第一部分请看： http://vipcowrie.iteye.com/blog/1561291
本文面向的是JAVA初学者，建议你们自己编译和运行例子程序。
本文包含了NoClassDefFoundError的原因分析和例子程序，并且给出了建议的处理策略。
NoClassDefFoundError 问题原因1：缺少jar包
首先最常见的原因是classpath的配置问题。例子程序：
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：
程序
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!");
}
}
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();
}
}
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...
}
}
打印classloader工具类：
public class ClassloaderUtil {
public static String getCurrentClassloaderDetail() {
StringBuffer classLoaderDetail = new StringBuffer();
Stack classLoaderStack = new Stack();
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();
}
}
正常运行：
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!
异常重现：
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.(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
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。
classloader分析
注意：
Classloading of NoClassDefFoundError.CallerClassA in progress... ----------------------------------------------------------------- sun.misc.Launcher$ExtClassLoader@addbf1 --- delegation --- sun.misc.Launcher$AppClassLoader@19821f **Current ClassLoader** -------------------------
sun.misc.Launcher$AppClassLoader是系统的classloader，负责根据classpath设置在启动的时候加载应用需要的class。
sun.misc.Launcher$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。
从打印结果可以看出，sun.misc.Launcher$ExtClassLoader是系统classloader的实际父类。
建议处理策略
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。