在EJB容器上的CDI EntityManager注入VS @PersistenceContext实体管理器

Question

When we use the @PersistenceContext annotation on an EntityManager in a JAVA EE environment, the container will create the entityManagerFactory (one for the whole session i guess) and will create a new EntityManager for each request ( by proxying it).

But using CDI without a JAVA EE container i saw something like this:

public class EntityManagerProducer {
private static EntityManagerFactory emf = Persistence.createEntityManagerFactory("livraria");

    @Produces  
    public EntityManager getEntityManager() {
        return emf.createEntityManager();
    }

    public void close(EntityManager em) {
    em.close();
}

}

Using that approach with CDI, will have the same effect and performance? Thanks in advance for any help

Answer 1

"Is using that approach with CDI will have the same effect"

No it won't, using @PersistenceContext the java EE container will provide you a different entityManager instance for each transaction.

Here you will retrieve an unique instance at each injection point which can be problematic (em is not thread safe, moreover the persistence context state will not be consistent with the database one if your data are updated somewhere else since the related entity have been loaded).

Answer 2

No, the effect is not the same. Like Gab said in a previous answer, @PersistenceContext by default injects a separate entity manager per transaction. There is also an option to use an EXTENDED persistence context , but its beyond the scope of your question.

With the code that you provided using CDI you would get an instance per injection point which is not the same unless you create the injecting bean for each transaction (most likely not).

However, what you can do is to use a RequestScoped producer for your entity manager, like so:

 public class EntityManagerProducer {
     private static EntityManagerFactory emf = Persistence.createEntityManagerFactory("livraria");

     @Produces
     @RequestScoped
     public EntityManager getEntityManager() {
         return emf.createEntityManager();
     }
 }

This will give you a similar semantics in a web application, but there may be some pitfalls if you plan to mix EJB and non-EJB code, since this entity manager is not aware of the transactions in progress. Some of this is covered here .

You should also check this similar question Getting a reference to EntityManager in Java EE applications using CDI where you can find a lot of excellent discussion on the matter.