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`XML in a Nutshell
`
`by Elliotte Rusty Harold and W. Scott Means
`
`Copyright © 2001 O’Reilly & Associates, Inc. All rights reserved.
`Printed in the United States of America.
`
`Published by O’Reilly 8:. Associates, Inc., 101 Morris Street, Sebastopol, CA 95472.
`
`Contributor: Stephen Spainhour
`
`Editors: Laurie Petrycki and John Posner
`
`Production Editor: Ann Schirmer
`
`Cover Designer: Ellie Volckhausen
`
`Printing History:
`
`January 2001:
`
`First Edition.
`
`Nutshell Handbook, the Nutshell Handbook logo, and the O‘Reilly logo are registered
`trademarks of O’Reilly 8: Associates, Inc. The association of the image of a peafowl
`and the topic of XML is a trademark of O’Reilly 8: Associates, Inc.
`
`Many of the designations used by manufacturers and sellers to distinguish their
`products are claimed as trademarks. Where those designations appear in this book,
`and O’Reilly 8: Associates, Inc. was aware of a trademark claim, the designations
`have been printed in caps or initial caps.
`
`While every precaution has been taken in the preparation of this book, the publisher
`assumes no responsibility for errors or omissions, or for damages resulting from the
`use of the information contained herein.
`
`Library of Congress Catalogt‘ng-in—Publtcation Data can befound at:
`him-flwww.orez‘ibmom/camtog/mtnut.
`
`ISBN: 0-596-00058—8
`[M]
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`Table of Contents
`
`Preface ....................................................................................................... 362'
`
`Part 1: XML Concepts
`
`Chapter I —In troducz‘ng XML ........................................................... 3
`
`What XML Offers .................................................................................... 3
`
`Portable Data .......................................................................................... 6
`How XML Works _.................................................................................... 6
`
`The Evolution of XML ............................................................................ 8
`
`Chapter 2-—XML Fundamentals ................................................... I 1
`
`XML Documents and XML Files .......................................................... 11
`
`Elements, Tags, and Character Data .................................................... 12
`Attributes ............................................................................................... 15
`
`XML Names ........................................................................................... 17
`
`Entity References .................................................................................. 18
`CDATA Sections ................................................................................... 19
`
`Comments ............................................................................................. 20
`
`Processing Instructions ......................................................................... 20
`The XML Declaration ........................................................................... 21
`
`Checking Documents for Well-Formedness ....................................... 25
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`Chapter 3—Docamem‘ Type Defintttons .................................... 26
`
`Validation .............................................................................................. 26
`
`Element Declarations ........................................................................... 34
`Attribute Declarations ........................................................................... 39
`General Entity Declarations ................................................................. 46
`
`External Parsed General Entities .......................................................... 48
`
`External Unparsed Entities and Notations ........................................... 49
`
`Parameter Entities ................................................................................. 51
`Conditional Inclusion ........................................................................... 55
`Two DTD Examples ............................................................................. 54
`
`Locating Standard DTDs ...................................................................... 56
`
`Chapter 4—Namespaces .................................................................. 58
`
`The Need for Namespaces ................................................................... 58
`
`Namespace Syntax ................................................................................ 61
`
`How Parsers Handle Namespaces ....................................................... 66
`
`Namespaces and DTDs ........................................................................ 67
`
`Chapter 5—Internattonalz‘zatz‘on ................................................. 69
`
`The Encoding Declaration ................................................................... 69
`Text Declarations .................................................................................. 7O
`XML-Defined Character Sets ................................................................ 71
`
`Unicode ................................................................................................. 72
`
`ISO Character Sets ................................................................................ 74
`Platform-Dependent Character Sets ..................................................... 75
`
`Converting Between Character Sets .................................................... 76
`
`The Default Character Set for XML Documents .................................. 77
`Character References ............................................................................ 78
`xml:lang ................................................................................................ 81
`
`Part II: Narrative-Centric Documents
`
`Chapter 6——XML as a Document Format ................................. 85
`
`SGML’s Legacy ...................................................................................... 85
`
`Narrative Document Structures ............................................................ 86
`
`TEI ......................................................................................................... 88
`
`DocBook ............................................................................................... 91
`Document Permanence ........................................................................ 94
`Transformation and Presentation ......................................................... 96
