SOT: Why JVM's Have No Class (.. or do they?)

So last week I was working with an external jar from ColdFusion and ran into what I thought was your typical ClassNotFoundException. It was perplexing because I knew the jar was accessible to the Classloader. So I could not figure out why the jvm was saying the class could not be found. At the time I was mainly focused on fixing the problem, but something about the exception struck me as odd. Rather than the typical ClassNotFoundException, the error message showed a chained exception that looked something like this:

java.lang.ExceptionInInitializerError
...
...
Caused by: java.lang.NoClassDefFoundError
...
...

With chained exceptions, I have found the first message is often a throw-away error with the true cause being further down the line. So I ignored the initial message and focused on what I thought was a ClassNotFoundException. A great article from the archives of javaworld.com explained why that was a mistake in this case. (Now keep in mind the entry is dated. So not everything in it still applies to later jvm's)

The article explains there are different ways classes can be loaded, each with different implications. When a class is loaded with the Class.forName(..) method, the jvm attempts to initialize the class. That means any static initialization code will be executed as well. If for some reason an error occurs, like during the execution of the static code blocks, the jvm may throw a misleading NoClassDefFoundError. It does necessarily not mean the class cannot be found, rather that it cannot be successfully loaded for some reason.

Perhaps a more elegant and intuitive name for this exception would have been ClassDefinitionLoadError . Oh well. I guess the lesson of the day is, pay attention to those Class exceptions. Sometimes they are tad misleading ;-)