I've been working on a webapp for my Dad's dental office. It is my Christmas gift to him this year. I made significant progress on the project this past weekend but then I hit a wall last night when JBoss started throwing a NoClassDefFoundError. My platform is Debian Linux using JDK 1.5.0_04. My environment is JBoss 4.0.2 + included Hibernate + Struts.

The error comes when I request an action URL. The action was working fine previously. The only changes I made between the time it worked and the time it broke was 1) I changed the underlying DB schema via the .hbm.xml files of 2 classes. 2) I added some new classes which are not referred to in any way from the offending action class.

The error is coming from my SessionManager class which is a wrapper for a Hibernate session. My Hibernate session is managed in the SessionManager in the ThreadLocal style. The ThreadLocal object and SessionFactory are instantiated in a class-scope static block. Inside the same static block I do a JNDI lookup to retrieve the Hibernate session. When I step through the code with a debugger I can see that the moment a reference to the SessionManager's static method getCurrentSession is made, the NoClassDefFoundError occurs.

This setup had been working fine for months, but now it's killing my app, there are only 6 days until Christmas, and I still have a lot of work to do! I've never had such trouble resolving ClassLoader/NoClassDefFoundErrors. I've checked my .har and the unfound class, of course, is where it should be (where it has always been). Why the ClassLoader can't find it all of a sudden is beyond me. I'm out of debugging ideas.

Please please please can someone offer advice on how to debug this problem. Time is ticking and this Santa Claus has some coding to do..

Thanks in advance to all who reply.

If static initializer fails then JVM throws "NoClassDefFoundError" on the next attemt to load "dead" class. It is better to initialize stuff in context listener for this reason, it helps to debug.

Thanks for the reply! Could you please elaborate on context listener initialization. I am unfamiliar with that approach. Thanks.

Great read, thanks. Some println debugging showed me that when jboss starts it does initialize my SessionManager class well before the hibernate session factory is bound to JNDI. Therefore, SessionManager initialization fails, and the class definition is not made, due to the fact that it looks up "java:/hibernate/SessionFactory" before the session factory is bound. However, the question still remains as to why this started happening all of a sudden.

Is it guaranteed that ServletContextListener.contextInitialized will be called at the very end of application start up? The article claims this method is called with the webapp is ready to service its first request, so I assume this is true. I'm hoping that I can get rid of the NoClassDefFoundError by initializing my SessionFactory in the contextInitialized method.

JNDI lookups inside a ServletContextListener was key. That works, thanks.