Lecture Outline

• We shall continue to study Extensible Markup Language (XML) 1.0 (Fourth Edition) which is a W3C Recommendation published on August 16, 2006.
  Link: http://www.w3.org/TR/2006/REC-xml-20060816

• We shall study Namespaces in XML 1.0 (Second Edition) which is a W3C Recommendation published on August 16, 2006.
  Link: http://www.w3.org/TR/REC-xml-names/

• Subjects to be covered this week:

  1. Document type declarations
  2. Namespace declarations

• Case studies:

  1. Rich Site Summary (RSS) 0.91
  2. Question and Answer Markup Language (QAML)
  3. XHTML (some part only)
  4. XML Bookmarks Exchange Language (XBEL)

Document Type Declarations

In an XML Document Type Definition (DTD), one may use the following document type declarations to specify the structure of the XML documents that conform to the DTD:

• element type declarations

• attribute-list declarations

• entity declarations which include

  • (general) entity declarations
  • parameter entity declarations

• notation declarations

Parameter Entity Declarations

• Parameter entities are declared and used like (general and character) entities. They start with a "%" sign instead of a "&" sign, and can only be used in a DTD.
• Parameter entities are used for DTDs in which some parts are parameterized, and for conditional inclusion of other document type declarations.

• An example:

  <!ENTITY % customer "name, street, city, state, zipcode">
  <!ELEMENT invoice (%customer;, item, price, date)>
  

  The above has the same effect as the following element declaration.

  <!ELEMENT invoice (name, street, city, state, zipcode, item, price, date)>  
  

Namespaces in XML

• XML namespaces is a mechanism to avoid clashes between names from different markup vocabularies.

• In the following example, title and number are from different vocabularies:

  <?xml version="1.0"?>
  <!-- both namespace prefixes are available throughout -->
  <bk:book xmlns:bk='urn:loc.gov:books'
           xmlns:isbn='urn:ISBN:0-395-36341-6'>
      <bk:title xml:lang="en">Cheaper by the Dozen</bk:title>
      <isbn:number>1568491379</isbn:number>
  </bk:book>
  

• URIs (Uniform Resource Identifiers) are used as namespace names. A namespace binding is declared by using an attribute whose name begins with xmlns. It binds a namespace prefix (bk) to a namespace name (urn:loc.gov:books).
• The element name bk:book is called a qualified name. It has a prefix bk and a local name book. The attribute name xml:lang is a qualified name too; it has a prefix xml and a local name lang.
• The two namespace prefixes xml and xmlns are reserved, and need not be declared.

Default Namespaces

• The declaration xmlns='urn:loc.gov:books' introduces a default namespace for element book and its children.

  <?xml version="1.0"?>
  <!-- initially, the default namespace is "books" -->
  <book xmlns='urn:loc.gov:books'
        xmlns:isbn='urn:ISBN:0-395-36341-6'>
      <title>Cheaper by the Dozen</title>
      <isbn:number>1568491379</isbn:number>
      <notes>
        <!-- make HTML the default namespace for some commentary -->
        <p xmlns='http://www.w3.org/1999/xhtml'>
            This is a <i>funny</i> book!
        </p>
      </notes>
  </book>
  

• The scope of a namespace declaration extends from the start-tag in which it appears to the corresponding end-tag, excluding the scope of any inner namespace declarations with the same prefix. It applies to all element and attribute names which are within its scope and have the matching prefix.
• The scope of a default namespace declaration extends from the start-tag in which it appears to the corresponding end-tag, excluding the scope of any inner default namespace declarations. It applies to all unprefixed element names within its scope. It do not apply directly to attribute names; the interpretation of unprefixed attributes is determined by the element on which they appear.