Sumesh,
I am not sure if you were disagreeing with me or not. But if you
were, then I might point out that COBOL is not relevant for RMI-
over-IIOP. I fully agree (and previously stated) that there are
needs for the narrow operation in CORBA, but in the case of RMI-
over-IIOP (which is, by definition, a Java only case) the narrow
should not be required. The fact that the current EJB spec requires
it is an oversight.
-jkw (the original developer of VisiBroker for Java)
sumesh wrote:
>
> the javax.naming.Context.lookup() will return an Object.
> In ejb1.0 it ws enough to cast it as teh protocol used was RMI .
> but in ejb1.1 it is required that it be compatible with RMI-IIOP..
> Since CORBA is programming language independent it is not possible to use
> casting. Because of the fact that some languages doesn't have the concept of
> polymorphism or inheritance(like COBOL) implicit casting is not at all aceptable
>
> "Jonathan K. Weedon" wrote:
>
> > Tibo,
> >
> > Although your answer is 100% correct, I thought it would be useful
> > to reiterate Evan's comment:
> >
> > > Actually nothing prevents a vendor from supporting a simple cast even when
> > > using IIOP (I know that because we implemented it in Sybase EAServer and it
> > > was easy).
> >
> > <vendor>
> > I second the fact that this is possible. In fact, an early version
> > of Borland AppServer also did the "automagic" casting, over IIOP. But
> > our users complained about non-compliance with the spec, and asked
> > that we require the narrow by default, thereby forcing users to write
> > portable code.
> > </vendor>
> >
> > So, strictly speaking, the narrow is not required for the EJB use
> > case of RMI-over-IIOP (although there are some very obscure pure-CORBA
> > use cases that do require the narrow).
> >
> > As such, I would love to see a future version of the EJB spec lift
> > this rather bothersome requirement, although until that happens we
> > will require users to write portable EJB code.
> >
> > -jkw
> >
> > Thibault Cuvillier wrote:
> > > narrow is a CORBAism, which cannot be hidden by the RMI API.
> > > To understand the narrow method, you need to understand how CORBA proxies
> > > are managed.
> > >
> > > CORBA proxies
> > > -------------
> > >
> > > Example:
> > > The remote interface B extends the remote interface A
> > > The remote object implementing the interface B (and so, A)
> > >
> > > When you get a remote reference from the naming service, CORBA will create a
> > > Corbae object stub, which is the super class of all the CORBA proxies. If
> > > you try to cast it down to a proxy to B, you will get a ClassCastException.
> > >
> > > With CORBA, the proxy may not implemented all the remote interfaces of the
> > > remote object.
> > >
> > > So, you have to call the narrow method to create a proxy implementing the
> > > right interface.
> > >
> > > The narrow method do the following job:
> > > 1- Test if the proxy implements the requested interface
> > > If yes, the proxy is only casted-down and returned as-is.
> > > 2- If No, the method test if the remote object implements the
> > > requested interface
> > > Call the CORBA method is_a which check if the cast is
> > > correct on the server side (IIOP request)
> > > If not, an exception is send (Corba BAD_PARAM exception)
> > > 3- Create a new proxy
> > > The narrow method return a new proxy.
> > >
> > > Here is an example of the narrow method generated by Javaidl:
> > > public static A narrow (org.omg.CORBA.Object obj)
> > > {
> > > if (obj == null)
> > > return null;
> > > else if (obj instanceof A)
> > > return (A)obj;
> > > else if (!obj._is_a (id ()))
> > > throw new org.omg.CORBA.BAD_PARAM ();
> > > else
> > > {
> > > org.omg.CORBA.portable.Delegate delegate =
> > > ((org.omg.CORBA.portable.ObjectImpl)obj)._get_delegate ();
> > > return new AStub (delegate);
> > > }
> > > }
> > > Remark:
> > > - the Delegate object contains ORB specific code.
> > > - The AStub do not implements the B interface.
> > >
> > > 2-- RMI over JRMP
> > > Much simpler. When you lookup an object with RMI, RMI will create a proxy
> > > consistant with the remote object interfaces.
> > > So you don't need narrow at all.
> > >
> > > 3-- So,
> > > With EJB, look at the following code:
> > > Object object = context.lookup("ejb.A");
> > > A a = (A)PortableRemoteObject.narrow(object, A.class);
> > >
> > > With CORBA, the proxy a implements the interface A, but not B !
> > > With RMI, the proxy a implements A and B, because the remote object
> > > support these interfaces.
> > >
> > > So if you cast down the proxy like that:
> > > B b = (A)a;
> > > With CORBA you will get an exception.
> > > With RMI it will works.
> > >
> > > To make your code portable, you need to write:
> > > B b = (B)PortableRemoteObject.narrow(a, B.class);
> > > With CORBA, narrow will create a new proxy, but the proxies a and b
> > > will contains a reference to the same remote object.
> > >
> > > Hope this help
> > > Tibo.
> > > http://www.valtech.com
> >
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