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Why are special attributes used to declare XML namespaces? I don't know the answer to this question, but the likely reason is that the hope that they would simplify the process of moving fragments from one document to another document.

XML Interviews Question page19

     

  1. Why are special attributes used to declare XML namespaces?
    I don't know the answer to this question, but the likely reason is that the hope that they would simplify the process of moving fragments from one document to another document. An early draft of the XML namespaces recommendation proposed using processing instructions to declare XML namespaces. While these were simple to read and process, they weren't easy to move to other documents. Attributes, on the other hand, are intimately attached to the elements being moved.
    Unfortunately, this hasn't worked as well as was hoped.
     For example, consider the following XML document:
    <google:A xmlns:google="http://www.google.org/">
    <google:B>
    <google:C>bar</google:C>
    </google:B>
    </google:A>
    Simply using a text editor to cut the fragment headed by the <B> element from one document and paste it into another document results in the loss of namespace information because the namespace declaration is not part of the fragment -- it is on the parent element (<A>) -- and isn't moved. Even when this is done programmatically, the situation isn't necessarily any better. For example, suppose an application uses DOM level 2 to "cut" the fragment from the above document and "paste" it into a different document. Although the namespace information is transferred (it is carried by each node), the namespace declaration (xmlns attribute) is not, again because it is not part of the fragment. Thus, the application must manually add the declaration before serializing the document or the new document will be invalid.
       
  2. How do different XML technologies treat XML namespace declarations?
    This depends on the technology -- some treat them as attributes and some treat them as namespace declarations. For example, SAX1 treats them as attributes and SAX2 can treat them as attributes or namespace declarations, depending on how the parser is configured. DOM levels 1 and 2 treat them as attributes, but DOM level 2 also interprets them as namespace declarations. XPath, XSLT, and XML Schemas treat them as namespaces declarations. The reason that different technologies treat these differently is that many of these technologies predate XML namespaces. Thus, newer versions of them need to worry both about XML namespaces and backwards compatibility issues.
       
  3. How do I use prefixes to refer to element type and attribute names in an XML namespace?
    Make sure you have declared the prefix and that it is still in scope . All you need to do then is prefix the local name of an element type or attribute with the prefix and a colon. The result is a qualified name, which the application parses to determine what XML namespace the local name belongs to.
    For example, suppose you have associated the serv prefix with the http://www.our.com/ito/servers namespace and that the declaration is still in scope. In the following, serv:Address refers to the Address name in the http://www.our.com/ito/servers namespace. (Note that the prefix is used on both the start and end tags.)
    <!-- serv refers to the http://www.our.com/ito/servers namespace. -->
    <serv:Address>127.66.67.8</serv:Address>
    Now suppose you have associated the xslt prefix with the http://www.w3.org/1999/XSL/Transform namespace. In the following, xslt:version refers to the version name in the http://www.w3.org/1999/XSL/Transform namespace:
    <!-- xslt refers to the http://www.w3.org/1999/XSL/Transform namespace. -->
    <html xslt:version="1.0">
       
  4. How do I use the default XML namespace to refer to element type names in an XML namespace?
    Make sure you have declared the default XML namespace and that that declaration is still in scope . All you need to do then is use the local name of an element type. Even though it is not prefixed, the result is still a qualified name ), which the application parses to determine what XML namespace it belongs to.
    For example, suppose you declared the http://www.w3.org/to/addresses namespace as the default XML namespace and that the declaration is still in scope. In the following, Address refers to the Address name in the http://www.w3.org/to/addresses namespace.
    <!-- http://www.w3.org/to/addresses is the default XML namespace. -->
    <Address>123.45.67.8</Address>
      
  5. How do I use the default XML namespace to refer to attribute names in an XML namespace?
    You can't. The default XML namespace only applies to element type names, so you can refer to attribute names that are in an XML namespace only with a prefix. For example, suppose that you declared the http://http://www.w3.org/to/addresses namespace as the default XML namespace. In the following, the type attribute name does not refer to that namespace, although the Address element type name does. That is, the Address element type name is in the http://http://www.fyicneter.com/ito/addresses namespace, but the type attribute name is not in any XML namespace.
    <!-- http://http://www.w3.org/to/addresses is the default XML namespace. -->
    <Address type="home">
    To understand why this is true, remember that the purpose of XML namespaces is to uniquely identify element and attribute names. Unprefixed attribute names can be uniquely identified based on the element type to which they belong, so there is no need identify them further by including them in an XML namespace. In fact, the only reason for allowing attribute names to be prefixed is so that attributes defined in one XML language can be used in another XML language.  
     
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Posted on: March 10, 2008

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