The E X tensible M arkup L anguage

1. The EXtensible Markup Language Mary C. Parmelee

2. What is XML? • XML is a text-based meta-language that was designed to: • structure and describe Web data and make it more human readable • be completely customizable and self describing • be platform independent for seamless interoperability • Facilitate the translation of a document from one language to another

3. Why is XML Important? • Two main reasons: • Data interchange: allows data to be shared between different applications and systems • Flexibility: allows data to be centrally stored and transformed into different medias and formats such as HTML

4. Evolution of XML • First there was Standard Generalized Markup Language (SGML) • provides a specification for the writing of markup languages • Developed by IBM researchers in 1965 • Became an ISO standard in 1986 • Powerful but complex

5. Evolution of XML • Then there was HypertextMarkup Language (HTML) • 1989 researcher Tim Berners-Lee proposed a simple system to share documents using hyperlinks • Named the hypertext system the WWW • Defined a simple SGML Document Type (HTML) • World Wide Web Consortium W3C founded in 1992 to standardize HTML

6. Evolution of XML • Finally eXtensible Markup Language (XML) • HTML was • Portable • Easy to understand and use • BUT • Fixed set of tags-not extensible • Designed to format and display data not describe its content • Does not address the transfer platform independent transfer of data • 1996 W3C began XML development • released XML 1.0 spec in 1998

7. The XML Family • 1998 CSS Cascading Style Sheets • 2000 XSL Extensible Style Language • 2000 XLL (XLINK) XML Linking Language • 2000 XPointer XML Pointing Language • 1999 XML Namespaces • 1999 XML Schema • 2000 XForms • XML Query

8. How Does XML Differ from HTML? Two main differences: Both derived from the SGML standard BUT • HTML is an application of SGML called a Document Type Definition (DTD) that specifies Markup • XML is a subset of SGML allows specification of custom DTDs and associated Markup • HTML specifies presentation • XML specifies data structure (content)

9. What Does XML Look Like? • Sample Data Title Author Price Introduction to XML Mark Torr 22.00 Advanced XML Mark Torr 32.00

10. What Does XML Look Like? Notice that there are NO presentation tags. XML simply defines a data structure <?xml version=“1.0”?> <books> <book name=“Introduction to XML” price=“22.00”> <author>Mark Torr</author> </book> <book name=“Advanced XML” price=“32.00”> <author>Mark Torr</author> </book> </books> It is pretty self describing. It is clear we are talking about books and that the data related to that book from simply reading the tags.

11. XML Structure: Declaration • XML declarations normally look as shown below: <?xml version=“1.0”?> • You can tell this is a declaration by the ? That follows immediately after the opening “<” and the “?” immediately prior to the closing “>”. • XML declarations, otherwise known as processing instructions, are used by parsers.

12. XML Structure: Document Root • Every XML document must contain a document root eg; <books></books> • The document root is simply the element that appears first after the processing instruction. • A document root cannot be empty.

13. XML Structure: Element • An XML element defines the content of an XML document eg; <books></books> • Elements can contain: • Other Elements • Text • Known as PCDATA (Parsable character data) • Nothing

14. XML Structure: Attribute • An XML attribute adds data about the elements. • Attributes can be considered similar to fields in a database. <book name=“Introduction to XML” price=“22.00”> </book>

15. XML Structure: Entities • Here is a list of predefined entities. There are only five and so it is pretty easy to remember them.

16. XML Structure Example Attribute. Attributes always appear with within Elements as shown in this example. Here name is an attribute of the element book and price is also an attribute of the element book. Declaration or processing instruction. Used by Parsers or XML editing tools. <?xml version=“1.0”?> <books> <book name=“Introduction to XML” price=“22.00”> <author> Mark Torr </author> </book> <book name=“Advanced XML” price=“32.00”> <author> Mark Torr &amp; His Friend</author> </book> </books> <?xml version=“1.0”?> <books> <book name=“Introduction to XML” price=“22.00”> <author>Mark Torr</author> </book> <book name=“Advanced XML” price=“32.00”> <author>Mark Torr &amp; His Friend</author> </book> </books> Document root Element. Other valid elements in this XML document are <book>, <author>. Notice that all Elements start with a “<” and end with a “>” Entity. This is the only entity in this XML document. Entities always start with an “&” and end with a “;”

17. What is Well Formed XML? • A well formed XML document must conform to these rules: • All element tags have been closed • All attribute values are enclosed in quotes • Nesting of components is correct • Element tags are of the same case.

18. Let’s Try Coding In XML <?xml version=“1.0”?> <books> <book name=“Introduction to XML” price=“22.00”> <author> Mark Torr </author> <personal age=“22”/> </book> <book name=“Advanced XML” price=“32.00”> <author> Mark Torr &amp; His Friend</author> <publisher> Harry Smiths </publisher> </book> </books> Save it into your C:temp file as sample.xml

19. Load this into IE5 • IE has parser that will tell you if your XML is Well Formed • Find the the C:temp directory • Find your file named sample.xml • It will have an icon like this • Double click on the file to open in Internet Explorer

20. Congratulations! :o) If your syntax is correct, the screen looks like this

21. Uh Oh! :o( If the screen looks like this, you made an error

22. Apply the Rules One At A Time • A well formed XML document must conform to these rules: • All element tags have been closed • All attribute values are enclosed in quotes • Nesting of components is correct • Element tags are of the same case.

23. What next? • Once you have a well formed document then you are able to start to use things such as the DOM (Document Object Model), XSL (EXtensible Style Language) to transform this into another markup: • HTML • XML (You can subset and combine XML documents documents to create new XML documents.)

24. What next? • Once a document is well formed you can also check if it is valid. A badly formed XML document is always invalid. • To check if an XML document is valid you need to make use of DTD’s. • DTDs

25. What is a DTD • DTD stands for Document Type Definition • Used to validate XML documents • Defines a set of rules that the content of one or more XML files must adhere to in order for the document to be valid

26. What is validation? • Validation is the process of checking: • Data is in the proper order • Data is in the proper format • Data is the correct type • Any mandatory fields are present

27. Book Example DTD <!ELEMENT books (book+)> <!ELEMENT book ( title, author+ , price, publisher+)> <!ELEMENT title (#PCDATA)> <!ELEMENT author (#PCDATA)> <!ELEMENT price (#PCDATA)> <!ELEMENT address (#PCDATA)> <!ELEMENT publisher (#PCDATA)> <!ATTLIST book Edition CDATA #REQUIRED> <!ATTLIST publisher Address CDATA #IMPLIED> <!ATTLIST price Currency (Dollar | Pound | Yen) "Pound">

28. Conclusion • XML is used to define the structure of the data. • XML does not define any aspect of presentation. • XML has strict rules that must be adhered to unlike HTML which is a little more forgiving. • Making an XML document well formed is the first step towards either doing transformation or ensuring it is a valid XML document. • IE5 can be used to root out anything that is making an XML document badly formed provided you give the file a .xml extension.

29. Further reading • Extensible Markup Language (XML) 1.0 • http://www.w3.org/TR/WD-xml • Defines the syntax of XML.