An introduction to basic XML concepts

Who is the twin brother of HTML? There is no doubt that what I want to say is XML. As a pair of twin brothers, their basic syntax is similar, but their appearance and interior are not equivalent. Although XML is very similar to HTML, there are essential differences between the two.

1. XML and HTML

XML (eXtensible Markup Language) is an extensible markup language, and HTML The language syntax is similar and both are markup languages. So how to distinguish them?

1. Design difference
XML is designed to transmit and save data, and its focus is the content of the data. While HTML is designed to display data, its focus is on the appearance of the data. It can be said that HTML is designed to display information and XML is designed to transmit information.

2. Syntax difference
(1) HTML has various formats, some of which can appear unpaired, while XML requires that all tags must appear in pairs;
( 2) HTML tags have limited fixed styles, XML can be extended in any way, and tag types can be defined by yourself;
(3) HTML can have multiple tag headers, XML only allows one tag header;
(4 ) HTML tags are not case-sensitive, XML is case-sensitive.

2. Detailed explanation of XML

Have you ever remembered the system configuration document used when developing the system? Yes, it is an xml document. As the system's configuration file is the main application of xml, it can also be used to transmit data in Ajax or Web Services.

1. History of XML

Before learning more about the basic usage of XML, let’s first take a look at the development history of markup languages.
The predecessor of XML is SGML (The Standard Generalized Markup Language), which is also optimized on the basis of HTML. SGML is a standard compiled and standardized by GML. However, SGML is a very rigorous text description method that is too large and complex to understand. At the same time, the W3C (World Wide Web Consortium) also discovered problems with HTML, such as the inability to interpret content in other forms such as audio and video files, chemical formulas, etc., and its poor performance and difficulty in expansion and flexibility. Problems such as difference. In order to solve the above problems, experts used SGML to make simple production and developed XML based on the development experience of HTML.

2. XML usage

Let’s look at an XML example first

XML/HTML Code复制内容到剪贴板
<?xml version="1.0" encoding="utf-8"?>
<?xml-stylesheet type=&#39;text/xsl&#39; href=&#39;content.xsl&#39;?>
<book>
    <书籍列表>
        <计算机书籍>
            <bookname isbn="xxx1234567"><<XML应用大全>></bookname>
            <price>39.00</price>
        </计算机书籍>
    </书籍列表>
<!-- 我加上的注释 -->
    <书籍列表>
        <计算机书籍>
            <bookname><![CDATA[Oracle应用大全《《《》》》‘’；；‘’“”]]></bookname>
            <price>39.00</price>
        </计算机书籍>
        <计算机书籍>
            <bookname>Java应用大全</bookname>
            <price>39.00</price>
        </计算机书籍>
    </书籍列表>
</book>

2.1 Declaration

Well-formed XML The file starts with an XML declaration, i.e. in the above example, which provides basic information about the document. The encoding represents the character encoding used in the XML file. The encoding format of the XML document to be saved must be the encoding

attribute value.

2.2 Elements

elements are the start tag, the end tag, and everything in between.

has the same basic composition as HTML. XML also has basic tags. The format of the tags is the same as
HTML tag . The difference is that XML tags must have end tags if they have a start tag, and they are strictly distinguished. Upper and lower case. It allows label formats to be nested, but only if they are nested correctly. XML structure is a tree structure, its content must have only one root element, and other elements must be nested within the root element. For example: in the above example is the root element.

2.3 EntityReference

Entity reference uses a specific format to print out some special symbols in the XML document, similar to The text in CDATA to be mentioned below.

XML/HTML Code复制内容到剪贴板
<book>
    <书籍列表>
        <计算机书籍>
            <bookname isbn="xxx1234567"><<XML应用大全>></bookname>
            <price>39.00</price>
        </计算机书籍>
    </书籍列表>
</book>
<!-- 使用IE打开上面的代码查看下结果 -->

##Basic entity reference Not going into details.

2.4 Special tags
CDATA is a special tag that enables all content within it to be printed out in the form of text. The form is as follows:

XML/HTML Code复制内容到剪贴板
<![CDATA[文本内容]]>
<?xml version=&#39;1.0&#39; encoding=&#39;utf-8&#39;?>
<书籍列表>
        <计算机书籍>
            <bookname><![CDATA[Oracle应用大全《《《》》》‘’；；‘’“”]]></bookname>
            <price>39.00</price>
        </计算机书籍>
<书籍列表>

#2.5 Hodgepodge

(1) The XML document is actually a tree structure when organized with pictures, which is often opened with IE when displayed; (2) Attributes in XML tags The value must be enclosed in quotation marks (single quotation marks or double quotation marks, but must be consistent);

(3) In XML documents containing complex internal structures, the xsl format can be used to beautify the display of xml, format: .

3. Note

1. There must be a statement:;
2. When saving, it must be based on The encoding type in encoding is saved into the corresponding type. Don't forget to select the saving type when saving;
3. There is only one top element;
4. Case-sensitive;
5. The value of the attribute must use quotation marks Enclosed (single quotation marks or double quotation marks);
6. Be sure to have a beginning and an end;
7. The nesting form must be correct.

The basic applications of XML have been discussed above. XML is very simple. Is this all there is to XML? Of course not, it can be seen from its basic name that it is also a programming language, so it should have some features of a programming language, such as templates in some languages ​​or API# in Java ##. Yes, XML can also define its own grammatical structure, and you can write templatesfunctions just like a programming language, but in XML it is called legal XML, which is what everyone calls DTDOr valid XML verified by Schema.

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