.286s28.828-64.285 64.286-64.285 64.286 28.828 64.286 64.285-28.829 64.286-64.286 64.286z"></path></svg><ul class="tags-wrapper"><li class="tag tag4 clickable" role="navigation">Java</li></ul><meta property="keywords" content="Java"></span><span class="reading-time-info" aria-label="阅读时间⌛" data-balloon-pos="down" isoriginal="false" pageview="false"><svg xmlns="http://www.w3.org/2000/svg" class="icon timer-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="timer icon"><path d="M799.387 122.15c4.402-2.978 7.38-7.897 7.38-13.463v-1.165c0-8.933-7.38-16.312-16.312-16.312H256.33c-8.933 0-16.311 7.38-16.311 16.312v1.165c0 5.825 2.977 10.874 7.637 13.592 4.143 194.44 97.22 354.963 220.201 392.763-122.204 37.542-214.893 196.511-220.2 389.397-4.661 5.049-7.638 11.651-7.638 19.03v5.825h566.49v-5.825c0-7.379-2.849-13.981-7.509-18.9-5.049-193.016-97.867-351.985-220.2-389.527 123.24-37.67 216.446-198.453 220.588-392.892zM531.16 450.445v352.632c117.674 1.553 211.787 40.778 211.787 88.676H304.097c0-48.286 95.149-87.382 213.728-88.676V450.445c-93.077-3.107-167.901-81.297-167.901-177.093 0-8.803 6.99-15.793 15.793-15.793 8.803 0 15.794 6.99 15.794 15.793 0 80.261 63.69 145.635 142.01 145.635s142.011-65.374 142.011-145.635c0-8.803 6.99-15.793 15.794-15.793s15.793 6.99 15.793 15.793c0 95.019-73.789 172.82-165.96 177.093z"></path></svg><span>大约 6 分钟</span><meta property="timeRequired" content="PT6M"></span></div><hr></div><div class="theme-hope-content"><h1 id="jvm到底是如何运行java代码的" tabindex="-1"><a class="header-anchor" href="#jvm到底是如何运行java代码的" aria-hidden="true">#</a> JVM到底是如何运行Java代码的？</h1><p>“二哥，看了上一篇 <a href="https://mp.weixin.qq.com/s/191I_2CVOxVuyfLVtb4jhg" target="_blank" rel="noopener noreferrer">Hello World<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a> 的程序后，我很好奇，它是怎么在 Run 面板里打印出‘三妹，少看手机少打游戏，好好学，美美哒’呢？”三妹咪了一口麦香可可奶茶后对我说。</p><p>“三妹，我们通常把 Java 分为编译期和运行时，弄清楚这两个阶段就知道原因了。”</p><p>贴一下 HelloWorld 这段代码：</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token doc-comment comment">/**

* <span class="token keyword">@author</span> 微信搜「沉默王二」，回复关键字 PDF

*/</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">HelloWorld</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">"三妹，少看手机少打游戏，好好学，美美哒。"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>点击 IDEA 工具栏中的锤子按钮（Build Project，编译整个项目），如下图所示。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-01.png" alt=""></p><p>这时候，就可以在 src 的同级目录 target 下找到一个名为 HelloWorld.class 的文件。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-02.png" alt=""></p><p>如果找不到的话，在目录上右键选择「Reload from Disk，从磁盘上重新加载」，如下图所示：</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-03.png" alt=""></p><p>可以双击打开它。</p><div class="language-java ext-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">//</span>

<span class="token comment">// Source code recreated from a .class file by IntelliJ IDEA</span>

<span class="token comment">// (powered by Fernflower decompiler)</span>

<span class="token keyword">package</span> <span class="token namespace">com<span class="token punctuation">.</span>itwanger<span class="token punctuation">.</span>five</span><span class="token punctuation">;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">HelloWorld</span> <span class="token punctuation">{</span>

