XML

1
XML

• eXtensible Markup Language
• CC292
• Simon M. Lucas

2
Overview

• Brief History
• Why XML?
• What is XML?
• Elements and attributes
• DTDs
• XHTML
• XML processing
• With Java
• With JavaScript E4X

3
What is XML?

• XML is a metadata language - a language for
  providing data about data
• W3C standard around 1998
• It looks a bit like HTML, but with XML the tags
  are user-defined and therefore extensible
• HTML marks up logical presentation
• CSS specifies presentation style
• XML marks up meaning (semantics)

4
Why XML?

• Separates content from presentation
• General - can be applied to anything
• Adds value to semi-structured data
• E.g. Product Catalogue
• Enables an enterprise to mark up all its data
• Using XML greatly simplifies encoding of data
• (c.f. ad hoc text representations)
• Ubiquitous - everybody is using it!

5
Where does XML fit? 

• Why not put everything in a relational or OO
  database?
• XML is a global standard
• offers better information transfer between
  different applications and enterprises than
  proprietary databases
• XML is flexible and easily applied
• (which also presents dangers - data does NOT
  become more valuable just because it is marked up
  in XML - the XML structures have to be well
  designed).

6
Data Centric or Document Centric?

• Data centric
• Used in web services
• Communication between applications
• Data export from databases
• Document centric
• To add meaning to semi-structured documents
• E.g. content for web pages, lecture notes,
  product catalogues
• Emerging XML databases such as Xindice
  http//xml.apache.org/xindice/ store XML directly
  (dont have to map to relational DB)

7
XML Basic Syntax

• An XML document consists of a number of
  declarations followed by a tree of elements. 
• Each element is delimited between begin and end
  tags.  
• Each element may contain attributes
• Elements may contain text or other elements (or a
  mixture of the two)
• Attributes may only contain text

8
XML Element

• Has a name
• Has a begin tag ltelementNamegt
• Then text and/or child elements
• Has an end tag lt/elementNamegt
• E.g. ltnamegt Simon lt/namegt
• Elements can also be empty
• E.g. ltperson nameSimon /gt

9
Well-Formed and Valid

• Elements tags must be properly nested
• E.g. ltagt ltbgt text lt/bgt lt/agt is ok
• But ltagt ltbgt text lt/agt lt/bgt is NOT
• Attribute values enclosed in string quotes
• A document where all the tags are properly nested
  is well-formed
• If a document is well-formed, and obeys the
  syntax rules of a specified DTD, then it is also
  Valid

10
Elements or Attributes

• Information can either be stored in elements or
  attributes
• Structured information is stored in elements
• Primitive information (i.e. a single atomic value
  or list of values) can either be stored in an
  element or an attribute
• Perhaps better to store primitives in attributes

11
XML Attributes

• Element start tags may also contain attributes
• An attribute consists of an attribute name
  followed by an attribute value
• Attributes are only allowed in the start tags
• E.g.
• ltperson emailsml_at_essex.ac.ukgt
• ltnamegtSimonlt/namegt
• lt/persongt

12
Document Type Definition (DTD)

• Provides a concise way to specify the syntax of a
  given document type
• Declares how the elements can include other
  elements
• And the attributes allowed for each element
• Special operators specify the order and
  cardinality of each item (see below)

13
DTD Symbols Elements

14
CDATA and PCDATA

• CDATA Character Data
• Attributes declared with CDATA may contain any
  text characters
• PCData Parsed Character Data
• Elements declared PCDATA do not contain other
  elements
• i.e. no other mark-up within them
• In tree-terms, these are LEAF-nodes

15
DTD for Address Book Example

• lt!-- DTD for simple address book --gt
• lt!ELEMENT AddressBook (Title, Person)gt
• lt!ELEMENT Title (PCDATA)gt
• lt!ELEMENT Person EMPTYgt
• lt!ATTLIST Person name CDATA REQUIREDgt
• lt!ATTLIST Person email CDATA IMPLIEDgt
• Tip Enter the Address Book DTD and XML as files
  in Intellij, then use the tools -gt validate
  command to perform validation on the document.
• Try to modify the DTD and/or XML document to make
  it invalid.

