Learning Java, 3rd Edition

Chapter 24. XML

Every now and then, an idea comes along that in retrospect seems just so simple and obvious that everyone wonders why it hadn’t been seen all along. Often when that happens, it turns out that the idea isn’t really all that new after all. The Java revolution began by drawing on ideas from all the programming languages that came before it. XML—the Extensible Markup Language—does for content what Java did for programming: providing a portable language for describing data.

XML is a simple, common format for representing structured information as text. The concept of XML follows the success of HTML as a universal document presentation format and generalizes it to handle any kind of data. In the process, XML has not only recast HTML but is transforming the way that businesses think about their information. In the context of a world driven more and more by documents and data exchange, XML’s time has come.

A Bit of Background

XML and HTML are called markup languages because of the way they add structure to plain-text documents—by surrounding parts of the text with tags that indicate structure or meaning, much as someone with a pen might highlight a sentence and add a note. While HTML predefines a set of tags and their structure, XML is a blank slate in which the author gets to define the tags, the rules, and their meanings.

Both XML and HTML owe their lineage to Standard Generalized Markup Language (SGML)—the mother of all markup languages. SGML has been used in the publishing ...