The EJB inheritance question crops up periodically in different guises and forums. A
good discussion is at:
http://www.theserverside.com/discussion/thread.jsp?thread_id=265
There are lots of gotchas, e.g. narrowing an inherited remote. And even if the
technical problems are overcome, the implicit data model lock-in will often break as
the system evolves.
I've reluctantly experimented with different flavors of EJB inheritance from time to
time. I always come to the same conclusion: the only situation where inheritance is
practical is an abstract non-distributed superclass of Entity Bean implementations for
simple housekeeping functionality, and then sparingly.
J2EE is complicated enough without the inheritance contortion. Even with expensive OR
mapping tools, at least as of the last time I used one in 2000.
I find it occasionally useful to inherit data object classes. E.g.:
CustomerBean {
String name;
CustomerData data;
...
}
EmployeeBean {
String name;
EmployeeData data;
...
}
PersonData {
String name;
...
}
CustomerData extends PersonData { ...
EmployeeData extends PersonData { ...
But even here a better solution is to use role aggregates:
abstract PersonRole {
PersonData person;
}
CustomerData extends PersonRole { ...
EmployeeData extends PersonRole { ...
A useful design check is: Could somebody who cares less about OO purity or system
integrity break this design? Because that Somebody always seems to surface the week
after go-live.
--
Fred Loney
Enterprise Java Consultant
Spirited Software, Inc.
[EMAIL PROTECTED]
ZHU Jia wrote:
> Many thanks for the detailed information!
> But I've some questions here:
> 1. I thought reflection is forbidden by the EJB spec, have you used it
> successfully in your code?
> 2. As far as I see, the way you do it is delegation and not inheritence,
> and you don't make this for conversion of individual Java class to EJB
> but as a general solution, right? So what's your experience with it
> performance-wise? And would it always work?
>
> Many thanks in advance!
>
> regards
> ZHU Jia
>
> "Schouten, Andreas" schrieb:
> >
> > Hello Jia!
> >
> > We migated the folowd situation to ejb
> >
> > Class ImplBasic implements InterfaceA
> >
> > Class Impl1 extendes ImplBasic
> > Class Impl2 extendes ImplBasic
> > Class Impl3 extendes ImplBasic
> > ...
> >
> > InterfaceA obj = (Inter...)new Implx(...)
> >
> > as
> >
> > MyRemote extends InterfaceA, EJBObject
> >
> > MyBean extends SessionBean {
> > private InterfaceA delegate;
> >
> > ejbCreate(String implementationClass){
> > delegate = (InterfaceA)
> > ImplBasic.getInstanceOf(implementationClass);
> > ....
> > aInterfaceMethod(...) {
> > delegate.aInterfaceMethod();
> > }
> >
> > where getInstaceOf creates a instance of the implementation class via
> > reflection.
> >
> > Thats the only way I saw to implement a remote interface with many
> > implementations. Maybe there is a better way?
> >
> > Andreas
> >
> > > -----Original Message-----
> > > From: ZHU Jia [SMTP:[EMAIL PROTECTED]]
> > > Sent: 03 July 2001 07:28
> > > To: EJBList; [EMAIL PROTECTED]
> > > Cc: jboss-user
> > > Subject: [JBoss-user] Help! - Can EJB inherit from normal classes?
> > > Packaging with normal classes?
> > >
> > > Hello,
> > >
> > > I've a question on inheritence in EJB.
> > > As I'm working on the adaption of an existing Java framework, I have to
> > > convert many normal Java classes to EJB. But I'm not sure what to do with
> > > the inheritence. Someone in the list suggested I let the RemoteInterface
> > > inherit from some super interface and let the BeanClass inherit from the
> > > super implementation. This way, if a type of the super interface is
> > > expected, then an EJB RemoteInterface can be returned, which looks quite
> > > reasonable to me, so I was trying to do just that.
> > > But the problem is, I didn't seem to be able to compile because I cann't
> > > have any constructor in the BeanClass (O'Reilly book), which I do have in
> > > the original class that the bean should substitute:-(
> > > I know, normlly the stuff in the constructor is done in the ejbCreate, but
> > > in my case, I have invokation of super() in the constructor, which is of
> > > course bad for EJB. But it didn't work even if I don't use the super() but
> > > move the init code from super class to the bean, compiler simply says it
> > > cann't deal with the constructor in the super class:-(
> > > Now I'm wondering, is it that I've done something wrong or is such thing
> > > not
> > > possible by definition? Anyone with experience here, comments are heartily
> > > welcome!
> > > Also, another question would be: If the purpose of an EJB is to substitute
> > > some normal class, should I put it in it's own package, or should I leave
> > > it
> > > in the original package? If I put it in a new package, then many
> > > "protected"
> > > access modifier has to be "cracked open" to "public" in the original
> > > package, which seems hairy to me. But I don't know whether it's
> > > possible/feasible to package EJBs with the normal classes and make them
> > > one
> > > big jar, will the app server have any problem with it?
> > > The problems are quite urgent and I'm looking forward to your reply!
> > > Many thanks in advance!
> > >
> > > regards
> > > ZHU Jia
> > >
> > >
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