eXtensible Stylesheet Language

Department of Computer Science Institute for System Architecture, Chair for Computer Networks

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Scenario of document generation

id name first name title room1 Hawking Stephen Professor 422 Newton Isaac Sir 433 Kepler Johannes Professor 71… … … … …

Web server

Data representation Data extraction

Transformation

InterpretationTransfer

Content + Style / Layout = Presentation

PDF

XHTML

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Layer Model

• The progression can be viewed as layer model• Data inside Storage Layer is extracted and forwarded to the

user after transformation to presentation format through processing layer

Storage Layer

Processing Layer

Presentation Layer

RDBMS File SystemXML Repository . . .

eXtensible Markup Language

Style sheetsLogic

(X)HTML PDF PostScript WMLRTF . . .

Browser Printer PDF Reader Mobile Application . . .

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Extensible Stylesheet Language

• The eXtensible Stylesheet Language (XSL) is a set of W3C recommendations for XML transformation and presentation that is widely used to generate arbitrary documents

• It consists out of:1. XML Path Language (XPath)

an expression language for addressing parts of an XML document

2. XSL Transformations (XSLT) a language for transforming one XML representation toanother

3. XSL Formatting Objects (XSL-FO) an XML vocabulary for specifying formatting semantics

Processing Layer

eXtensible Markup Language

XSL stylesheetsXSLT-, XSL-FO-Processors, XPATH, …

Used as integrative data representation

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XML Path Language

• The XML Path Language (XPath) is a non-XML syntax for addressing parts of an XML document

• To navigate in XML trees and select nodes or sets of nodes it uses path expressions

• A location step consists out of three parts:

Examples: • child::person[child::name] all children of type “person”

which have a child of type “name”• /descendant::person[position()=23] selects the twenty third

person in the document (absolute location path)

specifies the tree relationship between the nodes selected by the location step and the context node (e.g. “child”, “parent”, “attribute”, “self”)

specifies the node type

zero or more predicates, which use arbitrary expressions to further refine the set of nodes selected by the location step

Step = AxisName '::' NodeTest '[' Expression ']'

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XML Path Language

• Beside the expanded syntax there exists an abbreviated form • Some important abbreviated location path expressions are:

nodename Selects all child nodes of the node of type ‘nodename’

“title” selects all child elements “title”

nodename1/nodename2 Selects children of ‘nodename1’ which have type ‘nodename2’

“document/title” selects all “title” child nodes of “document” elements

../nodename Selects children of the parent of the current node

“../title” moves to the parent of the current node and selects its child element “title”

nodename/@attr Selects the attribute ‘attr’ of the children of type “nodename”

“document/@id” selects the attribute “id” of the element “document”

. Selects the current node ---

.//nodename Selects all element nodes of type ‘nodename’ that are successors of the current node (located in an arbitrary depth of the XML tree)

“.//title” selects all elements “title” that are successors of the current node (e.g. “document”)

* Selects all children of the current node ---

/path ‘/’ introduces an absolute path (root-node) ---

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XML Path Language

• XPath is the conceptual basis of further W3C specifications:– XQuery

Language for querying for information in XML documents

Example: Counts all elements “book” in an XML file

– XML Pointer Language (XPointer)Language for pointing to specific parts of an XML document

Example:Selects the second child element of the root element (= the first element)

in the file ‘list.xml’

– XML Linking Language (XLink)Language for creating hyperlinks in an XML document

Example:

XQuery

XSLT

XLinkXPointer

XPath

fn:count(//book)

xlink:href="list.xml#element(/1/2)"

<anchor xmlns:xlink="http://www.w3.org/1999/xlink" xlink:type="simple" xlink:href="http://www.w3.org/list.xml#id('msc').child(5,item)">text</anchor>

Defines a simple link that points via XPointer to the fifth item in a list with a unique id of “msc” in the file list.xml located at www.w3.org

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XSL Transformation

• XSLT is an XML based language used for transforming one XML tree to another XML tree (or e.g. another text based representation)

• It is based on a separation of content and style of the resulting tree:– Content is available as XML data– Style is available as an XSLT stylesheet file (valid XML)

• An XSLT processor takes the XML data as input and generates the output file whose structure is described in the XSLT stylesheet

XML Input file

XML Output file

XSLT ProcessorXSLT

declarations(stylesheet)

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XSL Transformation

• There exist two main areas of application for XSLT:1. Communication Oriented Transformation

• XML output file is a format for machine communication

• Often used in the context of Message Oriented Middleware or Service Oriented Architectures

