• HTML
• JavaScript

• 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 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

Tove Jani Reminder Don't forget me this weekend!

,

XML allows the author to define his/her own tags and his/her own document structure.

---

XML is Not a Replacement for HTML

XML is a complement to HTML.

It is important to understand that XML is not a replacement for HTML. In most web applications, XML is used to transport data, while HTML is used to format and display the data.

My best description of XML is this:

XML is a software- and hardware-independent tool for carrying information.

---

XML is a W3C Recommendation

XML became a W3C Recommendation 10. February 1998.

To read more about the XML activities at W3C, please read our W3C Tutorial.

---

XML is Everywhere

XML is now as important for the Web as HTML was to the foundation of the Web.

XML is the most common tool for data transmissions between all sorts of applications.

How Can XML be

XML Separates Data from HTML

If you need to display dynamic data in your HTML document, it will take a lot of work to edit the HTML each time the data changes.
With XML, data can be stored in separate XML files. This way you can concentrate on using HTML for layout and display, and be sure that changes in the underlying data will not require any changes to the HTML.
With a few lines of JavaScript code, you can read an external XML file and update the data content of your web page.

XML Simplifies Data Sharing

In the real world, computer systems and databases contain data in incompatible formats.
XML data is stored in plain text format. This provides a software- and hardware-independent way of storing data.
This makes it much easier to create data that can be shared by different applications.

XML Simplifies Data Transport

One of the most time-consuming challenges for developers is to exchange data between incompatible systems over the Internet.
Exchanging data as XML greatly reduces this complexity, since the data can be read by different incompatible applications.

XML Simplifies Platform Changes

Upgrading to new systems (hardware or software platforms), is always time consuming. Large amounts of data must be converted and incompatible data is often lost.
XML data is stored in text format. This makes it easier to expand or upgrade to new operating systems, new applications, or new browsers, without losing data.

XML Makes Your Data More Available

Different applications can access your data, not only in HTML pages, but also from XML data sources.
With XML, your data can be available to all kinds of "reading machines" (Handheld computers, voice machines, news feeds, etc), and make it more available for blind people, or people with other disabilities.

XML is Used to Create New Internet Languages

A lot of new Internet languages are created with XML.
Here are some examples:

• XHTML 
• WSDL for describing available web services
• WAP and WML as markup languages for handheld devices
• RSS languages for news feeds
• RDF and OWL for describing resources and ontology
• SMIL for describing multimedia for the web 

If Developers Have Sense

If they DO have sense, 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 XML format, without any conversion utilities in between.

XML Tree

An Example XML Document

XML documents use a self-describing and simple syntax:

Tove   Jani   Reminder   Don't forget me this weekend!

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 (like saying: "this document is a note"):
The next 4 lines describe 4 child elements of the root (to, from, heading, and body):

Tove Jani Reminder Don't forget me this weekend!

And finally the last line defines the end of the root element:
You can assume, from this example, that the XML document contains a note to Tove from Jani.
Don't you agree that XML is pretty self-descriptive?

XML Documents Form a Tree Structure

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:

Giada De Laurentiis     2005     30.00             J K. Rowling     2005     29.99             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:
    J K. Rowling
    2005
    29.99
 
 
   
    Erik T. Ray
    2003
    39.95
 

In the example above, and have element contents, because they contain other elements. has text content because it contains text.
In the example above only has an attribute (category="CHILDREN").

XML Naming Rules

XML elements must follow these naming rules:

• Names can contain letters, numbers, and other characters
• Names cannot start with a number or punctuation character
• Names cannot start with the letters xml (or XML, or Xml, etc)
• Names cannot contain spaces

Any name can be used, no words are reserved.

Best Naming Practices

Make names descriptive. Names with an underscore separator are nice: , .
Names should be short and simple, like this: not like this: .
Avoid "-" characters. If you name something "first-name," some software may think you want to subtract name from first.
Avoid "." characters. If you name something "first.name," some software may think that "name" is a property of the object "first."
Avoid ":" characters. Colons are reserved to be used for something called namespaces (more later).
XML documents often have a corresponding database. 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, but watch out for problems if your software vendor doesn't support them.

XML Elements are Extensible

XML elements can be extended to carry more information.
Look at the following XML example:

Tove Jani Don't forget me this weekend!

