Learning XML

277 Pages·2001·3.08 MB·English

Learning XML

Erik T. Ray

First Edition, January 2001
ISBN: 0-59600-046-4, 368 pages

XML (Extensible Markup Language) is a flexible way to create "self-describing data" -
and to share both the format and the data on the World Wide Web, intranets, and
elsewhere.
In Learning XML, the author explains XML and its capabilities succinctly and
professionally, with references to real-life projects and other cogent examples.
Learning XML shows the purpose of XML markup itself, the CSS and XSL styling
languages, and the XLink and XPointer specifications for creating rich link structures.

Release Team[oR] 2001
Preface 1
What's Inside
Style Conventions
Examples
Comments and Questions
Acknowledgments

1 Introduction 5
1.1 What Is XML ?
1.2 Origins of XML
1.3 Goals of XML
1.4 XML Today
1.5 Creating Documents
1.6 Viewing XML
1.7 Testing XML
1.8 Transformation

2 Markup and Core Concepts 25
2.1 The Anatomy of a Document
2.2 Elements: The Building Blocks of XML
2.3 Attributes: More Muscle for Elements
2.4 Namespaces: Expanding Your Vocabulary
2.5 Entities: Placeholders for Content
2.6 Miscellaneous Markup
2.7 Well-Formed Documents
2.8 Getting the Most out of Markup
2.9 XML Application: DocBook

3 Connecting Resources with Links 60
3.1 Introduction
3.2 Specifying Resources
3.3 XPointer: An XML Tree Climber
3.4 An Introduction to XLinks
3.5 XML Application: XHTML

4 Presentation: Creating the End Product 88
4.1 Why Stylesheets?
4.2 An Overview of CSS
4.3 Rules
4.4 Properties
4.5 A Practical Example

5 Document Models: A Higher Level of Control 119
5.1 Modeling Documents
5.2 DTD Syntax
5.3 Example: A Checkbook
5.4 Tips for Designing and Customizing DTD s
5.5 Example: Barebones DocBook
5.6 XML Schema: An Alternative to DTD s

6 Transformation: Repurposing Documents 156
6.1 Transformation Basics
6.2 Selecting Nodes
6.3 Fine-Tuning Templates
6.4 Sorting
6.5 Example: Checkbook
6.6 Advanced Techniques
6.7 Example: Barebones DocBook

7 Internationalization 206
7.1 Character Sets and Encodings
7.2 Taking Language into Account

8 Programming for XML 215
8.1 XML Programming Overview
8.2 SAX: An Event-Based API
8.3 Tree-Based Processing
8.4 Conclusion
A Resources 235
A.1 Online
A.2 Books
A.3 Standards Organizations
A.4 Tools
A.5 Miscellaneous

B A Taxonomy of Standards 241
B.1 Markup and Structure
B.2 Linking
B.3 Searching
B.4 Style and Transformation
B.5 Programming
B.6 Publishing
B.7 Hypertext
B.8 Descriptive/Procedural
B.9 Multimedia
B.10 Science

Glossary 252

Colophon 273

The arrival of support for XML - the Extensible Markup Language - in browsers and authoring tools has followed a
long period of intense hype. Major databases, authoring tools (including Microsoft's Office 2000), and browsers
are committed to XML support. Many content creators and programmers for the Web and other media are left
wondering, "What can XML and its associated standards really do for me?" Getting the most from XML requires
being able to tag and transform XML documents so they can be processed by web browsers, databases, mobile
phones, printers, XML processors, voice response systems, and LDAP directories, just to name a few targets.
In Learning XML, the author explains XML and its capabilities succinctly and professionally, with references to
real-life projects and other cogent examples. Learning XML shows the purpose of XML markup itself, the CSS and
XSL styling languages, and the XLink and XPointer specifications for creating rich link structures.
The basic advantages of XML over HTML are that XML lets a web designer define tags that are meaningful for the
particular documents or database output to be used, and that it enforces an unambiguous structure that supports
error-checking. XML supports enhanced styling and linking standards (allowing, for instance, simultaneous linking
to the same document in multiple languages) and a range of new applications.
For writers producing XML documents, this book demystifies files and the process of creating them with the
appropriate structure and format. Designers will learn what parts of XML are most helpful to their team and will
get started on creating Document Type Definitions. For programmers, the book makes syntax and structures
clear It also discusses the stylesheets needed for viewing documents in the next generation of browsers,
databases, and other devices.
Learning XML
Preface
Since its introduction in the late 90s, Extensible Markup Language (XML) has unleashed a torrent of new
acronyms, standards, and rules that have left some in the Internet community wondering whether it is all really
necessary. After all, HTML has been around for years and has fostered the creation of an entirely new economy
and culture, so why change a good thing? The truth is, XML isn't here to replace what's already on the Web, but
to create a more solid and flexible foundation. It's an unprecedented effort by a consortium of organizations and
companies to create an information framework for the 21st century that HTML only hinted at.
To understand the magnitude of this effort, we need to clear away some myths. First, in spite of its name, XML is
not a markup language; rather, it's a toolkit for creating, shaping, and using markup languages. This fact also
takes care of the second misconception, that XML will replace HTML. Actually, HTML is going to be absorbed into
XML, and will become a cleaner version of itself, called XHTML. And that's just the beginning, because XML will
make it possible to create hundreds of new markup languages to cover every application and document type.
The standards process will figure prominently in the growth of this information revolution. XML itself is an
attempt to rein in the uncontrolled development of competing technologies and proprietary languages that
threatens to splinter the Web. XML creates a playground where structured information can play nicely with
applications, maximizing accessibility without sacrificing richness of expression.
XML's enthusiastic acceptance by the Internet community has opened the door for many sister standards. XML's
new playmates include stylesheets for display and transformation, strong methods for linking resources, tools for
data manipulation and querying, error checking and structure enforcement tools, and a plethora of development
environments. As a result of these new applications, XML is assured a long and fruitful career as the structured
information toolkit of choice.
Of course, XML is still young, and many of its siblings aren't quite out of the playpen yet. Some of the subjects
discussed in this book are quasi-speculative, since their specifications are still working drafts. Nevertheless, it's
always good to get into the game as early as possible rather than be taken by surprise later. If you're at all
involved in web development or information management, then you need to know about XML.
This book is intended to give you a birds-eye view of the XML landscape that is now taking shape. To get the
most out of this book, you should have some familiarity with structured markup, such as HTML or TeX, and with
World Wide Web concepts such as hypertext linking and data representation. You don't need to be a developer to
understand XML concepts, however. We'll concentrate on the theory and practice of document authoring without
going into much detail about writing applications or acquiring software tools. The intricacies of programming for
XML are left to other books, while the rapid changes in the industry ensure that we could never hope to keep up
with the latest XML software. Nevertheless, the information presented here will give you a decent starting point
from which to jump in any direction you want to go with XML.

