JPA EntityManager, how does it work?

In a typical JPA/Hibernate application, you don't put persistence logic in the entity classes themselves. This is a big change in design philosophy compared to older EJB 2.x applications. Instead, many applications create a layer of Data Access Objects--separate from the entities--that use EntityManager instances to query, load, and save entities. Often, these are singletons, and the entity manager instances inside the DAOs are local to the thread.

If you use a framework like Spring, the management of the EntityManager instances and transactions is completely automatic. Same with EJB 3, although I have not used that on a large project. I would suggest reading the Spring documentation's chapter on Object-Relational Mapping data access. Even if you don't end up using Spring in your application, the chapter gives some good tips on how to structure your application in a layered way that separates persistence concerns from the entities being persisted. Good luck!

EntityManager is associated with a persistence context. Use a singleton pattern if all of yours entities are associated with one context. You use jpa on server side,right? If so you can initialized EntityManager in init methods, like init() on servlets.