ClassNotFoundException和NoClassDefFoundError的区别

在实践中添加一个可能的原因:

ClassNotFoundException:正如cletus所说，当接口的继承类不在类路径中时使用接口。例如，服务提供者模式(或服务定位器)试图定位一些不存在的类 NoClassDefFoundError:找到了给定的类，但没有找到给定类的依赖项

在实践中，Error可能会被无声地抛出，例如，你提交了一个计时器任务，在计时器任务中它抛出了Error，而在大多数情况下，你的程序只捕获Exception。然后Timer主循环结束，没有任何信息。与NoClassDefFoundError类似的错误是ExceptionInInitializerError，当您的静态初始化器或静态变量的初始化器抛出异常时。

当我需要复习的时候，我会一遍又一遍地提醒自己

ClassNotFoundException

类层次结构

ClassNotFoundException extends ReflectiveOperationException extends Exception extends Throwable

在调试

Required jar，类路径中缺少类。 验证所有必需的jar都在jvm的类路径中。

NoClassDefFoundError

类层次结构

NoClassDefFoundError extends LinkageError  extends Error extends Throwable

在调试

Problem with loading a class dynamically, which was compiled properly Problem with static blocks, constructors, init() methods of dependent class and the actual error is wrapped by multiple layers [especially when you use spring, hibernate the actual exception is wrapped and you will get NoClassDefError] When you face "ClassNotFoundException" under a static block of dependent class Problem with versions of class. This happens when you have two versions v1, v2 of same class under different jar/packages, which was compiled successfully using v1 and v2 is loaded at the runtime which doesn't has the relevant methods/vars& you will see this exception. [I once resolved this issue by removing the duplicate of log4j related class under multiple jars that appeared in the classpath]

当试图通过String引用该类来加载该类时，将抛出ClassNotFoundException。例如，Class.forName()中的参数to是一个字符串，这可能会将无效的二进制名称传递给类加载器。

当遇到可能无效的二进制名称时抛出ClassNotFoundException;例如，如果类名有'/'字符，你一定会得到一个ClassNotFoundException。当直接引用的类在类路径上不可用时，它也会被抛出。

另一方面，NoClassDefFoundError被抛出

when the actual physical representation of the class - the .class file is unavailable, or the class been loaded already in a different classloader (usually a parent classloader would have loaded the class and hence the class cannot be loaded again), or if an incompatible class definition has been found - the name in the class file does not match the requested name, or (most importantly) if a dependent class cannot be located and loaded. In this case, the directly referenced class might have been located and loaded, but the dependent class is not available or cannot be loaded. This is a scenario where the directly referenced class can be loaded via a Class.forName or equivalent methods. This indicates a failure in linkage.

简而言之，当类加载器无法找到或加载类定义时，通常会在new()语句或加载先前不存在的类的方法调用上抛出NoClassDefFoundError(与ClassNotFoundException基于字符串的类加载相反)。

最终，当ClassLoader无法加载类时，由ClassLoader实现抛出ClassNotFoundException实例。大多数自定义类加载器实现执行此操作，因为它们扩展了URLClassLoader。通常类加载器不会在任何方法实现上显式抛出NoClassDefFoundError——这个异常通常是由HotSpot编译器中的JVM抛出的，而不是由类加载器本身抛出的。

ClassNotFoundException和NoClassDefFoundError的区别

给定类装入器sussystem动作:

这是一篇帮助我理解差异的文章:http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-5.html

If an error occurs during class loading, then an instance of a subclass of LinkageError must be thrown at a point in the program that (directly or indirectly) uses the class or interface being loaded. If the Java Virtual Machine ever attempts to load a class C during verification (§5.4.1) or resolution (§5.4.3) (but not initialization (§5.5)), and the class loader that is used to initiate loading of C throws an instance of ClassNotFoundException, then the Java Virtual Machine must throw an instance of NoClassDefFoundError whose cause is the instance of ClassNotFoundException.

所以ClassNotFoundException是NoClassDefFoundError的根本原因。 NoClassDefFoundError是类型加载错误的一种特殊情况，发生在链接步骤。