getIdentifier(entity) - JPA PersistenceUnitUtil's method

Object getIdentifier(
  Object entity

Return the id of the entity. A generated id is not guaranteed to be available until after the database insert has occurred. Returns null if the entity does not yet have an id.
entity - entity instance
id of the entity
IllegalArgumentException - if the object is found not to be an entity
JPA 2.0

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.