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From Ilan Kirsh <ki...@objectdb.com>
Subject Re: A JDO-JPA Bridge
Date Sun, 08 Oct 2006 16:45:08 GMT
An interesting side effect of the suggested JDO-JPA bridge is that JDO gets Java EE integration
for free! Spring supports JDO but probably EJB based application servers will never do (at
least directly).

But look at this:

public class EmployeeServiceBean {
    @PersistenceContext(unitName="MyUnit") JEntityManager pm;

    public void addEmployee(String name)
        Employee employee = new Employee(name);


This is even better than the EntityManager's getDelegate hook method. Similar injection can
be used also for JEntityManagerFactory when necessary. I just hope that a field that is annotated
with @PersistenceContext can be of a subtype of EntityManager (if not an additional field
and a casting in a @PostConstruct method might be needed, maybe this can be inherited and
implemented once for the entire project).

Notice that also JDO implementations with no full JPA support can gain from this by implementing
JEntityManager and the other new interfaces and throwing NotSupported exceptions in most JPA
methods. Only support for initialization from persistence.xml file and a few JPA methods is
required (very simple) . Of course, such an implementation is not expected to claim for "javax.jdo.option.JPA"

It seems that the new pluggable provider architecture of EJB 3.0 can work great for JDO!

----- Original Message ----- 
  From: Ilan Kirsh 
  To: jdo-dev@db.apache.org ; JDO Expert Group 
  Sent: Saturday, October 07, 2006 8:08 PM
  Subject: A JDO-JPA Bridge

  Hi all,

  Following is an enhancement suggestion for JDO 2.1 (or maybe even for JDO 2.01?) that may
give maximum results in minimum effort.

  I suggest to add four new interfaces to javax.jdo (or to a sub package):

  public interface JEntityManagerFactory extends
      javax.persistence.EntityManagerFactory, javax.jdo.PersistenceManagerFactory

  public interface JEntityManager extends
      javax.persistence.EntityManager, javax.jdo.PersistenceManager

  public interface JEntityTransaction extends
      javax.persistence.EntityTransaction, javax.jdo.Transaction

  public interface JQuery extends
      javax.persistence.Query, javax.jdo.Query

  JDO 2.1 Implementations that will support the new optional "javax.jdo.option.JPA" feature
will have to return instances of these interfaces instead of the super interfaces when using
either the JDO API or the JPA API.

  For instance, a JDO 2.1 implementation with JPA 1.0 support will support datastore transactions
(currently only optimistic transactions are supported by JPA):

      EntityManagerFactory emf = Persistence.createEntityManagerFactory("myUnit");
      EntityManager em = emf.createEntityManager();

  And will support using Extent (no Extent in JPA and I think that there is no support for
filtering subclasses, at least not in an easy way):

      EntityManagerFactory emf = Persistence.createEntityManagerFactory("myUnit");
      EntityManager em = emf.createEntityManager();
      Extent extent = ((JEntityManager)em).getExtent(MyClass, false);

  As well as many other things that are missing in JPA, for example:

      boolean isDirty = JDOHelper.isDirty(entity);
      EntityManager em = (EntityManager)JDOHelper.getPersistenceManager(entity);

  These examples are for using JPA with JDO extensions which I think is more interesting.
Of course, similar examples can be written also for the other direction (using JPA from JDO).

  The names of the new four interfaces is based on the JPA interface names in a way that may
suggest that they are more powerful. Which do you consider more powerful AWT Frame, Panel
and Button or Swing JFrame, JPanel and JButton? If JPA will catch then every curious Java
developer will soon know that there is EntityManager but there is also JEntityManager that
supports additional operations and unlike Hibernate or TopLink extensions is supported by
many vendors and backed by a standard! I agree with David that some day the majority of the
JPA implementations might be also JDO implementations. In that case, JEntityManagerFactory,
JEntityManager, JEntityTransaction, JQuery, JDOHelper and Extent may be something significant
that cannot be ignored in the JPA market.

  The minimum that is required in order to support this addition in JDO 2.1 is:
  - Adding the four new interfaces.
  - Adding "javax.jdo.option.JPA" to the specification with a short explanation.

  Actually the maximum is not so far from the minimum:
  - New section in the specification that explains this addition with more details and maybe
some examples.
  - Handling possible conflicts (e,g, which exceptions are thrown JDO or JPA?).
  - TCK tests that check that an implementation that claims to support "javax.jdo.option.JPA"
indeed returns instances of the new four interfaces.

  Of course, no need to describe the JPA in the JDO spec or write JPA tests because all this
is already handled by the EJB 3 spec.

  Accepting this suggestion will enable using JDO in two modes:
  1. Standalone (for current users as well as for new users that are interested).
  2. As an extension to JPA (might be very useful if JPA will catch).

  Any comments will be welcomed.


  Ilan Kirsh
  ObjectDB Software

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