XML for the absolute beginner

A guided tour from HTML to processing XML with Java

Summary
In just a few short years, the World Wide Web and HTML have taken the world by storm. But HTML's limitations and the ever-increasing demand for more flexibility in Internet systems has XML, the Extensible Markup Language, brewing on the horizon. Further, Java applications that move data around need a data representation format as portable as Java itself. Developers who learn XML now will find it a powerful tool for data representation, storage, modelling, and interoperation.

Mark Johnson steps away from his popular JavaBeans column this month to introduce you to the world of XML: where it came from, why it's necessary, how it interoperates with existing Internet technology, and how to use it in your designs. You'll learn about Cascading Style Sheets and XSL, then follow up with a look at the XML and Java technology base at a promising Internet startup, with comments from that company's CEO and technical lead. By the time you've finished reading Mark's article, you'll understand why so many people are paying so much attention to this new data representation standard. (11,000 words)

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TML and the World Wide Web are everywhere. As an example of their ubiquity, I'm going to Central America for Easter this year, and if I want to, I'll be able to surf the Web, read my e-mail, and even do online banking from Internet cafés in Antigua Guatemala and Belize City. (I don't intend to, however, since doing so would take time away from a date I have with a palm tree and a rum-filled coconut.)

And yet, despite the omnipresence and popularity of HTML, it is severely limited in what it can do. It's fine for disseminating informal documents, but HTML now is being used to do things it was never designed for. Trying to design heavy-duty, flexible, interoperable data systems from HTML is like trying to build an aircraft carrier with hacksaws and soldering irons: the tools (HTML and HTTP) just aren't up to the job.

The good news is that many of the limitations of HTML have been overcome in XML, the Extensible Markup Language. XML is easily comprehensible to anyone who understands HTML, but it is much more powerful. More than just a markup language, XML is a metalanguage -- a language used to define new markup languages. With XML, you can create a language crafted specifically for your application or domain.

XML will complement, rather than replace, HTML. Whereas HTML is used for formatting and displaying data, XML represents the contextual meaning of the data.

This article will present the history of markup languages and how XML came to be. We'll look at sample data in HTML and move gradually into XML, demonstrating why it provides a superior way to represent data. We'll explore the reasons you might need to invent a custom markup language, and I'll teach you how to do it. We'll cover the basics of XML notation, and how to display XML with two different sorts of style languages. Then, we'll dive into the Document Object Model, a powerful tool for manipulating documents as objects (or manipulating object structures as documents, depending upon how you look at it). We'll go over how to write Java programs that extract information from XML documents, with a pointer to a free program useful for experimenting with these new concepts. Finally, we'll take a look at an Internet company that's basing its core technology strategy on XML and Java.

Is XML for you?
Though this article is written for anyone interested in XML, it has a special relationship to the JavaWorld series on XML JavaBeans. (See Resources for links to related articles.) If you've been reading that series and aren't quite "getting it," this article should clarify how to use XML with beans. If you are getting it, this article serves as the perfect companion piece to the XML JavaBeans series, since it covers topics untouched therein. And, if you're one of the lucky few who still have the XML JavaBeans articles to look forward to, I recommend that you read the present article first as introductory material.

A note about Java
There's so much recent XML activity in the computer world that even an article of this length can only skim the surface. Still, the whole point of this article is to give you the context you need to use XML in your Java program designs. This article also covers how XML operates with existing Web technology, since many Java programmers work in such an environment.

XML opens the Internet and Java programming to portable, nonbrowser functionality. XML frees Internet content from the browser in much the same way Java frees program behavior from the platform. XML makes Internet content available to real applications.

Java is an excellent platform for using XML, and XML is an outstanding data representation for Java applications. I'll point out some of Java's strengths with XML as we go along.

Let's begin with a history lesson.

The origins of markup languages
The HTML we all know and love (well, that we know, anyway) was originally designed by Tim Berners-Lee at CERN (le Conseil Européen pour la Recherche Nucléaire, or the European Laboratory for Particle Physics) in Geneva to allow physics nerds (and even non-nerds) to communicate with each other. HTML was released in December 1990 within CERN, and became publicly available in the summer of 1991 for the rest of us. CERN and Berners-Lee gave away the specifications for HTML, HTTP, and URLs, in the fine old tradition of Internet share-and-enjoy.