Let's imagine that we created an application that extracted the , , and 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:

2008-01-10 Tove Jani Reminder Don't forget me this weekend!

Should the application break or crash?
No. The application should still be able to find the , , and elements in the XML document and produce the same output.
One of the beauties of XML, is that it can be extended without breaking applications.

XML Attributes

XML Attributes

In HTML, attributes provide additional information about elements:
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 can be important to the software that wants to manipulate the element:

computer.gif

XML Attributes Must be Quoted

Attribute values must always be quoted. Either single or double quotes can be used. For a person's sex, the person element can be written like this:
or like this:
If the attribute value itself contains double quotes you can use single quotes, like in this example:
or you can use character entities:

XML Elements vs. Attributes

Take a look at these examples:

Anna   Smith

female   Anna   Smith

In the first example sex is an attribute. In the last, sex is an element. Both examples provide the same information.
There are no rules about when to use attributes or when to use elements. Attributes are handy in HTML. In XML my advice is to avoid them. Use elements instead.

My Favorite Way

The following three XML documents contain exactly the same information:
A date attribute is used in the first example:

Tove   Jani   Reminder   Don't forget me this weekend!

A date element is used in the second example:

10/01/2008   Tove   Jani   Reminder   Don't forget me this weekend!

An expanded date element is used in the third: (THIS IS MY FAVORITE):

10     01     2008     Tove   Jani   Reminder   Don't forget me this weekend!

Avoid XML Attributes?

Some of the problems with using attributes are:

• attributes cannot contain multiple values (elements can)
• attributes cannot contain tree structures (elements can)
• attributes are not easily expandable (for future changes)

Attributes are difficult to read and maintain. Use elements for data. Use attributes for information that is not relevant to the data.
Don't end up like this:

to="Tove" from="Jani" heading="Reminder" body="Don't forget me this weekend!">

XML Attributes for Metadata

Sometimes ID references are assigned to elements. These IDs can be used to identify XML elements in much the same way as the id attribute in HTML. This example demonstrates this:

Tove     Jani     Reminder     Don't forget me this weekend!         Jani     Tove     Re: Reminder     I will not

The id attributes above are for identifying the different notes. It is not a part of the note itself.
What I'm trying to say here is that metadata (data about data) should be stored as attributes, a

XML Validation

Well Formed XML Documents

A "Well Formed" XML document has correct XML syntax.
The syntax rules were described in the previous chapters:

• XML documents must have a root element
• XML elements must have a closing tag
• XML tags are case sensitive
• XML elements must be properly nested
• XML attribute values must be quoted

Tove Jani Reminder Don't forget me this weekend!

Valid XML Documents

A "Valid" XML document is a "Well Formed" XML document, which also conforms to the rules of a Document Type Definition (DTD):

Tove Jani Reminder Don't forget me this weekend!

The DOCTYPE declaration in the example above, is a reference to an external DTD file. The content of the file is shown in the paragraph below.

XML DTD

The purpose of a DTD is to define the structure of an XML document. It defines the structure with a list of legal elements:

[ ]>

If you want to study DTD, you will find our DTD tutorial on our 

XML Schema

W3C supports an XML-based alternative to DTD, called XML Schema:
If you want to study XML Schema, you will find our Schema tutorial on our homepage.

A General XML Validator

To help you check the syntax of your XML files, we have created an XML validator to syntax-check your XML.
Please see the next chapter.

XML Validator

XML Errors Will Stop You

Errors in XML documents will stop your XML applications.
The W3C XML specification states that a program should stop processing an XML document if it finds an error. The reason is that XML software should be small, fast, and compatible.
HTML browsers will display documents with errors (like missing end tags). HTML browsers are big and incompatible because they have a lot of unnecessary code to deal with (and display) HTML errors.
With XML, errors are not allowed.

Syntax-Check Your XML

To help you syntax-check your XML, we have created an XML validator.
Paste your XML into the text area below, and syntax-check it by clicking the "Validate" button.

<?xml version="1.0" ?> <note> <to>Tove</to> <from>Jani</Ffrom> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note>  

Note: This only checks if your XML is "Well formed". If you want to validate your XML against a DTD, see the last paragraph on this page.

