<p>在JAVA开发中，main线程中抛出java.lang.NoClassDefFoundError是一个非常普遍且比较难解决的问题。解决这个问题的复杂性主要取决于你的软件大小和中间件部署情况，尤其要考虑在应用中出现的数量众多的classloader的情况。</p><p>本文将从一个比较高的角度看这个问题，主要是介绍java classloader机制。</p><p><span style="font-size: 18px;"><strong>java.lang.NoClassDefFoundError是什么呢？</strong></span></p><p>我们先简单的看一下这个问题，这个runtime异常是JVM抛出的，当JVM发现一个classloader试图去Load一个class，而此class在当前的classloader tree中找不到的时候，就会抛出此异常。</p><p>很明显，这个问题是运行期的问题，在编译期一切正常。</p><p>那么，解决起来很简单，就是把jar包放到classpath下不就行了么？</p><p>ok，到这里还不行，这个问题解决起来不是那么容易的，在运行期的程序classpath中加入缺少的jar包仅仅是一种解决方法。关键是，我们必须掌握此种异常的根本原因，以后解决此问题就可以以不变应万变。这就是我写这个文章的初衷。</p><p>现在，先记住，此问题不一定是由于在classpath中缺少class的定义。</p><p><strong>java classloader概述</strong></p><p>在深入分析之前，我们必须掌握java classloader的基本原理。class loader是一个java对象，它负责load所有的class，负责查找、加载、生成一个class的基本定义信息。classloader自身采用了委托代理机制来查询class，每一个classloader的实例都有一个父classloader，所以，当一个应用的classloader去加载class A的时候，首先发生的事情是classloader委托其父classloader去加载class A，经过一串链式查找后，最终任务会落在JVM的系统启动classloader上。</p><p>那哪里会出问题？当你期望你的应用classloader能加载class A，但是当class A被其任意一个父classloader查询到并加载，那么就可能会出现java.lang.NoClassDefFoundError。当所有的父classloader都找不到class A的时候，才会由应用自己的classloader尝试加载。</p><p>下面给大家说说说NoClassDefFoundError的原因分析和例子程序，并且给出了建议的处理策略。<br/></p><p><strong>NoClassDefFoundError 问题原因：缺少jar包</strong></p><p>首先最常见的原因是classpath的配置问题。例子程序：</p><p>本例子程序尝试创建一个新的CallerClassA实例，然后执行他的一个方法，此方法引用了类ReferencingClassA，本例子演示了classpath问题导致的NoClassDefFoundError ，本例子还打印了当前的classloader chain的情况，以便进一步的分析。这个打印信息对你以后分析此类问题也很有帮助的：</p><pre class="brush:java;toolbar:false">public class NoClassDefFoundErrorSimulator { public static void main(String[] args) { System.out.println("java.lang.NoClassDefFoundError Simulator"); // Print current Classloader context System.out.println("\nCurrent ClassLoader chain: "+ ClassloaderUtil.getCurrentClassloaderDetail()); // 1. Create a new instance of CallerClassA CallerClassA caller = new CallerClassA(); // 2. Execute method of the caller caller.doSomething(); System.out.println("done!"); } }</pre><pre class="brush:java;toolbar:false">public class CallerClassA { private final static String CLAZZ = CallerClassA.class.getName(); static { System.out.println("Classloading of " + CLAZZ + " in progress..."+ ClassloaderUtil.getCurrentClassloaderDetail()); } public CallerClassA() { System.out.println("Creating a new instance of "+ CallerClassA.class.getName() + "..."); } public void doSomething() { // Create a new instance of ReferencingClassA ReferencingClassA referencingClass = new ReferencingClassA(); } }</pre><pre class="brush:java;toolbar:false">public class ReferencingClassA { private final static String CLAZZ = ReferencingClassA.class.getName(); static { System.out.println("Classloading of " + CLAZZ + " in progress..."+ ClassloaderUtil.getCurrentClassloaderDetail()); } public ReferencingClassA() { System.out.println("Creating a new instance of "+ ReferencingClassA.class.getName() + "..."); Maps.newHashMap(); } public void doSomething() { // nothing to do... } }</pre><p>打印classloader工具类：</p><pre class="brush:java;toolbar:false">public class ClassloaderUtil { public static String getCurrentClassloaderDetail() { StringBuffer classLoaderDetail = new StringBuffer(); Stack<ClassLoader> classLoaderStack = new Stack<ClassLoader>(); ClassLoader currentClassLoader = Thread.currentThread().getContextClassLoader(); classLoaderDetail.append("\n------------------------------------\n"); // Build a Stack of the current ClassLoader chain while (currentClassLoader != null) { classLoaderStack.push(currentClassLoader); currentClassLoader = currentClassLoader.getParent(); } // Print ClassLoader parent chain while (classLoaderStack.size() > 0) { ClassLoader classLoader = classLoaderStack.pop(); // Print current classLoaderDetail.append(classLoader); if (classLoaderStack.size() > 0) { classLoaderDetail.append("\n--- delegation ---\n"); } else { classLoaderDetail.append(" **Current ClassLoader**"); } } classLoaderDetail.append("\n------------------------------------\n"); return classLoaderDetail.toString(); } }</pre><p>正常运行：</p><pre class="brush:java;toolbar:false">java -classpath .;../guava-12.0.jar NoClassDefFoundError.NoClassDefFoundErrorSimulator java.lang.NoClassDefFoundError Simulator Current ClassLoader chain: ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** ----------------------------------------------------------------- Classloading of NoClassDefFoundError.CallerClassA in progress... ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** ----------------------------------------------------------------- Creating a new instance of NoClassDefFoundError.CallerClassA... Classloading of NoClassDefFoundError.ReferencingClassA in progress... ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** ----------------------------------------------------------------- Creating a new instance of NoClassDefFoundError.ReferencingClassA... done!</pre><p>异常重现：</p><pre class="brush:java;toolbar:false">java -classpath . NoClassDefFoundError.NoCl java.lang.NoClassDefFoundError Simulator Current ClassLoader chain: ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** ----------------------------------------------------------------- Classloading of NoClassDefFoundError.CallerClassA in progress... ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** ----------------------------------------------------------------- Creating a new instance of NoClassDefFoundError.CallerClassA... Classloading of NoClassDefFoundError.ReferencingClassA in progress... ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** ----------------------------------------------------------------- Creating a new instance of NoClassDefFoundError.ReferencingClassA... Exception in thread "main" java.lang.NoClassDefFoundError: com/google/common/collect/Maps at NoClassDefFoundError.ReferencingClassA.<init>(ReferencingClassA.java:28) at NoClassDefFoundError.CallerClassA.doSomething(CallerClassA.java:31) at NoClassDefFoundError.NoClassDefFoundErrorSimulator.main(NoClassDefFoundErrorSimulator.jav Caused by: java.lang.ClassNotFoundException: com.google.common.collect.Maps at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:306) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:247) ... 3 more</pre><p>发生了什么？当你在classpath中不包含guava的引用的时候，由于ReferencingClassA在运行期引用了此类，导致了classloader报告找不到此类，从而出现NoClassDefFoundError。</p><p><strong>classloader分析</strong></p><p>注意：</p><pre class="brush:java;toolbar:false">Classloading of NoClassDefFoundError.CallerClassA in progress... ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader**</pre><p>sun.misc.Launcher$AppClassLoader是系统的classloader，负责根据classpath设置在启动的时候加载应用需要的class。</p><p>sun.misc.Launcher$ExtClassLoader是扩展classloader，负责从java_home/lib/etc以及其他使用java.ext.dirs配置的目录从加载扩展java class。</p><p>从打印结果可以看出，sun.misc.Launcher$ExtClassLoader是系统classloader的实际父类。</p><p><strong>建议处理策略</strong></p><p>分析异常堆栈，找到缺少的java类名称，在classpath中验证，确保编译和运行期都能找到此类。</p>
