Mark Johnson
HTML 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.
HTML: All form and no substance
HTML is a language designed to "talk about" documents: headings,
titles, captions, fonts, and so on. It's heavily document structure-
and presentation-oriented.
Admittedly, artists and hackers have been able to work miracles with
the relatively dull tool called HTML. But HTML has serious drawbacks
that make it a poor fit for designing flexible, powerful, evolutionary
information systems. Here a few of the major complaints:
HTML isn't extensible
An extensible markup language would allow application developers to
define custom tags for application-specific situations. Unless you're a
600-pound gorilla (and maybe not even then) you can't require all
browser manufacturers to implement all the markup tags necessary for
your application. So, you're stuck with what the big browser makers, or
the W3C (World Wide Web Consortium) will let you have. What we need is
a language that allows us to make up our own markup tags without having
to call the browser manufacturer.
HTML is very display-centric
HTML is a fine language for display purposes, unless you require a lot
of precise formatting or transformation control (in which case it
stinks). HTML represents a mixture of document logical structure
(titles, paragraphs, and such) with presentation tags (bold, image
alignment, and so on). Since almost all of the HTML tags have to do
with how to display information in a browser, HTML is useless for other
common network applications -- like data replication or application
services. We need a way to unify these common functions with display,
so the same server used to browse data can also, for example, perform
enterprise business functions and interoperate with legacy systems.
HTML isn't usually directly reusable
Creating documents in word-processors and then exporting them as HTML
is somewhat automated but still requires, at the very least, some
tweaking of the output in order to achieve acceptable results. If the
data from which the document was produced change, the entire HTML
translation needs to be redone. Web sites that show the current weather
around the globe, around the clock, usually handle this automatic
reformatting very well. The content and the presentation style of the
document are separated, because the system designers understand that
their content (the temperatures, forecasts, and so on) changes
constantly. What we need is a way to specify data presentation in terms
of structure, so that when data are updated, the formatting can
be "reapplied" consistently and easily.
HTML only provides one 'view' of data
It's difficult to write HTML that displays the same data in different
ways based on user requests. Dynamic HTML is a start, but it requires
an enormous amount of scripting and isn't a general solution to this
problem. (Dynamic HTML is discussed in more detail below.) What we need
is a way to get all the information we may want to browse at once, and
look at it in various ways on the client.
HTML has little or no semantic structure
Most Web applications would benefit from an ability to represent data
by meaning rather than by layout. For example, it can be very difficult
to find what you're looking for on the Internet, because there's no
indication of the meaning of the data in HTML files (aside from META
tags, which are usually misleading). Type red into a search engine, and
you'll get links to Red Skeleton, red herring, red snapper, the red
scare, Red Letter Day, and probably a page or two of "Books I've Red."
HTML has no way to specify what a particular page item means. A more
useful markup language would represent information in terms of its
meaning. What we need is a language that tells us not how to display
information, but rather, what a given block of information is so we
know what to do with it.
SGML has none of these weaknesses, but in order to be general, it's
hair-tearingly complex (at least in its complete form). The language
used to format SGML (its "style language"), called DSSSL (Document
Style Semantics and Specification Language), is extremely powerful but
difficult to use. How do we get a language that's roughly as easy to
use as HTML but has most of the power of SGML?
