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XSLT Programmer's Reference

XSLT Programmer's Reference

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by Michael Kay, Michael H. Kay, Michael Kay

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This book is for programmers already using XML to organize their data in applications and for those who want to use the power and compatibility of XSLT to improve the display of their data. The book is in three parts: a detailed introduction to the concepts of the language, a reference section giving comprehensive specifications and working examples of every


This book is for programmers already using XML to organize their data in applications and for those who want to use the power and compatibility of XSLT to improve the display of their data. The book is in three parts: a detailed introduction to the concepts of the language, a reference section giving comprehensive specifications and working examples of every feature, and an exploitation guide giving advice and case studies for the advanced user.

Author Biography: Michael Kay is the author of the best selling XSLT Programmer's Reference. He has spent most of his career as a software designer and systems architect with ICL, the IT services supplier. In the XML world he is known as the developer of the open source Saxon product, the first fully-conformant implementation of the XSLT standard.

Editorial Reviews

The Barnes & Noble Review
As we've often said, the true power of XSL lies in its ability to transform content for myriad uses. The language XSL uses to perform this magic is XSLT. It's flexible, customizable, cross-platform, and (as author Michael Kay observes), notoriously difficult to understand. That's why you need Kay's XSLT 1.1 Programmer's Reference, Second Edition.

Kay begins by offering needed context -- helping you understand what XSLT can do, how it evolved from the even more bewildering DSSSL, and its key attributes (for example, its reliance on rules and on XML syntax). Kay reviews the XSLT processing model, as well as the structure of XSLT stylesheets. Next, he offers an example-rich reference to XSLT's key elements, expressions, patterns, and functions.

All the examples in this new Second Edition have been updated to reflect improvements in the XSLT 1.1 standard and the latest generation of XSLT parsers. You'll also find a new chapter on writing extension functions, which enable you to call custom C functions from an XSL stylesheet.

The book includes appendices covering the latest parsers, including Microsoft's MSXML3, Xalan, Oracle, and Kay's own well-regarded SAXON processor. (Bill Camarda)

Bill Camarda is a consultant and writer with nearly 20 years' experience in helping technology companies deploy and market advanced software, computing, and networking products and services. His 15 books include Special Edition Using Word 2000 and Upgrading & Fixing Networks For Dummies®, Second Edition.

This reference defines the syntax rules, attributes, effect, and usage of the key language features of XSLT, and offers design advice and case studies for the advanced user. Elements, expressions, patterns, and functions are listed alphabetically in separate sections. The second edition reflects the new working draft of XSLT 1.1., and adds more descriptions of XSLT products. Annotation c. Book News, Inc., Portland, OR

Product Details

Wiley, John & Sons, Incorporated
Publication date:
Programmer's Reference Series
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6.11(w) x 9.02(h) x 2.09(d)

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1: XSLT in Context

This chapter is designed to put XSLT in context. It's about the purpose of XSLT and the task it was designed to perform. It's about what kind of language it is, and how it came to be that way; and it's about how XSLT fits in with all the other technologies that you are likely to use in a typical web-based application. I won't be saying much in this chapter about what an XSLT stylesheet actually looks like or how it works: that will come later, in Chapters 2 and 3.

I shall begin by describing the task that XSLT is designed to perform – transformation – and why there is the need to transform XML documents. I'll then present a trivial example of a transformation in order to explain what this means in practice.

The chapter then moves on to discuss the relationship of XSLT to other standards in the growing XML family, to put its function into context and explain how it complements the other standards.

I'll describe what kind of language XSLT is, and delve a little into the history of how it came to be like that. If you're impatient you may want to skip the history and get on with using the language, but sooner or later you will ask "why on earth did they design it like that?" and at that stage I hope you will go back and read about the process by which XSLT came into being.

Finally, I'll have a few things to say about the different ways of using XSLT within the overall architecture of an application, in which there will inevitably be many other technologies and components each playing their own part.

What is XSLT?

XSLT, which stands for eXtensible Stylesheet Language: Transformations, is a language which, according to the very first sentence in the specification (found at http://www.w3.org/TR/xslt), is primarily designed for transforming one XML document into another. However, XSLT is more than capable of transforming XML to HTML and many other text-based formats, so a more general definition might be as follows:

XSLT is a language for transforming the structure of an XML document.

Why should you want to do that? In order to answer this question properly, we first need to remind ourselves why XML has proved such a success and generated so much excitement.

Why Transform XML?

