JPA controller & database. Web service - web-services

Question

I have created a restful web service on netbeans that accesses a local database.
I have looked at a sample project from netbeans and they use a JPA controller.
This question may be basic, but I don't have alotted time to investigate very deeply on JPAs.
Can someone explain why the use of JPA controllers is necessary?
Also, I read this previous question,"Database table access via JPA Vs. EJB in a Web-Application" and it advises to use an EJB.
Again, can this be explained.
public class CustomerJpaController implements Serializable {
public CustomerJpaController(UserTransaction utx, EntityManagerFactory emf) {
this.utx = utx;
this.emf = emf;
}
private UserTransaction utx = null;
private EntityManagerFactory emf = null;
public EntityManager getEntityManager() {
return emf.createEntityManager();
}
public void create(Customer customer) throws PreexistingEntityException, RollbackFailureException, Exception {
EntityManager em = null;
try {
utx.begin();
em = getEntityManager();
em.persist(customer);
utx.commit();
} catch (Exception ex) {
try {
utx.rollback();
} catch (Exception re) {
throw new RollbackFailureException("An error occurred attempting to roll back the transaction.", re);
}
if (findCustomer(customer.getCustomerId()) != null) {
throw new PreexistingEntityException("Customer " + customer + " already exists.", ex);
}
throw ex;
} finally {
if (em != null) {
em.close();
}
}
}

Answer 1

I've never heard of a "JPA controller", but one typically embeds JPA code within some kind of Service class in order to handle transactions.
Stateless EJB beans are perfectly suited for this. Without them you'd have to start, commit and rollback a transaction manually, which is tedious, verbose and error prone. With EJB this becomes trivial as they transparently manage transactions for you.