addTransformer(transformer) - JPA PersistenceUnitInfo's method

void addTransformer(
  ClassTransformer transformer

Add a transformer supplied by the provider that will be called for every new class definition or class redefinition that gets loaded by the loader returned by the PersistenceUnitInfo.getClassLoader method. The transformer has no effect on the result returned by the PersistenceUnitInfo.getNewTempClassLoader method. Classes are only transformed once within the same classloading scope, regardless of how many persistence units they may be a part of.
transformer - provider-supplied transformer that the container invokes at class-(re)definition time
JPA 1.0

This documentation page is derived (with some adjustments) from the open source JPA 2 RI (EclipseLink)
and is available under the terms of the Eclipse Public License, v. 1.0 and Eclipse Distribution License, v. 1.0.
Object Relational Mapping (ORM) JPA 2 providers include Hibernate, EclipseLink, TopLink, OpenJPA and DataNucleus.
ObjectDB is not an ORM JPA implementation but an Object Database for Java with built in JPA 2 support.