Introduction to XML: www.w3schools.com

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Introduction to XMLwww.w3schools.com

• Yong Choi
• CSU
• Bakersfield

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What is XML?

• XML stands for EXtensible Markup Language
• XML is a markup language much like HTML
• XML was designed to describe data
• XML will be the most common tool for all data
  manipulation and data transmission.
• XML tags are not predefined in XML. You must
  define your own tags
• XML uses a Document Type Definition (DTD) or an
  XML Schema to describe the data
• XML with a DTD or XML Schema is designed to be
  self-descriptive

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What is DTD?

• It defines the document structure with a list of
  legal elements.
• A DTD can be declared inline in your XML
  document, or as an external reference.
• Why use a DTD?
• With DTD, each of your XML files can carry a
  description of its own format with it.
• With a DTD, independent groups of people can
  agree to use a common DTD for interchanging data.
  
• Your application can use a standard DTD to verify
  that the data you receive from the outside world
  is valid.
• You can also use a DTD to verify your own data.

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The main difference between XML and HTML

• XML is not a replacement for HTML.
• XML and HTML were designed with different goals
• XML was designed to describe data and to focus on
  what data is.
• HTML was designed to display data and to focus on
  how data looks.
• HTML is about displaying information, while XML
  is about describing information.

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Example A note to Tove from Jani

• ltnotegt
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt
• ltheadinggtReminderlt/headinggt
• ltbodygtDon't forget me this weekend!lt/bodygt
• lt/notegt
• Just pure information wrapped in XML tags

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XML is free and extensible

• XML tags are not predefined. You must "invent"
  your own tags.
• The tags used to mark up HTML documents and the
  structure of HTML documents are predefined. The
  author of HTML documents can only use tags that
  are defined in the HTML standard (like ltpgt, lth1gt,
  etc.).
• XML allows the author to define his own tags and
  his own document structure.
• The tags in the example above (like lttogt and
  ltfromgt) are not defined in any XML standard.
  These tags are "invented" by the author of the
  XML document.

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XML is a complement to HTML

• XML is not a replacement for HTML.
• It is important to understand that XML is not a
  replacement for HTML. In future Web development
  it is most likely that XML will be used to
  describe the data, while HTML will be used to
  format and display the same data.
• Best description of XML
• XML is a cross-platform, software and hardware
  independent tool for transmitting information.

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XML can Separate Data from HTML

• With XML, your data is stored outside your HTML.
• When HTML is used to display data, the data is
  stored inside your HTML. With XML, data can be
  stored in separate XML files. This way you can
  concentrate on using HTML for data layout and
  display, and be sure that changes in the
  underlying data will not require any changes to
  your HTML.
• XML data can also be stored inside HTML pages as
  "Data Islands". You can still concentrate on
  using HTML only for formatting and displaying the
  data.

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XML is used to Exchange Data

• With XML, data can be exchanged between
  incompatible systems.
• In the real world, computer systems and databases
  contain data in incompatible formats. One of the
  most time-consuming challenges for developers has
  been to exchange data between such systems over
  the Internet.
• Converting the data to XML can greatly reduce
  this complexity and create data that can be read
  by many different types of applications.
• Especially in B2B (financial institutions)

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An example XML document

• lt?xml version"1.0" encoding"ISO-8859-1"?gt
• ltnotegt
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt
• ltheadinggtReminderlt/headinggt
• ltbodygtDon't forget me this weekend!lt/bodygt
• lt/notegt

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First Line

• The XML declaration
• Defines the XML version and the character
  encoding used in the document.
• Conforms to the 1.0 specification of XML
• Uses the ISO-8859-1 (Latin-1/West European)
  character set.

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Second and other lines

• The second line describes the root element of the
  document (
• like it was saying "this document is a note"
• Lines from 3 to 6 describe 4 child elements of
  the root
• to, from, heading, and body
• The last line defines the end of the root element

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XML Editors
This figure shows available XML editors
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XML Parsers

• An XML processor (also called XML parser)
  evaluates the document to make sure it conforms
  to all XML specifications for structure and
  syntax.
• XML parsers are strict and ensure XML code
  accepted by the parser will work the same
  everywhere.
• Microsofts parser is called MSXML and is built
  directly in IE versions 5.0 and above.
• Netscape developed its own parser, called
  Mozilla, which is built into version 6.0 and
  above.

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Well-Formed and Valid XML Documents

• There are two categories of XML documents
• Well-formed
• Valid
• An XML document is well-formed if it contains no
  syntax errors and fulfills all of the
  specifications for XML code as defined by the
  W3C.
• An XML document is valid if it is well-formed and
  also satisfies the rules laid out in the DTD or
  schema attached to the document.

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The Document Creation Process
This figure shows the document creation process
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XML Applications
This figure shows some XML applications
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XML syntax vs. HTML syntax

• With XML, it is illegal to omit the closing tag.
• Paragraph tag ltPgt in HTML
• XML tags are case sensitive
• Unlike HTML, XML tags are case sensitive
• ltMessagegtThis is incorrectlt/messagegt
• ltmessagegtThis is correctlt/messagegt
• All XML elements must be properly nested
• Improper nesting of tags makes no sense to XML.
• In HTML some elements can be improperly nested
• ltbgtltigtThis text is bold and italiclt/igtlt/bgt

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XML syntax vs. HTML syntax

• All XML documents must have a root element
• All XML documents must contain a single tag pair
  to define a root element.
• All other elements must be within this root
  element.
• All elements can have sub elements (child
  elements). Sub elements must be correctly nested
  within their parent element
• ltrootgt
• ltchildgt
• ltsubchildgt.....lt/subchildgt
• lt/childgt
• lt/rootgt