<span class="token keyword">public</span> <span class="token class-name">HelloWorld</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>IDEA 默认会用 Fernflower 反编译工具将字节码文件（后缀为 .class 的文件，也就是 Java 源代码编译后的文件）反编译为我们可以看得懂的 Java 源代码。但实际上，字节码文件并不是这样的，而是：</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>// class version 58.0 (58)

// access flags 0x21

public class com/itwanger/five/HelloWorld {

// compiled from: HelloWorld.java

// access flags 0x1

public <init>()V

LINENUMBER 6 L0

ALOAD 0

INVOKESPECIAL java/lang/Object.<init> ()V

RETURN

LOCALVARIABLE this Lcom/itwanger/five/HelloWorld; L0 L1 0

MAXSTACK = 1

MAXLOCALS = 1

// access flags 0x9

public static main([Ljava/lang/String;)V

LINENUMBER 8 L0

GETSTATIC java/lang/System.out : Ljava/io/PrintStream;

LDC "\u4e09\u59b9\uff0c\u5c11\u770b\u624b\u673a\u5c11\u6253\u6e38\u620f\uff0c\u597d\u597d\u5b66\uff0c\u7f8e\u7f8e\u54d2\u3002"

INVOKEVIRTUAL java/io/PrintStream.println (Ljava/lang/String;)V

LINENUMBER 9 L1

RETURN

LOCALVARIABLE args [Ljava/lang/String; L0 L2 0

MAXSTACK = 2

MAXLOCALS = 1

}

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>是不是就有点懵逼了？新手看到这个很容易头大，不过不要担心，后面我再和大家一块深入研究一下，这里就是感受一下字节码的魅力。</p><p>那这个字节码文件是怎么看到的呢？可以通过 IDEA 菜单栏中的「View」→「Show Bytecode」查看，如下图所示。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-04.png" alt=""></p><p>PS：字节码并不是机器码，操作系统无法直接识别，需要在操作系统上安装不同版本的 Java 虚拟机（JVM）来识别。通常情况下，我们只需要安装不同版本的 JDK（Java Development Kit，Java 开发工具包）就行了，它里面包含了 JRE（Java Runtime Environment，Java 运行时环境），而 JRE 又包含了 JVM。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-05.png" alt=""></p><p>Windows、Linux、MacOS 等操作系统都有相应的 JDK，只要安装好了 JDK 就有了 Java 语言的运行时环境，就可以把一份字节码文件在不同的平台上运行了。可以在 <a href="https://www.oracle.com/java/technologies/javase-jdk11-downloads.html" target="_blank" rel="noopener noreferrer">Oracle 官网<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>上下载不同版本的 JDK。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-06.png" alt=""></p><p>PPS：为什么要查看字节码呢？查看字节码文件更容易让我们搞懂 Java 代码背后的原理，比如搞懂 Java 中的各种语法糖的本质。</p><p>相比于 IDEA 自带的「Show Bytecode」功能，我更推荐 <code>jclasslib</code> 这款插件，可以在插件市场中安装（我已经安装过了）。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-07.png" alt=""></p><p>安装完成之后，点击 View -> Show Bytecode With jclasslib 即可通过 jclasslib 查看字节码文件了（点击之前，光标要停留在对应的类文件上），如下图所示。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-08.png" alt=""></p><p>使用 jclasslib 不仅可以直观地查看类对应的字节码文件，还可以查看类的基本信息、常量池、接口、字段、方法等信息，如下图所示。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-09.png" alt=""></p><p>也就是说，在编译阶段，Java 会将 Java 源代码文件编译为字节码文件。在这个阶段，编译器会进行一些检查工作，比如说，某个关键字是不是写错了，语法上是不是符合预期了，不能有很明显的错误，否则带到运行时再检查出来就会比较麻烦了。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/overview/five-10.png" alt=""></p><p>Java 字节码是沟通 JVM 与 Java 代码的桥梁，下面使用 javap 来稍微看一下字节码到底长什么样子。</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>0 getstatic #2 <java/lang/System.out>

