When should EntityManagerFactory instance be created/opened?

Question

Ok, I read bunch of articles/examples how to write Entity Manager Factory in singleton.

One of them easiest for me to understand a bit:

http://javanotepad.bl

Answer 1

• 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.

• More than one entity manager factory instance may be available simultaneously in the JVM. Methods of the EntityManagerFactory interface are threadsafe.