Syntax-Check an XML File

You can syntax-check an XML file by typing the URL of the file into the input field below, and then click the "Validate" button:

Filename:
 

Note: If you get an "Access denied" error, it's because your browser security does not allow file access across domains.
The file "note_error.xml" demonstrates your browsers error handling. If you want see an error free message, substitute the "note_error.xml" with "cd_catalog.xml".

Validate Your XML Against a DTD

If you know DTD, you can validate your XML in the text area below.
Just add the DOCTYPE declaration to your XML and click the "Validate" button:

Viewing XML Files

Viewing XML Files

- <note>        <to>Toveto>        <from>Janifrom>        <heading>Reminderheading>        <body>Don't forget me this weekend!body>    note>

Look at this XML file: note.xml
The XML document will be displayed with color-coded root and child elements. A plus (+) or minus sign (-) to the left of the elements can be clicked to expand or collapse the element structure. To view the raw XML source (without the + and - signs), select "View Page Source" or "View Source" from the browser menu.
Note: In Chrome, Opera, and Safari, only the element text will be displayed. To view the raw XML, you must right click the page and select "View Source"

Viewing an Invalid XML File

If an erroneous XML file is opened, the browser will report the error.
Look at this XML file: note_error.xml

Other XML Examples

Viewing some XML documents will help you get the XML feeling.
An XML CD catalog
This is a CD collection, stored as XML data.
An XML plant catalog
This is a plant catalog from a plant shop, stored as XML data.
A Simple Food Menu
This is a breakfast food menu from a restaurant, stored as XML data.

Why Does XML Display Like This?

XML documents do not carry information about how to display the data.
Since XML tags are "invented" by the author of the XML document, browsers do not know if a tag like
describes an HTML table or a dining table.
Without any information about how to display the data, most browsers will just display the XML document as it is.
In the next chapters, we will take a look at different solutions to the display problem, using CSS, XSLT and JavaScript.

Note: Only Internet Explorer will actually check your XML against the DTD. Firefox, Mozilla, Netscape, and Opera will not.

Displaying XML with CSS

Displaying your XML Files with CSS?

It is possible to use CSS to format an XML document.
Below is an example of how to use a CSS style sheet to format an XML document:
Take a look at this XML file: The CD catalog
Then look at this style sheet: The CSS file
Finally, view: The CD catalog formatted with the CSS file
Below is a fraction of the XML file. The second line links the XML file to the CSS file:

Bob Dylan     USA     Columbia     10.90     1985             Bonnie Tyler     UK     CBS Records     9.90     1988   . . .

Formatting XML with CSS is not the most common method.
W3C recommend using XSLT instead. See the next chapter.

Displaying XML with XSLT

Displaying XML with XSLT

XSLT is the recommended style sheet language of XML.
XSLT (eXtensible Stylesheet Language Transformations) is far more sophisticated than CSS.
XSLT can be used to transform XML into HTML, before it is displayed by a browser:
Display XML with XSLT
If you want to learn more about XSLT, find our XSLT tutorial on our homepage.

Transforming XML with XSLT on the Server

In the example above, the XSLT transformation is done by the browser, when the browser reads the XML file.
Different browsers may produce different result when transforming XML with XSLT. To reduce this problem the XSLT transformation can be done on the server.
View the result.
Note that the result of the output is exactly the same, either the transformation is done by the web server or by the web browser.

The XMLHttpRequest Object

The XMLHttpRequest Object

The XMLHttpRequest object is used to exchange data with a server behind the scenes.
The XMLHttpRequest object is the developers dream, because you can:

• Update a web page without reloading the page
• Request data from a server after the page has loaded
• Receive data from a server after the page has loaded
• Send data to a server in the background

T

XMLHttpRequest Example

When you type a character in the input field below, an XMLHttpRequest is sent to the server - and name suggestions are returned (from a file on the server):

Type a letter in the input box: First Name Suggestions:

Create an XMLHttpRequest Object

All modern browsers (IE7+, Firefox, Chrome, Safari, and Opera) have a built-in XMLHttpRequest object.
Syntax for creating an XMLHttpRequest object:

xmlhttp=new XMLHttpRequest();

Old versions of Internet Explorer (IE5 and IE6) uses an ActiveX Object:

xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");

In the next chapter, we will use the XMLHttpRequest object to retrieve XML information from a