PersistenceUnit vs PersistenceContext

Question

In few project I have been successfully using

@PersistenceUnit(unitName = "MiddlewareJPA")
EntityManagerFactory emf;
...
EntityManager entityManager = emf.createEntityManager();

to obtain EntityManager for Database connection, but some days ago I was trying to move my project to Jboss EAP 6.2 and it couldn't create EntityManager. I was googling it and I found that I should try change @PersistenceUnit to

@PersistenceContext(unitName = "MiddlewareJPA")
private EntityManager entityManager;

to obtain EntityManager. It worked but I don't know why. What is the difference bettween PersistenceUnit and PersistenceContext? What are pros and cons of each one? Where should we be using one of them?

Consider adopting the CDI spec (new in JEE 6) to do your dependency injections so you have a more "universal" way of doing it in stead of a big pile of API/framework specific annotations. That doesn't really provide support for entity manager injections out of the box in JEE 6 itself, but there are documented ways of dealing with that. mastertheboss.com/cdi/cdi-and-jpa-tutorial — Gimby
As an addition to the available answers: here is an extremely useful article that can also help answer the OP's questions: JPA Concepts — informatik01

Alexey Andreev Alexey Andreev · Accepted Answer · 2014-01-10T08:50:43

I don't know how it works exactly in the Java EE, but in Spring, when you specify @PersistenceContext annotation, it injects EntityManager. Where does it get EntityManager? It is wrong to create one EntityManager for the whole application lifetime by calling EntityManagerFactory.createEntityManager(). So instead a special implementation of EntityManager interface is used and instantiated directly. It has an internal mutable thread-local reference to a real EntityManager. Implementations of methods just redirect calls to this real EntityManager. And there is a servlet listener, that before each request obtain EM by calling EMF.createEntityManager() and assign it to that inner reference of special EM. Also this listener manages transactions by calling getTransaction().begin(), .commit() and .rollback() on the real EM. It is very simplified description of performed work. And I believe, that JEE container does the same thing, as Spring does.

In general case it is better to inject EntityManager, because with EntityManagerFactory and @PersistenceUnit you should create/destroy EntityManager every time by hands and manage transactions too.

PersistenceUnit vs PersistenceContext

3 Answers