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Programming XML in Java, Part 1

Create Java apps with SAX appeal

By Mark Johnson, JavaWorld.com, 03/13/00

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So, you understand (more or less) how you would represent your data in XML, and you're interested in using XML to solve many of your data-management problems. Yet you're not sure how to use XML with your Java programs.

TEXTBOX: TEXTBOX_HEAD: Programming XML in Java: Read the whole series!

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This article is a follow-up to my introductory article, "XML for the absolute beginner", in the April 1999 issue of JavaWorld (see the Resources section below for the URL). That article described XML; I will now build on that description and show in detail how to create an application that uses the Simple API for Java (SAX), a lightweight and powerful standard Java API for processing XML.

The example code used here uses the SAX API to read an XML file and create a useful structure of objects. By the time you've finished this article, you'll be ready to create your own XML-based applications.

The virtue of laziness

Larry Wall, mad genius creator of Perl (the second-greatest programming language in existence), has stated that laziness is one of the "three great virtues" of a programmer (the other two being impatience and hubris). Laziness is a virtue because a lazy programmer will go to almost any length to avoid work, even going so far as creating general, reusable programming frameworks that can be used repeatedly. Creating such frameworks entails a great deal of work, but the time saved on future assignments more than makes up for the initial effort invested. The best frameworks let programmers do amazing things with little or no work -- and that's why laziness is virtuous.

XML is an enabling technology for the virtuous (lazy) programmer. A basic XML parser does a great deal of work for the programmer, recognizing tokens, translating encoded characters, enforcing rules on XML file structure, checking the validity of some data values, and making calls to application-specific code, where appropriate. In fact, early standardization, combined with a fiercely competitive marketplace, has produced scores of freely available implementations of standard XML parsers in many languages, including C, C++, Tcl, Perl, Python, and, of course, Java.

The SAX API is one of the simplest and most lightweight interfaces for handling XML. In this article, I'll use IBM's XML4J implementation of SAX, but since the API is standardized, your application could substitute any package that implements SAX.

SAX is an event-based API, operating on the callback principle. An application programmer will typically create a SAX Parser object, and pass it both input XML and a document handler, which receives callbacks for SAX events. The SAX Parser converts its input into a stream of events corresponding to structural features of the input, such as XML tags or blocks of text. As each event occurs, it is passed to the appropriate method of a programmer-defined document handler, which implements the callback interface org.xml.sax.DocumentHandler. The methods in this handler class perform the application-specific functionality during the parse.

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Resources
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