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Introduction to XMLHow can XML be
Used?
It is important to know
that XML was designed to store, carry and exchange data. It was not designed to
display data.
XML can Separate Data from HTML
With XML, your data is
stored outside your HTML.
When HTML is used to
display data, the data is stored inside your HTML. With XML, data can be stored
in separate XML files. This way you can concentrate on using HTML for data
layout and display, and be sure that changes in the underlying data will not
require any changes to your HTML.
XML data can also be
stored inside HTML pages as "Data Islands". You can still concentrate on using
HTML only for formatting and displaying the data.
XML is used to Exchange Data
With XML, data can be
exchanged between incompatible systems.
In the real world,
computer systems and databases contain data in incompatible formats. One of the
most time-consuming challenges for developers has been to exchange data between
such systems over the Internet.
Converting the data to XML
can greatly reduce this complexity and create data that can be read by many
different types of applications.
XML and B2B
With XML, financial
information can be exchanged over the Internet.
Expect to see a lot about
XML and B2B (Business To Business) in the near future.
XML is going to be the
main language for exchanging financial information between businesses over the
Internet. A lot of interesting B2B applications are under development.
XML can be used to Share Data
With XML, plain text
files can be used to share data.
Since XML data is stored
in plain text format, XML provides a software- and hardware-independent way of
sharing data.
This makes it much easier
to create data that different applications can work with. It also makes it
easier to expand or upgrade a system to new operating systems, servers,
applications, and new browsers.
XML can be used to Store Data
With XML, plain text
files can be used to store data.
XML can also be used to
store data in files or in databases. Applications can be written to store and
retrieve information from the store, and generic applications can be used to
display the data.
XML can make your Data more Useful
With XML, your data is
available to more users.
Since XML is independent
of hardware, software and application, you can make your data available to more
than only standard HTML browsers.
Other clients and
applications can access your XML files as data sources, like they are accessing
databases. Your data can be made available to all kinds of "reading
machines" (agents), and it is easier to make your data available for blind
people, or people with other disabilities.
XML can be used to Create new
Languages
XML is the mother of
WAP and WML.
The Wireless Markup
Language (WML), used to markup Internet applications for handheld devices like
mobile phones, is written in XML.
You can read more about
WML in W3Schools' WML School.
If Developers have Sense
If they DO have sense,
all future applications will exchange their data in XML.
The future might give us
word processors, spreadsheet applications and databases that can read each
other's data in a pure text format, without any conversion utilities in
between.
We can only pray that
Microsoft and all the other software vendors will agree.
XML was designed to
describe data, and to focus on what data is.
HTML was designed to
display data, and to focus on how data looks.
What you should already know
Before you continue you
should have some basic understanding of the following:
- WWW,
HTML and the basics of building Web pages
- Web
scripting languages like JavaScript or VBScript
If you want to study these
subjects first, before you start reading about XML, you can find the tutorials
you need at W3Schools'
Home Page.
What is XML?
- XML
stands for EXtensible Markup Language
- XML
is a markup language much like HTML.
- XML
was designed to describe data.
- XML
tags are not predefined in XML. You must define your own tags.
- XML
uses a DTD (Document Type Definition) to describe the data.
- XML
with a DTD is designed to be self-descriptive.
The main difference between XML and
HTML
XML was designed to
carry data.
XML is not a replacement
for HTML.
XML and HTML were designed with different goals:
XML was designed to
describe data and to focus on what data is.
HTML was designed to display data and to focus on how data looks.
HTML is about displaying
information, XML is about describing information.
XML does not DO anything
XML was not designed to
DO anything.
Maybe it is a little hard
to understand, but XML does not DO anything. XML was not made to DO anything.
XML is created as a way to structure, store and send information.
The following example is a
note to Tove from Jani, stored as XML:
<note> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
The note has a header, and
a message body. It also has sender and receiver information. But still, this
XML document does not DO anything. It is just pure information wrapped in XML tags.
Someone must write a piece of software to send it, receive it or display it.
XML is free and extensible
XML tags are not
predefined. You must "invent" your own tags.
The tags used to markup
HTML documents and the structure of HTML documents are predefined. The author
of HTML documents can only use tags that are defined in the HTML standard (like
<p> and <h1>.....).
