Sorry for starting a new thread on this. But I believe this needs som new input.

I've read some hundreds posts/articles about this subject - but not yet found a good example/solution. The EJB 1.1 spec (and 2.0) says it cannot be done. Maybe this is true?

What I want to do is to make an J2EE component that can be expanded without having to make changes in the component.

So, let's say, for an example:

This is a Report component.
To simplify, this component has got a CMP EJB called Report.

Users of this component may want a more specialized Report - they want to make a new EJB - lets say TaxReport.

How to implement this without component inheritance?

You can use inheritance in ejb spec 1.1 or 2.0

I have an open source example on this.

- Please download the developer's manual of OpenUSS at:
ftp://openuss.sourceforge.net/pub/openuss/openuss-developer-manual.pdf

- Download the sourcecode at:
ftp://openuss.sourceforge.net/pub/openuss/openuss-discussion.zip

- And read my comment on that stuffs (from OpenUSS
  mailinglist). This is how I want to reach a
  plug&play component architecture with ejb.
  It also uses inheritance.

- You have to give me your e-mail address, if you
  want to have the uml diagram. It's easier to
  understand if you can take a look at the uml
  diagram.

Have fun!
Lofi.

Hi Mike,

Thanx for your question. I sent you the developer
manual of OpenUSS and the sourcecode for the
newsgroup directly to your email address,
so the other list member won't be angry with me
(It's about 1,5 MB) ;-)

For all of you, don't download the current version
of OpenUSS. It's too old. Wait until 1st April
and I'll update the download.

This documentation could be a good base for
developing the "plug-in" Extension Components.

Important in this architecture is this structure:
- Business Components (EJBs)
  + Foundation Components (BFC)
  + Extension Components (BEC)

- Presentation Components (Enhydra's PO)
  + Foundation Components (PFC)
  + Extension Components (PEC)

These are the steps writing an Business Extension
Component (BEC):

"Business Extension Components (BEC) - EJBs"
This is the most important part. Because the reuse
in this area should be the bigest part in the
OpenUSS architecture.

- First of all you just define your BEC totally
  independent from the BFC. Let's say we want
  to build a newsgroup component. It's important
  that you write this component as if no BFC existed.
  This is really important because I also want
  to develop this newsgroup component as
  a standalone component. A newsgroup component
  can be used overall on the web sites not only
  in a e-Learning platform.
- After you finish your BEC (test, etc.)
  you create the "glue code" betweeen
  your BEC and the existed BFC.
  This means that you just use "inheritance".
  You can extend your BEC and put some properties to
  link it to the existed BFC.
- That's it.

You can see this in the UML diagram enclosed in
this mail.

Okay, it's sound really easy. The problem is that
inheritance is not that easy by using EJBs. But
actually it is also not sooo complicated ;-)

These are the steps:
- This is the code snippets of the BEC for the newsgroup
  component. You just first define the interface and
  afterwards you can write the home, bean and object
  classes respectively. (The sourcecode of this
  newsgroup is also included - Actually I call this
  component, discussion component ;-))

...
public interface DiscussionItemBase {
  public String getSubject() throws RemoteException;
  public void setSubject(String subject) throws RemoteException;
...

- You write the "glue code". In this case we want to link
  the Enrollment component (this is on of the Business
  Foundation Components). This means that we want
  to have a newsgroup for each enrollment.

...
public interface DiscussionItem extends DiscussionItemBase {
  public Enrollment getEnrollment() throws RemoteException;
  public void setEnrollment(Enrollment enrollment) throws
    RemoteException;
...

  You have to write the home, bean and object classes for
  this glue code as well.
 
That's it. It's really easy to write such a "plug-in"
component within the EJB architecture.

Some problems you encounter here:
- If I delete an enrollment, how can I know
  that I also have to delete the newsgroup belongs
  to that enrollment? Or How can I know that
  there is a newsgroup already exists for
  this enrollment? Because this is a one direction
  relationship between the newsgroup/discussion
  and the enrollment. The enrollment component
  cannot know, what kind of Extension Components
  are installed at one time. I wonder whether
  an observer pattern can be done in this case?
- I use the Facade Pattern for Session Beans and
  Entity Beans. Building a glue code for Session
  Beans is not that useful :-( You can see it
  in the sourcecode.

With this architecture I can provide many standalone
components to the OpenUSS Foundation Components.
You can write any standalone Extension Components.
After you finish it you can plug it in into
the Foundation Components. You can take a ready
to use component (must be an ejb one) as well and
build the glue code to plug it into the Foundation
Components.

I would like to hear your opinion about this and
next time we can talk about the presentation layer,
because the presentation layer should be reusable
as well.

Greets,
--
---------------------------------------------------
Blasius Lofi Dewanto
---------------------------------------------------
OpenUSS - Open University Support System
http://openuss.sourceforge.net
---------------------------------------------------

Hi,

I would also be interested in the UML diagram. My email-id is kgupta at covad dot com.

Thanks
Gautam

Your comments seem interesting, but I couldn't get the file at

ftp://openuss.sourceforge.net/pub/openuss/openuss-developer-manual.pdf

It's there a problem?

Thanks!

::Omar

Hi,
I can not get all the files you mentioned,any suggestion?

Thanks!

hehe, my response won't be as long...

Take a look at my reponse to previous thread
href"http://www2.theserverside.com/discussion/thread.jsp?thread_id=5303

If you have any questions I'll be happy to elaborate.

-JC

my e-mail is:
b dot e at pedersen dot cc

I would be greatful for the UML diagrams.

Done!

Lofi.

Hi Lofi,

Is it possible for you to send me the UML diagram as well as I also trying to figure out EJB inheritance..

My email address is : murali dot muralidharan at etc dot co dot uk

Regards,
Murali