XML for the absolute beginner

A guided tour from HTML to processing XML with Java

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So, what good is made-up markup?
Here are some benefits of representing information in XML:

• XML is at least as readable as HTML and probably more so
  Anyone who understands, more or less, what HTML is probably understands just about everything in Listing 4. They're also likely to have a good idea what the markup means, since the markup uses fairly intuitive terms (<Ingredient><OBJECT CLASSID="000DDA23432...">).

• The tags don't have anything to do with how the document is displayed
  Listing 4 is pure content: It's information. The markup indicates what the information means, not how to display it. The formatting information for an XML file (if there is any need for formatting) is usually written in a style language and stored separately from the XML. (See the sections on CSS and XSL below for more on formatting XML.) Separation of content and presentation is a key concept inherited from SGML.

• A lot of the programming is already done for you
  If you write a DTD and use a validating parser, much of the error checking for the validity of your input is done by the parser. There's no need to write the parser yourself, since there are so many high-quality parsers available for free. If you want to change the language, you simply change the DTD; the parser then obeys your new rules. Moreover, if your system needs to interoperate with other systems, you can choose a standard DTD (like XML/EDI, for example), so that other systems will automatically understand your system's vocabulary, and vice versa.

• XML is more versatile than HTML
  Let's think about all the ways a document like Listing 4 could be used:
  • You could display this recipe in an online recipe database, with a page style easily modifiable across all recipes

  • The recipes are automatically scalable, convenient if you're planning a dinner party for 200

  • The recipe is already in a standard recipe format for transmission to the database

  • Online recipe servers could exchange recipes using this format, or recipe applications could share data

  • Such recipes would be much easier to search accurately (for example, "all recipes with lime Jello and Tabasco sauce") than HTML would be

  • It would be easy, based on the contents of your on your "legacy" pantry inventory database, to produce a shopping list

In fact, CSS (Cascading Style Sheets) and XSL (the Extensible Stylesheet Language) do precisely that: They're the style languages for XML. Let's take a quick look at these two technologies.

In Listing 3 above, you've seen what may be your first XML document. You've got a problem with that document, though: It's going to be pretty difficult to convince the browser manufacturers (not to mention the W3C) to add the <Ingredient> tag to their browsers. What if there were a way to turn this XML into a text file, a PostScript document, a photo-typesetting file, or input to a text-to-speech system for the hearing-impaired? Or what if the XML could somehow be transformed into HTML and displayed in a browser?

The members of the appropriate committees at the W3C have addressed these concerns with two specifications: CSS and XSL. While both are declarative languages (meaning that there are no instructions in the first-do-this, then-do-that sense), they serve different functions. CSS exists as a current recommendation from the W3C, usable with HTML or XML, is simpler to use and less powerful than XSL, and is supported by most current-generation browsers (to varying degrees). XSL is used exclusively to format XML or SGML and is more complex and powerful than CSS.

Great strides have been made with XSL in the past year. While XSL is still just a "working draft" (meaning its design isn't yet complete), you can experiment today with working implementations of the draft. Just this month (March 18, 1999), Microsoft released Internet Explorer 5.0, which includes support for part of the XSL specification. And Mozilla (the open source project based on the Netscape source code) can display XML using CSS. At the XTech '99 conference in San Jose, CA, in early March, Sun Microsystems "pre-announced" a request for proposals (for a grant) and a contest relating to the implementation of an XSL batch-processor and the addition of full XSL to Mozilla. (See Resources.)

Again, the purpose of creating these new standards is to make most things very simple for most people, just like HTML has made hypertext and structured documents attainable to your grandma (or your nine-year-old).

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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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