Java中的NoClassDefFoundError

定义:

Java虚拟机无法在运行时找到在编译时可用的特定类。 如果一个类在编译时存在，但在运行时在java类路径中不可用。

例子:

The class is not in Classpath, there is no sure shot way of knowing it but many times you can just have a look to print System.getproperty("java.classpath") and it will print the classpath from there you can at least get an idea of your actual runtime classpath. A simple example of NoClassDefFoundError is class belongs to a missing JAR file or JAR was not added into classpath or sometimes jar's name has been changed by someone like in my case one of my colleagues has changed tibco.jar into tibco_v3.jar and the program is failing with java.lang.NoClassDefFoundError and I were wondering what's wrong. Just try to run with explicitly -classpath option with the classpath you think will work and if it's working then it's a sure short sign that someone is overriding java classpath. Permission issue on JAR file can also cause NoClassDefFoundError in Java. Typo on XML Configuration can also cause NoClassDefFoundError in Java. when your compiled class which is defined in a package, doesn’t present in the same package while loading like in the case of JApplet it will throw NoClassDefFoundError in Java.

可能的解决方式:

The class is not available in Java Classpath. If you are working in J2EE environment than the visibility of Class among multiple Classloader can also cause java.lang.NoClassDefFoundError, see examples and scenario section for detailed discussion. Check for java.lang.ExceptionInInitializerError in your log file. NoClassDefFoundError due to the failure of static initialization is quite common. Because NoClassDefFoundError is a subclass of java.lang.LinkageError it can also come if one of it dependency like native library may not available. Any start-up script is overriding Classpath environment variable. You might be running your program using jar command and class was not defined in manifest file's ClassPath attribute.

资源:

3种方法解决NoClassDefFoundError

noclassdeffounderror问题模式

我发现，当使用运行时发现的不兼容的类版本编译代码时，有时会得到NoClassDefFound错误。我记得的具体实例是apache axis库。在我的运行时类路径上实际上有2个版本，它正在挑选过时和不兼容的版本，而不是正确的版本，导致NoClassDefFound错误。这是在一个命令行应用程序中，我正在使用类似的命令。

set classpath=%classpath%;axis.jar

我能够让它拿起正确的版本使用:

set classpath=axis.jar;%classpath%;

同一项目的两个不同的签出副本

In my case, the problem was Eclipse's inability to differentiate between two different copies of the same project. I have one locked on trunk (SVN version control) and the other one working in one branch at a time. I tried out one change in the working copy as a JUnit test case, which included extracting a private inner class to be a public class on its own and while it was working, I open the other copy of the project to look around at some other part of the code that needed changes. At some point, the NoClassDefFoundError popped up complaining that the private inner class was not there; double-clicking in the stack trace brought me to the source file in the wrong project copy.

关闭项目的主干副本并再次运行测试用例可以解决这个问题。

Java ClassNotFoundException vs NoClassDefFoundError

黑ClassLoader铝

静态与动态类加载

静态(隐式)类加载——引用、实例化或继承的结果。

MyClass myClass = new MyClass();

动态(显式)类加载是class . forname ()， loadClass()， findSystemClass()的结果

MyClass myClass = (MyClass) Class.forName("MyClass").newInstance();

每个类都有一个ClassLoader，它使用loadClass(字符串名);这就是为什么

explicit class loader uses implicit class loader

NoClassDefFoundError是显式类装入器的一部分。Error是为了保证在编译期间出现这个类，但现在(在运行时)它不存在。

ClassNotFoundException是隐式类装入器的一部分。Exception对于可以额外使用的场景(例如反射)具有弹性。

我通过禁用所有模块的preDexLibraries来解决我的问题:

dexOptions {
        preDexLibraries false
        ...

这也可能是因为您从具有特定包名的IDE复制了代码文件，并且希望尝试使用终端运行它。您必须首先从代码中删除包名。 这种事发生在我身上。