Specify the transactional
persistence contexts for EntityManager lookups,
as a Map from persistence unit name to persistence context JNDI name
(which needs to resolve to an EntityManager instance).
JNDI names specified here should refer to
persistence-context-refentries in the Java EE deployment descriptors, matching the target persistence unit
and being set up with persistence context type
In case of no unit name specified in the annotation, the specified value
#setDefaultPersistenceUnitNamewill be taken (by default, the value mapped to the empty String),
or simply the single persistence unit if there is only one.
This is mainly intended for use in a Java EE environment, with all
lookup driven by the standard JPA annotations, and all EntityManager
references obtained from JNDI. No separate EntityManagerFactory bean
definitions are necessary in such a scenario, and all EntityManager
handling is done by the Java EE server itself.