The problem is that Websphere comes with a javassist version that just can't be replaced in any way and which is missing the ProxyFactory class. When Hibernate updated the javassist version, it started using ProxyFactory which makes versions 4.2.7+ unusable on Websphere. The use of that class can be avoided easily and would result in binary compatibility with the version that is shipped with Websphere, thus making the latest versions of the 4.2 branch available to the poor Websphere users like me without having to use a custom patched version of Hibernate.