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`Chapter 7—le/IL on the Web .......................................................... 98
`
`XHTML .................................................................................................. 99
`
`Direct Display of XML in Browsers .................................................. 105
`Authoring Compound Documents with Modular XE-ITML ............... 110
`Prospects for Improved Web Search Methods ................................. 124
`
`Chapter 8—XSL Transformations .............................................. 129
`
`An Example Input Document ............................................................ 129
`xsl:stylesheet and xslztransform ......................................................... 130
`Stylesheet Processors .......................................................................... 152
`Templates ............................................................................................ 153
`Calculating the Value of an Element with xsl:value—of .................... 134
`Applying Templates with xsl:apply-templates .................................. 135
`The Built-in Template Rules .............................................................. 158
`Modes .................................................................................................. 142
`Attribute Value Templates .................................................................. 144
`XSLT and Namespaces ....................................................................... 144
`Other XSLT Elements
`........................................................... 146
`
`Chapter 9—XPath ............................................................................. I4 7
`
`The Tree Structure of an XML Document ......................................... 147
`
`Location Paths ..................................................................................... 150
`
`Compound Location Paths ................................................................. 155
`Predicates ............................................................................................ 157
`
`Unabbreviated Location Paths ........................................................... 158
`General XPath Expressions ................................................................ 160
`)G’ath Functions .................................................................................. 165
`
`Chapter I0—XLz‘nks ........................................................................ I 68
`Simple Links ........................................................................................ 169
`Link Behavior ..................................................................................... 170
`
`Link Semantics .................................................................................... 173
`
`Extended Links ................................................................................... 173
`
`Linkbases ............................................................................................ 180
`
`DTDs for XLinks ................................................................................. 181
`
`Chapter 1 1 —XP0 in ters ................................................................... 182
`
`XPointers on URLs .............................................................................. 182
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`XPointers in Links ............................................................................... 184
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`Bare Names ......................................................................................... 185
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`Child Sequences ................................................................................. 186
`
`Points .................................................................................................. 186
`
`Ranges ................................................................................................. 189
`
`Chapter 12— Cascading Stylesbeets (CSS) ............................... 19]
`
`The Three Levels of CSS .................................................................... 193
`
`CSS Syntax .......................................................................................... 193
`
`Associating Stylesheets with XML Documents .................................. 195
`
`Selectors .............................................................................................. 197
`
`The Display Property ......................................................................... 200
`
`Pixels, Points, Picas, and Other Units of Length .............................. 201
`Font Properties ................................................................................... 202
`
`Text Properties ................................................................................... 203
`
`Colors .................................................................................................. 204
`
`Chapter 13 —XSL Formatting Objects (XSL-FO) ................... 206
`
`XSL Formatting Objects ..................................................................... 208
`
`The Structure of an XSL-FO Document ............................................. 209
`
`Master Pages ....................................................................................... 210
`
`XSL—FO Properties .............................................................................. 216
`
`Choosing Between CSS and XSL-FO ................................................. 221
`
`Part III: Data-Centric Documents
`
`Chapter I 4—XML as a Data Format ........................................ 225
`
`Programming Applications of XML ................................................... 225
`
`Describing Data .................................................................................. 227
`
`Support for Programmers ................................................................... 229
`
`Chapter I5—Programmz°ng Models .......................................... 230
`
`Event— Versus Object-Driven Models ................................................ 230
`
`Programming Language Support ....................................................... 251
`Non-Standard Extensions ................................................................... 232
`
`Transformations .................................................................................. 232
`
`Processing Instructions ....................................................................... 255
`
`Links and References ......................................................................... 233
`
