What about non Java clients? I thought one of the reason for using IIOP was
to better support interoperability.
Cheers
Lawrence
> ----------
> From: Jonathan K. Weedon[SMTP:[EMAIL PROTECTED]]
> Reply To: A mailing list for Enterprise JavaBeans development
> Sent: 27 February 2001 07:46
> To: [EMAIL PROTECTED]
> Subject: Re: narrow
>
> 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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