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www/public/blogs/java/jvm/JVM-类加载.html
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="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="active 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之类加载</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之类加载</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>2022/12/9</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-类加载阶段"><a href="#_1-类加载阶段" class="header-anchor">#</a> 1 类加载阶段</h2> <h3 id="_1-1-加载"><a href="#_1-1-加载" class="header-anchor">#</a> 1.1 加载</h3> <ul><li><p>将类的字节码载入方法区中，内部采用 C++ 的 <code>instanceKlass</code> 描述 java 类，它的重要 <code>field</code> 有：</p> <ul><li><p><code>_java_mirror</code> 即 java 的类镜像，例如对 String 来说，就是 String.class，作用是把 klass 暴露给 java 使用，是一个桥梁</p></li> <li><p><code>_super</code> 即父类</p></li> <li><p><code>_fields</code> 即成员变量</p></li> <li><p><code>_methods</code> 即方法</p></li> <li><p><code>_constants</code> 即常量池</p></li> <li><p><code>_class_loader</code> 即类加载器</p></li> <li><p><code>_vtable</code> 虚方法表</p></li> <li><p><code>_itable</code> 接口方法表</p></li></ul></li> <li><p>如果这个类还有父类没有加载，先加载父类</p></li> <li><p>加载和链接可能是交替运行的</p></li></ul> <blockquote><p><strong>注意</strong></p> <ul><li><code>instanceKlass</code> 这样的【元数据】是存储在方法区（1.8 后的元空间内），但 <code>_java_mirror</code> 是存储在堆中</li> <li>可以通过前面介绍的 <code>HSDB</code> 工具查看</li></ul></blockquote> <p><img src="/jvm/1670419734596.png" alt="1670419734596"></p> <h3 id="_1-2-链接"><a href="#_1-2-链接" class="header-anchor">#</a> 1.2 链接</h3> <h4 id="_1-验证"><a href="#_1-验证" class="header-anchor">#</a> 1) 验证</h4> <p>验证类是否符合 JVM规范，安全性检查</p> <p>修改 <code>HelloWorld.class</code> 的魔数，在控制台运行</p> <ul><li><code>Incompatible magic value</code>：JVM 检查到了不兼容的魔数</li></ul> <div class="language-shell line-numbers-mode"><pre class="language-shell"><code>G:<span class="token punctuation">\</span>Idea_workspace<span class="token punctuation">\</span>JVM<span class="token punctuation">\</span>src<span class="token punctuation">\</span>main<span class="token punctuation">\</span>java<span class="token operator">&gt;</span>java HelloWorld
Error: A JNI error has occurred, please check your installation and try again
Exception <span class="token keyword">in</span> thread <span class="token string">&quot;main&quot;</span> java.lang.ClassFormatError: Incompatible magic value <span class="token number">1514689864</span> <span class="token keyword">in</span> class <span class="token function">file</span> HelloWorld
at java.lang.ClassLoader.defineClass1<span class="token punctuation">(</span>Native Method<span class="token punctuation">)</span>
at java.lang.ClassLoader.defineClass<span class="token punctuation">(</span>ClassLoader.java:756<span class="token punctuation">)</span>
at java.security.SecureClassLoader.defineClass<span class="token punctuation">(</span>SecureClassLoader.java:142<span class="token punctuation">)</span>
at java.net.URLClassLoader.defineClass<span class="token punctuation">(</span>URLClassLoader.java:473<span class="token punctuation">)</span>
at java.net.URLClassLoader.access<span class="token variable">$100</span><span class="token punctuation">(</span>URLClassLoader.java:74<span class="token punctuation">)</span>
at java.net.URLClassLoader<span class="token variable">$1</span>.run<span class="token punctuation">(</span>URLClassLoader.java:369<span class="token punctuation">)</span>
at java.net.URLClassLoader<span class="token variable">$1</span>.run<span class="token punctuation">(</span>URLClassLoader.java:363<span class="token punctuation">)</span>
at java.security.AccessController.doPrivileged<span class="token punctuation">(</span>Native Method<span class="token punctuation">)</span>
at java.net.URLClassLoader.findClass<span class="token punctuation">(</span>URLClassLoader.java:362<span class="token punctuation">)</span>
at java.lang.ClassLoader.loadClass<span class="token punctuation">(</span>ClassLoader.java:418<span class="token punctuation">)</span>
at sun.misc.Launcher<span class="token variable">$AppClassLoader</span>.loadClass<span class="token punctuation">(</span>Launcher.java:355<span class="token punctuation">)</span>
at java.lang.ClassLoader.loadClass<span class="token punctuation">(</span>ClassLoader.java:351<span class="token punctuation">)</span>
at sun.launcher.LauncherHelper.checkAndLoadMain<span class="token punctuation">(</span>LauncherHelper.java:601<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><h4 id="_2-准备"><a href="#_2-准备" class="header-anchor">#</a> 2) 准备</h4> <p>为 static 变量分配空间，设置默认值</p> <ul><li>static 变量在 <code>JDK 7</code> 之前存储于 <code>instanceKlass</code> 末尾，从<code>JDK 7</code> 开始，存储于 <code>_java_mirror</code>末尾</li> <li>static 变量<em>分配空间和赋值是两个步骤</em>，分配空间在准备阶段完成，赋值在初始化阶段完成</li> <li>如果 static 变量是 <code>final</code> 的<em>基本类型，以及字符串常量</em>，那么编译阶段值就确定了，赋值在<strong>准备阶段</strong>完成</li> <li>如果 static 变量是 <code>final</code> 的，但属于<em>引用类型</em>，那么赋值也会在<strong>初始化阶段</strong>完成</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">Load1</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> a<span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> b <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> c <span class="token operator">=</span> <span class="token number">30</span><span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> d <span class="token operator">=</span> <span class="token string">&quot;qwe&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> e <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;asd&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>以上代码的字节码为，印证以下结果</p> <ul><li>static 关键字声明的常量在 <strong>编译阶段完成</strong> 空间分配，在 <strong>初始化阶段</strong> 完成赋值</li> <li>static + final 关键字声明的常量且是 <em>基本类型或字符串常量</em>，在 <strong>编译阶段</strong> 就有了确定的值，且在 <strong>准备阶段</strong> 完成赋值</li> <li>static + final 关键字声明的常量且是 <em>引用类型</em>，需要在 <strong>初始化阶段</strong> 完成赋值</li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>static int a;
descriptor: I
flags: ACC_STATIC
static int b;
descriptor: I
flags: ACC_STATIC
static final int c;
descriptor: I
flags: ACC_STATIC, ACC_FINAL
ConstantValue: int 30
static final java.lang.String d;
descriptor: Ljava/lang/String;
flags: ACC_STATIC, ACC_FINAL
ConstantValue: String qwe
static final java.lang.String e;
descriptor: Ljava/lang/String;
flags: ACC_STATIC, ACC_FINAL
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=3, locals=0, args_size=0
0: bipush 20
2: putstatic #2 // 20 -&gt; b
5: new #3 // class java/lang/String
8: dup
9: ldc #4 // &lt;- &quot;asd&quot;
11: invokespecial #5 // Method java/lang/String.&quot;&lt;init&gt;&quot;:(Ljava/lang/String;)V
14: putstatic #6 // &quot;asd&quot; -&gt; e
17: 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><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></div></div><h4 id="_3-解析"><a href="#_3-解析" class="header-anchor">#</a> 3) 解析</h4> <p>将常量池中的符号引用解析为直接引用</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">package</span> <span class="token namespace">com<span class="token punctuation">.</span>zhuhjay<span class="token punctuation">.</span>demo4</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Load2</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">ClassLoader</span> classLoader <span class="token operator">=</span> <span class="token class-name">Load2</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 使用 loadClass 方法加载的类不会触发类的解析和初始化</span>
<span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> aClass <span class="token operator">=</span> classLoader<span class="token punctuation">.</span><span class="token function">loadClass</span><span class="token punctuation">(</span><span class="token string">&quot;com.zhuhjay.demo4.C&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 punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">C</span> <span class="token punctuation">{</span>
<span class="token class-name">D</span> d <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">D</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">D</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><p>接下来打开 <code>HSDB</code> 工具（在 JDK 目录下执行命令） <code>java -cp ./lib/sa-jdi.jar sun.jvm.hotspot.HSDB</code></p> <p>查看是否存在类 D，以及类 C 中的常量池信息</p> <ul><li>以上代码执行后，只会存在类C对象，而类D对象只存在字节码信息，不会进行解析</li></ul> <p><img src="/jvm/1670427087000.png" alt="1670427087000"></p> <p>而如果直接创建了对象，那么就会存在类C和类D对象，此时类C和类D将会被解析完</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">package</span> <span class="token namespace">com<span class="token punctuation">.</span>zhuhjay<span class="token punctuation">.</span>demo4</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Load2</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">ClassLoader</span> classLoader <span class="token operator">=</span> <span class="token class-name">Load2</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 使用 loadClass 方法加载的类不会触发类的解析和初始化</span>