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Learning XML
What's Inside
The book is organized into the following chapters:
Chapter 1
is an overview of XML and some of its common uses. It's a springboard to the rest of the book, I
ntroducing the main concepts that will be explained in detail in following chapters.
Chapter 2
describes the basic syntax of XML, laying the foundation for understanding XML applications and
technologies.
Chapter 3
shows how to create simple links between documents and resources, an important aspect of XML.
Chapter 4
introduces the concept of stylesheets with the Cascading Style Sheets language.
Chapter 5
covers document type definitions (DTDs) and introduces XML Schema. These are the major techniques
for ensuring the quality and completeness of documents.
Chapter 6
shows how to create a transformation stylesheet to convert one form of XML into another.
Chapter 7
is an introduction to the accessible and international side of XML, including Unicode, character
encodings, and language support.
Chapter 8
gives you an overview of writing software to process XML.
In addition, there are two appendixes and a glossary:
Appendix A
contains a bibliography of resources for learning more about XML.
Appendix B
lists technologies related to XML.
The Glossary explains terms used in the book.

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Learning XML
Style Conventions
Items appearing in the book are sometimes given a special appearance to set them apart from the regular text.
Here's how they look:
Italic
Used for citations to books and articles, commands, email addresses, URLs, filenames, emphasized text,
and first references to terms.
Constant width
Used for literals, constant values, code listings, and XML markup.
Constant width italic
Used for replaceable parameter and variable names.
Constant width bold
Used to highlight the portion of a code listing being discussed.

Examples
The examples from this book are freely downloadable from the book's web site at
http://www.oreilly.com/catalog/learnxml.

Comments and Questions
We have tested and verified the information in this book to the best of our ability, but you may find that features
have changed (or even that we have made mistakes!). Please let us know about any errors you find, as well as
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We have a web page for this book, where we list errata, examples, or any additional information. You can access
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For more information about our books, conferences, software, Resource Centers, and the O'Reilly Network, see
our web site at:
http://www.oreilly.com