XML allows the author to
define his own tags and his own document structure
The tags in the example
above (like <to> and <from>), are not defined in any XML standard.
These tags are "invented" by the author of the XML document.
XML is a complement to HTML
XML is not a
replacement for HTML.
It is important to
understand that XML is not a replacement for HTML. In future Web development it
is most likely that XML will be used to describe the data, while HTML will be
used to format and display the same data.
My best description of XML
is as a cross-platform, software and hardware independent tool for
transmitting information.
XML in future Web development
XML is going to be
everywhere.
We have been participating
in XML development since its creation. It has been amazing to see how quickly
the XML standard has been developed, and how quickly a large number of software
vendors have adopted the standard.
We strongly believe that
XML will be as important to the future of the Web as HTML has been to the
foundation of the Web, and that XML will be the most common tool for all data
manipulation and data transmission.
XML Joke
Question: When should I
use XML?
Answer: When you need a
buzzword in your resume.
XML Syntax
The syntax rules of XML
are very simple and very strict. The rules are very easy to learn, and very
easy to use.
Because of this, creating
software that can read and manipulate XML is very easy to do.
An example XML document
XML documents use a
self-describing and simple syntax.
<?xml version="1.0"?> <note> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
The first line in the
document - the XML declaration - defines the XML version of the document. In
this case the document conforms to the 1.0 specification of XML.
The next line describes
the root element of the document (like it was saying: "this document is a
note"):
The next 4 lines describe
4 child elements of the root (to, from, heading, and body):
<to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> |
And finally the last line
defines the end of the root element:
Can you detect from this
example that the XML document contains a Note to Tove from Jani? Don't you
agree that XML is pretty self-descriptive?
All XML elements must have a closing
tag
With XML, it is illegal
to omit the closing tag.
In HTML some elements do
not have to have a closing tag. The following code is legal in HTML:
<p>This is a paragraph <p>This is another paragraph |
In XML all elements must
have a closing tag like this:
<p>This is a paragraph</p> <p>This is another paragraph</p> |
Note: 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. It is not an
XML element, and it should not have a closing tag.
XML tags are case sensitive
Unlike HTML, XML tags
are case sensitive.
With XML, the tag
<Letter> is different from the tag <letter>.
Opening and closing tags
must therefore be written with the same case:
<Message>This is incorrect</message> <message>This is correct</message> |
All XML elements must be properly
nested
Improper nesting of
tags makes no sense to XML.
In HTML some elements can
be improperly nested within each other like this:
<b><i>This text is bold and italic</b></i> |
In XML all elements must
be properly nested within each other like this:
<b><i>This text is bold and italic</i></b> |
All XML documents must have a root
tag
The first tag in an XML
document is the root tag.
All XML documents must
contain a single tag pair to define the root element. All other elements must
be nested within the root element.
All elements can have sub
elements (children). Sub elements must be correctly nested within their parent
element:
<root> <child> <subchild>.....</subchild> </child> </root> |
Attribute values must always be
quoted
With XML, it is illegal
to omit quotation marks around attribute values.
XML elements can have
attributes in name/value pairs just like in HTML. In XML the attribute value
must always be quoted. Study the two XML documents below. The first one is
incorrect, the second is correct:
<?xml version="1.0"?> <note date=12/11/99> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
<?xml version="1.0"?> <note date="12/11/99"> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
The error in the first document is that the date attribute in the note element
is not quoted.
This is correct:
date="12/11/99". This is incorrect: date=12/11/99.
With XML, White Space is Preserved
With XML, the white
space in your document is not truncated.
This is unlike HTML. With
HTML, a sentence like this:
Hello
my name is Tove, will be displayed like this: Hello my name is Tove, because
HTML strips off the white space.
With XML, CR / LF is converted to LF
With XML, a new line is
always stored as LF.
Do you know what a
typewriter is?. Well, a typewriter is a type of mechanical device they used in
the previous century :-)
After you have typed one
line of text on a typewriter, you have to manually return the printing carriage
to the left margin position and manually feed the paper up one line.
In Windows applications, a
new line in the text is normally stored as a pair of CR LF (carriage return,
line feed) characters. In Unix applications, a new line is normally stored as a
LF character. Some applications use only a CR character to store a new line.