<span class="token comment">// Class&lt;?&gt; aClass = classLoader.loadClass(&quot;com.zhuhjay.demo4.C&quot;);</span>
<span class="token keyword">new</span> <span class="token class-name">C</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></div></div><h3 id="_1-3-初始化"><a href="#_1-3-初始化" class="header-anchor">#</a> 1.3 初始化</h3> <h4 id="clinit-v"><a href="#clinit-v" class="header-anchor">#</a> <code>&lt;clinit&gt;()V</code></h4> <p>初始化即调用 <code>&lt;clinit&gt;()V</code> ，虚拟机会保证这个类的『构造方法』的线程安全</p> <h4 id="发生时机"><a href="#发生时机" class="header-anchor">#</a> 发生时机</h4> <p>概括得说，类初始化是【懒惰的】</p> <ul><li>main 方法所在的类，总会被首先初始化</li> <li>首次访问这个类的静态变量或静态方法时</li> <li>子类初始化，如果父类还没初始化，会引发</li> <li>子类访问父类的静态变量，只会触发父类的初始化</li> <li>Class.forName</li> <li>new 会导致初始化</li></ul> <p>不会导致类初始化的情况</p> <ul><li>访问类的 static final 静态常量（基本类型和字符串）不会触发初始化</li> <li>类.class 不会触发初始化</li> <li>创建该类的数组不会触发初始化</li> <li>类加载器的 loadClass 方法</li> <li>Class.forName 的参数 2 为 false</li></ul> <p>使用以下代码对以上进行验证（依次执行）</p> <ol start="0"><li>当只有一个空的 mian 方法进行执行时，会有 <code>main init</code> ：main 方法所在的类，总会被首先初始化</li> <li><code>final static double b = 5.0</code> 不会导致初始化过程产生：访问类的 static final 静态常量（基本类型和字符串）不会触发初始化</li> <li><code>B.class</code> 不会触发初始化</li> <li><code>new B[0]</code> 不会触发初始化</li> <li><code>c1.loadClass(&quot;com.zhuhjay.demo4.B&quot;)</code> 会加载类B以及父类A，但不会触发初始化</li> <li><code>Class.forName(&quot;com.zhuhjay.demo4.B&quot;, false, c2)</code> 会加载类B以及父类A，但不会触发初始化</li> <li><code>static int a = 0</code> 在初始化阶段才进行赋值，此时会触发初始化</li> <li><code>static boolean c = false</code> 在初始化阶段进行赋值，子类初始化触发会引发父类初始化</li> <li>使用子类访问父类 <code>static int a = 0</code>，只会触发父类的初始化</li> <li><code>Class.forName(&quot;com.zhuhjay.demo4.B&quot;)</code> 会触发初始化，并且父类先进行初始化</li></ol> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">package</span> <span class="token namespace">com<span class="token punctuation">.</span>zhuhjay<span class="token punctuation">.</span>demo4</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Load3</span> <span class="token punctuation">{</span>
<span class="token keyword">static</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;main init&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</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 comment">// 1. 静态常量不会触发初始化</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 class-name">B</span><span class="token punctuation">.</span>b<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 2. 类.class 不会触发初始化</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 class-name">B</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 3. 创建类的数组不会触发初始化</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 keyword">new</span> <span class="token class-name">B</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 punctuation">;</span>
<span class="token comment">// 4. 不会初始化类B，但会加载 B、A</span>
<span class="token class-name">ClassLoader</span> c1 <span class="token operator">=</span> <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getContextClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
c1<span class="token punctuation">.</span><span class="token function">loadClass</span><span class="token punctuation">(</span><span class="token string">&quot;com.zhuhjay.demo4.B&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 5. 不会初始化类B， 但会加载 B、A</span>
<span class="token class-name">ClassLoader</span> c2 <span class="token operator">=</span> <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getContextClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">&quot;com.zhuhjay.demo4.B&quot;</span><span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">,</span> c2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 6. 首次访问这个类的静态变量或静态方法时</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 class-name">A</span><span class="token punctuation">.</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 7. 子类初始化，如果父类还没初始化，会引发</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 class-name">B</span><span class="token punctuation">.</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 8. 子类访问父类的静态变量，只会触发父类的初始化</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 class-name">B</span><span class="token punctuation">.</span>a<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 9. 会初始化类B，并先初始化类A</span>
<span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">&quot;com.zhuhjay.demo4.B&quot;</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">A</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> a <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">static</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;a init&quot;</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">B</span> <span class="token keyword">extends</span> <span class="token class-name">A</span> <span class="token punctuation">{</span>
<span class="token keyword">final</span> <span class="token keyword">static</span> <span class="token keyword">double</span> b <span class="token operator">=</span> <span class="token number">5.0</span><span class="token punctuation">;</span>
<span class="token keyword">static</span> <span class="token keyword">boolean</span> c <span class="token operator">=</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token keyword">static</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;b init&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><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></div></div><h3 id="_1-4-练习"><a href="#_1-4-练习" class="header-anchor">#</a> 1.4 练习</h3> <p>判断以下代码哪些会触发E的初始化</p> <ul><li><code>public static final Integer c = 20</code> 在编译阶段会转换为 <code>public static final Integer c = Integer.valueOf(20)</code>，访问变量c会触发初始化方法</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">Load4</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 class-name">E</span><span class="token punctuation">.</span>a<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 class-name">E</span><span class="token punctuation">.</span>b<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 class-name">E</span><span class="token punctuation">.</span>c<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">E</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> a <span class="token operator">=</span> <span class="token number">10</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">String</span> b <span class="token operator">=</span> <span class="token string">&quot;qwe&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">Integer</span> c <span class="token operator">=</span> <span class="token number">20</span><span class="token punctuation">;</span>
<span class="token keyword">static</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;E init&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><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><p>懒惰初始化单例模式</p> <ul><li>懒惰的、线程安全的</li> <li>只有在调用 <code>getInstance()</code> 方法的时候， 静态内部类 <code>LazyHolder</code> 才会被初始化</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">Load5</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">Singleton</span><span class="token punctuation">.</span><span class="token function">test</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">class</span> <span class="token class-name">Singleton</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token class-name">Singleton</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">class</span> <span class="token class-name">LazyHolder</span> <span class="token punctuation">{</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token class-name">Singleton</span> <span class="token constant">INSTANCE</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Singleton</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">static</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;LazyHolder init&quot;</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">public</span> <span class="token keyword">static</span> <span class="token class-name">Singleton</span> <span class="token function">getInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token class-name">LazyHolder</span><span class="token punctuation">.</span><span class="token constant">INSTANCE</span><span class="token punctuation">;</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">test</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;test method&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><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><h2 id="_2-类加载器"><a href="#_2-类加载器" class="header-anchor">#</a> 2 类加载器</h2> <p>以 JDK 8 为例：</p> <table><thead><tr><th>名称</th> <th>加载哪的类</th> <th>说明</th></tr></thead> <tbody><tr><td>Bootstrap ClassLoader</td> <td>JAVA_HOME/jre/lib</td> <td>无法直接访问</td></tr> <tr><td>Extension ClassLoader</td> <td>JAVA_HOME/jre/lib/ext</td> <td>上级为 Bootstrap，显示为 null</td></tr> <tr><td>Application ClassLoader</td> <td>classpath</td> <td>上级为 Extension</td></tr> <tr><td>自定义类加载器</td> <td>自定义</td> <td>上级为 Application</td></tr></tbody></table> <p>在加载一个类时，首先会向上级的类加载器&quot;询问&quot;是否可以进行类的加载，如果上级加载器可以加载，则由上级加载器加载，否则由 Application ClassLoader 进行类的加载。这就是<strong>双亲委派机制</strong></p> <ul><li>由于 Bootstrap ClassLoader 是由 C++ 运行的，所以该类加载器无法直接访问，以 null 存在</li></ul> <h3 id="_2-1-启动类加载器"><a href="#_2-1-启动类加载器" class="header-anchor">#</a> 2.1 启动类加载器</h3> <p>用 Bootstrap 类加载器加载类：</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">package</span> <span class="token namespace">com<span class="token punctuation">.</span>zhuhjay<span class="token punctuation">.</span>demo4</span><span class="token punctuation">;</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Load6</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">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> aClass <span class="token operator">=</span> <span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">&quot;com.zhuhjay.demo4.F&quot;</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>aClass<span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">class</span> <span class="token class-name">F</span> <span class="token punctuation">{</span>
