Introduction to XML

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Introduction to XML

• Babak Esfandiari

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What is XML?

• introduced by W3C in 98
• Stands for eXtensible Markup Language
• it is more general than HTML, but simpler than
  SGML
• it is used to to describe metadata
• you can define your own set of tags!
• an XML document does not do anything on its own

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XML example

• lt?xml version"1.0" ?gt
• lt!-- a simple tagset for museumsgt
• ltMuseum name"Louvre"gt
• ltcitygt Paris
• lt/citygt
• lt/Museumgt

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XML - what for?

• content is independent from rendering
• meta-data makes search easier
• standard tags enable data interchange across
  tools
• format for data and object persistence, human
  readable and editable
• no need for a custom parser anymore

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XML concepts and syntax

• Elements
• can be nested
• must have a closing tag
• Attributes
• XML declaration
• Comments

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XML concepts (2)

• An XML document that follows the syntax rules is
  considered well-formed
• But there is no restriction on the nature, order
  and number of tags in a well-formed XML document!
• in order to impose some restrictions, you need to
  define validity criteria in a separate document

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DTD

• Document Type Definition
• Describes the XML tagset
• lt!DOCTYPE Museum
• lt!ELEMENT Museum (city?)gt
• lt!ATTLIST Museum name CDATA REQUIREDgt
• lt!ELEMENT city (PCDATA)gt
• gt
• An XML document that is compliant to its DTD is
  valid

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DTD Syntax

• Defining elements
• lt!ELEMENT Museum (city?, genre)
• Character data types
• PCDATA (is parsed)
• CDATA (is not parsed)

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DTD

• DTDs are hard to read
• DTD has its own syntax
• DTD has very limited support for data types

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XML Schema

• a 2001 W3C recommendation
• allows the definition of elements and attributes
  using the XML syntax
• supports many primitive types
• allows the creation of complex types
• uses namespaces to
• allow reuse of types and schemas
• avoid naming clashes

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XML Schema Example

• http//chat.carleton.ca/narthorn/project/communit
  y.xsd

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Some XML-based standards

• MathML
• CML
• MusicXML
• XMI

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XMI Example

• ltClassgt
• ltnamegtMuseumlt/namegt
• ltfeaturegt
• ltAttributegt
• ltnamegtnamelt/namegt
• lt/Attributegt
• lt/featuregt
• lt/Classgt

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XML Parsing

• Many XML parsers are available JAXP, XERCES
• Two standardized parsing methods
• SAX
• event-driven
• serial-access
• element-by-element processing
• DOM
• creates a tree structure of objects
• stores it in memory
• easier to navigate, but more memory needed

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SAX

• good to use if you are consuming XML data from
  a stream
• see Echo.java example (from JAXP)

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DOM

• use it if you need random access to various
  elements of the document
• see EchoDom.java example

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XSLT

• eXtensible Stylesheet Language Templates
• allows the transformation of one XML document
  into another by specifying transformation rules

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XSLT example

• ltxslstylesheetgt
• ltxsltemplate match"Class"gt
• blah ltxslvalue-of select"name"/gt blah
• ltxslapply-templates select"feature/Attribute"/gt
  
• lt/xsltemplategt
• ltxsltemplate match"feature/Attribute"gt
• ltxslvalue-of select"name"/gt
• lt/xsltemplategt
• lt/xslstylesheetgt

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Semantic Web

• Tim Berners-Lees idea of the future of the Web
• The goal is to make information accessible to
  non-humans(ie agents)
• Therefore information should be structured and
  use metadata
• RDF is proposed as such structure

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RDF Example

• See Software Agents course example

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Refs

• W3C specs http//www.w3.org/TR/REC-xml