Exception in thread "main" java.lang.NoClassDefFoundError is one of the common and difficult problems that you can face when developing Java EE enterprise or standalone Java applications. The complexity of the root cause analysis and resolution process mainly depend of the size of your Java EE middleware environment; especially given the high number of ClassLoaders present across the various Java EE applications.
The part 1 of the series will focus on a high level overview of this Java runtime error along with a Java ClassLoader overview.
java.lang.NoClassDefFoundError – what is it?
Now let’s begin what a simple overview of this problem. This runtime error is thrown by the JVM when there is an attempt by a ClassLoader to load the definition of a Class (Class referenced in your application code etc.) and when such Class definition could not be found within the current ClassLoader tree.
Basically, this means that such Class definition was found at compiled time but is not found at runtime.
Simple enough, what about adding the missing Class to the classpath?
For now, just keep in mind that this error does not necessarily mean that you are missing this Class definition from your “expected” classpath or ClassLoder so please do not assume anything at this point.
Java ClassLoader overview
Before going any further, it is very important that you have a high level of understanding of the Java ClassLoader principles. Quite often individuals debugging NoClassDefFoundError problems are struggling because they are lacking proper knowledge and understanding of Java ClassLoader principles; preventing them to pinpoint the root cause.
What is the problem? Well if Class A is found from a particular parent class loader then it will be loaded by such parent which open the doors for NoClassDefFoundError if you are expecting Class A to be loaded by your application (child) class loader. For example, third part JAR file dependencies could only be present to your application child class loader.
Now let’s visualize this whole process in the context of a Java EE enterprise environment so you can better understand.
Please note that Java EE containers such as Oracle Weblogic have mechanisms to override this default class loader delegation behavior. I will get back to this in the future articles.
Please feel free to post any comment or question of what you learned so far.
The part 2 will follow shortly.