Berners-Lee defined HTML in SGML, the Standard Generalized Markup Language. SGML, like XML, is a metalanguage -- a language used for defining other languages. Each so-defined language is called an application of SGML. HTML is an application of SGML.

SGML emerged from research done primarily at IBM on text document representation in the late '60s. IBM created GML ("General Markup Language"), a predecessor language to SGML, and in 1978 the American National Standards Institute (ANSI) created its first version of SGML. The first standard was released in 1983, with the draft standard released in 1985, and the first standard was published in 1986. Interestingly enough, the first SGML standard was published using an SGML system developed by Anders Berglund at CERN, the organization that, as we have seen, gave us HTML and the Web.

SGML is widely used in large industries and governments such as in large aerospace, automotive, and telecommunications companies. SGML is used as a document standard at the United States Department of Defense and the Internal Revenue Service. (For readers outside of the US, the IRS are the tax guys.)

Albert Einstein said everything should be made as simple as possible, and no simpler. The reason SGML isn't found in more places is that it's extremely sophisticated and complex. And HTML, which you can find everywhere, is very simple; for a lot of applications, it's too simple.

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Resources
There are so many XML resources on the Web, I've had to categorize. The first section here is the most useful, since the documents are either high-level summaries or excellent link sites. Apologies to anyone who was omitted.

XML and Java: General XML resources


• "XML, Java and the Future of the Web," Jon Bosak. The paper that started it all, at least from a Java programmer's point of view. Definitely worth a read, even if it's a bit dated. Jon is commonly considered to be the father of XML. Funny how all of these technologies seem to have paternity:
  http://metalab.unc.edu/pub/sun-info/standards/xml/why/xmlapps.html
  
• "Media-Independent Publishing: Four Myths about XML" Jon Bosak:
  http://metalab.unc.edu/pub/sun-info/standards/xml/why/4myths.htm
  
• Robin Cover's XML-SGML site is, according to my SGML buddies, the bible of XML resources:
  http://www.oasis-open.org/cover/
  
• The W3C's XML resource page lets you cheer from the sidelines as XML technology proposals develop into recommendations, or join in the fray on their active mailing lists:
  http://www.w3.org/XML/
  
• OASIS, the Web site of the Organization for the Advancement of Structured Information Standards, offers general news and information about XML:
  http://www.oasis-open.org/
  
• The Graphics Communications Association, host of the XTech '99 conference (March 11 to 13, 1999, San Jose, CA) and the upcoming XML Europe '99 conference in Granada, Spain, (April 26 to 30, 1999) has a Web site packed with XML information:
  http://www.gca.org/
  
• XML.com is great for watching trends and digging up XML news:
  http://www.xml.com/
  
• Textuality hosts Tim Bray's site. Check it out for a look at the "big picture" of how XML fits into the structured document universe -- and for a look at Lark, Tim's nonvalidating XML processor:
  http://www.textuality.com/
  
• The XML FAQ:
  http://www.ucc.ie/xml/
  
• IBM's XML Website is an outstanding supplement to alphaWorks:
  http://www.software.ibm.com/xml/index.html
  

  XML and Java
  

• "XML and Java: The Perfect Pair" by Ken Sall (Internet.com, November 1998) provides information about XML, Java, and why these two are a match made in heaven:
  http://wdvl.com/Authoring/Languages/XML/Java/index.html
  

  Tutorials and training
  

• Generally Markup, Richard Lander's Web site may be of interest to you if you haven't yet read enough about markup languages:
  http://pdbeam.uwaterloo.ca/~rlander/
  
• The Mulberry Technologies Web site is a good resource for commercial training in XML, as well as general XML and SGML consulting by seasoned SGML experts:
  http://www.mulberrytech.com/
  
• The Web Developer's Virtual Library Series on XML offers good summaries of various XML technologies, as well as annotated indices of XML software:
  http://wdvl.com/Software/XML
  
• Microsoft's Site Builder Network provides a series of articles called "Extreme XML," one of which appears in the following link. While some of it focuses on Microsoft-only, Windows-only technology, there's still some great stuff here:
  http://www.microsoft.com/sitebuilder/magazine/xml.asp
  
