Best practice to get EntityManagerFactory

问题:

What is the best approach to get EntityManagerFactory in web app(jsp/servlets)? Is this a good way When should EntityManagerFactory instance be created/opened?, or is it better to get it from JNDI, or something else?

回答1:

They're heavyweight and they're supposed to be in the application scope. So, you need to open them on application startup and close them on application shutdown.

How to do that depends on your target container. Does it support EJB 3.x (Glassfish, JBoss AS, etc)? If so, then you don't need to worry about opening/closing them (neither about transactions) at all if you just do the JPA job in EJBs with @PersistenceContext the usual way:

@Stateless
public class FooService {

    @PersistenceContext
    private EntityManager em;

    public Foo find(Long id) {
        return em.find(Foo.class, id);
    }

    // ...
}

If your target container doesn't support EJBs (e.g. Tomcat, Jetty, etc) and an EJB add-on like OpenEJB is also not an option for some reason, and you're thus manually fiddling with creating EntityManagers (and transactions) yourself, then your best bet is a ServletContextListener. Here's a basic kickoff example:

@WebListener
public class EMF implements ServletContextListener {

    private static EntityManagerFactory emf;

    @Override
    public void contextInitialized(ServletContextEvent event) {
        emf = Persistence.createEntityManagerFactory("unitname");
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        emf.close();
    }

    public static EntityManager createEntityManager() {
        if (emf == null) {
            throw new IllegalStateException("Context is not initialized yet.");
        }

        return emf.createEntityManager();
    }

}

(note: before Servlet 3.0, this class needs to be registered by <listener> in web.xml instead of @WebListener)

Which can be used as:

EntityManager em = EMF.createEntityManager();
// ...