What Does Namespace URI Tell You?

When working with XML, you'll often see namespace definitions like this, and wonder what significance the “http” URI might have.

<beans xmlns="http://www.springframework.org/schema/beans"
             xmlns:int="http://www.springframework.org/schema/integration"
             xmlns:ctx="http://www.springframework.org/schema/context"
             …

The answer: absolutely none. It's simply a string that the parser associates with each namespaced element or attribute. As far as the parser is concerned, it could be “foo”.

I can already hear people saying “no, you're wrong! put that URL into your browser!” And, in the case of Spring namespaces, the URL will take you to a page where you can see the relevant schemas. But this is simply a convenience provided by SpringSource, a way to make developers' lives easier.

It's not mandated by specification, nor suggested by convention. And most of us don't have the control over our corporate website necessary to make it happen. If you do, great; if not, then don't feel that you need to use a “http” namespace URL.

I personally prefer a URN — a Uniform Resource Name, rather than a Uniform Resource Locator — because it doesn't give the impression that there might be something that could be located.

The rules for constructing a URN are simple:

<URN> ::= "urn:" <NID> ":" <NSS>

The NID portion is the “namespace identifier,” and there are also rules for registering your namespace with the Internet Assigned Numbers Authority. But those same rules allow “experimental” namespaces, which are anything starting with “X-”. So you can construct perfectly legal URNs using your company's domain name (reversed here, in keeping with the Java package naming convention).

urn:X-com.mycompany:NSS

The NSS portion is the “namespace specific string,” and can be anything you want. If you have corporate coding standards, you can follow them; otherwise, something like “PROJECT:PURPOSE” works well:

urn:X-com.mycompany:frobulator:schemas

And that's it. You have a namespace URI that won't confuse anybody as to its purpose: it's used only to uniquely group elements and attributes.