• Webmonkey has a good series of articles introducing readers to XML. The index is at:
  http://www.hotwired.com/webmonkey/xml/?tw=xml
  
• "What the ?xml!" by L.C. Rees offers an interesting take on XML and why it's necessary -- nicely written and entertaining to boot:
  http://www.geocities.com/SiliconValley/Peaks/5957/wxml.html
  
• "The XML Revolution" by Dan Connolly is a quick backgrounder on XML (Nature):
  http://helix.nature.com/webmatters/xml.html
  

  Cascading Style Sheets
  

• W3C's CSS page will get your started learning about CSS:
  http://www.w3.org/Style/CSS/
  
• "Cascading Style Sheets Designing for the Web" by Hakom Wium Lie and Bert Bos (Addison-Wesley, 1997) Sample chapters from the book appear at:
  http://www.awl.com/cseng/titles/0-201-41998-X/liebos/
  

  Extensible Style Language (XSL)
  

• The W3C's XSL page:
  http://www.w3.org/Style/XSL/
  
• Read (and comment on) the W3C's XSL Working Draft (currently dated December 16, 1998):
  http://www.w3.org/TR/WD-xsl
  
• "The Extensible Style Language: Styling XML Documents" (WebTechniques Magazine) XSL tutorial information and examples:
  http://www.webtechniques.com/features/1999/01/walsh/walsh.shtml
  
• Microsoft's XML and XSL tutorial site is especially interesting because of the recent release of client-side XSL in Internet Explorer 5.0. Extensive and excellent:
  http://www.microsoft.com/xml
  
• If you're still using IE 4.0, you can still experiment with XML, using Microsoft's internal DOM:
  http://www.microsoft.com/xml/articles/xmlmodel.asp
  
• If you want to experiment with XSL, try downloading IBM's LotusXSL. It's all Java, and for the time being, it's free:
  http://www.alphaworks.ibm.com/tech/LotusXSL
  
• Or, you can try James Clark's XT XSL engine, downloadable from:
  http://www.jclark.com/xml/xt.html
  

  Upcoming XSL contest

  Though the details aren't yet worked out, Sun Microsystems will soon announce a call for proposals for a $30,000 grant to develop a client-side processor for full XSL implementation in Mozilla. It will also announce, in conjunction with Adobe, a contest (first prize $40,000, second prize $20,000) to develop a pure-Java, server-side processor of the entire XSL language, to format XML to PDF (Adobe's document format). Keep watching the Java Developer Connection (requires free registration), and Mozilla sites for the eventual announcements.

• "XTech '99: Java and the XML wave" by Mark Johnson (JavaWorld, April 1999) offers the most current information on the contest:
  http://www.javaworld.com/javaworld/jw-04-1999/jw-04-xtech.html
  

  Simple API for XML (SAX)
  

• The definitive description of SAX is available online. You can also download free SAX software here:
  http://www.megginson.com/SAX/index.html
  

  Document Object Model (DOM)
  

• The W3C information page for the Document Object Model appears on the W3C site:
  http://www.w3c.org/DOM/
  
• Among other things, you'll find the W3C Recommendation for DOM Level 1:
  http://www.w3.org/TR/REC-DOM-Level-1/
  
• The Java bindings for DOM, for both XML and HTML, are in this Recommendation appendix:
  http://www.w3.org/TR/REC-DOM-Level-1/java-language-binding.html
  
• A great DOM tutorial by William Robert Stanek appears on PC Magazine Online in "Object-Based Web Design." This tutorial includes a discussion of using DOM with IDL, CORBA's Interface Definition Language:
  http://www8.zdnet.com/pcmag/pctech/content/17/13/tf1713.001.html
  

  Dynamic HTML

• The Dynamic HTML Resource page contains several links to DHTML articles:
  http://www.hotwired.com/webmonkey/dynamic_html/?tw=dynamic_html
  

  Software
  

• Epicentric, Inc.:
  http://www.epicentric.com/
  
• More XML (and other Java) technology than you can shake a stick at is available at IBM's alphaWorks:
  http://alphaworks.ibm.com/
  
