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Introduction to XML

 

The Extensible Markup Language

 

Introduction

The eXtensible Markup Language or XML is a technique of using a document, such as a text file, to describe information and making that information available to whatever and whoever can take advantage of it. The description is done so the document can be created by one person or company and used by another person or another company. This document created is not a program, it is not an application: it is just a text-based document.

XML is very flexible> It can be used in regular Windows applications, in databases, in web-based systems (Internet), in communication applications, in computer networks, in scientific applications, etc. To make sure that XML can be universally used without one person or group owning it, it is standardized by the W3C organization. XML is released through an XML Recommendation document with a version.

In our lessons, we will learn how to create XML documents. To create an XML file, in the document, you type units of codes using normal characters of the English language. The XML document is made of units called entities. These entities are spread on various lines of the document as you judge them necessary and as we will learn. XML has strict rules as to how the content of the document should or must be structured.

After an XML document has been created and is available, in order to use it, you need a program that can read, analyze, and interpret it. This program is called a parser. The most popular parser used in Microsoft Windows applications is MSXML, published by Microsoft.

Markup

A markup is an instruction that defines XML. The fundamental formula of a markup is:

<tag>

The left angle bracket "<" and the right angle bracket ">" are required. Inside of these symbols, you will type a word or a group of words of your choice, using regular characters of the alphabet and sometimes non-readable characters such as ?, !, or [. The combination of a left angle bracket "<", the right angle bracket ">", and what is inside of these symbols is called a markup. There are various types of markups we will learn.

An XML File

 

The Document Type Declaration

As mentioned above, XML is released as a version. Because there can be various versions, the first line that can be processed in an XML file must specify the version of XML you are using. At the time of this writing, the widely supported version is 1.0. When creating an XML file, you should (should in 1.0 but must in 1.1) specify what version your file is conform with, especially if you are using a version higher than 1.0. For this reason, an XML file should start (again, must, in 1.1), in the top section, with a line known as an XML declaration. It starts with <?xml version=, followed by the version you are using, assigned as a string, and followed by ?>. An example of such a line is:

<?xml version="1.0"?>

Under the XML declaration line, you can then create the necessary tags of the XML file.

Encoding Declaration

As mentioned already, the tags are created using characters of the alphabet and conform to the ISO standard. This is known as the encoding declaration. For example, most of the characters used in the English language are known as ASCII. These characters use a combination of 7 bits to create a symbol (because the computer can only recognize 8 bits, the last bit is left for other uses). Such an encoding is specified as UTF-8. There are other standards such as UTF-16 (for wide, 2-Byte, characters).

To specify the encoding you are using, type encoding followed by the encoding scheme you are using, which must be assigned as a string. The encoding is specified in the the first line. Here is an example:

<?xml version="1.0" encoding="utf-8"?>
 

Creating an XML File

There are various ways you can create an XML file. The most common technique consists of using a simple text editor. In Microsoft Windows, this would be Notepad. An XML file is first of all a normal text-based document that has a .xml extension. Therefore, however you create it, you must specify that extension.

Many other applications allow creating an XML file or generating one from an existing file. There are also commercial editors you can get or purchase to create the XML file.

Probably the most common way to create an XML file in Microsoft Windows consists of using Notepad or any other text editor. After opening the text editor, you can enter the necessary lines of code. After creating the file, you must save it. When saving it, you can include the name of the file in double-quotes. You can also first set the Save As Type combo box to All Files and then enter the name of the file with the .xml extension.

After creating an XML file, you can open it in another application. For example, you can open it in a browser, such as Internet Explorer or Netscape.

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