There is nothing Special about XML
There is nothing special
about XML. It is just plain text with the addition of some XML tags enclosed in
angle brackets.
Software that can handle
plain text can also handle XML. In a simple text editor, the XML tags will be
visible and will not be handled specially.
In an XML-aware
application however, the XML tags can be handled specially. The tags may or may
not be visible, or have a functional meaning, depending on the nature of the
application.
XML Elements
XML Elements are
extensible and they have relationships.
XML Elements have simple
naming rules.
XML Elements are Extensible
XML documents can be
extended to carry more information.
Look at the following XML
NOTE example:
<note> <to>Tove</to> <from>Jani</from> <body>Don't forget me this weekend!</body> </note> |
Let's imagine that we
created an application that extracted the <to>, <from>, and
<body> elements from the XML document to produce this output:
|
MESSAGE
To: Tove
From: Jani
Don't forget me this
weekend!
|
Imagine that the author of
the XML document added some extra information to it:
<note> <date>1999-08-01</date> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
Should the application
break or crash?
No. The application should
still be able to find the <to>, <from>, and <body> elements
in the XML document and produce the same output.
XML documents are
Extensible.
XML Elements have Relationships
Elements are related as
parents and children.
To understand XML
terminology, you have to know how relationships between XML elements are named,
and how element content is described.
Imagine that this is a
description of a book:
|
Book Title: My First
XML
Chapter 1: Introduction
to XML
Chapter 2: XML Syntax
- Elements must have a closing tag
- Elements must be properly nested
|
Imagine that this XML
document describes the book:
<book> <title>My First XML</title> <prod id="33-657" media="paper"></prod> <chapter>Introduction to XML <para>What is HTML</para> <para>What is XML</para> </chapter> <chapter>XML Syntax <para>Elements must have a closing tag</para> <para>Elements must be properly nested</para> </chapter> </book> |
Book is the root
element. Title and chapter are child elements of book. Book is the parent
element of both title and chapter. Title and chapter are siblings
(or sister elements) because they have the same parent.
Elements have Content
Elements can have
different content types.
An XML element is
everything from (including) the element's start tag to (including) the
element's end tag.
An element can have element
content, mixed content, simple content, or empty content.
An element can also have attributes.
In the example above, book
has element content, because it contains other elements. Chapter has mixed
content because it contains both text and other elements. Para has simple
content (or text content) because it contains only text. Prod has empty
content, because it carries no information.
In the example above only
the prod element has attributes. The attribute named id has the value
"33-657". The attribute named media has the value
"paper".
Element Naming
XML elements must
follow these naming rules:
- Names
can contain letters, numbers, and other characters
- Names
must not start with a number or other punctuation characters
- Names
must not start with the letters xml (or XML or Xml ..)
- Names
cannot contain spaces
Take care when you
"invent" element names and follow these simple rules:
Any name can be used, no
words are reserved, but the idea is to make names descriptive. Names with an
underscore separator are nice.
Examples:
<first_name>, <last_name>.
Avoid "-" and
"." in names. It could be a mess if your software tried to subtract
name from first (first-name) or think that "name" is a property of
the object "first" (first.name).
Element names can be as
long as you like, but don't exaggerate. Names should be short and simple, like
this: <book_title> not like this: <the_title_of_the_book>.
XML documents often have a
corresponding database, in which fields exist corresponding to elements in the
XML document. A good practice is to use the naming rules of your database for
the elements in the XML documents.
Non-English letters like
éòá are perfectly legal in XML element names, but watch
out for problems if your software vendor doesn't support them.
The ":" should
not be used in element names because it is reserved to be used for something
called namespaces (more later).
XML Attributes
XML elements can have
attributes in the start tag, just like HTML.
Attributes are used to
provide additional information about elements.
XML Attributes
XML elements can have
attributes.
From HTML you will
remember this: <IMG SRC="computer.gif">. The SRC attribute
provides additional information about the IMG element.
In HTML (and in XML)
attributes provide additional information about elements:
<img src="computer.gif"> <a href="demo.asp"> |
Attributes often provide
information that is not a part of the data. In the example below, the file type
is irrelevant to the data, but important to the software that wants to
manipulate the element:
<file type="gif">computer.gif</file> |
Quote Styles, "female" or
'female'?