`Notations ............................................................................................. 234
`
`What You Get 15 Not What You Saw ................................................ 254
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`Chapter I 6—D0cament Object Mode! (DOM) ...................... 23 6
`
`DOM Core ........................................................................................... 237
`
`DOM Strengths and Weaknesses ....................................................... 237
`Parsing a Document with DOM ........................................................ 238
`The Node Interface ............................................................................ 238
`
`Specific Node Types ........................................................................... 240
`The DOMImplementation Interface .................................................. 245
`A Simple DOM Application ............................................................... 245
`
`Chapter 1 7—SAX .............................................................................. 250
`
`The ContentHandler Interface ........................................................... 252
`
`SAX Features and Properties .............................................................. 259
`
`Part IV: Reference
`
`Chapter 18—XML 1.0 Reference ................................................. 265
`How to Use This Reference ............................................................... 26S
`Annotated Sample Documents .......................................................... 265
`Key to XML Syntax ............................................................................. 266
`Well-Formedness ................................................................................ 266
`
`Validity ................................................................................................ 275
`Global Syntax Structures .................................................................... 279
`DTD (Document Type Definition) .................................................... 285
`Document Body ................................................................................. 294
`XML Document Grammar .................................................................. 295
`
`Chapter IQ—XPath Reference ..................................................... 299
`
`The XPath Data Model
`
`....................................................................... 299
`
`Datatype .............................................................................................. 300
`Location Paths ..................................................................................... 301
`
`Predicates ............................................................................................ 305
`
`XPath Functions .................................................................................. 305
`
`Chapter 20—XSLT Reference ........................................................ 3 15
`
`The XSLT Namespace ...................'..................................................... 3 15
`XSLT Elements .................................................................................... 515
`
`XSLT Functions ................................................................................... 339
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`Chapter 21—DOM Reference ....................................................... 345
`
`Object Hierarchy ................................................................................ 346
`
`Object Reference ................................................................................ 346
`
`Chapter 22—SAX Reference .......................................................... 400
`
`The 0rg.xml.sax Package ................................................................... 400
`
`The org.xml.sax.helpers Package ...................................................... 407
`SAX Features and Properties .............................................................. 413
`
`The org.xml.sax.ext Package ............................................................. 415
`
`Chapter 23—Character Sets .......................................................... 41 7
`
`Character Tables ................................................................................. 419
`
`HTML-4 Entity Sets .............................................................................. 424
`
`Other Unicode Blocks ........................................................................ 432
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`Index ...................................................................................................... 459
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`
`
`Preface
`
`XML is one of the most important developments in document syntax in the history
`of computing. In the last few years it has been adopted in fields as diverse as law,
`aeronautics,
`finance,
`insurance,
`robotics, multimedia, hospitality,
`travel,
`art,
`construction, telecommunications, software design, agriculture, physics, journalism,
`theology, retail, and medieval literature. XML has become the syntax of choice for
`newly designed document formats across almost all computer applications. It's used
`on Linux, Windows, Macintosh, and many other computer platforms. Mainframes
`on Wall Street trade stocks with one another by exchanging XML documents. Chil-
`dren playing games on their home PCs save their documents in XML. Sports fans
`receive real-time game scores on their cell phones in XML. XML is simply the most
`robust, reliable, and flexible document syntax ever invented.
`
`XML in a Number? is a comprehensive guide to the rapidly growing world of XML.
`It covers all aspects of XML, from the most basic syntax rules.
`to the details of
`DTD creation,
`to the APIs you can use to read and write XML documents in a
`variety of programming languages.
`
`What This Book Covers
`
`There are hundreds of formally established XML applications from the W3C and
`other standards bodies, such as OASIS and the Object Management Group. There
`are even more informal, unstandardized applications from individuals and corpora-
`tions, such as Microsoft‘s Channel Definition Format and John Guajardo’s Mind
`Reading Markup Language. This book cannot cover them all, any more than a
`book on Java could discuss every program that has ever been or might ever be
`written in Java. This book focuses primarily on XML itself. It covers the funda-
`mental rules that all XML documents and authors must adhere to, whether a web
`designer uses SMIL to add animations to web pages or a C++ programmer uses
`SOAP to serialize objects into a remote database.
