unwrap(cls) - JPA EntityManager's method

T unwrap(
  Class<T> cls

Return an object of the specified type to allow access to the provider-specific API. If the provider's EntityManager implementation does not support the specified class, the PersistenceException is thrown.
cls - the class of the object to be returned. This is normally either the underlying EntityManager implementation class or an interface that it implements.
an instance of the specified class
PersistenceException - if the provider does not support the call
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.