An Introduction to XML

Patrice Bonhomme & Laurent RomaryLucid-IT LORIA

bonhomme@lucid-it.comromary@loria.fr

eXtensible Markup Language version 1.0Recommendation, February 1998

Objectives

Understanding the basic concepts of XML Elements, attributes and content DTD (, Schemas) Namespaces

An overview of the main associated recommendations: XML path language (XPath) XML pointers and links (Xpointer and XLink) The transformation language of XSL (eXtensible Stylesheet Language)

XML in the document chain

Edition

XML

Data

Transformation

XML

XSL/XSLT

Data processing

Consultation

HTMLXHTML

User perspective

Conception

DTD/Schema

Structures

A quick historical overview

1986 SGML (Standard Generalized Markup Language) ISO standard: ISO:8879:1986

1987 TEI (Text Encoding Initiative)

1990 HTML 1.0 (HyperText Markup Language)

1997/1998 XML 1.0 (eXtensible Markup Language)

What XML is:

XML: eXtended Markup Language A W3C (World Wide Web Consortium)

Recommendation A meta-language: it allows one to define his

own markup language A simplification of the SGML standard

SGML was intended to represent the “logical” structure of a document

HTML was conceived as an application of SGML

A simplified SGML

An XML document is an SGML document With some slight (but essential) differences...

XML has the expressive power of SGML without its complexity

Opens the door to the transmission of structured documents on the web Databases also entered the game...

What can we do with it?

Data modeling (in complement to UML for instance)

Publication of structured data on the web Separation of the logical structure of a

document from its actual presentation Distributed applications (cf. well-formed vs.

valid documents) Integrating data from heterogeneous sources

Why can’t we avoid it? Simplicity, which makes it simple to integrate into any

kind of application XML specifications = 36 pages SGML standard, ISO-8879 = 250 pages

Wide variety of application already implemented Industry: Publishing, Databases, Cataloguing, e-business etc. Science, research: genomics, astronomy, maths, etc.

Consequence: a lot of software available: editors, parsers, bridges from and to

existing editing environment or DBMSs

From HTML to XML - 1 A simple HTML document:

<B> Patrice Bonhomme </B><P>Patrice.Bonhomme@loria.fr <BR>tél : 03 83 59 30 52 <BR>fax : 03 83 41 30 79 <BR>équipe : Langue et Dialogue (<I>LORIA</I>)<BR>

From HTML to XML - 2

The XML way:<?xml version="1.0" encoding="iso-8859-1"?><!DOCTYPE MEMBRE SYSTEM "http://…/MEMBRE.dtd"><!-- Un membre du LORIA --><MEMBRE TYPE="IE" ID="M28"><NOM> BONHOMME </NOM><PRENOM> Patrice </PRENOM><MEL> Patrice.Bonhomme@loria.fr </MEL><TEL> 03 83 59 30 52 </TEL><FAX> 03 83 41 30 79 </FAX><EQUIPE LAB="LORIA">Langue et Dialogue</EQUIPE>

</MEMBRE>

Some properties of XML

Emphasis should be put on the “semantics” of a document

Underlying model: tree structure Possibility to imagine a script language to

access any part of an XML documente.g.: DB/MEMBRE[28]/MEL/text()

XML supports Unicode character encodings

Elements and their content

<MEMBRE TYPE="IE" ID="M28"> <LOGIN ID="bonhomme"/> <NOM> BONHOMME </NOM>

<PRENOM> Patrice </PRENOM><MEL> Patrice.Bonhomme@loria.fr </MEL><TEL> 03 83 59 30 52 </TEL><FAX> 03 83 41 30 79 </FAX><EQUIPE LAB="LORIA">Langue et Dialogue</EQUIPE>

</MEMBRE>

Opening tag

Closing tagTextual content

Empty element

Element

Elements and their attribute

<MEMBRE TYPE="IE" ID="M28"> <LOGIN ID="bonhomme"/> <NOM> BONHOMME </NOM>

<PRENOM> Patrice </PRENOM><MEL> Patrice.Bonhomme@loria.fr </MEL><TEL> 03 83 59 30 52 </TEL><FAX> 03 83 41 30 79 </FAX><EQUIPE LAB="LORIA">Langue et Dialogue</EQUIPE>

</MEMBRE>

Attribut name Attribut value

Other features XML declaration

<?xml version=“1.0"?><?xml version="1.0" encoding="UTF-8" standalone="yes"?>

Commentaries<!-- ceci est un commentaire -->

CDATA section<![CDATA[Langue & Dialogue]]>

Processing instruction (application specific)<?edit line="wrap"?>

From one document to a class…How do I know the

structure of my

document?

How may I share this

structure with others?

Document Type Definition

Expresses constraints on: Allowed element and attribute names Possible content of a given element (“content

model”) To which elements a given attribute can be

attached Similar to the traditional SGML approach, but:

Simplified syntax The DTD is optional for a document

Example<!ELEMENT

MEMBRE (LOGIN, NOM?, PRENOM?,MEL, TEL+, FAX*, EQUIPE)>

<!ELEMENT LOGIN EMPTY><!ATTLIST LOGIN ID ID #REQUIRED>

<!ELEMENT NOM (#PCDATA)>...<!ENTITY W3C "World Wide Web Consortium"><!ENTITY chap1 SYSTEM "http://…/chapitre-1.xml"><!ENTITY img2 SYSTEM "image2.gif" NDATA gif>...