`
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`This book also covers generic supporting technologies that have been layered on
`top of XML and are used across a wide range of XML applications. These technol-
`ogies include:
`
`XLmks
`
`An attribute-based syntax for hyperlinks between XML and non-XML docu-
`ments that provide the simple, one-directional
`links familiar from HTML,
`multidirectional
`links between many documents, and links between docu—
`ments you don't have write access to.
`
`XSLT
`
`An XML application that describes transformations from one document to
`another, in either the same or different XML vocabularies.
`
`XPoimers
`
`A syntax for identifying particular parts of an XML document referred to by a
`URI; often used in conjunction with an XLink.
`
`XPatb
`
`A non-XML syntax used by both XPointers and XSLT for identifying particular
`pieces of XML documents. For example, an XPath can locate the third
`address element in the document, or all elements with an email attribute
`whose value is elhaxotilmetalabmnc . edu.
`
`Namespaces
`A means of distinguishing between elements and attributes from different XML
`vocabularies that have the same name; for instance, the title of a book and
`the title of a web page in a web page about books.
`
`SAX
`
`The Simple API for XML, an event~based Java application programming inter-
`face implemented by many XML parsers.
`
`DOM
`
`The Document Object Model, a tree-oriented API that treats an XML docu-
`ment as a set of nested objects with various properties.
`
`All these technologies, whether defined in XML (XLinks, XSLT, and Name-Spaces)
`or in another syntax (XPointers, XPath, SAX, and DOM), are used in many
`different XML applications.
`
`This book does not specifically cover XML applications that are relevant to only
`some users of XML. These include:
`
`5 VG
`
`Scalable Vector Graphics is a Wfic-endorsed standard used for encoding line
`drawings in XML.
`
`Mathi
`
`The Mathematical Markup Language is a WSC-endorsed standard XML appli-
`cation used for embedding equations in web pages and other documents.
`
`xii Preface
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`CML
`
`The Chemical Markup Language was one of the first XML applications. It
`describes chemistry, solid-state physics, molecular biology, and the other
`molecular sciences.
`
`RDF
`
`The Resource Description Framework is a WSC-standard XML application
`used for describing resources, with a particular focus on the sort of metadata
`one might find in a library card catalog.
`
`GDP
`The Channel Definition Framework is a nonstandard, Microsoft-defined XML
`application used to publish web sites to Internet Explorer for offline browsing.
`
`Occasionally we use one or more of these applications in an example, but we do
`not cover all aspects of the relevant vocabulary in depth. While interesting and
`important, these applications (and hundreds more like them) are intended prima-
`rily for use with special software that knows their format intimately. For instance,
`graphic designers do not work directly with SVG. Instead, they use their customary
`tools, such as Adobe Illustrator,
`to create SVG documents. They may not even
`know they’re using XML.
`
`This book focuses on standards that are relevant to almost all developers working
`with XML. We investigate XML technologies that span a wide range of XML appli-
`cations, not those that are relevant only within a few restricted domains.
`
`Organization of the Book
`
`Part I, XML Concepts, introduces you to the fundamental standards that form the
`essential core that all XML applications and software must adhere to. It teaches
`you about well-formed XML, DTDs, namespaces, and Unicode as quickly as
`possible.
`
`Part II, Narrative-Centric Documents, explores technologies that are used mostly
`for narrative XML documents, such as web pages, books, articles, diaries, and
`plays. You’ll learn about XSLT, CSS, XSL-FO, )flinks, XPointers, and XPath.
`
`One of the most unexpected developments in XML was its enthusiastic adoption
`of data-heavy structured documents such as spreadsheets, financial statistics, math-
`ematical tables, and software file formats. Part III, Dara-Centric Xflfl, explores the
`use of XML for such data-intensive documents. This part focuses on the tools and
`APIs needed to write software that process XML, including SAX, the Simple API for
`XML, and the W305 Document Object Model.