16
Address Book XML

• lt!DOCTYPE AddressBook SYSTEM "AddressBook.dtd"gt
• ltAddressBookgt
• ltTitlegtSimon's address booklt/Titlegt
• ltPerson name"Simon
• email"sml_at_essex.ac.uk" /gt
• ltPerson name"Anna" /gt
• lt/AddressBookgt

17
Alternative Address Book

• What about this version
• ltAddressBookgt
• ltSimon emailsml_at_essex.ac.uk /gt
• ltAnna emailthewife_at_gmail.com /gt
• lt/AddressBookgt
• Is it well formed?
• Is it valid (with respect to previous DTD?)
• Is it well designed?

18
Creating XML with JDOM(JDOM Java API for XML)

• public static void main(String args) throws
  Exception
• Element root new Element("AddressBook")
  
• Element title new Element("Title")
• title.setText("Simon's address book")
• Element e1 new Element("Person")
• Element e2 new Element("Person")
• e1.setAttribute("name", "Simon")
• e1.setAttribute("email",
  "sml_at_essex.ac.uk")
• e2.setAttribute("name", "Anna")
• root.addContent(title)
• root.addContent(e1)
• root.addContent(e2)
• XMLOutputter out new
  XMLOutputter( Format.getPrettyFormat())
• out.output(root, System.out)

19
Produced the following

• ltAddressBookgt
• ltTitlegtSimon's address booklt/Titlegt
• ltPerson name"Simon"
• email"sml_at_essex.ac.uk" /gt
• ltPerson name"Anna" /gt
• lt/AddressBookgt

20
Reading and Processing XML

• public static void main(String args) throws
  Exception
• String infile args0
• SAXBuilder builder new SAXBuilder()
• InputStream is new FileInputStream(infil
  e)
• Document doc builder.build(is)
• Element root doc.getRootElement()
• // now print the names and emails in
  plain text
• for (Element el
  (ListltElementgt) root.getChildren())
• System.out.println(el.getAttribute("na
  me"))
• ----------- Produces -----------------------
  ---
• null
• Attribute name"Simon"
• Attribute name"Anna"

21
XHTML

• XHTML is a stricter version of HTML
• Tags must have begin/end pairs
• E.g. ltpgt lt/pgt and not just ltpgt
• Tags must be properly nested
• Attribute values must be in string quotes
• Document must have a single root element
• MS Frontpage can apply XML formatting rules to
  comply with this
• Then makes info very easy to manipulate

22
Example Benefit of XHTML

• Web site construction
• If all pages are in XHTML
• Can be edited with WYSIWYG editor
• And manipulated with JSP / JDOM / E4X
• I use this method for web site construction
• Example http//cigames.org
• This allows a single master page
• To include selected parts of content pages
• Less effort than adding ltjspincludegt tags to
  each content page
• Simple instance of MVC architecture

23
XML and JavaScript

• Can use XML within JavaScript
• JavaScript is also known as ECMAScript
• Easiest to use E4X
• ECMAScript 4 XML
• Can treat XML fragments as native parts of the
  document
• Supported in Rhino (JavaScript implemented in
  Java)
• Hence can be executed on the server / stand-alone
• But on server, does not have access to Browser
  DOM
• And in Firefox (e.g. 2.1)
• Enables concise generation of HTML

24
E4X Native XML

• Fantastic!
• Write XML mark-up
• Then directly instantiate object models of the
  XML
• And navigate using dot notation etc.
• Following examples adapted from the
  e4x_example.js file that comes with the Rhino
  distribution
• Can be executed on Server using Rhino
• Or in a compatible web browser (e.g. Firefox)
• Note that print is a utility method defined in
  the Shell program that comes with Rhino