Using a DTD<!DOCTYPE MEMBRE SYSTEM "http://…/MEMBRE.dtd"><MEMBRE TYPE="IE" ID="M28"> … </MEMBRE>

<!DOCTYPE MEMBRE [<!ELEMENT MEMBRE … >…]><MEMBRE TYPE="IE" ID="M28"> … </MEMBRE>

Valid vs. Well-formed

Well-formed documents Syntactic bracketing is preserved, without a DTD Empty element:

<toto></toto> = <toto/> Valid documents

With a DTD (à la SGML) Essential difference with SGML

Extracting and re-using document fragments One usually produce valid document and distribute well-

formed ones

XML namespaces Objectives: avoid conflicts between element and

attribute names coming from various sources Composite documents XSLT instructions, Schema declarations

Declaration:<DOC xmlns:mml="http://www.w3.org/Math/MathML/" xmlns="http://www.ua99.net/DOC/1.0"> <P>blah blah : <mml:fn mml:definitionURL="mydef.xml"> … </mml:fn> re blah blah</P></DOC>

Reserved namespaces The xml: prefix is reserved by the W3C for specific

attributes:<title xml:space="default">...</title><p xml:lang="FR">…</p>

XPath

XML Path Language 1.0 REC 29012000 Wide purpose syntax for addressing sub-parts of an

XML document Joint specification used by XML Pointers

(XPointer recommendation) and the XSLT transformation language

Allows one to access, select and filter XML fragments (cf. Tree representation of an XML document)

Addressing nodes in XPath

Absolute addressing Given: a URL id(M28), root()

Relative addressing along axes Given: a node ancestor, child descendant psibling, fsibling

An XML document represents a hierarchical structure

LOGINid="bonhomme"

BONHOMME

NOM ...

Langue et Dialogue

EQUIPELAB="LORIA"

MEMBRETYPE="IE" ID="M28"

The only view youshould ever, ever haveof an XML document

XPath - Exemples<DB> <MEMBRE TYPE="IE" ID="M28"> <LOGIN ID="bonhomme"/> ...

<EQUIPE LAB="LORIA">Langue et Dialogue</EQUIPE> </MEMBRE> <MEMBRE TYPE="CR" ID="M14"> <LOGIN ID="romary"/> ... </MEMBRE></DB>

/DB/MEMBRE[@ID=‘M28’]/EQUIPE[1]/text()/DB/MEMBRE[2]

/DB/MEMBRE/LOGIN[@ID=‘romary’]/../@ID

/ ou /DB /DB/MEMBRE

XPointer

Cf. HTML, anchors are needed:<A NAME="TOTO">http://www.titi.fr/index.html#toto

In XML, pointers can directly address a document component:http://…/doc.xml#xptr(id(M28))http://…/doc.xml#xptr(/DB/MEMBRE[28]/MEL)

Advantage: no need to modify the target document (notion of primary source)

XLink In HTML: the elements which may carry links are

known:<A>, <IMG>, ...

In XML: any element may carry a simple or complex link This is done by using pre-defined attributes:<a xlink:type="simple" xlink:href="http://www.w3.org/">W3C</a>

Visualizing XML documents

Basically, an XML document does not provide any information about its presentation

Visualizing a document may depend on the target audience, device etc.

Stylesheets: Casdading Style Sheets (CSS 1 et 2) Extensible Style Language (XSL) >> XSLT

eXtensible Style Language Describes the way a

document will be shown, printed or verbalized…XML

XSL+

XSL: a two-fold proposal

XSL = Transformations + Visualizing properties XSLT : Transformation of XML documents

Allows one to transform an XML document into another XML document

Use this to produce well-formed (!) HTML documents XSL FO: formatting XML data

FO = Formatting Objects Is supposed to be application independent (Word/RTF, PS,

PDF, MIF, …) Not a recommendation yet :-(

General structure of an XSL document

<?xml version="1.0"?><xsl:stylesheet

xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

… <xsl:template match="/"> … </xsl:template>

<xsl:template match="NOM"> … </xsl:template></xsl:stylesheet>

Declarative approach

Sequence of rules (templates) specifying: The pattern (XPath) of nodes to which the rule can

be applied Actions to be undertaken:

Elements to be generated in the target document Selection of the elements to be further explored in the

source document Additional functionalities: testing, sorting, etc.

A simple rule

<xsl:template match='/DB/MEMBRE/NOM'> <B> <xsl:apply-templates/> </B></xsl:template>

pattern (XPath)

The content of <B>will be the one

produced by the instruction

HTML element to be produced

Creating a HTML core document<xsl:template match=“/”> <HTML> <HEAD> <TITLE>My directory</TITLE>

</HEAD> <BODY> <xsl:apply-templates/> </BODY></HTML>

</xsl:template>

Selecting the nodes to be explored

<xsl:template match=“MEMBRE”> <P>

<xsl:apply-templatesselect=“NOM”/>

<xsl:text> - </xsl:text>

<xsl:apply-templatesselect=“EQUIPE”/>

</P></xsl:template>

Conclusion

XML - a practical format (protocol) Next steps:

Sharing DTD, resources tools Generic mechanisms for handling families of

documents (cf. Nancy’s presentation)

References

www.oasis-open.org/cover/www.w3.org/XML/www.w3.org/TRwww.w3.org/TR/REC-xmlbabel.alis.com/web_ml/xml/REC-xml.fr.htmlwww.xml.comwww.xmlinfo.comxml.apache.org