`
`is a series of quick-reference chapters that form the
`Finally, Part IV, Reference,
`core of any Nutshell handbook. These chapters give you detailed syntax rules for
`the core XML technologies, including XML, D'I‘Ds, XPath, XSLT, SAX, and DOM.
`Turn to this section when you need to quickly find out the precise syntax for
`something you know you can do but don‘t remember exactly how to do.
`
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`Conventions Used in This Book
`
`Body text, like the text you‘re reading now,
`
`is written in Garamond.
`
`Constant width is used for:
`
`'
`
`-
`
`0
`
`Code examples and fragments.
`
`Anything that might appear in an XML document, including element names,
`tags, attribute values, entity references, and processing instructions.
`
`Anything that might appear in a program,
`method names, class names, and literals.
`
`including keywords, operators,
`
`Cmstant —width hold
`
`I!
`
`'-
`
`User input.
`
`Signifies emphasis should be deleted.
`
`Constant—width italic is used for:
`
`-
`
`Replaceable elements in code statements.
`
`Jrah‘c is used for:
`
`0
`
`0
`
`0
`
`.
`
`New terms where they are defined.
`
`Pathnames. filenames, and program names. (However, if the program name is
`also the name of a Java class,
`it is written in constant-width font,
`like other
`class names.)
`
`Host and domain names (wwwxmicom).
`
`URLs(bt¥1-//z'bz'blio.o7g/xmi/).
`
`Significant code fragments, complete programs, and documents are generally
`placed into a separate paragraph like this:
`
`<?xml version="1.0"?>
`
`<?xml—stylesheet href="person.css” type="text/css"?>
`<person>
`
`Alan Turing
`<fperson>
`
`XML is case sensitive. The PERSON element is not the same thing as the person or
`Person element. Case-sensitive languages do not always allow authors to adhere
`to standard English grammar. It is usually possible to rewrite the sentence _so the
`two do not conflict, and when pOSSible we have endeavored to do so. However,
`on rare occasions when there is simply no way around the problem, we let stan-
`. dard English come up the loser.
`
`Finally, although most of the examples used here are toy examples unlikely to be
`reused, a few have real value. Please feel free to reuse them or any parts of them
`in your own code. No special permission is required. As far as we are concerned,
`they are in the public domain (though the same is definitely not
`true of the
`explanatory text).
`
`xiv prqace
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`Requestfor Comments
`
`We enjoy hearing from readers with general comments about how this book could
`be better, specific corrections, or topics you would like to see covered. You can
`reach
`the
`authors
`by
`sending
`email
`to
`elbam@metaiab.unc.edu
`and
`smeans®emetpnlsewebmacbinescom. Please
`realize, however,
`that we each
`receive several hundred pieces of email a day and cannot respond to every one
`personally. For the best chances of getting a. personal response, please identify
`yourself as a reader of this book. And please send the message from the account
`you want us to reply to and make sure that your Reply-to address is properly set.
`There's nothing quite so frustrating as spending an hour or more carefully
`researching the answer to an interesting question and composing a detailed
`response, only to have it bounce because the correspondent sent
`the message
`from a public terminal and neglected to set the browser preferences to include
`their actual email address.
`
`The information in this book has been tested and verified, but you may find that
`features have changed (or you may even find mistakes). We believe the old
`saying, “If you like this book, tell your friends. If you don‘t like it, tell us." We're
`especially interested in hearing about mistakes. As hard as the authors and editors
`worked on this book, inevitably there are a few mistakes and typographical errors
`that slipped by us. If you find a mistake or a typo, please let us know so we can
`correct
`it. You can send any errorsyou find, as well as suggestions for future
`editions, to:
`
`O‘Reilly & Associates, Inc.