<span class="token keyword">static</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;Bootstrap F init&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><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>将上述代码编译执行一次过后，打开对应的class文件输出目录，执行命令 <code>java -Xbootclasspath/a:. com.zhuhjay.demo4.Load6</code> 会出现以下输出结果，<code>aClass.getClassLoader()</code> 结果为 <code>null</code> 即为 Bootstrap 类加载器</p> <ul><li><code>-Xbootclasspath</code> 表示设置 <code>bootclasspath</code></li> <li>其中 <code>/a:.</code> 表示将当前目录追加至 <code>bootclasspath</code> 之后</li> <li>可以用这个办法替换核心类（修改 Bootstrap 类加载器 加载的目录）
<ul><li><code>java -Xbootclasspath:&lt;new bootclasspath&gt;</code></li> <li><code>java -Xbootclasspath/a:&lt;追加路径&gt;</code>：后追加</li> <li><code>java -Xbootclasspath/p:&lt;追加路径&gt;</code>：前追加</li></ul></li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>Bootstrap F init
null
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-2-扩展类加载器"><a href="#_2-2-扩展类加载器" class="header-anchor">#</a> 2.2 扩展类加载器</h3> <p>编译执行以下代码</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">G</span> <span class="token punctuation">{</span>
<span class="token keyword">static</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;ext classpath G init&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><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">Load7</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">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> aClass <span class="token operator">=</span> <span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">&quot;com.zhuhjay.demo4.G&quot;</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>aClass<span class="token punctuation">.</span><span class="token function">getClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-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>移步至 class 输出目录下，使用命令 <code>jar -cvf my.jar com/zhuhjay/demo4/G.class</code> 对类G生成jar包，将生成后的jar包复制到 JDK 目录下的 <code>jre/lib/ext</code> 中，然后将 G 改为以下</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">G</span> <span class="token punctuation">{</span>
<span class="token keyword">static</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;classpath G init&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>再次执行 Load7 得到结果</p> <ul><li>执行的类G是在 JDK 目录下的，并且由 Extension ClassLoader 进行该类的创建，使得自己写的类G失效，印证了双亲委派（启动&gt;扩展&gt;应用&gt;自定义）</li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>ext classpath G init
sun.misc.Launcher$ExtClassLoader@7f31245a
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br></div></div><h3 id="_2-3-双亲委派模式"><a href="#_2-3-双亲委派模式" class="header-anchor">#</a> 2.3 双亲委派模式</h3> <p>所谓的双亲委派，就是指调用类加载器的 <code>loadClass</code> 方法时，查找类的规则</p> <ul><li>双亲委派并没有<strong>继承关系</strong>，而是一种委托</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">protected</span> <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> <span class="token function">loadClass</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">,</span> <span class="token keyword">boolean</span> resolve<span class="token punctuation">)</span>
<span class="token keyword">throws</span> <span class="token class-name">ClassNotFoundException</span>
<span class="token punctuation">{</span>
<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token function">getClassLoadingLock</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// First, check if the class has already been loaded</span>
<span class="token comment">// 1. 检查该类是否已经加载</span>
<span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> c <span class="token operator">=</span> <span class="token function">findLoadedClass</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>c <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">long</span> t0 <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 keyword">if</span> <span class="token punctuation">(</span>parent <span class="token operator">!=</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 2. 如果存在上级加载器，那么将委托执行 loadClass</span>
c <span class="token operator">=</span> parent<span class="token punctuation">.</span><span class="token function">loadClass</span><span class="token punctuation">(</span>name<span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
<span class="token comment">// 3. 如果没有上级（ExtClassLoader），则委派 BootstrapClassLoader</span>
c <span class="token operator">=</span> <span class="token function">findBootstrapClassOrNull</span><span class="token punctuation">(</span>name<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">ClassNotFoundException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// ClassNotFoundException thrown if class not found</span>
<span class="token comment">// from the non-null parent class loader</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>c <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// If still not found, then invoke findClass in order</span>
<span class="token comment">// to find the class.</span>
<span class="token keyword">long</span> t1 <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 comment">// 4. 都找不到，那么就调用 findClass（每个类加载器自己扩展）来加载</span>
c <span class="token operator">=</span> <span class="token function">findClass</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// this is the defining class loader; record the stats</span>
<span class="token comment">// 5. 记录耗时</span>
<span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>PerfCounter</span><span class="token punctuation">.</span><span class="token function">getParentDelegationTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">addTime</span><span class="token punctuation">(</span>t1 <span class="token operator">-</span> t0<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>PerfCounter</span><span class="token punctuation">.</span><span class="token function">getFindClassTime</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">addElapsedTimeFrom</span><span class="token punctuation">(</span>t1<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name"><span class="token namespace">sun<span class="token punctuation">.</span>misc<span class="token punctuation">.</span></span>PerfCounter</span><span class="token punctuation">.</span><span class="token function">getFindClasses</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">increment</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">if</span> <span class="token punctuation">(</span>resolve<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">resolveClass</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> c<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></div></div><h3 id="_2-4-线程上下文类加载器"><a href="#_2-4-线程上下文类加载器" class="header-anchor">#</a> 2.4 线程上下文类加载器</h3> <p>我们在使用 JDBC 的时候，都需要加载 Driver 驱动，发现不写 <code>Class.forName(&quot;com.mysql.jdbc.Driver&quot;);</code> 也能正确加载该驱动并进行使用（需要引入驱动jar）</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token comment">// 注册驱动</span>
<span class="token comment">// Class.forName(&quot;com.mysql.jdbc.Driver&quot;);</span>
<span class="token comment">// 使用 DriverManager 获取连接</span>
<span class="token class-name">Connection</span> connection <span class="token operator">=</span> <span class="token class-name">DriverManager</span><span class="token punctuation">.</span><span class="token function">getConnection</span><span class="token punctuation">(</span><span class="token string">&quot;jdbc:mysql:///test?useSSL=false&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;root&quot;</span><span class="token punctuation">,</span> <span class="token string">&quot;root&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Statement</span> statement <span class="token operator">=</span> connection<span class="token punctuation">.</span><span class="token function">createStatement</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">ResultSet</span> resultSet <span class="token operator">=</span> statement<span class="token punctuation">.</span><span class="token function">executeQuery</span><span class="token punctuation">(</span><span class="token string">&quot;select * from user&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
resultSet<span class="token punctuation">.</span><span class="token function">next</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>resultSet<span class="token punctuation">.</span><span class="token function">getString</span><span class="token punctuation">(</span><span class="token string">&quot;username&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></div></div><p>看看 DriverManager 源码，可以发现它好像自动去加载了驱动。</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">DriverManager</span> <span class="token punctuation">{</span>
<span class="token comment">// 注册驱动的集合</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token keyword">static</span> <span class="token class-name">CopyOnWriteArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">DriverInfo</span><span class="token punctuation">&gt;</span></span> registeredDrivers <span class="token operator">=</span>
<span class="token keyword">new</span> <span class="token class-name">CopyOnWriteArrayList</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">static</span> <span class="token punctuation">{</span>
<span class="token comment">// 初始化驱动</span>
<span class="token function">loadInitialDrivers</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;JDBC DriverManager initialized&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><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></div></div><p>先获取看看 DriverManager 的类加载器，可以发现打印结果为 null，证明说 DriverManager 是通过 Bootstrap ClassLoader 进行加载的，会到 <code>JAVA_HOME/jre/lib</code> 目录下进行类的搜索，但是显然 mysql驱动 文件并不是出现在 <code>JAVA_HOME/jre/lib</code> 目录下面，那为什么它却可以加载到 <code>com.mysql.jdbc.Driver</code>？</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><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 class-name">DriverManager</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getClassLoader</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></div></div><p>看看源码中 <code>loadInitialDrivers()</code> 方法如何进行工作的</p> <ul><li><code>Class.forName(aDriver, true, ClassLoader.getSystemClassLoader());</code> 打破双亲委派机制，直接使用应用类加载器来进行驱动的类加载</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">loadInitialDrivers</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">String</span> drivers<span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
drivers <span class="token operator">=</span> <span class="token class-name">AccessController</span><span class="token punctuation">.</span><span class="token function">doPrivileged</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PrivilegedAction</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><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> <span class="token class-name">System</span><span class="token punctuation">.</span><span class="token function">getProperty</span><span class="token punctuation">(</span><span class="token string">&quot;jdbc.drivers&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
drivers <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token comment">// 1) 使用 ServiceLoader 机制加载驱动，即 SPI</span>
<span class="token class-name">AccessController</span><span class="token punctuation">.</span><span class="token function">doPrivileged</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">PrivilegedAction</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Void</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">public</span> <span class="token class-name">Void</span> <span class="token function">run</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">ServiceLoader</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Driver</span><span class="token punctuation">&gt;</span></span> loadedDrivers <span class="token operator">=</span> <span class="token class-name">ServiceLoader</span><span class="token punctuation">.</span><span class="token function">load</span><span class="token punctuation">(</span><span class="token class-name">Driver</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Driver</span><span class="token punctuation">&gt;</span></span> driversIterator <span class="token operator">=</span> loadedDrivers<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">try</span><span class="token punctuation">{</span>
<span class="token keyword">while</span><span class="token punctuation">(</span>driversIterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
driversIterator<span class="token punctuation">.</span><span class="token function">next</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">Throwable</span> t<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// Do nothing</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;DriverManager.initialize: jdbc.drivers = &quot;</span> <span class="token operator">+</span> drivers<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 2) 使用 jdbc.drivers 定义的驱动名加载驱动</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>drivers <span class="token operator">==</span> <span class="token keyword">null</span> <span class="token operator">||</span> drivers<span class="token punctuation">.</span><span class="token function">equals</span><span class="token punctuation">(</span><span class="token string">&quot;&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> driversList <span class="token operator">=</span> drivers<span class="token punctuation">.</span><span class="token function">split</span><span class="token punctuation">(</span><span class="token string">&quot;:&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;number of Drivers:&quot;</span> <span class="token operator">+</span> driversList<span class="token punctuation">.</span>length<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">String</span> aDriver <span class="token operator">:</span> driversList<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
<span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;DriverManager.Initialize: loading &quot;</span> <span class="token operator">+</span> aDriver<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 这里的 ClassLoader.getSystemClassLoader() 就是应用程序加载器</span>
<span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span>aDriver<span class="token punctuation">,</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
<span class="token class-name">ClassLoader</span><span class="token punctuation">.</span><span class="token function">getSystemClassLoader</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> ex<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;DriverManager.Initialize: load failed: &quot;</span> <span class="token operator">+</span> ex<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><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></div></div><p><code>1)</code> 中使用的就是大名鼎鼎的 <code>Service Provider Interface (SPI)</code></p> <ul><li><p>约定如下，在 jar 包的 <code>META-INF/services</code> 包下，以接口全限定名名为文件，文件内容是实现类名称（就像 <code>SpringBoot</code> 中使用的 <code>spring.factories</code> 文件）</p> <img src="/jvm/1670493540488.png" alt="1670493540488" style="zoom:50%;"></li></ul> <p>之后就可以使用以下 ServiceLoader 加载器来加载信息</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token class-name">ServiceLoader</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Driver</span><span class="token punctuation">&gt;</span></span> loadedDrivers <span class="token operator">=</span> <span class="token class-name">ServiceLoader</span><span class="token punctuation">.</span><span class="token function">load</span><span class="token punctuation">(</span><span class="token class-name">Driver</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Driver</span><span class="token punctuation">&gt;</span></span> driversIterator <span class="token operator">=</span> loadedDrivers<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">while</span><span class="token punctuation">(</span>driversIterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
driversIterator<span class="token punctuation">.</span><span class="token function">next</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>体现的是【面向接口编程+解耦】的思想，在下面一些框架中都运用了此思想：</p> <ul><li>JDBC</li> <li>Servlet 初始化器</li> <li>Spring 容器</li> <li>Dubbo（对 SPI 进行了扩展）</li></ul> <p>接着看看 <code>ServiceLoader.load(Driver.class)</code> 方法</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">S</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">ServiceLoader</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">S</span><span class="token punctuation">&gt;</span></span> <span class="token function">load</span><span class="token punctuation">(</span><span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">S</span><span class="token punctuation">&gt;</span></span> service<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// 获取线程上下文类加载器</span>
<span class="token class-name">ClassLoader</span> cl <span class="token operator">=</span> <span class="token class-name">Thread</span><span class="token punctuation">.</span><span class="token function">currentThread</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getContextClassLoader</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> <span class="token class-name">ServiceLoader</span><span class="token punctuation">.</span><span class="token function">load</span><span class="token punctuation">(</span>service<span class="token punctuation">,</span> cl<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>线程上下文类加载器是当前线程使用的类加载器，默认就是应用程序类加载器，它内部又是由 <code>Class.forName</code> 调用了线程上下文类加载器完成类加载，具体代码在 <code>ServiceLoader</code> 的内部类 <code>LazyIterator</code> 中：</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token class-name">S</span> <span class="token function">nextService</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token function">hasNextService</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">NoSuchElementException</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">String</span> cn <span class="token operator">=</span> nextName<span class="token punctuation">;</span>
nextName <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> c <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">{</span>
c <span class="token operator">=</span> <span class="token class-name">Class</span><span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span>cn<span class="token punctuation">,</span> <span class="token boolean">false</span><span class="token punctuation">,</span> loader<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">ClassNotFoundException</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">fail</span><span class="token punctuation">(</span>service<span class="token punctuation">,</span>
<span class="token string">&quot;Provider &quot;</span> <span class="token operator">+</span> cn <span class="token operator">+</span> <span class="token string">&quot; not found&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>service<span class="token punctuation">.</span><span class="token function">isAssignableFrom</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">fail</span><span class="token punctuation">(</span>service<span class="token punctuation">,</span>
<span class="token string">&quot;Provider &quot;</span> <span class="token operator">+</span> cn <span class="token operator">+</span> <span class="token string">&quot; not a subtype&quot;</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">S</span> p <span class="token operator">=</span> service<span class="token punctuation">.</span><span class="token function">cast</span><span class="token punctuation">(</span>c<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
providers<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span>cn<span class="token punctuation">,</span> p<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">return</span> p<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> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">fail</span><span class="token punctuation">(</span>service<span class="token punctuation">,</span>
<span class="token string">&quot;Provider &quot;</span> <span class="token operator">+</span> cn <span class="token operator">+</span> <span class="token string">&quot; could not be instantiated&quot;</span><span class="token punctuation">,</span>
x<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">Error</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// This cannot happen</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></div></div><h3 id="_2-5-自定义类加载器"><a href="#_2-5-自定义类加载器" class="header-anchor">#</a> 2.5 自定义类加载器</h3> <p>什么时候需要自定义类加载器</p> <ol><li>想加载非 classpath 随意路径中的类文件</li> <li>都是通过接口来使用实现，希望解耦时，常用在框架设计</li> <li>这些类希望予以隔离，不同应用的同名类都可以加载，不冲突，常见于 tomcat 容器</li></ol> <p>步骤：</p> <ol><li>继承 ClassLoader 父类</li> <li>要遵从双亲委派机制，重写 findClass 方法 注意不是重写 loadClass 方法，否则不会走双亲委派机制</li> <li>读取类文件的字节码</li> <li>调用父类的 defineClass 方法来加载类</li> <li>使用者调用该类加载器的 loadClass 方法</li></ol> <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">CustomClassLoader</span> <span class="token keyword">extends</span> <span class="token class-name">ClassLoader</span><span class="token punctuation">{</span>
<span class="token annotation punctuation">@Override</span>
<span class="token keyword">protected</span> <span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> <span class="token function">findClass</span><span class="token punctuation">(</span><span class="token class-name">String</span> name<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">ClassNotFoundException</span> <span class="token punctuation">{</span>
<span class="token comment">// 类加载路径</span>
<span class="token class-name">String</span> path <span class="token operator">=</span> <span class="token string">&quot;G:\\classpath\\&quot;</span> <span class="token operator">+</span> name <span class="token operator">+</span> <span class="token string">&quot;.class&quot;</span><span class="token punctuation">;</span>
<span class="token keyword">try</span> <span class="token punctuation">(</span><span class="token class-name">ByteArrayOutputStream</span> os <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ByteArrayOutputStream</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">Files</span><span class="token punctuation">.</span><span class="token function">copy</span><span class="token punctuation">(</span><span class="token class-name">Paths</span><span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>path<span class="token punctuation">)</span><span class="token punctuation">,</span> os<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">// 获取字节码数据的 byte 数组</span>
<span class="token keyword">byte</span><span class="token punctuation">[</span><span class="token punctuation">]</span> bytes <span class="token operator">=</span> os<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">// 加载 Class 文件</span>
<span class="token keyword">return</span> <span class="token function">defineClass</span><span class="token punctuation">(</span>name<span class="token punctuation">,</span> bytes<span class="token punctuation">,</span> <span class="token number">0</span><span class="token punctuation">,</span> bytes<span class="token punctuation">.</span>length<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">IOException</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 keyword">throw</span> <span class="token keyword">new</span> <span class="token class-name">ClassNotFoundException</span><span class="token punctuation">(</span><span class="token string">&quot;类文件未找到&quot;</span><span class="token punctuation">,</span> e<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<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></div></div><blockquote><p><strong>Tip</strong></p> <p>​ 判断两个类是否是同一个类：<strong>同样的包名、类名和使用相同的类加载器</strong></p></blockquote> <h2 id="_3-运行期优化"><a href="#_3-运行期优化" class="header-anchor">#</a> 3 运行期优化</h2> <h3 id="_3-1-即时编译"><a href="#_3-1-即时编译" class="header-anchor">#</a> 3.1 即时编译</h3> <h4 id="_1-分层编译"><a href="#_1-分层编译" class="header-anchor">#</a> 1) 分层编译</h4> <p>TieredCompilation</p> <p>现来执行以下代码</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">JIT1</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">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>
<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">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">1000</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 keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token 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">printf</span><span class="token punctuation">(</span><span class="token string">&quot;%d\t%d\n&quot;</span><span class="token punctuation">,</span> i<span class="token punctuation">,</span> <span class="token punctuation">(</span>end <span class="token operator">-</span>start<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-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>输出一下结果（进行了截断获取），发现</p> <ul><li>执行到差不多 69 次，发现已经快了将近4倍</li> <li>执行到差不多 147 次，发现已经快了将近100倍</li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>0 30200
1 23800
...
67 18400
68 9100
...
145 9600
146 400
</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><p>原因是什么呢？</p> <p>JVM 将执行状态分成了 5 个层次：</p> <ul><li>0 层，解释执行（Interpreter）</li> <li>1 层，使用 C1 即时编译器编译执行（不带 profiling）</li> <li>2 层，使用 C1 即时编译器编译执行（带基本的 profiling）</li> <li>3 层，使用 C1 即时编译器编译执行（带完全的 profiling）</li> <li>4 层，使用 C2 即时编译器编译执行</li></ul> <blockquote><p>profiling 是指在运行过程中收集一些程序执行状态的数据，例如【方法的调用次数】，【循环的 回边次数】等</p></blockquote> <p>即时编译器（JIT）与解释器的区别</p> <ul><li>解释器是将字节码解释为机器码，下次即使遇到相同的字节码，仍会执行重复的解释</li> <li>JIT 是将一些字节码编译为机器码，并存入 Code Cache，下次遇到相同的代码，直接执行，无需再编译</li> <li>解释器是将字节码解释为针对所有平台都通用的机器码</li> <li>JIT 会根据平台类型，生成平台特定的机器码</li></ul> <p>对于占据大部分的不常用的代码，我们无需耗费时间将其编译成机器码，而是采取解释执行的方式运 行；另一方面，对于仅占据小部分的热点代码，我们则可以将其编译成机器码，以达到理想的运行速 度。 执行效率上简单比较一下 <code>Interpreter &lt; C1 &lt; C2</code>，总的目标是发现热点代码（<code>hotspot</code>名称的由来），优化之</p> <p>刚才的一种优化手段称之为【逃逸分析】，发现新建的对象是否逃逸。可以使用 <code>-XX:-DoEscapeAnalysis</code> 关闭逃逸分析，再运行刚才的示例观察结果</p> <p><a href="https://docs.oracle.com/en/java/javase/12/vm/java-hotspot-virtual-machine-performance-enhancements.html#GUID-D2E3DC58-D18B-4A6C-8167-4A1DFB4888E4" target="_blank" rel="noopener noreferrer">参考资料: Java HotSpot Virtual Machine Performance Enhancements (oracle.com)<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> <h4 id="_2-方法内联"><a href="#_2-方法内联" class="header-anchor">#</a> 2) 方法内联</h4> <p>Inlining</p> <p>定义一个计算方法 square</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">square</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> i <span class="token operator">*</span> i<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></div></div><p>将其调用</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><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 function">square</span><span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token number">9</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></div></div><p>如果发现 square 是热点方法，并且长度不太长时，会进行内联，所谓的内联就是把方法内代码拷贝、 粘贴到调用者的位置：</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><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 number">9</span> <span class="token operator">*</span> <span class="token number">9</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></div></div><p>还能够进行常量折叠 (Constant Folding) 的优化</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><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 number">81</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></div></div><p>进行实验代码测试</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">JIT2</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">int</span> <span class="token function">square</span><span class="token punctuation">(</span><span class="token keyword">final</span> <span class="token keyword">int</span> i<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">return</span> i <span class="token operator">*</span> i<span class="token punctuation">;</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> x <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token 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">500</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 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">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">1000</span><span class="token punctuation">;</span> j<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
x <span class="token operator">=</span> <span class="token function">square</span><span class="token punctuation">(</span><span class="token number">9</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">printf</span><span class="token punctuation">(</span><span class="token string">&quot;%d\t%d\n&quot;</span><span class="token punctuation">,</span> i<span class="token punctuation">,</span> <span class="token punctuation">(</span>end <span class="token operator">-</span>start<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-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></div></div><p>输出一下结果（进行了截断获取），发现</p> <ul><li>越后面的执行越快，已经被 JVM 优化了</li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>0 1335400
1 38700
2 83100
...
64 9800
65 6400
...
252 0
253 0
</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><p>接下来进行添加虚拟机参数来查看一些优化信息</p> <ul><li><p><code>-XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining</code>：打印方法内联的信息</p> <p>执行信息如下(只截取了一部分)</p> <ul><li>可以看见 square 方法被优化了三次，第三次甚至被标记为热点代码 <code>inline (hot)</code></li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>...
@ 28 com.zhuhjay.demo5.JIT2::square (4 bytes)
@ 38 java.lang.System::nanoTime (0 bytes) intrinsic
@ 11 java.lang.System::nanoTime (0 bytes) intrinsic
@ 28 com.zhuhjay.demo5.JIT2::square (4 bytes)
@ 38 java.lang.System::nanoTime (0 bytes) intrinsic
@ 28 com.zhuhjay.demo5.JIT2::square (4 bytes) inline (hot)
@ 38 java.lang.System::nanoTime (0 bytes) (intrinsic)
@ 11 java.lang.System::nanoTime (0 bytes) (intrinsic)
</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><code>-XX:CompileCommand=dontinline,*JIT2.square</code>：禁止某个方法的方法内联，因为 Java 中也有很多使用了方法内联的，全部禁用会影响效率</p> <p>执行信息如下(只截取部分进行展示)</p> <ul><li>到了最后一次执行，也不能够优化为 0</li></ul> <div class="language- line-numbers-mode"><pre class="language-text"><code>CompilerOracle: dontinline *JIT2.square
0 106000
1 24000
...
65 6300
66 6600
...
498 5500
499 4800
</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><code>-XX:+PrintCompilation</code>：打印编译信息</p></li></ul> <h4 id="_3-字段优化"><a href="#_3-字段优化" class="header-anchor">#</a> 3) 字段优化</h4> <p>JMH 基准测试请参考：<a href="http://openjdk.java.net/projects/code-tools/jmh/" target="_blank" rel="noopener noreferrer">http://openjdk.java.net/projects/code-tools/jmh/<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>为了保证配置的正确性，建议使用 archetype 生成 JMH 配置项目。cmd 运行下面这段代码：</p> <ul><li>解读：创建一个文件夹 jmh-test，里面包含完整的 jmh 配置 pom 文件</li></ul> <div class="language-shell line-numbers-mode"><pre class="language-shell"><code>mvn archetype:generate ^
<span class="token parameter variable">-DinteractiveMode</span><span class="token operator">=</span>false ^
<span class="token parameter variable">-DarchetypeGroupId</span><span class="token operator">=</span>org.openjdk.jmh ^
<span class="token parameter variable">-DarchetypeArtifactId</span><span class="token operator">=</span>jmh-java-benchmark-archetype ^
<span class="token parameter variable">-DarchetypeVersion</span><span class="token operator">=</span><span class="token number">1.25</span> ^
<span class="token parameter variable">-DgroupId</span><span class="token operator">=</span>com.zhuhjay ^
<span class="token parameter variable">-DartifactId</span><span class="token operator">=</span>jmh-test ^
<span class="token parameter variable">-Dversion</span><span class="token operator">=</span><span class="token number">1.0</span>.0
</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><p>将以上创建的项目导入到 IDEA，并进行 Maven 加载，可能需要将 Maven 中的两个 jar 包使用 <a href="https://mvnrepository.com/" target="_blank" rel="noopener noreferrer">mvnrepository<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> 进行下载，手动使用maven 命令添加到仓库中，如果需要则使用以下命令进行安装</p> <div class="language-shell line-numbers-mode"><pre class="language-shell"><code>mvn install:install-file ^
<span class="token parameter variable">-Dfile</span><span class="token operator">=</span>/filepath/jmh-generator-annprocess-1.25.jar ^
<span class="token parameter variable">-DgroupId</span><span class="token operator">=</span>org.openjdk.jmh ^
<span class="token parameter variable">-DartifactId</span><span class="token operator">=</span>jmh-generator-annprocess ^
<span class="token parameter variable">-Dversion</span><span class="token operator">=</span><span class="token number">1.25</span> ^
<span class="token parameter variable">-Dpackaging</span><span class="token operator">=</span>jar
</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><div class="language-shell line-numbers-mode"><pre class="language-shell"><code>mvn install:install-file ^
<span class="token parameter variable">-Dfile</span><span class="token operator">=</span>/filepath/jmh-core-1.25.jar ^
<span class="token parameter variable">-DgroupId</span><span class="token operator">=</span>org.openjdk.jmh ^
<span class="token parameter variable">-DartifactId</span><span class="token operator">=</span>jmh-core ^
<span class="token parameter variable">-Dversion</span><span class="token operator">=</span><span class="token number">1.25</span> ^
<span class="token parameter variable">-Dpackaging</span><span class="token operator">=</span>jar
</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>可能还需要以下 maven 坐标</p> <div class="language-xml line-numbers-mode"><pre class="language-xml"><code><span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>dependency</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>groupId</span><span class="token punctuation">&gt;</span></span>org.apache.commons<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>groupId</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>artifactId</span><span class="token punctuation">&gt;</span></span>commons-math3<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>artifactId</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;</span>version</span><span class="token punctuation">&gt;</span></span>3.6.1<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>version</span><span class="token punctuation">&gt;</span></span>
<span class="token tag"><span class="token tag"><span class="token punctuation">&lt;/</span>dependency</span><span class="token punctuation">&gt;</span></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>以下为基准测试代码，三个测试方法都是对数字进行累加的操作</p> <ul><li><code>@Warmup(iterations = 2, time = 1)</code> <ul><li>进行几轮热身，让程序预热，使得对其进行充分的优化</li></ul></li> <li><code>@Measurement(iterations = 5, time = 1)</code> <ul><li>设置进行几轮测试，然后取平均值，较为直观</li></ul></li> <li><code>@Benchmark</code> 未来对该注解标识的方法进行测试</li> <li><code>@CompilerControl(CompilerControl.Mode.INLINE)</code> 控制被调用方法使用方法内联</li> <li>三个测试方法区别：
<ol><li>直接使用成员变量进行累加</li> <li>使用局部变量进行累加</li> <li>使用 foreach 对成员变量进行累加</li></ol></li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token annotation punctuation">@Warmup</span><span class="token punctuation">(</span>iterations <span class="token operator">=</span> <span class="token number">2</span><span class="token punctuation">,</span> time <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@Measurement</span><span class="token punctuation">(</span>iterations <span class="token operator">=</span> <span class="token number">5</span><span class="token punctuation">,</span> time <span class="token operator">=</span> <span class="token number">1</span><span class="token punctuation">)</span>
<span class="token annotation punctuation">@State</span><span class="token punctuation">(</span><span class="token class-name">Scope<span class="token punctuation">.</span>Benchmark</span><span class="token punctuation">)</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Benchmark1</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> elements <span class="token operator">=</span> <span class="token function">randomInts</span><span class="token punctuation">(</span><span class="token number">1_000</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">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> <span class="token function">randomInts</span><span class="token punctuation">(</span><span class="token keyword">int</span> size<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Random</span> random <span class="token operator">=</span> <span class="token class-name">ThreadLocalRandom</span><span class="token punctuation">.</span><span class="token function">current</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> values <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token keyword">int</span><span class="token punctuation">[</span>size<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> size<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
values<span class="token punctuation">[</span>i<span class="token punctuation">]</span> <span class="token operator">=</span> random<span class="token punctuation">.</span><span class="token function">nextInt</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> values<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token annotation punctuation">@Benchmark</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test1</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> elements<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">doSum</span><span class="token punctuation">(</span>elements<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>
<span class="token annotation punctuation">@Benchmark</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">int</span><span class="token punctuation">[</span><span class="token punctuation">]</span> local <span class="token operator">=</span> <span class="token keyword">this</span><span class="token punctuation">.</span>elements<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> local<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">doSum</span><span class="token punctuation">(</span>local<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>
<span class="token annotation punctuation">@Benchmark</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test3</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> element <span class="token operator">:</span> elements<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token function">doSum</span><span class="token punctuation">(</span>element<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token keyword">static</span> <span class="token keyword">int</span> sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
<span class="token annotation punctuation">@CompilerControl</span><span class="token punctuation">(</span><span class="token class-name">CompilerControl<span class="token punctuation">.</span>Mode</span><span class="token punctuation">.</span><span class="token constant">INLINE</span><span class="token punctuation">)</span>
<span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">doSum</span><span class="token punctuation">(</span><span class="token keyword">int</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
sum <span class="token operator">+=</span> x<span class="token punctuation">;</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">RunnerException</span> <span class="token punctuation">{</span>
<span class="token class-name">Options</span> opt <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">OptionsBuilder</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">include</span><span class="token punctuation">(</span><span class="token class-name">Benchmark1</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getSimpleName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">forks</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">build</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">new</span> <span class="token class-name">Runner</span><span class="token punctuation">(</span>opt<span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">run</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><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>首先启用 doSum 的方法内联，测试结果如下（每秒吞吐量，分数越高的更好）</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>Benchmark Mode Cnt Score Error Units
Benchmark1.test1 thrpt 5 3109424.624 ± 253596.436 ops/s
Benchmark1.test2 thrpt 5 3354988.133 ± 91779.242 ops/s
Benchmark1.test3 thrpt 5 3192089.287 ± 811172.283 ops/s
</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>接下来禁用 doSum 方法内联</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token annotation punctuation">@CompilerControl</span><span class="token punctuation">(</span><span class="token class-name">CompilerControl<span class="token punctuation">.</span>Mode</span><span class="token punctuation">.</span><span class="token constant">DONT_INLINE</span><span class="token punctuation">)</span>
<span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">doSum</span><span class="token punctuation">(</span><span class="token keyword">int</span> x<span class="token punctuation">)</span> <span class="token punctuation">{</span>
sum <span class="token operator">+=</span> x<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>此时测试结果如下，跟以上结果对比，发现吞吐量小了10倍左右</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>Benchmark Mode Cnt Score Error Units
Benchmark1.test1 thrpt 5 357337.402 ± 131498.372 ops/s
Benchmark1.test2 thrpt 5 394812.201 ± 434387.776 ops/s
Benchmark1.test3 thrpt 5 377967.137 ± 377008.086 ops/s
</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>分析：</p> <p>在刚才的示例中，doSum 方法是否内联会影响 elements 成员变量读取的优化：</p> <p>如果 doSum 方法内联了，刚才的 test1 方法会被优化成下面的样子（伪代码）：</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token annotation punctuation">@Benchmark</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token comment">// elements 首次读取会缓存起来 -&gt; int[] local（机器码级别缓存，后续就不需要再访问 elements 成员变量了）</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> elements<span class="token punctuation">.</span>length<span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span> <span class="token punctuation">{</span> <span class="token comment">// 后续 999 次 求长度 &lt;- local</span>
sum <span class="token operator">+=</span> elements<span class="token punctuation">[</span>i<span class="token punctuation">]</span><span class="token punctuation">;</span> <span class="token comment">// 1000 次取下标 i 的元素 &lt;- local</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>可以节省 1999 次 Field 读取操作</p> <p>但如果 doSum 方法没有内联，则不会进行上面的优化</p> <blockquote><p>练习：在内联情况下将 elements 添加 volatile 修饰符会发生什么？</p> <p>运行结果如下，发现方法一的吞吐量大大降低了</p> <div class="language- line-numbers-mode"><pre class="language-text"><code>Benchmark Mode Cnt Score Error Units
Benchmark1.test1 thrpt 5 660532.339 ± 41637.563 ops/s
Benchmark1.test2 thrpt 5 3356062.311 ± 32060.956 ops/s
Benchmark1.test3 thrpt 5 3359952.483 ± 30073.397 ops/s
