Chapter 1. Introduction

XML. These three letters have brought shivers to almost every developer in the world today at some point in the last two years. While those shivers were often fear at another acronym to memorize, excitement at the promise of a new technology, or annoyance at another source of confusion for today's developer, they were shivers all the same. Surprisingly, almost every type of response was well merited with regard to XML. It is another acronym to memorize, and in fact brings with it a dizzying array of companions: XSL, XSLT, PI, DTD, XHTML, and more. It also brings with it a huge promise: what Java did for portability of code, XML claims to do for portability of data. Sun has even been touting the rather ambitious slogan "Java + XML = Portable Code + Portable Data" in recent months. And yes, XML does bring with it a significant amount of confusion. We will seek to unravel and demystify XML, without being so abstract and general as to be useless, and without diving in so deeply that this becomes just another droll specification to wade through. This is a book for you, the Java developer, who wants to understand the hype and use the tools that XML brings to the table.

Today's web application now faces a wealth of problems that were not even considered ten years ago. Systems that are distributed across thousands of miles must perform quickly and flawlessly. Data from heterogeneous systems, databases, directory services, and applications must be transferred without a single decimal place being lost. Applications must be able to communicate not only with other business components, but other business systems altogether, often across companies as well as technologies. Clients are no longer limited to thick clients, but can be web browsers that support HTML, mobile phones that support the Wireless Application Protocol (WAP), or handheld organizers with entirely different markup languages. Data, and the transformation of that data, has become the crucial centerpiece of every application being developed today.