Processing Xml With Java

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One

night five developers, all of whom wore very thick glasses and had

recently been hired by Elephants, Inc., the world's largest purveyor of

elephants and elephant supplies, were familiarizing themselves with the

company's order processing system when they stumbled into a directory

full of XML documents on the main server. "What's this?" the team

leader asked excitedly. None of them had ever heard of XML before so

they decided to split up the files between them and try to figure out

just what this strange and wondrous new technology actually was.

  

The

first developer, who specialized in optimizing Oracle databases,

printed out a stack of FMPXMLRESULT documents generated by the

FileMaker database where all the orders were stored, and began poring

over them. "So this is XML! Why, it's nothing novel. As anyone can see

who's able, an XML document is nothing but a table!"

  

"What do

you mean, a table?" replied the second programmer, well versed in

object oriented theory and occupied with a collection of XMI documents

that encoded UML diagrams for the system. "Even a Visual Basic

programmer could see that XML documents aren't tables. Duplicates

aren't allowed in a table relation, unless this is truly some strange

mutation. Classes and objects is what these document are. Indeed, it

should be obvious on the very first pass. An XML document is an object

and a DTD is a class."

  

"Objects? A strange kind of object,

indeed!" said the third developer, a web designer of some renown, who

had loaded the XHTML user documentation for the order processing system

into Mozilla. "I don't see any types at all. If you think this is an

object, then it's your software I refuse to install. But with all those

stylesheets there, it should be clear to anyone not sedated, that XML

is just HTML updated!"

  

"HTML? You must be joking" said the

fourth, a computer science professor on sabbatical from MIT, who was

engrossed in an XSLT stylesheet that validated all the other documents

against a Schematron schema. "Look at the clean nesting of hierarchical

structures, each tag matching its partner as it should. I've never seen

HTML that looks this good. What we have here is S-expressions, which is

certainly nothing new. Babbage invented this back in 1882!"

  

"An

S expression?" queried the technical writer, who was occupied with

documentation for the project written in DocBook. "Maybe that means

something to those in your learned profession. But to me, this looks

just like a FrameMaker MIF file. However, locating the GUI is taking me

awhile."

  

And so they argued into the night, none of them

willing to give an inch, all of them presenting still more examples to

prove their points, none of them bothering to look at the others'

examples. Indeed, they're probably still arguing today. You can even

hear their shouts from time to time on xml-dev. Their mistake, of

course, was in trying to force XML into the patterns of technologies

they were already familiar with rather than taking it on its own terms.

XML can store data, but it is not a database. XML can serialize

objects, but an XML document is not an object. Web pages can be written

in XML, but XML is not HTML. Functional (and other) programming

languages can be written in XML, but XML is not a programming language.

Books are written in XML, but that doesn't make XML desktop publishing

software.

  

XML is something truly new that has not been seen

before in the world of computing. There have been precursors to it, and

there are always fanatics who insist on seeing XML through database (or

object, or functional, or S-expression) colored glasses. But XML is

none of these things. It is something genuinely unique and new in the

world of computing; and it can only be understood when you're willing

to accept it on its own terms, rather than forcing it into yesterday's

pigeon holes.

  

There are a lot of tools, APIs, and

applications in the world that try to pretend XML is something more

familiar to programmers; that it's just a funny kind of database, or

just like an object, or just like remote procedure calls. These APIs

are occasionally useful in very restricted and predictable

environments. However, they are not suitable for processing XML in its

most general format. They work well in their limited domains, but they

fail when presented with XML that steps outside the artificial

boundaries they've defined. XML was designed to be extensible, but it's

a sad fact that many of the tools designed for XML aren't nearly as

extensible as XML itself.

  

This book is going to show you how

to handle XML in its full generality. It pulls no punches. It does not

pretend that XML is anything except XML, and it shows you how to design

your programs so that they handle real XML in all its messiness: valid

and invalid, mixed and unmixed, typed and untyped, and both all and

none of these at the same time. To that end, this book focuses on those

APIs that don't try to hide the XML. In particular, there are three

major Java APIs that correctly model XML, as opposed to modeling a

particular class of XML documents or some narrow subset of XML. These

are:

   

  
•   

  SAX, the Simple API for XML

    
•   

  DOM, the Document Object Model

    
•   

  JDOM, a Java native API

  

These

APIs are the core of this book. In addition I cover a number of

preliminaries and supplements to the basic APIs including:

   

  
•   

  XML syntax

    
•   

  DTDs, schemas, and validity

    
•   

  XPath

    
•   

  XSLT and the TrAX API

    
•   

  JAXP, a combination of SAX, DOM, and TrAX with a few factory classes

  

And,

since we're going to need a few examples of XML applications to

demonstrate the APIs with, I also cover XML-RPC, SOAP, and RSS in some

detail. However, the techniques this book teaches are hardly limited to

just those three applications.

   

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