XML Basics
White Paper on XML Basics
XML BasicsDocument Control
Change RecordDateAuthorVersionChange Reference
04-Apr-09Anoosha Burlakanti,Preethi Phani Mummaleti1.0.0Initial
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Krishna Mohan Adavi
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Table of Contents:TopicPage No
1. Introduction To XML4
2. What is XML4
3. Why XML?4
4. Difference Between XML and HTML5
5. Characteristics of XML5
6. XML Tree6
7. XML Syntax Rules8
8. XML Elements10
9. XML Attributes10
10. Well-formed XML versus Valid XML11
11. XML Schema13
12. References13
Extensible Markup Language1. Introduction to XML: Markup
languages evolved from early, private company and government forms
into Standard Generalized Markup Language (SGML), Hypertext Markup
Language (HTML), and eventually into XML. SGML can seem complex,
and HTML (which was really just an element set) was just not
powerful enough to identify information. XML is designed as an
easy-to-use and easy-to-extend markup language. The Extensible
Markup Language (XML) came into existence as a result of an attempt
to facilitate the sharing of information (data) across different
information systems working on different technology platforms, via
the internet. The XML is a simplified subset of Standard
Generalized Markup Language (SGML). XML stands for Extensible
Markup Language .XML is a markup language much like HTML .It was
designed to carry data, not to display data .XML tags are not
predefined. You must define your own tags. It is designed to be
self-descriptive. XML uses a DTD (Document Type Definition) to
formally describe the data.
XML is a complement to HTML. It is important to understand that
XML is not a replacement for HTML. In the future development of the
Web, it is most likely that XML will be used to structure and
describe the Web data, while HTML will be used to format and
display the same data.
XML was designed to transport and store data.HTML was designed
to display data.2. What is XML?
XML is a software- and hardware-independent tool for carrying
information. XML stands for Extensible Markup Language
XML is a markup language much like HTML
XML was designed to carry data, not to display data
XML tags are not predefined. You must define your own tags
XML is designed to be self-descriptive
XML is a W3C Recommendation
XML (Extensible Markup Language) is a general-purpose
specification for creating custom markup languages. It is
classified as an extensible language, because it allows the user to
define the mark-up elements. You can create content and mark it up
with delimiting tags, making each word, phrase, or chunk into
identifiable, sortable information.
XML is recommended by the World Wide Web Consortium (W3C). It is
a fee-free open standard. The recommendation specifies lexical
grammar and parsing requirements.
3. Why XML?
XML was created so that richly structured documents could be
used over the web. The only viable alternatives, HTML and SGML, are
not practical for this purpose.
HTML comes bound with a set of semantics and does not provide
arbitrary structure.
SGML provides arbitrary structure, but is too difficult to
implement just for a web browser. Full SGML systems solve large,
complex problems that justify their expense. Viewing structured
documents sent over the web rarely carries such justification.
This is not to say that XML can be expected to completely
replace SGML. While XML is being designed to deliver structured
content over the web, some of the very features it lacks to make
this practical, make SGML a more satisfactory solution for the
creation and long-time storage of complex documents. In many
organizations, filtering SGML to XML will be the standard procedure
for web delivery.Designed for ease-of-use with Standard Generalized
Markup Language (SGML).Goal is to enable SGML to be served,
received and processed beyond what is now possible with HTML.4.
Difference between XML and HTML
XML is not a replacement for HTML.XML was designed to transport
and store data, with focus on what data is.
HTML was designed to display data, with focus on how data
looks.
HTML is about displaying information, while XML is about
carrying information.
5. Characteristics of XML1) XML was created to structure, store,
and transport information.
The following example is a note to Anoosha from Preethi, stored
as XML:
AnooshaPreethiUrgentPlease call me!
The above example is self descriptive. It has sender and
receiver information, it also has a heading and a message body.
This XML document does not do anything. It is just pure
information wrapped in tags.