• Version 2 of IBM's excellent XML parser package, xml4j, is available for download. This package includes several parsers, both validating and nonvalidating:
  http://www.alphaworks.ibm.com/tech/xml4j
  
• See also IBM's exciting Bean Markup Language project, which uses XML to represent and manipulate JavaBeans:
  http://www.alphaworks.ibm.com/tech/bml
  
• Another free Java XML parser was written by the indefatiguable James Clark, download at:
  http://www.jclark.com/xml/xp/index.html
  
• XEENA is IBM alphaWorks's DTD-guided XML editor. You want it, you need it, you gotta have it:
  http://www.alphaworks.ibm.com/tech/xeena
  
• Mozilla.org is the open source community's effort to extend the Netscape source code. Find out about it at:
  http://www.mozilla.org/
  
• Information about XML and CSS in Mozilla appears at:
  http://www.mozilla.org/rdf/doc/xml.html
  
• You can read about Sun's XML and Java initiatives at:
  http://www.sun.com/990310/java_xml.jhtml
  
• In addition, Java Project X includes source code downloadable from:
  http://developer.java.sun.com/developer/earlyAccess/xml/index.html
  
• ArborText has a suite of sophisticated tools for editing SGML, XML, and XSL:
  http://www.arbortext.com/Products/products.html
  
• Oracle8i from Oracle corporation uses XML inside the Oracle core:
  http://www.oracle.com/xml/
  
• Download Oracle's free XML for Java parser:
  http://technet.oracle.com/direct/3xml.htm
  
• Microsoft's Internet Explorer 5.0, released this month, implements part of the XSL spec. You can find it on Microsoft's Web site -- and also just about anywhere else:
  http://www.microsoft.com/windows/ie/default.htm
  
• You can also download a beta release of Microsoft's XML Notepad editor (limited to running only on Microsoft Windows):
  http://www.microsoft.com/xml/notepad/download.asp
  
• Vervet Logic of Bloomington, IN, has announced XML <PRO>, a commercial XML editor:
  http://www.vervet.com/
  
• Majix, to transform XML to HTML via XSL, is available at:
  http://www.tetrasix.com/
  
• If your French is rusty, you might want to try the English-language site at:
  http://www.tetrasix.com/english/default.htm
  

  History
  

• Read about the history of HTML here. It's part of an online book, so there's no telling for how long it will be available:
  http://ei.cs.vt.edu/~wwwbtb/hardcopy/book/chap4/origins.html
  The two chapters listed below (of the book "HTML Unleashed" by Rick Darnell, et al., also cover some of the technical background of these languages.

• SGML history
  http://www.webreference.com/dlab/books/html/3-2.html
  
• XML history (such as it is):
  http://www.webreference.com/dlab/books/html/38-0.html
  
• Nothing to do on Friday night? Why not read up on the history of SGML? Charles Goldfarb, considered by many to be the "father of SGML," reminisces publicly at:
  http://www.sgmlsource.com/Goldfarb/history/index.htm
  
• Useful XML and SGML information appears at Goldfarb's Web site, including a comprehensive XML book list:
  http://www.sgmlsource.com/
  

  Miscellaneous links
  

• Uche Ogbuji has written an interesting article in LinuxWorld about using XML on Linux in the Enterprise. It's at:
  http://www.linuxworld.com/linuxworld/lw-1999-03/lw-03-xml.html
  
• Bluestone Software has recently made a splash with pure-Java XML application servers, and a freely downloadable Swing package called XwingML:
  http://www.bluestone.com/
  
• Everyone (except Microsoft) is pretty freaked out about the US Patent Office awarding Microsoft a patent for certain kinds of functionality in style sheets. What happens with this patent, and its impact on developing technology, remains to be seen. Judge for yourself by reading the patent at:
  http://www.patents.ibm.com/patlist?icnt=US&patent_number=5860073
  
• The title of the sample recipe is actually the title of a very funny song by William Bolcom. Similar recipes may be found at:
  http://www.b4uby.com/granny/gsoup.htm
  
• The song appears on a compact disc (with other odd songs) available from the Public Radio Music Source at:
  http://75music.org/best/docs/keepers.htm
  

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