25
Making an XML Structure

• var order ltordergt
• ltcustomergt
• ltfirstnamegtJohnlt/firstnamegt
• ltlastnamegtDoelt/lastnamegt
• lt/customergt
• ltitemgt
• ltdescriptiongtBig Screen Televisionlt/descript
  iongt
• ltpricegt1299.99lt/pricegt
• ltquantitygt1lt/quantitygt
• lt/itemgt
• lt/ordergt

26
Accessing with . Notation

• // Construct the full customer name
• var name order.customer.firstname " "
• order.customer.lastname
• // Calculate the total price
• var total order.item.price order.item.quantity
  

27
Construction with Expressions

• Contents of curly braces are evaluated as
  expressions e.g.
• var tagname "name"
• var attributename "id"
• var attributevalue 5
• var content "Fred"
• var x lttagname attributenameattributevalue
  gtcontentlt/tagnamegt
• Exercise write the XML that this produces (i.e.
  that x is bound to after executing the above).

28
Data Selection

• var e ltemployeesgt
• ltemployee id"1"gtltnamegtJoelt/namegtltagegt20lt/agegtlt
  /employeegt
• ltemployee id"2"gtltnamegtSuelt/namegtltagegt30lt/agegtlt
  /employeegt
• ltemployee id"3"gtltnamegtSimonlt/namegtltagegt25lt/age
  gtlt/employeegt
• lt/employeesgt
• // get all the names in e
• print("All the employee names are\n" e..name)
• // employees with name Joe
• print("The employee named Joe is\n"
  e.employee.(name "Joe"))
• // employees with id's 1 2
• print("Employees with ids 1 2\n"
  e.employee.(_at_id 1 _at_id 2))
• // name of employee with id 1
• print("Name of the employee with ID1 "
  e.employee.(_at_id 1).name)

29
Produced

• All the employee names are
• ltnamegtJoelt/namegt
• ltnamegtSuelt/namegt
• ltnamegtSimonlt/namegt
• The employee named Joe is
• ltemployee id"1"gt
• ltnamegtJoelt/namegt
• ltagegt20lt/agegt
• lt/employeegt
• Employees with ids 1 2
• ltemployee id"1"gt
• ltnamegtJoelt/namegt
• ltagegt20lt/agegt
• lt/employeegt
• ltemployee id"2"gt
• ltnamegtSuelt/namegt
• ltagegt30lt/agegt
• lt/employeegt
• Name of the employee with ID1 Joe

30
Iteration

• // calculate the average age of all employees
• // based on previous employee data
• var totalAge 0.0
• var nEmps 0.0
• for each (i in e.employee) totalAge 1.0
  i.age nEmps
• print("Average age of all employees "
  totalAge / nEmps)
• Produces
• Average age of all employees 25

31
Simplified HTML Generation(Tested in Firefox
Browser)

• ltscript type"text/javascript"gt
• function myFunc()
• var el document.getElementById('test')
• var table lttablegtlttrgt
• ltthgtCelsiuslt/thgt
• ltthgtFahrenheitlt/thgtlt/trgtlt/tablegt
• var from 10
• var to 12
• for (var ifrom iltto i)
• table.tr lttrgt lttdgt i lt/tdgt
• lttdgt i 9.0/5 32 lt/tdgt lt/trgt
• el.innerHTML table
• lt/scriptgt

32
Some Exciting XML Applications

• Word processing
• (e.g. Syntext Serna)
• Web Application Programming
• XForms
• News feeds
• RSS (Really Simple Syndication)
• Mathematics
• MathML

33
Summary

• XML
• Simple yet powerful
• Will become more and more widespread
• Java APIs such as JDOM allow easy XML processing
  in Java
• E4X even easier!
• Also see AJAX
• Many challenges ahead
• In order to get greatest benefit
• Common standards are required
• Is there a common XML standard for a delivery
  address yet? UK? Globally?

34
Exercise

• Design an XML markup for a simple product
  catalogue
• Each product has a name, price and manufacturer
• Each manufacturer has a name and homepage-URL
• Include sample XML DTD in your solution