`101 Morris Street
`
`Sebastopol, CA 95472
`1—800—998-9938 (in the United States or Canada)
`
`1-707-829-051 5 (international/ local)
`1-707-829-0104 (fax)
`
`We have a web site for the book, where we list errata, examples, and any addi-
`tional information. You can access this site at:
`
`farm/Aurora.oreilly.corn/cataiog/xminu:
`
`Before reporting errors, please check this web site to see if we already posted a
`fix. To ask technical questions or comment on the book, send email to:
`
`bookquestions®orerfly com
`
`For more information about our books, conferences, software, Resource Centers,
`
`and the O‘Reilly Network, see our web site at:
`
`brm://www.oreifly.com
`
`Acknowledgments
`
`Many people were involved in the production of this book. The original editor,
`John Posner, got
`this book rolling and provided many helpful comments that
`substantially improved the book. When John moved on, Laurie Petrycki shep-
`herded this book to its completion. Stephen Spainhour deserves special thanks for
`
`
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`helped create a better book. We‘d like to thank Matt Sergeant and Didier P. H.
`Martin for their thorough technical review of the manuscript and thoughtful
`suggestions.
`
`We‘d also like to thank everyone who has worked so hard to make XML such a
`success over the last few years and thereby give us something to write about. There
`are so many of these people that we can only list a few. In alphabetical order we’d
`like to thank Tim Berners-Lee, Jon Bosak, Tim Bray, James Clark, Charles Gold—
`farb, Jason Hunter, Michael Kay, Brett McLaughlin, David Megginson, David
`Orchard, Walter E. Perry, Simon St. Laurent, C. M. Sperberg-McQueen, James
`Tauber, B. Tommie Usdin, and Mark Wutka. Our apologies to everyone we unin-
`tentionally omitted.
`
`Elliotte would like to thank his agent, David Rogelberg, who convinced him that it
`was possible to make a living writing books like this rather than working in an
`office. The entire Sunsite crew (now ibz‘blz'oorg) has also helped him to communi-
`cate better with his readers in a variety of ways over the last several years. All
`these people deserve much thanks and credit. Finally, as always, he offers his
`largest thanks to his wife, Beth, without whose love and support this book would
`never have happened.
`
`Scott would most like to thank his lovely wife, Celia, who has already spent way
`too much time as a “computer widow.” He would also like to thank his daughter
`Selene for understanding why Daddy can’t play with her when he’s “working,”
`and Skyler for just being himself. Also, he‘d like to thank the team at Enterprise
`Web Machines for helping him make time to write. Finally, he would like to thank
`John Posner for getting him into this and Laurie Petrycki for working with him
`when things got tough.
`
`Elliotte Rusty Harold
`elbaro®metalab.unc.edu
`
`W. Scott Means
`
`meam©enterpnsmebmacbmescom
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`
`
`Introducing XML
`
`><
`E
`
`"n
`
`a..,
`I:
`:2.
`t:
`n"I-.
`:3
`
`(=1
`
`XML, the Extensible Markup Language, is a W3C—endorsed standard for document
`markup. It defines a generic syntax used to mark up data with simple, human-
`readable tags. It provides a standard format for computer documents. This format
`is flexible enough to be customized for domains as diverse as web sites, elec-
`tronic data interchange, vector graphics, genealogy,
`real estate listings, object
`serialization, remote procedure calls, and voice mail systems.
`
`You can write your own programs that interact with, massage, and manipulate
`data in XML documents. If you do, you'll have access to a wide range of free
`libraries in a variety of languages that can read and write XML so that you can
`focus on the unique needs of your program. Or you can use off-the—shelf software
`like web browsers and text editors to work with XML documents. Some tools are
`
`able to work with any XML document. Others are customized to support a partic-
`ular XML application in a particular domain like vector graphics and may not be of
`much use outside that domain. But in all cases, the same underlying syntax is
`used, even if it’s deliberately hidden by more user-friendly tools or restricted to a
`single application.
`
`What XML Ofiers
`
`XML is a meta-markup language for text documents. Data is included in XML
`documents as