Question

I am moving a JPA-Hibernate application from a Java EE 6 environment to a Tomcat 7 one.

The application has several DAO classes making queries on the EntityManager. In the Java EE environment I could just inject it using the @PersistenceContext annotation, and let the container manage the EntityManager. Now that I have to do it manually, I was wondering what is the way to go.

Should the entity manager be unique? If so can it be a static final field, created on startup and that every DAO class uses? Does it have a lifecycle that involves closing it and then re-opening it?

Answer 1

Unfortunately, you’ll have to do it manually. The way I usually do it is to define a special class:

public class EMF {
  private static EntityManagerFactory factory = Persistence.createEntityManagerFactory("name");

  public static EntityManager getEntityManager() {
    return factory.createEntityManager();
  }
}

So, every time you need EntityManager, you have to create it manually. You need to handle transactions as well:

EntityManager em = EMF.getEntityManager();
EntityTransaction et = em.getTransaction();
try {
  MyEntity my = new MyEntity();
  et.begin();
  try {
    em.persist(my);
    et.commit();
  } catch (Exception ex) { 
    if (et.isActive())
      et.rollback();
  }
} finally {
  em.close();
}