XML elements are extensible and are related to each other.
XML elements have simple naming rules.
XML elements are extensible
XML documents can be expanded to carry more information.
Look at the following XML note example:
<note> <to>Lin</to> <from>Ordm</from> <body>Don't forget me this weekend!</body> </note>
Let's imagine a program that can read this XML document and interpret the XML elements (
MESSAGE
To: Lin
From: Ordm
Don't forget me this weekend!
Let's imagine again that if Ordm, the author of the note, adds some additional information to this XML document, as shown below:
<note> <date>2002-12-24</date> <to>Lin</to> <from>Ordm</from> <heading>Reminder</heading> <body>Don't forget me this weekend!</body> </note>
Will the original application interrupt or crash?
Won't. The application will still interpret the
XML documents are extensible!
XML elements are related to each other
The relationship between XML elements is the relationship between parent elements and child elements.
In order to better understand XML terminology, you must understand the relationship between XML elements and how the content of the elements is described.
Imagine there is such a book:
Signature: XML Guide
Chapter 1: Introduction to XML
What is HTML
What is XML
Chapter 2: XML Syntax
XML elements must have closing tags
XML elements must be nested correctly
We can use XML documents to describe this book:
<book> <title>XML 指南</title> <prod id="33-657" media="paper"></prod> <chapter>XML入门简介 <para>什么是HTML</para> <para>什么是XML</para> </chapter> <chapter>XML语法 <para>XML元素必须有结束标记</para> <para>XML元素必须正确的嵌套</para> </chapter> </book>
In the above In the code, the Book element is the root element of the XML document, and the title element and chapter element are child elements of the book element. The Book element is the parent element of the title element and chapter element. The title element, prod element and chapter element are horizontal elements because they all have the same parent element.
Contents of XML elements
XML elements have different contents.
XML element refers to the content from the start tag to the end tag of the element.
XML elements have element content, mixed content, simple content or empty content. Each element can have its own properties.
In the above example, the book element has element content, which should be because the book element contains other elements. The Chapter element has mixed content because it contains text and other elements. The para element has simple content because it only has simple text inside it. The prod element has empty content because it does not carry any information.
In the above example, only the prod element has attributes, the id attribute value is 33-657, and the media attribute value is paper.
XML element naming
XML element naming must comply with the following rules:
The name of the element can contain letters, numbers and other characters.
The name of the element cannot start with a number or punctuation mark.
The name of the element cannot start with XML (or xml, Xml, xMl...).
The name of the element cannot contain spaces.
You must also pay attention to the following simple rules for your own "invented" XML elements:
Any name can be used, no reserved words (except XML), but the name of the element should be readable, and the name should be used Underscore is a good choice.
For example:
Try to avoid using "-" and "." because it may cause confusion.
The name of the element can be as long as you want, but don't make it too exaggerated. Naming should follow the principle of being simple and easy to read. For example:
XML documents often correspond to data tables. We should try to keep the naming of fields in the database consistent with the naming in the corresponding XML document, so as to facilitate data transformation.
Non-English/characters/strings can also be used as names of XML elements, such as
Do not use ":" in XML element naming, because XML namespaces require this very special character.
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