XML

XML Tutorial
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XML stands for EXtensible Markup Language.

XML was designed to transport and store data.

In this tutorial you will learn about XML, and the difference between XML and HTML.

XML is important to know, and very easy to learn.

Start learning XML now!

Table of Contents

Introduction to XML
What is XML, and how does it differ from HTML?

How XML can be used
Some of the different ways XML can be used.

XML Tree
How an XML document forms a logical tree structure.

XML Syntax
The logical and very simple syntax rules of XML.

XML Elements
XML Elements, naming rules, and their parent and child relationships.

XML Attributes
How attributes can be used to provide additional information about elements.

XML Validation
The difference between a well-formed and a valid XML document, and how to specify the structure of an XML document.

XML Validator
A simple on-line tool for syntax-checking (validating) XML.

Viewing XML in Browsers
How to view XML files with your browser.

Displaying XML with CSS
How to use CSS to display an XML file.

Displaying XML with XSLT
How to use XSLT to display an XML file.
XML JavaScript

XML Parser
How to use your browser to read, update, create, and manipulate XML documents.

XML DOM
About XML Document Object Model (The DOM).

XML to HTML
How to display XML data inside HTML documents.

XML HTTP Requests
How to update a page with data from a server without reloading the page (AJAX).

XML Applications
How to use XML data and JavaScript to create a complete HTML application.
XML Advanced

XML Namespaces
How XML namespaces can be used to avoid element name conflicts.

XML CDATA
How store invalid XML data inside a valid XML document.

XML Encoding
How to use different character sets in your XML documents.

XML Server
How to output XML from the server.

XML DOM Advanced
More about XML Document Object Model (The DOM).

XML Don’t
This chapter describes some technologies you should try to avoid when using XML.

XML Technologies
The most common XML-related languages.

XML in Real Life
Real-life examples of how XML can be used.

XML Editors
Why you should use an XML editor when you edit your XML documents.

XML Summary
This chapter contains a summary on what you have learned in this tutorial and a recommendation on what to do next.
XML Examples

Learn by examples! With our editor, you can edit XML and click on a test button to view the result.

Try-It-Yourself!
XML Quiz Test

Test your XML skills at W3Schools!

Start the XML Quiz!

XML was designed to transport and store data.

HTML was designed to display data.
What You Should Already Know

Before you continue you should have a basic understanding of the following:

* HTML
* JavaScript

If you want to study these subjects first, find the tutorials on our Home page.
What is XML?

* 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

The Difference Between XML and HTML

XML is not a replacement for HTML.
XML and HTML were designed with different goals:

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.
XML Does not DO Anything

Maybe it is a little hard to understand, but XML does not DO anything. XML was created to structure, store, and transport information.

The following example is a note to Tove from Jani, stored as XML:

Tove
Jani
Reminder
Don’t forget me this weekend!

The note above is quite self descriptive. It has sender and receiver information, it also has a heading and a message body.

But still, this XML document does not DO anything. It is just pure information wrapped in tags. Someone must write a piece of software to send, receive or display it.
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.
With XML You 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.
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

The Extensible Markup Language (XML) became a W3C Recommendation 10. February 1998.
XML is Everywhere

We have been participating in XML development since its creation. It has been amazing to see how quickly the XML standard has developed and how quickly a large number of software vendors have adopted the standard.

XML 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 becomes more and more popular in the area of storing and describing information.
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XML is used in many aspects of web development, often to simplify data storage and sharing.
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, you can read an external XML file and update the data content of your HTML.

You will learn more about this in a later chapter of this tutorial.
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 different applications can share.
XML Simplifies Data Transport

With XML, data can easily be exchanged between incompatible systems.

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 very 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

Since XML is independent of hardware, software and application, XML can make your data more available and useful.

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 the latest version of HTML
* 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 a pure text format, without any conversion utilities in between.

We can only pray that all the software vendors will agree.

XML documents form a tree structure that starts at “the root” and branches to “the leaves”.
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:
DOM node tree

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: ,, , .

The syntax rules of XML are very simple and logical. The rules are easy to learn, and easy to use.
All XML Elements Must Have a Closing Tag

In HTML, you will often see elements that don’t have a closing tag:

This is a paragraph

This is another paragraph

In XML, it is illegal to omit the closing tag. All elements must have a closing tag:

This is a paragraph

This is another paragraph

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, and it has no closing tag.
XML Tags are Case Sensitive

XML 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 incorrect

This is correct

Note: “Opening and closing tags” are often referred to as “Start and end tags”. Use whatever you prefer. It is exactly the same thing.
XML Elements Must be Properly Nested

In HTML, you will often see improperly nested elements:

This text is bold and italic

In XML, all elements must be properly nested within each other:

This text is bold and italic

In the example above, “Properly nested” simply means that since the element is opened inside the element, it must be closed inside the element.
XML Documents Must Have a Root Element

XML documents must contain one element that is the parent of all other elements. This element is called the root element.

…..

XML Attribute Values Must be Quoted

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:

Tove
Jani

Tove
Jani

The error in the first document is that the date attribute in the note element is not quoted.
Entity References

Some characters have a special meaning in XML.

If you place a character like “<” inside an XML element, it will generate an error because the parser interprets it as the start of a new element.

This will generate an XML error:

if salary < 1000 then

To avoid this error, replace the “<” character with an entity reference:

if salary < 1000 then

There are 5 predefined entity references in XML:
< greater than
& & ampersand
' ‘ apostrophe
" ” quotation mark

Note: Only the characters “<” and “&” are strictly illegal in XML. The greater than character is legal, but it is a good habit to replace it.
Comments in XML

The syntax for writing comments in XML is similar to that of HTML.

With XML, White Space is Preserved

HTML reduces multiple white space characters to a single white space:
HTML: Hello my name is Tove
Output: Hello my name is Tove.

With XML, the white space in your document is not truncated.
XML Stores New Line as LF

In Windows applications, a new line is normally stored as a pair of characters: carriage return (CR) and line feed (LF). The character pair bears some resemblance to the typewriter actions of setting a new line. In Unix applications, a new line is normally stored as a LF character. Macintosh applications use only a CR character to store a new line.

An XML document contains XML Elements.
What is an XML Element?

An XML element is everything from (including) the element’s start tag to (including) the element’s end tag.

An element can contain other elements, simple text or a mixture of both. Elements can also have attributes.

Harry Potter
J K. Rowling
2005
29.99

Learning XML
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 must not start with a number or punctuation character
* Names must not 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 often be extended without breaking applications.

XML elements can have attributes in the start tag, just like HTML.

Attributes provide additional information about elements.
XML Attributes

From HTML you will remember this: . The “src” attribute provides additional information about the element.

In HTML (and in XML) attributes provide additional information about elements: