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<article id="post-java双亲委派模式" class="h-entry article article-type-post" itemprop="blogPost" itemscope itemtype="https://schema.org/BlogPosting">
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<a href="/2024/05/09/java%E5%8F%8C%E4%BA%B2%E5%A7%94%E6%B4%BE%E6%A8%A1%E5%BC%8F/" class="article-date">
<time class="dt-published" datetime="2024-05-09T12:37:36.000Z" itemprop="datePublished">2024-05-09</time>
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<a class="p-name article-title" href="/2024/05/09/java%E5%8F%8C%E4%BA%B2%E5%A7%94%E6%B4%BE%E6%A8%A1%E5%BC%8F/">java双亲委派模式</a>
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<h1 id="为什么需要双亲委派机制"><a href="#为什么需要双亲委派机制" class="headerlink" title="为什么需要双亲委派机制"></a>为什么需要双亲委派机制</h1><p><strong>双亲委派机制</strong>是Java虚拟机（JVM）中的一种类加载机制。</p>
<p>在Java中，类加载器（ClassLoader）负责将类的字节码加载到内存中，并创建对应的Class对象。当一个类被加载时，JVM会按照一定的顺序去搜索类的定义。双亲委派机制就是基于这种搜索顺序来工作的。</p>
<p>具体来说，当一个类加载器收到加载类的请求时，它首先会将这个请求委派给它的父类加载器去处理。如果父类加载器能够找到并加载这个类，那么加载过程就结束了。如果父类加载器无法找到这个类，那么子类加载器才会尝试加载这个类。</p>
<p>这种双亲委派机制的好处在于可以避免类的重复加载和冲突。当一个类被加载后，它会被缓存起来，下次再次加载时可以直接使用缓存的结果，提高了加载的效率。另外，由于父类加载器优先加载类，所以可以保证类的一致性，避免了不同类加载器加载同一个类的问题。</p>
<p>双亲委派机制还有助于保护核心类库的安全性。JVM中有一个根类加载器（Bootstrap ClassLoader），它负责加载核心类库，如java.lang包下的类。由于根类加载器是由JVM实现的，无法通过Java代码直接访问，因此可以防止恶意代码替换核心类库。</p>
<p><strong>总结</strong></p>
<ul>
<li><strong>解决类加载的冲突和重复加载的问题</strong>。</li>
<li><strong>沙箱安全机制：防止Java核心API库被随意篡改。通俗点说，当Java核心类库中的类（比如：java.lang.String）的类名与用户自定义类的类名相同有冲突时，Java不会加载用户自定义的类。</strong></li>
</ul>
<h1 id="java中双亲委派机制如何实现的"><a href="#java中双亲委派机制如何实现的" class="headerlink" title="java中双亲委派机制如何实现的"></a>java中双亲委派机制如何实现的</h1><img src="/2024/05/09/java%E5%8F%8C%E4%BA%B2%E5%A7%94%E6%B4%BE%E6%A8%A1%E5%BC%8F/%E7%B1%BB%E5%8A%A0%E8%BD%BD%E5%99%A8%E7%A4%BA%E6%84%8F%E5%9B%BE.png" class="" title="This is an example image">
<p>在java中的类加载器模型图如上，分为三层。</p>
<p>BootstrapClassLoader: 启动类加载器，最上层类加载器为本地方法，主要加载 jre/lib/rt.jar 路径内类文件</p>
<p>ExtClassLoader：扩展类加载器 主要加载 jre/lib/ext/*.jar</p>
<p>AppClassLoader： 用来加载CLASS_PATH 下所有jar，即项目所有class</p>
<p>UserClassLoader： 自定义类加载器，该类加载器是否遵循类加载机制，或特殊操作可以自己实现，打破类加载机机制也由该类加载器实现。</p>
<h2 id="源码解析"><a href="#源码解析" class="headerlink" title="源码解析"></a>源码解析</h2><p>测试代码如下</p>
<p> 尝试用AppClassLoader 去加载 java.lang.String</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="keyword">class</span> <span class="title class_">Main</span> {</span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title function_">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> ClassNotFoundException {</span><br><span class="line"></span><br><span class="line"> <span class="type">ClassLoader</span> <span class="variable">appClassLoader</span> <span class="operator">=</span> Main.class.getClassLoader();</span><br><span class="line"> <span class="comment">// AppClassLoader</span></span><br><span class="line"> System.out.println(<span class="string">"当前类加载器为："</span> + appClassLoader);</span><br><span class="line"></span><br><span class="line"> Class<?> aClass = appClassLoader.loadClass(<span class="string">"java.lang.String"</span>);</span><br><span class="line"> <span class="type">ClassLoader</span> <span class="variable">StringClassLoader</span> <span class="operator">=</span> aClass.getClassLoader();</span><br><span class="line"> System.out.println(<span class="string">"String类加载器为："</span> + StringClassLoader);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>测试结果：</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">当前类加载器为：sun.misc.Launcher$AppClassLoader@14dad5dc</span><br><span class="line">String类加载器为：null</span><br></pre></td></tr></table></figure>
<p>解释： </p>
<p>因为<code>String</code>类是由引导类加载器加载的，这个类加载器是JVM的一部分，不是一个标准的Java类加载器。由引导类加载器加载的类不会有一个对应的类加载器实例，因此返回<code>null</code>。</p>
<p>类加载机制关键方法为loadClass， 接下来进行刨析jkd源码，去探索jdk如何实现的双亲委派机制</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">static</span> <span class="keyword">class</span> <span class="title class_">AppClassLoader</span> <span class="keyword">extends</span> <span class="title class_">URLClassLoader</span> {</span><br><span class="line"> <span class="comment">// 获取当前项目lib 及本身项目jar包路径</span></span><br><span class="line"> <span class="keyword">final</span> <span class="type">URLClassPath</span> <span class="variable">ucp</span> <span class="operator">=</span> SharedSecrets.getJavaNetAccess().getURLClassPath(<span class="built_in">this</span>);</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> <span class="keyword">static</span> ClassLoader <span class="title function_">getAppClassLoader</span><span class="params">(<span class="keyword">final</span> ClassLoader var0)</span> <span class="keyword">throws</span> IOException {</span><br><span class="line"> <span class="keyword">final</span> <span class="type">String</span> <span class="variable">var1</span> <span class="operator">=</span> System.getProperty(<span class="string">"java.class.path"</span>);</span><br><span class="line"> <span class="keyword">final</span> File[] var2 = var1 == <span class="literal">null</span> ? <span class="keyword">new</span> <span class="title class_">File</span>[<span class="number">0</span>] : Launcher.getClassPath(var1);</span><br><span class="line"> <span class="keyword">return</span> (ClassLoader)AccessController.doPrivileged(<span class="keyword">new</span> <span class="title class_">PrivilegedAction</span><AppClassLoader>() {</span><br><span class="line"> <span class="keyword">public</span> AppClassLoader <span class="title function_">run</span><span class="params">()</span> {</span><br><span class="line"> URL[] var1x = var1 == <span class="literal">null</span> ? <span class="keyword">new</span> <span class="title class_">URL</span>[<span class="number">0</span>] : Launcher.pathToURLs(var2);</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">new</span> <span class="title class_">AppClassLoader</span>(var1x, var0);</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> AppClassLoader(URL[] var1, ClassLoader var2) {</span><br><span class="line"> <span class="built_in">super</span>(var1, var2, Launcher.factory);</span><br><span class="line"> <span class="built_in">this</span>.ucp.initLookupCache(<span class="built_in">this</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">public</span> Class<?> loadClass(String var1, <span class="type">boolean</span> var2) <span class="keyword">throws</span> ClassNotFoundException {</span><br><span class="line"> <span class="type">int</span> <span class="variable">var3</span> <span class="operator">=</span> var1.lastIndexOf(<span class="number">46</span>);</span><br><span class="line"> <span class="keyword">if</span> (var3 != -<span class="number">1</span>) {</span><br><span class="line"> <span class="type">SecurityManager</span> <span class="variable">var4</span> <span class="operator">=</span> System.getSecurityManager();</span><br><span class="line"> <span class="keyword">if</span> (var4 != <span class="literal">null</span>) {</span><br><span class="line"> var4.checkPackageAccess(var1.substring(<span class="number">0</span>, var3));</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// 判断当前项目 及 lib目录下有没有java.lang.String 显然没有</span></span><br><span class="line"> <span class="keyword">if</span> (<span class="built_in">this</span>.ucp.knownToNotExist(var1)) {</span><br><span class="line"> <span class="type">Class</span> <span class="variable">var5</span> <span class="operator">=</span> <span class="built_in">this</span>.findLoadedClass(var1);</span><br><span class="line"> <span class="keyword">if</span> (var5 != <span class="literal">null</span>) {</span><br><span class="line"> <span class="keyword">if</span> (var2) {</span><br><span class="line"> <span class="built_in">this</span>.resolveClass(var5);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">return</span> var5;</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> <span class="title class_">ClassNotFoundException</span>(var1);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// 调用父类loadClass， 父类loadClass 方法内就包含了双亲委派机制</span></span><br><span class="line"> <span class="keyword">return</span> <span class="built_in">super</span>.loadClass(var1, var2);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">protected</span> PermissionCollection <span class="title function_">getPermissions</span><span class="params">(CodeSource var1)</span> {</span><br><span class="line"> <span class="type">PermissionCollection</span> <span class="variable">var2</span> <span class="operator">=</span> <span class="built_in">super</span>.getPermissions(var1);</span><br><span class="line"> var2.add(<span class="keyword">new</span> <span class="title class_">RuntimePermission</span>(<span class="string">"exitVM"</span>));</span><br><span class="line"> <span class="keyword">return</span> var2;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">void</span> <span class="title function_">appendToClassPathForInstrumentation</span><span class="params">(String var1)</span> {</span><br><span class="line"> <span class="keyword">assert</span> Thread.holdsLock(<span class="built_in">this</span>);</span><br><span class="line"></span><br><span class="line"> <span class="built_in">super</span>.addURL(Launcher.getFileURL(<span class="keyword">new</span> <span class="title class_">File</span>(var1)));</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> AccessControlContext <span class="title function_">getContext</span><span class="params">(File[] var0)</span> <span class="keyword">throws</span> MalformedURLException {</span><br><span class="line"> <span class="type">PathPermissions</span> <span class="variable">var1</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">PathPermissions</span>(var0);</span><br><span class="line"> <span class="type">ProtectionDomain</span> <span class="variable">var2</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">ProtectionDomain</span>(<span class="keyword">new</span> <span class="title class_">CodeSource</span>(var1.getCodeBase(), (Certificate[])<span class="literal">null</span>), var1);</span><br><span class="line"> <span class="type">AccessControlContext</span> <span class="variable">var3</span> <span class="operator">=</span> <span class="keyword">new</span> <span class="title class_">AccessControlContext</span>(<span class="keyword">new</span> <span class="title class_">ProtectionDomain</span>[]{var2});</span><br><span class="line"> <span class="keyword">return</span> var3;</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">static</span> {</span><br><span class="line"> ClassLoader.registerAsParallelCapable();</span><br><span class="line"> }</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p>父类方法如下</p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">protected</span> Class<?> loadClass(String name, <span class="type">boolean</span> resolve)</span><br><span class="line"> <span class="keyword">throws</span> ClassNotFoundException</span><br><span class="line"> {</span><br><span class="line"> <span class="keyword">synchronized</span> (getClassLoadingLock(name)) {</span><br><span class="line"> <span class="comment">// First, check if the class has already been loaded</span></span><br><span class="line"> <span class="comment">// 首先检查是否已经被本类加载器加载过了，findLoadedClass 本地方法不去关注</span></span><br><span class="line"> Class<?> c = findLoadedClass(name);</span><br><span class="line"> <span class="keyword">if</span> (c == <span class="literal">null</span>) {</span><br><span class="line"> <span class="type">long</span> <span class="variable">t0</span> <span class="operator">=</span> System.nanoTime();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="comment">// 如果父类加载器不为空就调用 父类加载器的loadClass方法，即委托父类去加载</span></span><br><span class="line"> <span class="keyword">if</span> (parent != <span class="literal">null</span>) {</span><br><span class="line"> c = parent.loadClass(name, <span class="literal">false</span>);</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">// 如果父类加载器为空就调用 BootstraploadClass方法，即委托Bootstrap类加载器去加载</span></span><br><span class="line"> c = findBootstrapClassOrNull(name);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">catch</span> (ClassNotFoundException e) {</span><br><span class="line"> <span class="comment">// ClassNotFoundException thrown if class not found</span></span><br><span class="line"> <span class="comment">// from the non-null parent class loader</span></span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (c == <span class="literal">null</span>) {</span><br><span class="line"> <span class="comment">// If still not found, then invoke findClass in order</span></span><br><span class="line"> <span class="comment">// to find the class.</span></span><br><span class="line"> <span class="type">long</span> <span class="variable">t1</span> <span class="operator">=</span> System.nanoTime();</span><br><span class="line"> c = findClass(name);</span><br><span class="line"></span><br><span class="line"> <span class="comment">// this is the defining class loader; record the stats</span></span><br><span class="line"> sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);</span><br><span class="line"> sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);</span><br><span class="line"> sun.misc.PerfCounter.getFindClasses().increment();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (resolve) {</span><br><span class="line"> resolveClass(c);</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> c;</span><br><span class="line"> }</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p>从上述源码显而易见，双亲委派机制本质上就是一个责任链， 每个子类加载器中都包含父类加载器， 一层层递归调用上去实现了双亲委派机制，既然了解了该机制那么破坏双亲委派机制就很简单了，只需要重写该方法时候不去调用parent.loadClass(name, false)即可。</p>
<h1 id="为什么需要破坏双亲委派机制"><a href="#为什么需要破坏双亲委派机制" class="headerlink" title="为什么需要破坏双亲委派机制"></a>为什么需要破坏双亲委派机制</h1><p>目前我接触到的主要是为了解决类加载冲突：</p>
<p>例如：</p>
<p>1.在<a target="_blank" rel="noopener" href="https://www.baidu.com/s?rsv_idx=1&wd=Tomcat&fenlei=256&usm=1&ie=utf-8&rsv_pq=a7e3089400508c19&oq=%E4%B8%BA%E4%BB%80%E4%B9%88%E9%9C%80%E8%A6%81%E7%A0%B4%E5%9D%8F%E5%8F%8C%E4%BA%B2%E5%A7%94%E6%B4%BE%E6%9C%BA%E5%88%B6&rsv_t=72ec6RJWthdLwxwGhlDtXuAxsmOk2aRSy5IYSAV2SwMP9+HCDp8c9QQmHAI&sa=re_dqa_zy&icon=1">Tomcat<em></em></a>中，不同的Web应用程序可能使用不同版本的jar包</p>
<p>2.javaAgent技术中引入三方依赖极易产生jar包冲突等现象，所以需要破坏双亲委派机制 例如sermant <a target="_blank" rel="noopener" href="https://bbs.huaweicloud.com/blogs/410414">https://bbs.huaweicloud.com/blogs/410414</a> 讲解很不错</p>
<h1 id="如何破坏类加载机制"><a href="#如何破坏类加载机制" class="headerlink" title="如何破坏类加载机制"></a>如何破坏类加载机制</h1><p>暂无</p>
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