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Learning XML
Acknowledgments
This book would not have seen the light of day without the help of my top-notch editors Andy Oram, Laurie
Petrycki, John Posner, and Ellen Siever; the production staff, including Colleen Gorman, Emily Quill, and Ellen
Troutman-Zaig; my brilliant reviewers Jeff Liggett, Jon Udell, Anne-Marie Vaduva, Andy Oram, Norm Walsh, and
Jessica P. Hekman; my esteemed coworkers Sheryl Avruch, Cliff Dyer, Jason McIntosh, Lenny Muellner, Benn
Salter, Mike Sierra, and Frank Willison; Stephen Spainhour for his help in writing the appendixes; and Chris
Maden, for the enthusiasm and knowledge necessary to get this project started.
I am infinitely grateful to my wife Jeannine Bestine for her patience and encouragement; my family (mom1:
Birgit, mom2: Helen, dad1: Al, dad2: Butch, as well as Ed, Elton, Jon-Paul, Grandma and Grandpa Bestine, Mare,
Margaret, Gene, Lianne) for their continuous streams of love and food; my pet birds Estero, Zagnut, Milkyway,
Snickers, Punji, Kitkat, and Chi Chu; my terrific friends Derrick Arnelle, Mr. J. David Curran, Sarah Demb, Chris
"800" Gernon, John Grigsby, Andy Grosser, Lisa Musiker, Benn "Nietzsche" Salter, and Greg "Mitochondrion"
Travis; the inspirational and heroic Laurie Anderson, Isaac Asimov, Wernher von Braun, James Burke, Albert
Einstein, Mahatma Gandhi, Chuck Jones, Miyamoto Musashi, Ralph Nader, Rainer Maria Rilke, and Oscar Wilde;
and very special thanks to Weber's mustard for making my sandwiches oh-so-yummy.
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Learning XML
Chapter 1. Introduction
Extensible Markup Language (XML) is a data storage toolkit, a configurable vehicle for any kind of information, an
evolving and open standard embraced by everyone from bankers to webmasters. In just a few years, it has
captured the imagination of technology pundits and industry mavens alike. So what is the secret of its success?
A short list of XML's features says it all:
• XML can store and organize just about any kind of information in a form that is tailored to your needs.
• As an open standard, XML is not tied to the fortunes of any single company, nor married to any
particular software.
• With Unicode as its standard character set, XML supports a staggering number of writing systems
(scripts) and symbols, from Scandinavian runic characters to Chinese Han ideographs.
• XML offers many ways to check the quality of a document, with rules for syntax, internal link
checking, comparison to document models, and datatyping.
• With its clear, simple syntax and unambiguous structure, XML is easy to read and parse by humans
and programs alike.
• XML is easily combined with stylesheets to create formatted documents in any style you want. The
purity of the information structure does not get in the way of format conversions.
All of this comes at a time when the world is ready to move to a new level of connectedness. The volume of
information within our reach is staggering, but the limitations of existing technology can make it difficult to
access. Businesses are scrambling to make a presence on the Web and open the pipes of data exchange, but are
hampered by incompatibilities with their legacy data systems. The open source movement has led to an explosion
of software development, and a consistent communications interface has become a necessity. XML was designed
to handle all these things, and is destined to be the grease on the wheels of the information infrastructure.
This chapter provides a wide-angle view of the XML landscape. You'll see how XML works and how all the pieces
fit together, and this will serve as a basis for future chapters that go into more detail about the particulars of
stylesheets, transformations, and document models. By the end of this book, you'll have a good idea of how XML
can help with your information management needs, and an inkling of where you'll need to go next.

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Learning XML
1.1 What Is XML?
This question is not an easy one to answer. On one level, XML is a protocol for containing and managing
information. On another level, it's a family of technologies that can do everything from formatting documents to
filtering data. And on the highest level, it's a philosophy for information handling that seeks maximum usefulness
and flexibility for data by refining it to its purest and most structured form. A thorough understanding of XML
touches all these levels.
Let's begin by analyzing the first level of XML: how it contains and manages information with markup. This
universal data packaging scheme is the necessary foundation for the next level, where XML becomes really
exciting: satellite technologies such as stylesheets, transformations, and do-it-yourself markup languages.
Understanding the fundamentals of markup, documents, and presentation will help you get the most out of XML
and its accessories.
1.1.1 Markup
Note that despite its name, XML is not itself a markup language: it's a set of rules for building markup languages.
So what exactly is a markup language? Markup is information added to a document that enhances its meaning in
certain ways, in that it identifies the parts and how they relate to each other. For example, when you read a
newspaper, you can tell articles apart by their spacing and position on the page and the use of different fonts for
titles and headings. Markup works in a similar way, except that instead of space, it uses symbols. A markup
language is a set of symbols that can be placed in the text of a document to demarcate and label the parts of
that document.
Markup is important to electronic documents because they are processed by computer programs. If a document
has no labels or boundaries, then a program will not know how to treat a piece of text to distinguish it from any
other piece. Essentially, the program would have to work with the entire document as a unit, severely limiting the
interesting things you can do with the content. A newspaper with no space between articles and only one text
style would be a huge, uninteresting blob of text. You could probably figure out where one article ends and
another starts, but it would be a lot of work. A computer program wouldn't be able to do even that, since it lacks
all but the most rudimentary pattern-matching skills.
Luckily, markup is a solution to these problems. Here is an example of how XML markup looks when embedded in
a piece of text:
<message>
<exclamation>Hello, world!</exclamation>
<paragraph>XML is <emphasis>fun</emphasis> and
<emphasis>easy</emphasis> to use.
<graphic fileref="smiley_face.pict"/></paragraph>
</message>
This snippet includes the following markup symbols, or tags:
• The tags <message> and </message> mark the start and end points of the whole XML fragment.
• The tags <exclamation> and </exclamation> surround the text Hello, world!.
• The tags <paragraph> and </paragraph> surround a larger region of text and tags.
• Some <emphasis> and </emphasis> tags label individual words.
• A <graphic fileref="smiley_face.pict"/> tag marks a place in the text to insert a picture.
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