• One common target format is the SOAP protocol:

XMLdata XSLT Service

Service Provider

Network

Service Requestor

2. Presentation Oriented Publishing (POP) Output format is generated for purpose of presentation

(e.g. as PDF or Website)Content of this lecture

SOAPenvelope

SOAPstyle

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XSL Transformation

• The XSLT stylesheet is a collection of templates• Each template defines which actions (e.g. generating XML

tags) are performed if a particular node is examined• The XML source tree is processed recursively beginning with

the root node• If the processing order is not changed explicitly the children

of the current node are traversed from left to right• XSLT uses XPath to specify nodes (e.g. by ‘match’ or ‘select’)

root node

document

title chapter

source tree result (XHTML) tree

chapter

paragraph paragraph paragraph

root node

htmlhead

title

body

h1 p p h1 p

transformation

title titletext text text text text text

text text text text text

text

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<xsl:apply-templates select="chapter"/>. . . <xsl:template match="chapter"><html:h1><xsl:value-of select="title"/></html:h1><xsl:value-of select="paragraph"/></template>. . .

XSL Transformation

transformation

• Particular template is processed by ‘apply-templates’-instruction that results in a new set of selected nodes:

result (XHTML) tree

root node

htmlhead

title

body

h1 p p h1 p

text text text text text text

root node

document

title chapter

source tree

chapter

paragraph paragraph paragraphtitle title

text text text text text

text

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XSL Transformation example

<?xml version="1.0" ?><staff>

<staffMember><name>Hawking</name><firstname>Stephen</firstname><title>Professor</title><room>42</room>

</staffMember><staffMember>

<name>Newton</name><firstname>Isaac</firstname><title>Sir</title><room>43</room>

</staffMember></staff>

<table><tr>

<td>Hawking</td><td>Stephen</td><td>Professor</td><td>42</td>

</tr><tr>

<td>Newton</td><td>Isaac</td><td>Sir</td><td>43</td>

</tr></table>

<xsl:template match="staff"><table><xsl:apply-templates/></table>

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

<tr><xsl:apply-templates/></tr></xsl:template><xsl:template match="name">

<td><xsl:value-of select=". "/></td></xsl:template><xsl:template match="firstname">

<td><xsl:value-of select="."/></td></xsl:template><xsl:template match="title">. . .

+

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XSLT language constructs

• Beside the simple template mechanism XSLT features many powerful language constructs making it an expressive programming language

• Some of these constructs are:– <xsl:for-each select=“XPATH expression”>

• Selects every XML element of a specified node-set – <xsl:if test=“XPATH expression”>

• Puts a conditional test against the content of the XML file– <xsl:sort select=“XPATH expression”/>

• Selected nodes can be sorted in alphabetical or numeric order

– <xsl:variable name=“somename” select=“XPATH expression”/>• Declares a variable which is initialised with the value

specified by the XPATH expression and can be accessed by $somename

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XHTML generation

• An often used application of XSLT is the generation of XHTML documents

• By this a website can be generated dynamically out of XML content and thus be adapted to the characteristics of a requesting client

• In contrast to the XSLT stylesheet, which describes the resulting XHTML tree, Cascading Style Sheets (CSS) are used to describe the layout of the XHTML document in the web browser

XMLdocument

XHTMLdocument

XSLT

Presentationlayout+

CSSdocument

XSLStylesheet

+

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XHTML generation

XHTML Documents

1. Document has been generated before a client request occurs

2. Transformation done by server

3. Transformation done by client

web serverXHTML data XHTML data

XHTML dataXML data

XSLT stylesheet

XML data XML data

XSLT Stylesheet

web browser

web server web browser

web server web browser

generates XHTML

• There exist three different possibilities for the XHTML generation process:

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XHTML generation on server

Web browser

Web server

1

1

2

3

4

Request for a dynamically generated document

Application server

Script code

2

Web accessible logic (e.g. Script code in form of Java Server Pages or Active Server Pages) is executed on application server

3

Load XML and XSL data into memory

Pass the XML and XSL data to a XSLT processor

3

XML XSL

XSLT processor4

5 Generate the XHTML file

6 Send the XHTML file to the web browser

6

XHTMLfile

5

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XHTML generation on client

Web browser

Web server

1

2

3

4

Request for an XML document and delivery of this document

Web browser determines reference to XSL file that is included in the XML document

Request for XSL-document and delivery of this document

Out of the XML and XSL data the browser’s XSLT processor generates the XHTML document