2) XML is Just Plain Text
XML is nothing special. It is just plain text. Software that can
handle plain text can also handle XML.
However, XML-aware applications can handle the XML tags
specially. The functional meaning of the tags depends on the nature
of the application.
3) With XML You Can Invent Your Own Tags
The tags in the example above (like and ) are not defined in any
XML standard. These tags are "invented" by the author of the XML
document.
That is because the XML language has no predefined tags.
The tags used in HTML (and the structure of HTML) are
predefined. HTML documents can only use tags defined in the HTML
standard (like , , etc.).
XML allows the author to define his own tags and his own
document structure.
4) XML is Not a Replacement for HTML
It is important to understand that XML is not a replacement for
HTML but complement to HTML. In most web applications, XML is used
to transport data, while HTML is used to format and display the
data.
5) XML is a W3C Recommendation
XML became a W3C Recommendation on 10. February 1998.
6) XML is everywhereXML is now as important for the Web as HTML
was to the foundation of the Web.XML is everywhere. It is the most
common tool for data transmissions between all sorts of
applications, and is becoming more and more popular in the area of
storing and describing information.6. XML TreeXML documents form a
tree structure that starts at "the root" and branches to "the
leaves".
An Example XML Document
Consider the below example:
Anoosha Preethi Urgent Please call me!
The first line is the XML declaration. It defines the XML
version (1.0) and the encoding used (ISO-8859-1 = Latin-1/West
European character set).
The next line describes the root element of the document
The next 4 lines describe 4 child elements of the root (to,
from, heading, and body):
AnooshaPreethiUrgentPlease call me!And finally the last line
defines the end of the root element:
XML documents must contain a root element. This element is "the
parent" of all other elements.
The elements in an XML document form a document tree. The tree
starts at the root and branches to the lowest level of the
tree.
All elements can have sub elements (child elements):
.....
The terms parent, child, and sibling are used to describe the
relationships between elements. Parent elements have children.
Children on the same level are called siblings (brothers or
sisters).
All elements can have text content and attributes (just like in
HTML).
Example:
The image above represents one book in the XML below:
Everyday Italian Giada De Laurentiis 2005 30.00 Harry Potter J
K. Rowling 2005 29.99 Learning XML Erik T. Ray 2003 39.95
The root element in the example is . All elements in the
document are contained within .
The element has 4 children: ,< author>, , .
7. XML Syntax RulesThe syntax rules of XML are very simple and
logical. The rules are easy to learn, and easy to use.
1) All XML Elements Must Have a Closing TagIn HTML, you will
often see elements that don't have a closing tag:
This is a paragraphThis is another paragraphIn XML all elements
must have a closing tag:This is a paragraphThis is another
paragraphNote: You might have noticed from the previous example
that the XML declaration did not have a closing tag. This is not an
error. The declaration is not a part of the XML document itself,
and it has no closing tag.
2) XML Tags are Case SensitiveXML elements are defined using XML
tags.XML tags are case sensitive. With XML, the tag is different
from the tag .
Opening and closing tags must be written with the same case:This
is incorrectThis is correctNote: "Opening and closing tags" are
often referred to as "Start and end tags". Use whatever you prefer.
It is exactly the same thing.
3)XML Elements Must be properly NestedIn XML, all elements must
be properly nested within each other:This text is bold and
italic (this is wrong)This text is bold and
italic (this is correct)4)XML Documents Must Have a Root
ElementXML documents must contain one element that is the parent of
all other elements. This element is called the root element.
.....
5)XML Attribute Values must be quotedXML elements can have
attributes in name/value pairs just like in HTML.
In XML the attribute value must always be quoted. Consider the
two XML documents below. (This is wrong because date attribute in
the note element is not quoted) Anoosha Preethi
(This is correct) Anoosha Preethi
6)Entity References
Some characters have a special meaning in XML.
If you place a character like "
