See
http://docs.oracle.com/javaee/6/api/javax/persistence/spi/PersistenceUnitInfo.html#addTransformer%28javax.persistence.spi.ClassTransformer%29

This allows Hibernate to more smothly integrate in other frameworks (e.g. OSGi) where classloading/enhancing must take place in special places (e.g. WeavinHook) to prevent inconsistent state.

I debugged a problem related to this, and it seems Hibernate is never calling the method.

If this causes trouble in some circumstances. maybe an option can be provided to switch usage of this on or off.