When should EntityManagerFactory instance be created/opened?

EntityManagerFactory instances are
heavyweight objects. Each factory
might maintain a metadata cache,
object state cache, EntityManager
pool, connection pool, and more. If
your application no longer needs an
EntityManagerFactory, you should
close it to free these resources.

When an EntityManagerFactory closes,
all EntityManagers from that factory,
and by extension all entities managed
by those EntityManagers, become
invalid.

It is much better to keep a factory
open for a long period of time than
to repeatedly create and close new
factories. Thus, most applications
will never close the factory, or only
close it when the application is
exiting.

Only applications that require
multiple factories with different
configurations have an obvious reason
to create and close multiple
EntityManagerFactory instances.

Only one EntityManagerFactory is
permitted to be created for each
deployed persistence unit
configuration. Any number of
EntityManager instances may be
created from a given factory.