</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></blockquote> <blockquote><p><strong>Tip</strong>:</p> <p>这三种方法分别进行了优化</p> <ol><li>如果没有关闭方法内联，则虚拟机将自动进行优化</li> <li>使用了局部变量，避免多次向成员变量进行访问，进行了手动优化</li> <li>foreach 语法糖编译结果与 2 相同，也属于编译器优化</li></ol> <p><strong>注意</strong>：若要对这种类似的进行优化，可以考虑多使用局部变量。</p></blockquote> <h3 id="_3-2-反射优化"><a href="#_3-2-反射优化" class="header-anchor">#</a> 3.2 反射优化</h3> <p>运行以下反射代码，进行代码执行分析</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">Reflect1</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">foo</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;foo method&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</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">Method</span> method <span class="token operator">=</span> <span class="token class-name">Reflect1</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">.</span><span class="token function">getMethod</span><span class="token punctuation">(</span><span class="token string">&quot;foo&quot;</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">16</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 class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">printf</span><span class="token punctuation">(</span><span class="token string">&quot;%d\t&quot;</span><span class="token punctuation">,</span> i<span class="token punctuation">)</span><span class="token punctuation">;</span>
method<span class="token punctuation">.</span><span class="token function">invoke</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 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><span class="line-number">13</span><br><span class="line-number">14</span><br></div></div><p>在方法反射的 invoke 源码中，会发现反射调用是通过一个接口 <code>MethodAccessor</code> 来执行的</p> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token annotation punctuation">@CallerSensitive</span>
<span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">invoke</span><span class="token punctuation">(</span><span class="token class-name">Object</span> obj<span class="token punctuation">,</span> <span class="token class-name">Object</span><span class="token punctuation">.</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">IllegalAccessException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">,</span>
<span class="token class-name">InvocationTargetException</span>
<span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span>override<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">!</span><span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">quickCheckMemberAccess</span><span class="token punctuation">(</span>clazz<span class="token punctuation">,</span> modifiers<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token class-name">Class</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> caller <span class="token operator">=</span> <span class="token class-name">Reflection</span><span class="token punctuation">.</span><span class="token function">getCallerClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token function">checkAccess</span><span class="token punctuation">(</span>caller<span class="token punctuation">,</span> clazz<span class="token punctuation">,</span> obj<span class="token punctuation">,</span> modifiers<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">MethodAccessor</span> ma <span class="token operator">=</span> methodAccessor<span class="token punctuation">;</span> <span class="token comment">// read volatile</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>ma <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
ma <span class="token operator">=</span> <span class="token function">acquireMethodAccessor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> ma<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>obj<span class="token punctuation">,</span> args<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></div></div><p><code>method.invoke(null)</code> 前面 0 ~ 15 次调用使用的是 <code>MethodAccessor</code> 的 <code>NativeMethodAccessorImpl</code> 实现，是个本地方法</p> <ul><li>步入到 <code>ReflectionFactory.inflationThreshold()</code> 中可以发现该值为 15，是一个膨胀阈值，就是执行本地方法反射的最大执行次数</li> <li>当超过了膨胀阈值之后，就会使用 ASM 动态生成的新实现代替本地实现，执行速度较本地快 20 倍左右</li></ul> <div class="language-java line-numbers-mode"><pre class="language-java"><code><span class="token keyword">class</span> <span class="token class-name">NativeMethodAccessorImpl</span> <span class="token keyword">extends</span> <span class="token class-name">MethodAccessorImpl</span> <span class="token punctuation">{</span>
<span class="token keyword">private</span> <span class="token keyword">final</span> <span class="token class-name">Method</span> method<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token class-name">DelegatingMethodAccessorImpl</span> parent<span class="token punctuation">;</span>
<span class="token keyword">private</span> <span class="token keyword">int</span> numInvocations<span class="token punctuation">;</span>
<span class="token class-name">NativeMethodAccessorImpl</span><span class="token punctuation">(</span><span class="token class-name">Method</span> var1<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method <span class="token operator">=</span> var1<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token class-name">Object</span> <span class="token function">invoke</span><span class="token punctuation">(</span><span class="token class-name">Object</span> var1<span class="token punctuation">,</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> var2<span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">IllegalArgumentException</span><span class="token punctuation">,</span> <span class="token class-name">InvocationTargetException</span> <span class="token punctuation">{</span>
<span class="token comment">// 判断是否超过预设的膨胀阈值</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token operator">++</span><span class="token keyword">this</span><span class="token punctuation">.</span>numInvocations <span class="token operator">&gt;</span> <span class="token class-name">ReflectionFactory</span><span class="token punctuation">.</span><span class="token function">inflationThreshold</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token operator">&amp;&amp;</span>
<span class="token operator">!</span><span class="token class-name">ReflectUtil</span><span class="token punctuation">.</span><span class="token function">isVMAnonymousClass</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getDeclaringClass</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">// 使用 ASM 动态生成新的反射调用</span>
<span class="token class-name">MethodAccessorImpl</span> var3 <span class="token operator">=</span>
<span class="token punctuation">(</span><span class="token class-name">MethodAccessorImpl</span><span class="token punctuation">)</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">MethodAccessorGenerator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">generateMethod</span><span class="token punctuation">(</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getDeclaringClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getParameterTypes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getReturnType</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getExceptionTypes</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">.</span><span class="token function">getModifiers</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">this</span><span class="token punctuation">.</span>parent<span class="token punctuation">.</span><span class="token function">setDelegate</span><span class="token punctuation">(</span>var3<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">return</span> <span class="token function">invoke0</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>method<span class="token punctuation">,</span> var1<span class="token punctuation">,</span> var2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">void</span> <span class="token function">setParent</span><span class="token punctuation">(</span><span class="token class-name">DelegatingMethodAccessorImpl</span> var1<span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">this</span><span class="token punctuation">.</span>parent <span class="token operator">=</span> var1<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">native</span> <span class="token class-name">Object</span> <span class="token function">invoke0</span><span class="token punctuation">(</span><span class="token class-name">Method</span> var0<span class="token punctuation">,</span> <span class="token class-name">Object</span> var1<span class="token punctuation">,</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> var2<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></div></div><p>当调用到第 16 次（从0开始算）时，会采用运行时生成的类代替掉最初的实现，可以通过 debug 得到 类名为 <code>sun.reflect.GeneratedMethodAccessor1</code></p> <img src="/jvm/1670569560207.png" alt="1670569560207" style="zoom:67%;"> <p>可以使用 arthas 工具来进行查看</p> <ul><li>进入到对应的运行进程</li> <li>输入命令 <code>jad sun.reflect.GeneratedMethodAccessor1</code> 进行反编译</li></ul> <img src="/jvm/1670570534000.png" alt="1670569967783" style="zoom:67%;"> <blockquote><p><strong>注意</strong></p> <p>通过查看 <code>ReflectionFactory</code> 源码可知</p> <ul><li><code>sun.reflect.noInflation</code> 可以用来禁用膨胀（直接生成 <code>GeneratedMethodAccessor1</code>，但首 次生成比较耗时，如果仅反射调用一次，不划算）</li> <li><code>sun.reflect.inflationThreshold</code> 可以修改膨胀阈值</li></ul></blockquote></div></section> <footer class="page-edit"><!----> <!----></footer> <div class="page-nav"><p class="inner"><span class="prev"><a href="/blogs/java/jvm/JVM-字节码技术.html" class="prev">
JVM之字节码技术
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