XML for the absolute beginner

A guided tour from HTML to processing XML with Java

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Make up a markup
While a well-formed document is well-formed because it follows rules defined by the XML spec, a valid document is valid because it matches its document type definition (DTD). The DTD is the grammar for a markup language, defined by the designer of the markup language. For my little XML recipe in Listing 3, for example, that designer would be me. The DTD specifies what elements may exist, what attributes the elements may have, what elements may or must be found inside other elements, and in what order.

Nonvalidating parsers read the XML and, if it's well-formed, give you back the document structure as a tree of objects. We'll discuss the document structure you get from a parser in the section below entitled "The Document Object Model." If the document is well-formed but the elements are nonsensical (as was the case with the two <Qty> elements in the <Ingredient> above), that's your problem.

This is, in fact, how HTML browsers work. Generally, HTML parsers are nonvalidating. The various "HTML checking" parsers, which report sytax errors in HTML, are essentially validating HTML parsers (with additional functionality, like link checking).

Validating parsers read XML, verify that it's well-formed (just as nonvalidating parsers do), and then go on to determine whether the document's element tags are legal, whether the attribute names make sense, whether every element nested inside another element belongs there, and so on.

The DTD defines the document type. It accounts for the Extensible in XML. The DTD is how you actually define a new markup language -- what I often call a dialect of XML. DTDs currently are being written for an enormous number of different problem domains, and each DTD defines a new markup language. New markup languages now exist, or are being designed, to mark up the plays of Shakespeare; to define general data resources (RDF); to model information in the health care industry (HL7 SGML/XML); to typeset, display, and actively use mathematical equations (MathML); and to perform electronic data interchange (XML/EDI). There's even a proposal for a markup language for business data in the footwear industry (FDX). (No, I'm not joking.)

Central to each of these new languages is a DTD that describes what tags the markup language has, what those tags' attributes may be, and how they may be combined. A DTD specifies very clearly what information may or may not be included in a markup language. For instance, the DTD for HTML does not allow for markup tags to select paper size for printing.

Let's take a look at a DTD for the recipe XML in Listing 3. I'm going to call it JWSRML (JavaWorld Scary Recipe Markup Language). Apologies to anyone already using that acronym.


<!-- This is the example DTD for the example XML -->
<!ELEMENT Recipe (Name, Description?, Ingredients?, Instructions?)>
<!ELEMENT Name (#PCDATA)>
<!ELEMENT Description (#PCDATA)>
<!ELEMENT Ingredients (Ingredient)*>
<!ELEMENT Ingredient (Qty, Item)>
<!ELEMENT Qty (#PCDATA)>
<!ATTLIST Qty unit CDATA #REQUIRED>
<!ELEMENT Item (#PCDATA)>
<!ATTLIST Item optional CDATA "0"
                  isVegetarian CDATA "true">
<!ELEMENT Instructions (Step)+>


Listing 4. The DTD for JWSRML

The document type definition in Listing 4 defines a language for a validating parser to accept -- meaning, the parser will produce errors if the rules listed in the DTD aren't followed. To get a general idea of how a DTD works, let's look at what a few of the lines in this file mean.

• <!ELEMENT Recipe (Name, Description?, Ingredients?, Instructions?)>
  The <!ELEMENT...> statement defines a tag in the document. This tag defines a <Recipe> tag, stating that it can contain a <Name>, an optional <Description> (the question mark [?] denotes optionality), an optional<Ingredients> tag, and an optional<Instructions> tag.

• <!ELEMENT Name (#PCDATA)>
  This simply states that a <Name> tag can contain character data and nothing else.

• <!ATTLIST Item optional CDATA "0" isVegetarian CDATA "true">
  This section states that the <Item> tag has two possible attributes: optional, whose default value is 0; and isVegetarian, whose default value is true. Notice that attribute values aren't limited to numbers; they can be any text.

A DTD is associated with an XML document by way of a document type declaration, which appears at the top the XML file (after the <?xml...?> line). The document type declaration may contain either an inline copy of the document type definition or contain a reference to that document as a system filename or URI (universal resource ID). For example,


<!DOCTYPE Recipe SYSTEM "example.dtd">


tells the parser to start looking for a <Recipe> tag as the top-level tag of the document. It also declares that the DTD is in the system file example.dtd.

There are other characters and notations in the DTD, but writing DTDs is a topic unto itself. If you're interested in learning more, check out the DTD-related links in Resources.

You now know a lot about how XML is structured and controlled, but you haven't heard what it's good for. Why are people so excited about this technology?

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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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