Java Reference
In-Depth Information
@UserDatabase
private EntityManager em;
...
}
The class does not explicitly produce a persistence unit field, but the application has
a
persistence.xml
file that specifies a persistence unit. The class is annotated
javax.inject.Singleton
to specify that the injector should instantiate it only
once.
The
db.UserDatabaseEntityManager
class also contains commented-out code
that uses
create
and
close
methods to generate and remove the producer field:
/* @PersistenceContext
private EntityManager em;
@Produces
@UserDatabase
public EntityManager create() {
return em;
} */
public void close(@Disposes @UserDatabase EntityManager em) {
em.close();
}
You can remove the comment indicators from this code and place them around the field
declaration to test how the methods work. The behavior of the application is the same with
either mechanism.
The advantage of producing the
EntityManager
in a separate class rather than simply
injecting it into an enterprise bean is that the object can easily be reused in a typesafe way.
Also, a more complex application can create multiple entity managers using multiple per-
sistence units, and this mechanism isolates this code for easy maintenance, as in the fol-
lowing example:
@Singleton
public class JPAResourceProducer {
@Produces
@PersistenceUnit(unitName="pu3")
@TestDatabase
EntityManagerFactory customerDatabasePersistenceUnit;
