Adam Bien's Weblog
When Is PortableRemoteObject#narrow needed?
PortableRemoteObject#narrow was the default of casting of remote objects to the desired type:
Context context = new InitialContext(); Object raw = context.lookup(jndiName); MyRemoteType type = (MyRemoteType)PortableRemoteObject.narrow(raw, MyRemoteType.class);
According to JavaDoc, the method
narrow "narrows" the abstract remote type to the requested one:
"Checks to ensure that an object of a remote or abstract interface type can be cast to a desired type."
PortableRemoteObject#narrow was widely used within ServceLocator pattern implementations. In the early J2EE years most application servers relied on CORBA/IIOP (RMI over IIOP). In the CORBA / IIOP context casting and inheritance is challenging and requires the "narrowing".
Later,the popular appservers switched to optimized RMI, or even proprietary binary implementations where the PortableRemoteObject#narrow was no more required. However, for compatibility and portability reasons it was a good programming style to use always use PortableRemoteObject#narrow.
In modern Java EE architectures InitialContext#lookup of EJB / CDI components, as well as, IIOP-based communication are dying. "Don't distribute" becomes a best practice: applications are working as much as only possible locally and only exposing coarse grained interfaces, mostly via JAX-RS.
[See also an in-depth discussion in the "Real World Java EE Patterns--Rethinking Best Practices" book (Second Iteration, "Green Book"), page 343 in, chapter "Bean Locator"]
Java EE 7 + HTML 5 Workshops, 7th Edition: March 24th-28th, 2014, Airport Munich Tweet Follow @AdamBien