CP3024 Lecture 9 XML: Extensible Markup Language

What is a markup language?  Textual (i.e. person readable) language where significant elements are indicated by markers – XML  Examples are RTF, HTML, VRML, TEX etc.  Easy to process and can be manipulated by a variety of application programs

What does the Web use?  HTML –Hypertext Markup Language  Defined as the original Web language  Based on SGML (see later)  Suited for hypertext, multimedia, small simple documents  Currently at version 4.01 (the last?)

Why change? - 1  Change in Web usage –no longer a mechanism for exchanging scientific papers –presentational aspects are now seen as of greater importance –extracting the meaning of a document using a program will be a new growth area  HTML can't grow much more!

Why change? - 2  Extensibility –HTML does not allow users to specify their own tags  Structure –HTML cannot represent database schemas or object- oriented hierarchies  Validation –HTML does not allow applications to check that the structure of data is valid

What is SGML?  Standard Generalised Markup Language  ISO 8879  Can define any document format of any complexity  Enables, extensibility, structure and validation  Too many optional features for the Web

What is XML?  Simplified subset of SGML designed for Web applications  Differs from HTML –Can define new tags –Structures may be nested to any level of complexity –XML documents may define a grammar which enables structural validation of that document

Where has XML come from?  Emanates from the Word Wide Web consortium (W3C)  Developed by XML working group chaired by Jon Bosak (Sun Microsystems)  Group includes representatives from Microsoft, Netscape, HP, Adobe, etc.  Last bastion against proprietary markup and Web fragmentation

Design Goals for XML - 1  XML shall be straightforwardly usable over the Internet  XML shall support a wide variety of applications  XML shall be compatible with SGML  It shall be easy to write programs which process XML documents  The number of optional features is to be kept to the absolute minimum

Design Goals for XML - 2  XML documents should be human-legible  The XML design should be prepared quickly  The design of XML shall be formal and concise  XML documents shall be easy to create  Terseness in XML markup is of minimum importance

The XML View of a Document Taken from an example given by Jon Bosak

Structured Publishing Taken from an example given by Jon Bosak

XML Example Say Bye Bye, Sweep Bye Bye, Sweep

XML Markup  Elements  Entity references  Comments  Processing Instructions  Marked sections  Document type declarations (DTD)

Elements  Commonest form of markup  Delimited by angle brackets ( )  May be empty but normally consist of start tag and end tag  Start tag may contain attributes –

Entity References  In XML (and HTML) certain characters are reserved e.g. <  Entity references are used to insert these into documents  Entity references begin with an ampersand ( & ) and end with a semicolon ( ; )  You can define your own entities  Can be used to insert Unicode characters

Comments  Begin with <!--  End with -->  Can contain any data except --  XML processors are not required to pass comments to an application

Processing Instructions (PIs)  Provide information to an application  XML processors required to pass them on  Have the form  The name (PI target) identifies the PI  Data is optional and meaningful to an application that recognises the target

Marked Sections  Parsers ignore everything in CDATA sections <![CDATA[ if p ]]>  Only character string not allowed is ]]>  Data is passed on to the application

Document Type Declarations  Optional in XML (not in SGML)  Specify constraints on the sequence and nesting of tags  Communicates meta-information to the parser about content  Sequence and nesting of tags, attribute values, external files, entities

Kinds of Declaration  Element type declarations  Attribute list declarations  Entity declarations  Notation declarations

Element Type Declaration  A sweepjoke consists of a harry element followed by a sweep element and a laughter element  The harry element may be repeated (+) –+ indicates one or more  The laughter element is optional (?)

Sweepjoke Declaration  PCDATA indicates parseable character data  | indicates 'or'  * indicates 'zero or more'

Attribute List Declaration  Identifies –which elements may have attributes –what attributes they may have –what values are permitted for an attribute –what value is the default <!ATTLIST sweepjoke name ID #REQUIRED label CDATA #IMPLIED status ( funny | notfunny ) 'funny'>

Entity Declarations  Allow a name to be associated with some other content  Internal entities associate a name with a string of literal text (e.g. < )  External entities associate a name with the content of another file  Parameter entities enable text replacement within the DTD

Adding a DTD to an XML File  Inline  External –

Links in XML  HTML anchors are a very limited form of hypertext  XML introduces –XPointers –XLinks  These standards are outside the scope of the XML standard

Presentation Issues  Use of a stylesheet is implicit  Possible standards: –DSSSL Document Style and Semantics Specification Language (ISO 10179) –CSS Cascading Stylesheet Specification –XSL Extensible Style Language (uses XML syntax)

XSL  XSL is an XML sylesheet language –XSLT is a language for transforming XML documents –XSL formatting objects specify formatting semantics  A set of rules to transform a document  XML can be transformed into HTML

XML Application Areas  Mediation between heterogeneous databases on the Web  Client centric web applications  Applications requiring different views of the same data  Information discovery tailored to the needs of differing individuals

Languages based on XML  MathML  SMIL  RDF  XHTML  CML

RDF  Resource Description Framework  Integrates a variety of web-based metadata activities  Provides interoperability between applications that exchange metadata  Allows machine readable description of Web resources

RDF Example <?xml:namespace ns = " prefix ="RDF" ?> <?xml:namespace ns = " prefix = "DC" ?> <RDF:Description RDF: HREF = " John Smith

XHTML  New Web languages are defined using XML  HTML 4.0 cannot be defined using XML  XHTML is XML compliant HTML

Major Changes  Documents must be well-formed  Elements and attributes must have lower case names  End tags required in non-empty elements  Attribute values must be in quotes  Empty tags must be terminated  Scripts will be processed by XHTML

XHTML Compatibility  Current browsers unlikely to understand all XHTML  E.g. may cause an error  Compatibility guidelines defined in XHTML standard  See Appendix C

Summary  XML significantly expands what is possible on the Web  XML preserves the basic Web ideas  Using XML is an order of magnitude more difficult than writing HTML  Software is out there and more will soon follow  The opportunities are endless!