Overview of XML

CS 284 (CSA), Spring 2005

In this page, we present an outline of markup languages in general and XML in particular, providing a framework for programming with XML. We make no particular assumptions, but assume general prior exposure to HTML.

Markup

• Terms: markup, markup languages, rendition ("source" text with markup), presentation (formatted view), style sheet (determines how to present rendition).

• Markup languages such as HTML and XML use tags for the markup

  • General form: <name attrib="value" ...> content... </name>.

  • Tags such as <name attrib="value" ...> are open tags; </name> is the corresponding close tag

  • Elements of the document are indicated by tag (pair)s <name ...> content... </name>.
    The content of such an element is the (marked up) text between the open and close tags.

  • Attributes are options of the form attrib="value" ... within an open tag.

  • Entities are additional objects within the rendition. For example, the entity   represents a "non-breaking space", and < represents the less-than character. Other ideas for entities: an entity used to insert a company's logo graphic; and an entity used to insert a standard body of text, such as a copyright notice.

History of markup languages

• SGML, Standard Generalized Markup Language, Goldfarb et al since 1960's, beginning at IBM.

  • Goldfarb coined term markup in 1970

  • Standards in '86, '91

  • Tags of form <name attrib="value" ...> content... </name>. Properly bracketed, i.e, every open tag in the rendition has a close tag.

  • Fundamental goals:

    • Common rendition representation

    • Extensibility, i.e., ability to define new tags, etc.

    • Document type rules

  • Document type rules represented in a separate DTD (Document Type Definition) language, using regular expressions. See below

• LaTeX, Leslie Lamport published 1985

  • Implemented as a macro package over TeX (Donald Knuth, 1978-81) typesetting language.

  • Some proper bracketing: e.g., "environments" have the form \begin{name}...\end{name}

  • Common rendition representation, extensible, but no explicit document type rules.

  • commonly used in Mathematics and CS research publications.

• HTML, Tim Berners-Lee (creator of WWW) and Anders Berglund, '89

  • Tags <name attrib="value" ...> content... </name> as in predecessor language SGML

  • Common rendition, but no extensibility (at first), no (modifyable) document type rules.

  • Designed as a simplification of SGML for WWW authoring.

• XML, Berners-Lee et al (WWW consortium, w3c.org) '96, standard '98.

  • Simplified subset of SGML, but with extensibility and document type rules

  • Document types may be expressed using DTDs or XML Schema (an XML form for specifying document types).

DTDs

• Examples: Note.dtd, SpecML.dtd.

• Uses a form of regular expressions to represent patterns:

  symbol	meaning	example
  ,	sequence: items expected in order	(to,from,message)
  |	OR	(nosuperclass | (superclass | interface)+)
  *	0 or more	var*
  +	1 or more	var+
  ?	0 or 1	var?
  ( )	grouping	(nosuperclass | (superclass | interface)+)

Processing XML

• There are two main approaches to processing XML in a language such as Java:

  • SAX, the Simple API for XML, performs actions as an XML document is parsed (input as a rendition and prepared for processing as elements, attributes, and entities).

  • DOM, the Document Object Model, creates an internal data structure (a DOM tree) during parsing, atructure that can be manipulated later by the program code.

• SAX

  • The API for SAX provides a parse() method for performing the parsing of XML input, and a class ContentHandler with methods for specifying actions to be performed when certain tags (elements) are encountered in the XML input stream.

  • Example ContentHandler methods:

    ContentHandler_____

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