Java Reference
In-Depth Information
XML
XML
, or the e
X
tensible
M
arkup
L
anguage to give it its full title, is a system and hardware independent
language for expressing data and its structure within an
XML document
. An
XML document
is a
Unicode text file that contains the data together with
markup
that defines the structure of the data.
Because an XML document is a text file, you can create XML using any plain text editor, although an
editor designed for creating and editing XML will obviously make things easier. The precise definition
of XML is in the hands of the World Wide Web Consortium (W3C), and if you want to consult the
XML 1.0 specification, you can find it at
http://www.w3.org/XML
.
The term 'markup' derives from a time when the paper draft of a document to be printed was
marked up
by hand to indicate to the typesetter how the printed form of the document should look. Indeed the
ancestry of XML can be traced back to a system that was originally developed by IBM in the 1960s to
automate and standardize markup for system reference manuals for IBM hardware and software
products. XML markup looks similar to HTML in that it consists of tags and attributes added to the text
in a file. However, the superficial appearance is where the similarity between XML and HTML ends.
XML and HTML are profoundly different in purpose and capability.
Firstly, although an XML document can be created, read, and understood by a person, XML is
primarily for communicating data from one computer to another. XML documents will therefore more
typically be generated and processed by computer programs. An XML document defines the structure
of the data it contains so a program that receives it can properly interpret it. Thus XML is a tool for
transferring information and its organization between computer programs. The purpose of HTML on
the other hand is solely the description of how data should look when it is displayed or printed. The
only structuring information that generally appears in an HTML document relates to the appearance of
the data as a visible image. The purpose of HTML is data presentation.
Secondly, HTML provides you with a set of tags that is essentially fixed and geared to the presentation of
data. XML is a language in which you can define new sets of tags and attributes to suit different kinds of
data - indeed to suit any kind of data including
your
particular data. Because XML is extensible, it is often
described as a
meta-language
- a language for defining new languages in other words. The first step in using
XML to exchange data is to define the language that you intend to use for that purpose - in XML.
Of course, if I invent a set of XML markup to describe data of a particular kind, you will need to know
the rules for creating XML documents of this type if you want to create, receive, or modify them. As we
shall see, the definition of the markup that has been used within an XML document can be included as
part of the document. It also can be provided as a separate entity, in a file identified by a URI for
instance, that can be referenced within any document of that type. The use of XML has already been
standardized for very diverse types of data. There are XML languages for describing the structures of
chemical compounds and, musical scores, as well as plain old text such as in this topic.
The
J
ava
A
PI for
X
ML
P
rocessing (
JAXP
) provides you with the means for reading, creating, and
modifying XML documents from within your Java programs. In order to understand and use this API
there are two basic topics you need to be reasonably familiar with:
What an XML document is for and what it consists of.
❑
What a DTD is and how it relates to an XML document.
❑