XML is a simple, standard way to interchange structured textual data between computer programs. Part of its success comes because it is also readable and writable by humans, using nothing more complicated than a text editor, but this doesn't alter the fact that it is primarily intended for communication between software systems. As such, XML satisfies two compelling requirements:
  • Separating data from presentation. The need to separate information (such as a weather forecast) from details of the way it is to be presented on a particular device. This need is becoming ever more urgent as the range of internet-capable devices grows. Organizations that have invested in creating valuable information sources need to be able to deliver them not only to the traditional PC-based web browser (which itself now comes in many flavors), but also to TV sets and WAP phones, not to mention the continuing need to produce print-on-paper.

  • Transmitting data between applications. The need to transmit information (such as orders and invoices) from one organization to another without investing in bespoke software integration projects. As electronic commerce gathers pace, the amount of data exchanged between enterprises increases daily and this need becomes ever more urgent.
Of course, these two ways of using XML are not mutually exclusive. An invoice can be presented on the screen as well as being input to a financial application package, and weather forecasts can be summarized, indexed, and aggregated by the recipient instead of being displayed directly. Another of the key benefits of XML is that it unifies the worlds of documents and data, providing a single way of representing structure regardless of whether the information is intended for human or machine consumption. The main point is that, whether the XML data is ultimately used by people or by a software application, it will very rarely be used directly in the form it arrives: it first has to be transformed into something else.

In order to communicate with a human reader, this something else might be a document that can be displayed or printed: for example an HTML file, a PDF file, or even audible sound. Converting XML to HTML for display is probably the most common application of XSLT today, and it is the one I will use in most of the examples in this book. Once you have the data in HTML format, it can be displayed on any browser.

In order to transfer data between different applications we need to be able to transform data from the data model used by one application to the model used in another. To load the data into an application, the required format might be a comma-separated-values file, a SQL script, an HTTP message, or a sequence of calls on a particular programming interface. Alternatively, it might be another XML file using a different vocabulary from the original. As XML-based electronic commerce becomes widespread, so the role of XSLT in data conversion between applications also becomes ever more important. Just because everyone is using XML does not mean the need for data conversion will disappear. There will always be multiple standards in use. For example, the newspaper industry is likely to use different formats for exchanging news articles from the format used in the broadcasting industry. Equally, there will always be a need to do things like extracting an address from a purchase order and adding it to an invoice. So linking up enterprises to do e-commerce will increasingly become a case of defining how to extract and combine data from one set of XML documents to generate another set of XML documents: and XSLT is the ideal tool for the job.

At the end of this chapter we will come back to specific examples of when XSLT should be used to transform XML. For now, I just wanted to establish a feel for the importance and usefulness of transforming XML. Before we move on to discuss XSLT in more detail and have a first look at how it works, let's take a look at an example that clearly demonstrates the variety of formats to which we can transform XML, using XSLT.

An Example: Transforming Music
There is an excellent registry of XML vocabularies and schemas at http://www.xml.org/xmlorg_registry/index.shtml.

If you look there, you will find half a dozen different XML schemas for describing music. These were all invented with different purposes in mind: a markup language used by a publisher for printing sheet music has different requirements from one designed to let you listen to the music from a browser. MusicML, for example, is oriented to displaying music notation graphically; ChordML is designed for encoding the harmonic accompaniment to vocal lyrics; MusicXML is designed to represent musical scores, specifically western musical notation from the 17th century onwards, while the rather more academic Music Markup Language (MML) from the University of Pretoria is designed for serious musicological analysis, embracing Eastern and African as well as Western musical idioms....

...So you could use XSLT to process marked-up music in many different ways:

  • You could use XSLT to convert music from one of these representations to another, for example from MusicXML to MML.

  • You could use XSLT to convert music from any of these representations into visual music notation, by generating the XML-based vector graphics format SVG.

  • You could use XSLT to play the music on a synthesizer, by generating a MIDI (Musical Instrument Digital Interface) file.

  • You could use XSLT to perform a musical transformation, such as transposing the music into a different key.

  • You could use XSLT to extract the lyrics, into HTML or into a text-only XML document.
As you can see, XSLT is not just for converting XML documents to HTML!

How does XSLT transform XML?

By now you are probably wondering exactly how XSLT goes about processing an XML document in order to convert it into the required output. There are usually two aspects to this process:
  • The first stage is a structural transformation, in which the data is converted from the structure of the incoming XML document to a structure that reflects the desired output.

  • The second stage is formatting, in which the new structure is output in the required format such as HTML or PDF.
The second stage covers the ground we discussed in the previous section; the data structure that results from the first stage can be output as HTML, a text file or as XML. HTML output allows the information to be viewed directly in a browser by a human user or be input into any modern word processor. Plain text output allows data to be formatted in the way an existing application can accept, for example comma-separated values or one of the many text-based data interchange formats that were developed before XML arrived on the scene. Finally, XML output allows the data to be supplied to one of the new breed of applications that accepts XML directly. Typically this will use a different vocabulary of XML tags from the original document: for example an XSLT transformation might take the monthly sales figures as its XML input and produce a histogram as its XML output, using the XML-based SVG standard for vector graphics. Or you could use an XSLT transformation to generate VoxML output, for aural rendition of your data.

