XML for the absolute beginner

A guided tour from HTML to processing XML with Java

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Become a tree surgeon!
One final, somewhat more advanced topic, before we close. The SAX interface allows you to parse an XML file and execute particular actions whenever certain structures (like tags) appear in the input. That's great for a lot of applications. There are times, though, when you want to be able to cut and paste whole sections of XML documents, restructure them, or maybe even build from scratch an object structure like the one in Figure 3, and then save the whole structure as an XML file. For that, you need access to the DOM API.

The DOM API allows you to represent your XML document as a tree of nodes in your Java (or other language) program. While a SAX parser reads an XML file, doing callbacks to a user-defined class, a DOM parser reads an XML file and returns a representation of the file as a tree of objects, most of which are of type org.w3c.dom.Node This gives you immense power in manipulating structured documents. Figure 4 is an example of what I'm talking about.

The Document Object Model, in the package org.w3c.dom, defines interfaces for document elements (that is, tags), DTD elements, text nodes (where the actual text inside the tags is kept), and quite a few other things we haven't even discussed. Figure 4 is a schematic of a general system that can transform one XML document to some other form programmatically. Your program uses a DOM parser to parse an XML file, and the parser returns a tree that is an exact representation of the XML in the file. Note that, at this point, you've read an input file, checked it for formatting and semantic validity, and built a complex hierarchical object structure, all in just a few lines of code. You can then traverse the document tree in software, doing whatever you like to the tree structure. Add nodes, delete them, update their values, read or set their attributes -- basically anything you like. When your tree has the new structure you desire, tell the top node to print itself to another XML file, and the new document is created.

XML-Java synergy
One of the reasons Java and XML are so well-suited for one another is that Java and XML are both extensible: Java through its class loaders, XML through its DTD. Imagine a server, reading and writing XML, where the DTD for the system input can change. When a new element is added to the input language, a running server (written in Java) could automatically load new Java classes to handle the new tags. You would not only have an extensible application server -- you wouldn't even have to take the server down to add the extensions!

One small idea points to the possible implementations of XML and Java together. The next section is about a company whose combination of XML and Java is its core technology.

XML with Java in the real world
You now have a handle on XML technology, including how it's implemented in Java. You understand that a document can be viewed as a tree of objects and manipulated using SAX or DOM. Let's have a look at a real company that is using all of these technologies to provide solutions for its clients.

DOM interfaces exist not only for XML, but for HTML, as well. This means that the leftmost document in Figure 4 could be a Web page from which you wish to extract information for manipulation in Java.

In fact, Epicentric, an Internet startup in San Francisco, does just that. Epicentric uses Java and XML in its turnkey systems to allow creation of custom portal sites. Portal sites, like the front pages of Netscape Netcenter and Excite!, are integrated aggregations of information from various Internet sources. In a corporate Internet environment, a portal may contain information gleaned from external Web pages (for example, weather reports), alongside internal enterprise data. Portals are also often customizable by each user.

Epicentric's systems read HTML from the Internet as DOM documents, extract information from those documents, and store that information in a standard XML format. Other information sources are also converted into this same XML format and stored on Epicentric's server. The company then uses the XML with XSL and Java Server Pages to create custom portals for its clients.

"A lot of good work has been done on the basics ... like parsers and XSL processors," says Ed Anuff, CEO of Epicentric. One benefit of using XML is that it makes designers think through the system structure in a very structured way, Anuff says.

When asked about concerns with XML, Anuff states that many of the problems he runs into are architectural, such as which DTD to use, and designating the appropriate places in the system to use XML. Systems designers are still working out how to use this new technology most effectively in an enterprise environment.

Also, since the technology is so new, it's often hard to know what pieces of the system to build in-house. For example, quite a few companies built their own XML parsers but now have little return on investment because larger companies are developing superior XML technology and giving it away for free. "The biggest challenge today is figuring out when you're reinventing the wheel, and when you're adding value," says Anuff.

Despite these challenges, the future looks bright for Epicentric, which has several "pretty decent-sized customers" using the company's software in beta. With clients and advertisers that include the likes of Eastman Kodak Company, Sun Microsystems, Chase Bank, and LIFE Magazine, Epicentric is using XML to aggregate and redistribute information in novel ways.

Conclusion
XML is a powerful data representation technology for which Java is uniquely well-suited. You're going to be hearing a lot about XML in the coming months and years. Anyone working with information systems that communicate with other systems (and what systems don't, these days?) has a lot to gain by understanding XML technology and using it to its full advantage.

Using XML with XSL or CSS, you can manage your Web site's content and style, and change style in one place (the style sheet) instead of editing piles of HTML files or, worse, editing the scripts that produce HTML dynamically. Using SAX or DOM, you can treat Web documents as object structures and process them in a general and clean way. Or, you can leave browsers behind entirely and write pure-Java clients and servers that talk to each other -- and other systems -- in XML, the new lingua franca of the Internet. Sun Microsystems, the creator of Java, has perhaps best described the power of XML and Java together in its slogan: Portable Code -- Portable Data. Start experimenting with XML in Java, and you'll soon wonder how you ever lived without it.

Thanks to Dave Orchard for his comments on drafts of this article, and to the many helpful people I met in San Jose, CA.

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About the author
Mark Johnson lives in Fort Collins, CO, and is a C++ programmer by day and Java columnist by night. Very late night.

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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