Attribute values must
always be enclosed in quotes, but either single or double quotes can be used.
For a person's sex, the person tag can be written like this:
or like this:
Double quotes are the most
common, but sometimes (if the attribute value itself contains quotes) it is
necessary to use single quotes, like in this example:
<gangster name='George "Shotgun" Ziegler'> |
Use of Elements vs. Attributes
Data can be stored in
child elements or in attributes.
Take a look at these
examples:
<person sex="female"> <firstname>Anna</firstname> <lastname>Smith</lastname> </person> |
<person> <sex>female</sex> <firstname>Anna</firstname> <lastname>Smith</lastname> </person> |
In the first example sex
is an attribute. In the last, sex is a child element. Both examples provide the
same information.
There are no rules about
when to use attributes, and when to use child elements. My experience is that
attributes are handy in HTML, but in XML you should try to avoid them. Use
child elements if the information feels like data.
My Favorite Way
I like to store data in
child elements.
The following three XML
documents contain exactly the same information:
A date attribute is used
in the first example:
<note date="12/11/99"> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
A date element is used in
the second example:
<note> <date>12/11/99</date> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
An expanded date element
is used in the third: (THIS IS MY FAVORITE):
<note> <date> <day>12</day> <month>11</month> <year>99</year> </date> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
Avoid using attributes?
Should you avoid using
attributes?
Here are some of the problems
using attributes:
- attributes
cannot contain multiple values (child elements can)
- attributes
are not easily expandable (for future changes)
- attributes
cannot describe structures (child elements can)
- attributes
are more difficult to manipulate by program code
- attribute
values are not easy to test against a DTD
If you use attributes as
containers for data, you end up with documents that are difficult to read and
maintain. Try to use elements to describe data. Use attributes only to
provide information that is not relevant to the data.
Don't end up like this (
if you think this looks like XML, you have not understood the point):
<note day="12" month="11" year="99" to="Tove" from="Jani" heading="Reminder" body="Don't forget me this weekend!"> </note> |
An Exception to my Attribute rule
Rules always have
exceptions.
My rule about attributes
has one exception:
Sometimes I assign ID
references to elements. These ID references can be used to access XML elements
in much the same way as the NAME or ID attributes in HTML. This example
demonstrates this:
<messages> <note ID="501"> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> <note ID="502"> <to>Jani</to> <from>Tove</from> <heading>Re: Reminder</heading> <body>I will not!</body> </note> </messages> |
The ID in these examples
is just a counter, or a unique identifier, to identify the different notes in
the XML file, and not a part of the note data.
What I am trying to say
here is that metadata (data about data) should be stored as attributes, and
that data itself should be stored as elements.
XML Validation
XML with correct syntax is
Well Formed XML.
XML validated against a
DTD is Valid XML.
"Well Formed" XML
documents
A "Well
Formed" XML document has correct XML syntax.
A "Well Formed"
XML document is a document that conforms to the XML syntax rules that were
described in the previous chapters:
<?xml version="1.0"?> <note> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
"Valid" XML documents
A "Valid" XML
document also conforms to a DTD.
A "Valid" XML
document is a "Well Formed" XML document, which also conforms to the
rules of a Document Type Definition (DTD):
<?xml version="1.0"?> <!DOCTYPE note SYSTEM "InternalNote.dtd"> <note> <to>Tove</to> <from>Jani</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note> |
XML DTD
A DTD defines the legal
elements of an XML document.
The purpose of a DTD is to
define the legal building blocks of an XML document. It defines the document
structure with a list of legal elements. You can read more about DTD, and how
to validate your XML documents in W3Schools'
DTD School.
XML Schema
XSchema is an XML based
alternative to DTD.
W3C supports an
alternative to DTD called XML Schema. You can read more about XML Schema in W3Schools' Schema School.
Errors will Stop you
Errors in XML documents
will stop the XML program.
The W3C XML specification
states that a program should not continue to process an XML document if it
finds a validation error. The reason is that XML software should be easy to
write, and that all XML documents should be compatible.
With HTML it was possible
to create documents with lots of errors (like when you forget an end tag). One
of the main reasons that HTML browsers are so big and incompatible, is that
they have their own ways to figure out what a document should look like when
they encounter an HTML error.
With XML this should not
be possible.