应用程序与容器管理的EntityManager

Question

I am currently having a problem with understanding a concept of JPA.

I am currently using/developing recent EclipseLink, Glassfish, Derby database to demonstrate a project.

Before I develop something in much bigger picture, I need to be absolutely sure of how this PersistingUnit work in terms of different scopes.

I have bunch of servlets 3.0 and currently saving user's associated entity classes in the request.session object (everything in the same war file). I am currently using Application-managed EntityManager using EntityManagerFactory and UserTransaction injection. It works smooth when it is tested by myself. The different versions of entities occur when 2 people accessing the same entities at the same time. I want to work with managed beans cross the same WAR, same persistence unit if possible.

I have read http://docs.oracle.com/javaee/6/tutorial/doc/bnbqw.html and bunch of explanations of those scopes which don't make sense at all for me.

Long story short , what are the usage and difference of app and container managed EntityManagers?

Answer 1

When you say application managed transaction it means its your code which is supposed to handle the transaction. In a nutshell it means:

You call:

entityManager.getTransaction().begin(); //to start a transaction

then if success you will ensure to call

entityManager.getTranasaction().commit(); //to commit changes to database

or in case of failure you will make sure to call:

entityManager.getTransaction().rollBack();

Now imagine you have a container, which knows when to call begin() , commit() or rollback() , thats container managed transaction. Someone taking care of transaction on your behalf.

You just need to specify that.

Answer 2

Container managed transaction(CMT) could be regarded as a kind of declarative transaction, in which case, transaction management is delegated to container (normally EJB container), and much development work could be simplified.

If we are in a Java EE environment with an EJB container, we could use CMT directly.

If we are in a Java SE environment, or a Java EE environment without an EJB container, we could still take advantage of CMT, one way is to use Spring, which uses AOP to implement declarative transaction management; Another way is to use Guice, which uses a PersistFilter to implement declarative transaction.

In CMT, a container (whatever an EJB container, Spring or Guice) will take care of the transaction propagation and commit/rollback stuff;

Application managed transaction (AMT) differs from CMT in that we need to handle transactions programmatically in our code.