NoClassDefFoundError和ClassNotFoundException之间的区别是什么?
是什么导致它们被抛出?如何解决这些问题?
在修改现有代码以包含新的jar文件时,我经常遇到这些可抛出的文件。
我已经在客户端和服务器端为一个通过webstart分发的java应用程序击中了它们。
我发现的可能原因是:
未包含在代码客户端build.xml中的包
我们正在使用的新jar缺少运行时类路径
版本与上一个jar冲突
当我今天遇到这些问题时,我采取了一种反复尝试的方法来让事情正常运行。我需要更多的清晰和理解。
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]
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。
ClassNotFoundException是一个检查异常,当我们告诉JVM使用class . forname()或ClassLoader.findSystemClass()或ClassLoader.loadClass()方法根据类的字符串名加载类,并且在类路径中没有找到所提到的类时,就会发生这种异常。
大多数情况下,当您试图运行应用程序而不使用所需的JAR文件更新类路径时,会发生此异常。例如,你可能已经看到这个异常时,做JDBC代码连接到你的数据库,即mysql,但你的类路径没有JAR。
NoClassDefFoundError发生错误当JVM试图加载一个特定的类的代码执行(作为一个正常的一部分方法调用或使用新的关键字创建一个实例的一部分),这类在类路径中不存在,但在编译时,因为为了执行程序需要编译它,如果你正在使用一个类不存在编译器会提高编译错误。
下面是简短的描述
你可以阅读关于ClassNotFoundException Vs NoClassDefFoundError的一切来了解更多细节。