RuntimeException: class is frozen

I’ve used quite extensively Javassist since when it was used for pretty much only Aspect Oriented Programming. Today, I would say that the strength of Javassist is to assist and implement magic under the hood of persistency layers, like Hibernate.

And it is from such a persisten layer, that a couple of times, luckily very rarely, I came across a RuntimeException: class is frozen.

What is a frozen class?

Javassist is a tool to modify at run-time Java classes. Once a class is loaded, Javassist freezes it, indicating the class should not be modified any more because it has been already loaded. In short, in order to modify a class at run-time you need to re-compile and use a different class loader.
Javassist prevents you to do “stupid” things marking a class as sealed, but in order to manipulate it you need to defrost the class, that is always possible in order to proceed with changes at the bytecode level.

Why is a frozen class throwing an exception?

I don’t know, I can only guess that what is happening inside Hibernate is that the class need to be changed to insert *hooks** for Hibernate to handle persistency, but somehow Javassist prevents the system to modify the class.
If I had to guess, I would say it is a class loader problem.

How to fix the problem?

Really, I don’t know!

However, restarting the application server (in my case an Apache Tomcat), fixed the problem. That lead me thinking it is a problem of caching of the classloaders.