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

更新[https://www.infoq.com/articles/single-file-execution-java11/]:

在Java SE 11中，可以选择启动单个源代码文件 直接，不需要中间编译。为了方便大家， 所以像你这样的新手不需要运行javac + Java(当然， 让他们感到困惑)。

我从SRC库中删除了两个文件后得到了这个消息，当我把它们带回来时，我一直看到这个错误消息。

我的解决方案是:重新启动Eclipse。从那以后我就再没见过这条消息了:-)

虽然这可能是由于编译时和运行时之间的类路径不匹配造成的，但这并不一定是真的。

在这种情况下，记住两到三个不同的异常是很重要的:

java.lang.ClassNotFoundException This exception indicates that the class was not found on the classpath. This indicates that we were trying to load the class definition, and the class did not exist on the classpath. java.lang.NoClassDefFoundError This exception indicates that the JVM looked in its internal class definition data structure for the definition of a class and did not find it. This is different than saying that it could not be loaded from the classpath. Usually this indicates that we previously attempted to load a class from the classpath, but it failed for some reason - now we're trying to use the class again (and thus need to load it, since it failed last time), but we're not even going to try to load it, because we failed loading it earlier (and reasonably suspect that we would fail again). The earlier failure could be a ClassNotFoundException or an ExceptionInInitializerError (indicating a failure in the static initialization block) or any number of other problems. The point is, a NoClassDefFoundError is not necessarily a classpath problem.

这是迄今为止我找到的最好的解决办法。

假设我们有一个名为org的包。Mypackage包含的类:

HelloWorld(主类) SupportClass UtilClass

定义这个包的文件物理存储在目录D:\myprogram (Windows)或/home/user/myprogram (Linux)下。

文件结构如下所示:

当调用Java时，我们指定要运行的应用程序的名称:org.mypackage.HelloWorld。但是，我们还必须告诉Java在哪里查找定义包的文件和目录。所以要启动程序，我们必须使用下面的命令:

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。