Links to information about Motorola's VoxML Voice Markup Language can be found at http://www.oasis-open.org/cover/voxML.html

Let's now delve into the first stage, transformation – the stage with which XSLT is primarily concerned and which makes it possible to provide output in all of these formats. This stage might involve selecting data, aggregating and grouping it, sorting it, or performing arithmetic conversions such as changing centimeters to inches.

So how does this come about? Before the advent of XSLT, you could only process incoming XML documents by writing a custom application. The application wouldn't actually need to parse the raw XML, but it would need to invoke an XML parser, via a defined Application Programming Interface (API), to get information from the document and do something with it. There are two principal APIs for achieving this: the Simple API for XML (SAX) and the Document Object Model (DOM).

The SAX API is an event-based interface in which the parser notifies the application of each piece of information in the document as it is read. If you use the DOM API, then the parser interrogates the document and builds a tree-like object structure in memory. You would then write a custom application (in a procedural language such as C++, Visual Basic, or Java, for example), which could interrogate this tree structure. It would do so by defining a specific sequence of steps to be followed in order to produce the required output. Thus, whatever parser you use, this process has the same principal drawback: every time you want to handle a new kind of XML document, you have to write a new custom program, describing a different sequence of steps, to process the XML.

Both the DOM and the SAX APIs are fully described in the Wrox Press book Professional XML, ISBN 1-861003-11-0.

So how is using XSLT to perform transformations on XML better than writing "custom applications"? Well, the design of XSLT is based on a recognition that these programs are all very similar, and it should therefore be possible to describe what they do using a high-level declarative language rather than writing each program from scratch in C++, Visual Basic, or Java. The required transformation can be expressed as a set of rules. These rules are based on defining what output should be generated when particular patterns occur in the input. The language is declarative, in the sense that you describe the transformation you require, rather than providing a sequence of procedural instructions to achieve it. XSLT describes the required transformation and then relies on the XSLT processor to decide the most efficient way to go about it.

XSLT still relies on an XML parser – be it a DOM parser or a SAX-compliant one – to convert the XML document into a tree structure. It is the structure of this tree representation of the document that XSLT manipulates, not the document itself. If you are familiar with the DOM, then you will be happy with the idea of treating every item in an XML document (elements, attributes, processing instructions etc.) as a node in a tree. With XSLT we have a high-level language that can navigate around a node tree, select specific nodes and perform complex manipulations on these nodes.

The XSLT tree model is similar in concept to the DOM but it is not the same. The full XSLT processing model is discussed in Chapter 2.

The description of XSLT given thus far (a declarative language that can navigate to and select specific data and then manipulate that data) may strike you as being similar to that of the standard database query language, SQL. Let's take a closer look at this comparison....

Meet the Author

Michael Kay has spent most of his career as a software designer and systems architect with ICL, the IT services supplier. As an ICL Fellow, he divides his time between external activities and mainstream projects for clients, mainly in the area of electronic commerce and publishing. His background is in database technology: he has worked on the design of network, relational, and object-oriented database software products as well as a text search engine. In the XML world he is known as the developer of the open source Saxon product, the first fully-conformant implementation of the XSLT standard.

Michael lives in Reading, Berkshire with his wife and daughter. His hobbies, as you might guess from the examples in this book, include genealogy and choral singing.

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XSLT Programmer's Reference 5 out of 5 based on 0 ratings. 5 reviews.
Anonymous More than 1 year ago
Guest More than 1 year ago
This is probably the best ' quick reference' book covering XSLT out there. It is short and quick to the point. This is a keeper !!!
Guest More than 1 year ago
This reference becomes more and more valuable as you gain skill in XSLT. I find myself going back over sections that I previously skimmed as unimportant and finding solid new practical insights
Guest More than 1 year ago
Great explanation of all topics and excellent excellent examples. I'd like to thank the author for writing this book.
Guest More than 1 year ago
It's a real shame to leave such a nice book attributed to wrong single author (and page count is 777, not 400). This is a rare occasion that Wrox published a regular book and not a hefty compendium and it definitely shows in a much higher quality. OK it is from a guy who wrote one XSLT engine but surely we shuld expect Wrox to put more production effort into this kind of high quality expert books. If I would have to pick a chapter it would definitely be number 10 since it gave me exact solution to a problem I needed to solve (generation of multiple files from one XML). I would almost call this book a C&R of XSL since it is rare to see a programmer capable of effectively communicating his knowledge in a written form. Hope to see Kay writing a 'Nutshell' and 'Pocket Reference' for XML and XSL combined.