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XML syntax vs. HTML syntax

• Attribute values must always be quoted
• With XML, it is illegal to omit quotation marks
  around attribute values. 
• lt?xml version"1.0" encoding"ISO-8859-1"?gt
• ltnote date"12/11/2002"gt
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt
• lt/notegt

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XML syntax vs. HTML syntax

• With XML, white space is preserved
• With XML, the white space in your document is not
  truncated.
• Using XML
• Hello              my name is Tove,
• Using HTML
• Hello my name is Tove,
• HTML strips off the white space.
• The syntax for writing comments in XML is similar
  to that of HTML.
• lt!-- This is a comment --gt

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XML Elements

• XML Elements are Extensible
• XML documents can be extended to carry more
  information.
• XML Elements have Relationships
• Elements are related as parents and children.
• Elements have Content
• Elements can have different content types.

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Extensible

• ltnotegt
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt
• ltheadinggtReminderlt/headinggt
• ltbodygtDon't forget me this weekend!lt/bodygt
• lt/notegt
• Extracted output by an application
• MESSAGE
• To Tove
• From Jani
• Don't forget me this weekend!

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Extensible

• ltnotegt
• ltdategt2002-08-01lt/dategt // new added information
• lttogtTovelt/togt
• ltfromgtJanilt/fromgt
• ltheadinggtReminderlt/headinggt
• ltbodygtDon't forget me this weekend!lt/bodygt
• lt/notegt
• Should the application break or crash because we
  added new information?

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Relationships

• A description of a book
• My First XML
• Introduction to XML
• What is HTML
• What is XML
• XML Syntax
• Elements must have a closing tag
• Elements must be properly nested

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Relationships

• A description of the book in XML
• ltbookgt
• lttitlegtMy First XMLlt/titlegt
• ltprod id"33-657" media"paper"gtlt/prodgt
• ltchaptergtIntroduction to XML
• ltparagtWhat is HTMLlt/paragt
• ltparagtWhat is XMLlt/paragt
• lt/chaptergt
• ltchaptergtXML Syntax
• ltparagtElements must have a closing taglt/paragt
• ltparagtElements must be properly nestedlt/paragt
• lt/chaptergt
• lt/bookgt

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Relationships

• Book is the root element.
• Title, prod, and chapter are child elements of
  book.
• Book is the parent element of title, prod, and
  chapter.
• Title, prod, and chapter are siblings (or sister
  elements) because they have the same parent.

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Content

• In previous example,
• book has element content, because it contains
  other elements.
• Chapter has mixed content because it contains
  both text and other elements.
• Para has simple content (or text content) because
  it contains only text.
• Prod has empty content, because it carries no
  information.
• Only the prod element has attributes.
• The attribute named id has the value "33-657".
• The attribute named media has the value "paper". 

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Element Naming

• Names can contain letters, numbers, and other
  characters
• Names must not start with a number or punctuation
  character
• Names must not start with the letters xml (or XML
  or Xml ..)
• Names cannot contain spaces
• Any name can be used, no words are reserved, but
  the idea is to make names descriptive. Names with
  an underscore separator are nice.

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XML Attributes

• XML elements can have attributes.
• From HTML ltIMG SRC"computer.gif"gt. The SRC
  attribute provides additional information about
  the IMG element.
• ltfile type"gif"gtcomputer.giflt/filegt
• Attribute values must always be enclosed in
  quotes
• either single or double quotes can be used
• If the attribute value itself contains double
  quotes, it is necessary to use single quotes or
  vice versa.

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Use of Elements vs. Attributes

• Data can be stored in child elements or in
  attributes both below examples provide same
  information.

Sex is an attribute ltperson sex"female"gt ltfirstnamegtAnnalt/firstnamegt ltlastnamegtSmithlt/lastnamegt lt/persongt Sex is an element ltpersongt ltsexgtfemalelt/sexgt ltfirstnamegtAnnalt/firstnamegt ltlastnamegtSmithlt/lastnamegt lt/persongt
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Problems using Attributes

• attributes cannot contain multiple values (child
  elements can)
• attributes are not easily expandable (for future
  changes)
• attributes cannot describe structures (child
  elements can)
• attributes are more difficult to manipulate by
  program code
• attribute values are not easy to test against a
  DTD
• ltnote day"12" month"11" year"2002" to"Tove"
  from"Jani" heading"Reminder" body"Don't forget
  me this weekend!"gt lt/notegt

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Linking to a Style Sheet

• There are two main style sheet languages used
  with XML
• Cascading Style Sheets (CSS)
• Extensible Style Sheets (XSL)

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CSS

• Because HTML uses predefined tags, the meanings
  of these tags are well understood
• The ltpgt element defines a paragraph and the lth1gt
  element defines a heading and the browser knows
  how to display them.
• Adding styles to HTML elements with CSS is
  simple.
• Telling a browser to display each element in a
  special font or color, is easy to do and easy for
  a browser to understand. 

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XSL

• Because XML does not use predefined tags (we can
  use any tags we want), the meanings of these tags
  are not understood
• lttablegt could mean an HTML table, a piece of
  furniture, or something else. A browser does not
  know how to display an XML document.
• Therefore there must be something in addition to
  the XML document that describes how the document
  should be displayed and that is XSL!

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XSL

• XSL is the preferred style sheet language of XML.
• XSL (the eXtensible Stylesheet Language) is far
  more sophisticated than CSS.
• Below is a fraction of the XML file, with an
  added XSL reference.
• lt?xml-stylesheet type"text/xsl"
  href"simple.xsl"?gt