Class loading: JBoss AS vs. WAS
Recently I got a task - take the "JEE" application from JBoss AS 5.1 and run it on WAS 7.0. It shouldn't be difficult - we have JEE standard after all 
Of course it's easier said than done. The first issue that I had to struggle with a little was the class loading.
"By default JBoss (prior to version 3.2) uses a flat class loading model that avoids the need to redundantly include classes in different layers. " (http://community.jboss.org/wiki/ClassLoadingConfiguration)
Basically, if you need a class, just drop it into lib/ or deploy/ and that's it. Of course this is the simplistic approach, comfortable for development time, but not so good for production.
Websphere on the other hand uses different approach (by default) - it uses a hierarchy of classloaders. So trying just to move the application from JBoss to WAS ended with a tone of ClassNotFoundException and ClassCastException. The later one is cause by the fact that classes in Java are identified by their fully qualified names AND their classloaders. So if the same class is loaded by two different classloaders it is in fact considered as a different one.
In theory it is possible to configure JBoss to work like WAS, or the other way. I haven't tried that. Anyone?
Resources
http://publib.boulder.ibm.com/infocenter/wasinfo/v7r0/index.jsp?topic=/com.ibm.websphere.base.doc/info/aes/ae/crun_classload.html
http://community.jboss.org/wiki/jbossclassloadingusecases
http://community.jboss.org/wiki/ClassLoadingConfiguration
http://community.jboss.org/wiki/JBossClassLoaderOverviewandGoals
http://community.jboss.org/docs/DOC-13268?uniqueTitle=false
WAS sux:P I still didn’t figure out how to replace JSF implementation. parent last/first option does not work for me ;-/
Maybe you have somewhere link for tutorial with solution how to change standard implementation of framework in WAS (OpenJPA/JSF)?
Łukasz