XML Interviews Question page28

1. What is an XML namespace prefix?
   An XML namespace prefix is a prefix used to specify that a local element type or attribute name is in a particular XML namespace. For example, in the following, the serv prefix specifies that the Address element type name is in the http://www.foo.com/ito/addresses namespace:
   <serv:Addresses xmlns:serv="http://www.foo.com/ito/addresses">
    
2. What characters are allowed in an XML namespace prefix?
   The prefix can contain any character that is allowed in the Name [5] production in XML 1.0 except a colon.
    
3. Can I use the same prefix for more than one XML namespace?
   Yes.
    
4. What happens if there is no prefix on an element type name?
   If a default XML namespace declaration is in scope, then the element type name is in the default XML namespace. Otherwise, the element type name is not in any XML namespace.
       
5. What does the URI used as an XML namespace name point to?
   The URI used as an XML namespace name is simply an identifier. It is not guaranteed to point to anything and, in general, it is a bad idea to assume that it does. This point causes a lot of confusion, so we'll repeat it here:
   URIs USED AS XML NAMESPACE NAMES ARE JUST IDENTIFIERS. THEY ARE NOT GUARANTEED TO POINT TO ANYTHING. While this might be confusing when URLs are used as namespace names, it is obvious when other types of URIs are used as namespace names. For example, the following namespace declaration uses an ISBN URN:
   xmlns:xbe="urn:ISBN:0-7897-2504-5"
   and the following namespace declaration uses a UUID URN:
   xmlns:foo="urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6"
   Clearly, neither namespace name points to anything on the Web.
   NOTE: Namespace URIs that are URLs may point to RDDL documents, although this does not appear to be widely implemented. For details, see the next question.
   NOTE: An early version of the W3C's XML Schemas used namespace URIs to point to an XML Schema document containing the definitions of the element types and attributes named in the namespace. However, this proved very controversial and the idea has been withdrawn.
     
6. What is an XML namespace name?
   An XML namespace name is a URI that uniquely identifies the namespace. URIs are used because they are widely understood and well documented. Because people may only allocate URIs under their control, it is easy to ensure that no two XML namespaces are identified by the same URI.
    
7. Can I resolve the URI used as an XML namespace name?
   Yes.
    
8. Can I use a relative URI as a namespace name?
   Yes. However, such usage is deprecated, so you should never do it.
      
9. What is XPointer?
   XPointer is set of recommendations developed by the W3C. The core recommendations are the XPointer Framework which provides an extensible addressing behavior for fragment identifiers in XML media types.
   XPointer gains its extensibility through the XPointer Framework, which identifies the syntax and processing architecture for XPointer expressions and through an extensible set of XPointer addressing schemes. These schemes, e.g., element() or xpointer(), are actually QNames. The xmlns() scheme makes it possible for an XPointer to declare namespace bindings and thereby use third-party schemes as readily as W3C defined XPointer schemes.
      
10. How do I install the XPointer processor?
    Download the latest "cweb-xpointer" release from SourceForge. This project uses Apache Maven and Java 1.4+, so you will need to install those as well. Normally you will also want to download one of the XPointer Framework integrations, such as the xpointer+dom4j or the xpointer+jdom package. These "integration packages" provide support for a specific XML Document model. The project dependencies are explicitly declared in the Maven POM. This means that Maven can automagically download the required releases of dependent JARs. There are several release artifacts. The "uberjar" release provides an executable command line utility  and bundles all dependancies. If you want to integrate into an existing application, then you should use the cweb-xpointer JAR and also download copies of its dependencies. If you are using a Maven project, then this is all very, very easy.