Introduction to XML

1. Introduction to XML Babak Esfandiari

2. 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

3. XML example <?xml version="1.0" ?> <!-- a simple tagset for museums> <Museum name="Louvre"> <city> Paris </city> </Museum>

4. 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

5. XML concepts and syntax • Elements • can be nested • must have a closing tag • Attributes • XML declaration • Comments

6. 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…

7. DTD • Document Type Definition • Describes the XML tagset <!DOCTYPE Museum [ <!ELEMENT Museum (city?)> <!ATTLIST Museum name CDATA #REQUIRED> <!ELEMENT city (#PCDATA)> ]> • An XML document that is compliant to its DTD is valid

8. DTD Syntax • Defining elements: <!ELEMENT Museum (city?, genre+) • Character data types: • #PCDATA (is parsed) • #CDATA (is not parsed)

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

10. 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

11. XML Schema Example http://chat.carleton.ca/~narthorn/project/community.xsd

12. Some XML-based standards • MathML • CML • MusicXML • XMI

13. XMI Example <Class> <name>Museum</name> <feature> <Attribute> <name>name</name> </Attribute> </feature> </Class>

14. 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

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

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

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

18. XSLT example <xsl:stylesheet> <xsl:template match="Class"> blah <xsl:value-of select="name"/> blah <xsl:apply-templates select="feature/Attribute"/> </xsl:template> <xsl:template match="feature/Attribute"> <xsl:value-of select="name"/> </xsl:template> </xsl:stylesheet>

19. Semantic Web • Tim Berners-Lee’s 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

20. RDF Example • See Software Agents course example

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