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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