The Java EE 6 Tutorial

Non-Entity Superclasses

Entities may have non-entity superclasses, and these superclasses can be either abstract or concrete. The state of non-entity superclasses is nonpersistent, and any state inherited from the non-entity superclass by an entity class is nonpersistent. Non-entity superclasses may not be used in EntityManager or Query operations. Any mapping or relationship annotations in non-entity superclasses are ignored.