Sams Teach Yourself XML in 21 Days

About the Author

Steven Holzner is an award-winning author who has written 80 computing books. He has been writing about XML since it first appeared and is one of the foremost XML experts in the United States, having written several XML bestsellers and being a much-requested speaker on the topic. He's also been a contributing editor at PC Magazine, has been on the faculty of Cornell University and MIT, and teaches corporate programming classes around the United States.

Acknowledgments

A book like the one you're reading is the product of many people's hard work. I'd especially like to thank Todd Green, the acquisitions editor; Songlin Qiu, the development editor; Matt Purcell, the project editor; and Christian Kenyeres, the tech editor.

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Introduction

Welcome to Extensible Markup Language (XML), the most influential innovation the Internet has seen in years. XML is a powerful, very dynamic topic, spanning dozens of fields, from the simple to the very complex. This book opens up that world, going after XML with dozens of topics—and hundreds of examples.

Unlike other XML books, this book makes it a point to show how XML actually works, making sure that you see everything demonstrated with examples. The biggest problem with most XML books is that they discuss XML and its allied specifications in the abstract, which makes it very hard to understand what's going on. This book, however, illustrates every XML discussion with examples. It shows all that's in the other books and more besides, emphasizing seeing things at work to make it all clear.

Instead of abstract discussions, this book provides concrete working examples because that's the only way to really learn XML. You're going to see where to get a lot of free software on the Internet to run the examples you create—everything from XML browsers to XPath visualizers to XQuery processors to XForms handlers, which you don't find in other books. You'll create XML-based documents that display multimedia shows you can play in RealPlayer, use browser plug-ins to handle XML-based graphics in the popular Hypertext Markup Language (HTML) browsers, enable Web pages to load and handle XML, and much more. XML can get complicated, and seeing it at work is the best way to understand it.

Part I: At a Glance

Creating XML Documents

Part I provides an overview of XML and many of the popular ways it's used. You'll take a look at the various markup languages that have been created using XML and how they work.

You'll also begin creating your own XML documents in this part, and in the process you'll get all the basics down. you're going to see how to create both well-formed and valid XML documents.

Well-formed documents obey a number of rules, and before an XML document can be considered "official," it must be well-formed. To be valid, an XML document must specify a set of syntax rules, and XML processors can use these rules to check whether that document adheres to those rules. You're going to see the two ways of specifying the syntax of XML documents in this part—by using document type declarations (DTDs) and XML schemas.

Day 1. Welcome to XML

Welcome to Extensible Markup Language, XML, the language for handling data in compact, easy-to-manage form—not to mention the most powerful advance the Internet has seen for years. The XML world is a large and ever-expanding one, full of complex and unpredictable innovations, and this book is your guided tour to that world. We're going to go just about everywhere XML goes these days, and that's going to include some pretty amazing territory. Today, we'll get our start with XML and see what it's good for. Here are today's topics in overview:

• Markup languages
• Introducing XML
• Seeing XML in a browser
• Well-formed and valid XML documents
• Extracting data from XML documents
• Working with XML validators
• Seeing XML at work
• Finding XML resources on the Internet

The name of the game in XML is data, because XML is all about storing your data—phone directories, business orders, book lists, anything you like. Unlike HTML, XML is not about displaying your data—it's about packaging that data to transport it easily. The main reason XML has experienced such popularity is that it stores its data as text, meaning that XML documents can be transferred using the already-existing Web technology, which was built to transfer HTML documents as text.

We'll start today's work by taking a look at the languages designed to let you store and handle text, called markup languages, and there are plenty of them out there. As we're going to see, XML is both different and more powerful than most other markup languages.