3 ldc #3 <Hello World>

5 invokevirtual #4 <java/io/PrintStream.println>

8 return

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>Java 虚拟机采用基于栈的架构，其指令由操作码和操作数组成。这些<a href="https://tobebetterjavaer.com/jvm/zijiema-zhiling.html" target="_blank" rel="noopener noreferrer">字节码指令<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>，就叫作 opcode。其中，getstatic、ldc、invokevirtual、return 等，就是 opcode，可以看到是比较容易理解的。</p><p>我们继续使用 hexdump 看一下字节码的二进制内容。与以上字节码对应的二进制，就是下面这几个数字：</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>b2 00 02 12 03 b6 00 04 b1

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div></div></div><blockquote><p>注意：这里是二进制文件的16进制表示，也就是hex，一般分析二进制文件都是以hex进行分析。</p></blockquote><p>我们可以看一下它们的对应关系。</p><div class="language-text ext-text line-numbers-mode"><pre class="language-text"><code>0xb2 getstatic 获取静态字段的值

0x12 ldc 常量池中的常量值入栈

0xb6 invokevirtual 运行时方法绑定调用方法

0xb1 return void 函数返回

</code></pre><div class="line-numbers" aria-hidden="true"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>opcode 有一个字节的长度(0~255)，意味着指令集的操作码个数不能操作 256 条。而紧跟在 opcode 后面的是被操作数。比如 b2 00 02，就代表了 <code>getstatic #2 <java/lang/System.out></code>。</p><p>JVM 就是靠解析这些 opcode 和操作数来完成程序的执行的。当我们使用 Java 命令运行 .class 文件的时候，实际上就相当于启动了一个 JVM 进程。</p><p>然后 JVM 会翻译这些字节码，它有两种执行方式。常见的就是解释执行，将 opcode + 操作数翻译成机器代码；另外一种执行方式就是 <a href="https://tobebetterjavaer.com/jvm/jit.html" target="_blank" rel="noopener noreferrer">JIT<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>，也就是我们常说的即时编译，它会在一定条件下将字节码编译成机器码之后再执行。</p><p>这些 .class 文件会被加载、存放到 metaspace 中，等待被调用，这里会有一个<a href="https://tobebetterjavaer.com/jvm/class-load.html" target="_blank" rel="noopener noreferrer">类加载器<span><svg class="external-link-icon" xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewbox="0 0 100 100" width="15" height="15"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path><polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg><span class="external-link-icon-sr-only">open in new window</span></span></a>的概念。</p><p>而 JVM 的程序运行，都是在栈上完成的，这和其他普通程序的执行是类似的，同样分为堆和栈。比如我们现在运行到了 main 方法，就会给它分配一个栈帧。当退出方法体时，会弹出相应的栈帧。你会发现，大多数字节码指令，就是不断的对栈帧进行操作。</p><p>而其他大块数据，是存放在堆上的。</p><p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/jvm/how-run-java-code-91dac706-1c4e-4775-bc4e-b2104283aa04.png" alt=""></p><img src="http://cdn.tobebetterjavaer.com/tobebetterjavaer/images/xingbiaogongzhonghao.png"></div><footer class="page-meta"><div class="meta-item edit-link"><a href="https://github.com/itwanger/toBeBetterJavaer/edit/master/docs/jvm/how-run-java-code.md" rel="noopener noreferrer" target="_blank" aria-label="在 GitHub 上编辑此页" class="nav-link label"><svg xmlns="http://www.w3.org/2000/svg" class="icon edit-icon" viewbox="0 0 1024 1024" fill="currentColor" aria-label="edit icon"><path d="M430.818 653.65a60.46 60.46 0 0 1-50.96-93.281l71.69-114.012 7.773-10.365L816.038 80.138A60.46 60.46 0 0 1 859.225 62a60.46 60.46 0 0 1 43.186 18.138l43.186 43.186a60.46 60.46 0 0 1 0 86.373L588.879 565.55l-8.637 8.637-117.466 68.234a60.46 60.46 0 0 1-31.958 11.229z"></path><path 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