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本文介绍了XML的基础知识，包括XML的设计理念、与HTML的区别、基本语法、文档结构及常用规范等。

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大部分内容来自http://www.w3schools.com。

本文主要是个人学习笔记，将替代手写的笔记。以此记录自己的学习过程，记录不懂的，可以逐渐增加新知识点。

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.

An Example XML Document

XML documents use a self-describing and simple syntax:

<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>

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):

XML Documents Form a Tree Structure

<bookstore>
  <book category="COOKING">
    <title lang="en">Everyday Italian</title>
    <author>Giada De Laurentiis</author>
    <year>2005</year>
    <price>30.00</price>
  </book>
  <book category="CHILDREN">
    <title lang="en">Harry Potter</title>
    <author>J K. Rowling</author>
    <year>2005</year>
    <price>29.99</price>
  </book>
  <book category="WEB">
    <title lang="en">Learning XML</title>
    <author>Erik T. Ray</author>
    <year>2003</year>
    <price>39.95</price>
  </book>
</bookstore>

XML Syntax Rules

All XML Elements Must Have a Closing Tag

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

XML Tags are Case Sensitive

XML tags are case sensitive. The tag <Letter> is different from the tag <letter>.

XML Elements Must be Properly Nested

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

<b><i>This text is bold and italic</i></b>

In the example above, "Properly nested" simply means that since the <i> element is opened inside the <b> element, it must be closed inside the <b> 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.

<root>
  <child>
    <subchild>.....</subchild>
  </child>
</root>

XML Attribute Values Must be Quoted

XML elements can have attributes in name/value pairs just like in HTML.

In XML, the attribute values must always be quoted.

Study the two XML documents below. The first one is incorrect, the second is correct:

<note date=12/11/2007>
  <to>Tove</to>
  <from>Jani</from>
</note>

<note date="12/11/2007">
  <to>Tove</to>
  <from>Jani</from>
</note>

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:

<message>if salary < 1000 then</message>

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

<message>if salary < 1000 then</message>

There are 5 predefined entity references in XML:

<	<	less than
>	>	greater than
&	&	ampersand
'	'	apostrophe
"	"	quotation mark

Comments in XML

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

White-space is Preserved in XML

HTML truncates multiple white-space characters to one single white-space:

HTML:	Hello Tove
Output:	Hello Tove

With XML, the white-space in a 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). In Unix applications, a new line is normally stored as an LF character. Macintosh applications also use an LF to store a new line.

XML stores a new line as LF.