• Alternative 1: the browser requests an XML document that contains XSL stylesheet reference:

<?xml-stylesheet type="text/xsl" href="pathtofile.xsl"?>

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XML document

2

3

XSLT processor

XSLdocument

4 4

XHTML

4

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XHTML generation on client

Web browser

Web server

1

1

2

3

4

Request for a document and delivery of this document

Web browser executes JavaScript code

JavaScript code loads XML data JavaScript code loads XSL data

5 The transformation of XML data to XHTML tags is done inside the JavaScript-Engine

6 Generated XHTML-tags are included into the document that finally can be displayed

2

3

• Alternative 2: the requested document contains JavaScript code, that organises the request for the XML and XSL files

This method works in non XSLT aware browsers

<html>JavaScript-Code</html>

JavaScript-Engine

XML XSL

4

XHTMLtags

65

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Transformation on client

<html><body>

<script type="text/javascript">var xml = new ActiveXObject("Microsoft.XMLDOM")xml.async = falsexml.load("data.xml")var xsl = new ActiveXObject("Microsoft.XMLDOM")xsl.async = falsexsl.load("style.xsl")document.write(xml.transformNode(xsl))

</script></body>

</html>

• The following code fragment shows a possibility for a JavaScript based transformation on client

• In real applications it is necessary to detect the client’s web browser and depending on this information instantiate the right XML parser

Instantiating the Microsoft XML parser

Load the XML datawithout delay

Start the transformation

Load XSL data

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Client-dependent transformation

Web server

Categorizer

Regular web browser

MobileClient

XSLT workflow

WML Scheme

XHTML Schemechooses

Adapteddocument reply

• Depending on the requesting client a categorizer chooses the necessary stylesheet and forwards it to the XSLT procedure that generates the client adapted document

request

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XSL Formatting Objects

• XSLT can only transform one XML tree to another or alternatively to text based formats and thus is inapplicable for generating arbitrary result documents such as especially page oriented representations

• To specify complex page layout for a document the W3C has released the XSL Formatting Object (XSL-FO) standard

• XSL-FO is an XML based mark-up language describing the formatting of XML data for output to screen, printer or other media

• An XSL-FO file describes what the pages look like and where the content has to be placed in a very detailed way

• Further developments of the CSS specification (CSS Level 3) show an increasing convergence between CSS and XSL-FO though XSL-FO still offers a more powerful expressiveness regarding page-oriented layout

• Alternative for generating PDF files: iText (http://itextpdf.com/)

XSL-FOfile PDF

XSL-FOprocessor

RTF

Bitmap

. . .

External resources

(images etc.)

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XSL Formatting Objects

<fo:layout-master-set><fo:simple-page-master

master-name="example"page-width="210mm"page-height="297mm"margin-top=""margin-bottom=""margin-left=""margin-right=""><fo:region-body margin="2cm"/><fo:region-before extent="2cm"/><fo:region-after extent="2cm"/><fo:region-start extent="1cm"/><fo:region-end extent="1cm"/>

</fo:simple-page-master></fo:layout-master-set>

region-before

region-after

regi

on-s

tart region-end

region-body

• Different page layouts can be defined inside the “layout-master-set”

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XSL Formatting Objects

<?xml version="1.0" encoding="iso-8859-1"?><fo:root

xmlns:fo="http://www.w3.org/1999/XSL/Format"><fo:layout-master-set>

<!– definition of layout-master --></fo:layout-master-set><fo:page-sequence

master-reference="example"><fo:flow flow-name="xsl-region-body">

<fo:block font-family="Arial" font-size="12pt">

The more you sweat in training,</fo:block><fo:block font-family="Verdana"

font-size="18pt">the less you'll bleed in battle.

</fo:block></fo:flow>

</fo:page-sequence></fo:root>

The more you sweat in training,the less you'll bleed in battle.

• The content is embedded into text-blocks inside the page flow

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Generating XSL-FO

• In practise an XSL-FO file is generated dynamically out of an XML document by XSLT

• The input file’s markup has to be replaced entirely by XSL-FO-Markup except a few allowed objects that can be embedded into the XSL-FO data (e.g. vector graphics)

XSL-FOdata

PDF

XSL-FOprocessor

RTF

Bitmap

. . .

XML Input file

XSLT declarations(style sheet)

XSLT processor

External resources

(images etc.)

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Generating XML

• In general the content that should be presented is not stored in XML and thus must be transferred to XML at first

• The conversion of data from a relational database to XML is done in three steps:1. Query for data by SQL2. Storage of data in a temporary data structure (e.g. an

Java object)3. Transformation of this data structure to XML by using a

language specific Application Programming Interface (e.g. the Simple API for XML – SAX)

JavaObject

XMLdata

1. SQLLogic

3. API2.

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XSL-FO based document generation

Server Client

HTTP response

webbrowser

HTTP request

ServletProgram

LogicSQL

XMLrepresentation

of data

XSLTstylesheet

XSL-FOprocessor

External viewer

1

23

4

5

7 8

9

• A (Java) Servlet can function as central point to deliver a document to a client

• The following image shows an reference infrastructure for XSL-FO based document generation (e.g. PDF files) in the World Wide Web

XSLT processor

6

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XSL-FO based document generation1

2

3

Web browser sends an HTTP request to the Servlet

Servlet invokes program logic for accessing the database and for generating the document

Data is extracted via SQL from a relational database

4 Data is transferred to an XML representation

5 With the use of an XSLT processor the pure XSL-FO data is generated out of the XML data and the XSLT stylesheet

6 XSL-FO data is forwarded to an XSL-FO processor; document is generated

7 Document is passed as byte-stream to the Servlet

8 Servlet sets the http-response content type (“MIME-type”) to specific value (e.g. application/pdf or image/png) and sends the document to the client

9 If the document can not be directly presented in the web browser, an external application is invoked for this purpose

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Useful target formats

• Beside the already described document types some further useful target formats for an XSL transformation are:– DocBook

Open document standard that is used to generate Unix Manpages and computer documentation in general but also HTML and PDF files

– WorldML / SpreadsheetMLXML based Markup Languages that can be used by current office products as e.g. Microsoft Word and Microsoft Excel

– Java2D / Abstract Window Toolkit (AWT)XSL-FO description can be transformed to a window that displays the single pages

– Really Simple Syndication (RSS)/AtomFamous formats for web feeds used in web blogs etc.

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XSLT / XSL-FO processors

• Demands on XSLT / XSL-FO processors:– Implementation of at least large parts of the XSL

specification (because of the complexity of the spec. a full implementation is rare)

– Availability for desired programming language– Adequate documentation – Adequate set of example applications – Support for additional features– Modular structure which enables simple integration in

own software– Acceptable output generation speed– Support for the wanted XSLT version

• Special demand on XSLT processors:– Validating input and output XML trees

• Special demand on XSL-FO processors:– Support for a variety of output formats

XSLT processor

input

output

XMLValidator

Schemadefinition

valid / not valid

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XSLT processors

• In addition to the mentioned web browser embedded XSLT processors, there exist many free or commercial implementations of the XSLT specifications

• Examples of XSLT processors are:– XALAN

• Java and C++ open source software library that is part of the Apache project

– SAXON• Open Source basic processor and commercial

schema-aware solution and thus supporting input/output validation

• Includes XQuery processor– .NET Class System.Xml.Xsl.XslCompiledTransform

• XSLT processor that is integrated in the Microsoft .NET Framework

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XSL-FO processors

• XSL-FO processors differ highly in the degree of XSL-FO spec. implementation and supported output types

• Examples for XSL-FO processors are:– XSL Formatter

• Commercial processor with full XSL-FO spec. support and many additional features

• Offers interfaces for various programming languages (Java, C++, …)

– Apache Formatting Objects Processor (FOP)• Open source Java application that supports different

output formats and the redirection of the output directly to a printer

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Conclusion

Storage Layer

Processing Layer

Presentation Layer

RDBMS File SystemXML Repository . . .

Temporary data structure

XML APIXML dataXSLTstylesheet

XSL-FOstyle

XSLT processor

XHTML

XSL-FOprocessor

XML Text

resources(images, …)

PDF PNGPostScript. . .

Browser Printer PDF Reader Mobile Application . . .

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References

Links at W3C:

XSL homepage http://www.w3.org/Style/XSL/XSL spec. http://www.w3.org/TR/xsl11/XSLT 1.0 spec. http://www.w3.org/TR/xslt XSLT 2.0 spec. http://www.w3.org/TR/xslt20/XPath 1.0 spec. http://www.w3.org/TR/xpathXPath 2.0 spec. http://www.w3.org/TR/xpath20/

Further Links

FOP http://xmlgraphics.apache.org/fop/SAX http://www.saxproject.org/SAXON http://saxon.sourceforge.net/XALAN http://xalan.apache.org/XSL Formatter http://www.antennahouse.com/