当前位置：网站首页>Some introduction and precautions about XML

Some introduction and precautions about XML

2022-07-03 00:32:00 【Champion_ me】

Catalog

1、 What is? XML

Study XML First of all, of course, you should know XML What is it? , What is? XML？

  • XML Extensible markup language （Extensible Markup Language）
  • XML It's a kind of Markup language , Is very similar HTML
  • XML The purpose of the design is To transmit data , Instead of displaying data （XML Transmit information , have Cross platform features ）
  • XML The label is not predefined . You need to Define your own label .
  • XML Designed to have self-description .
  • XML yes W3C The recommended standard of .

2、XMl And HTML Major differences

  • XML No HTML An alternative .
  • XML and HTML Designed for different purposes
  • XML Designed to transmit and store data , The focus is on the content of the data .
  • HTML Designed to display data , The focus is on the appearance of the data .
  • HTML To show information , and XML Designed to transmit data

3、 Transmission data can also be used JSON,XML and JSON The difference between ：

JSON and XML The difference between ：
1、JSON yes JavaScript Object Notation;XML Is an extensible markup language .
2、JSON Is based on JavaScript Language ;XML Derived from SGML.
3、JSON Is a way of representing objects ;XML It's a markup language , Use tag structures to represent data items .
4、JSON There is no support for namespaces ;XML Namespace support .
5、JSON Support array ;XML Arrays are not supported .
6、XML The document is relatively difficult to read and interpret ; And XML comparison ,JSON The file is very easy to read .
7、JSON Do not use end tags ;XML There are start and end labels .
8、JSON It's less secure ;XML Than JSON More secure .
9、JSON Comments are not supported ;XML Support comments .
10、JSON Support only UTF-8 code ;XML Supports all kinds of coding .

4、XML Tree structure

XML Documents form a tree structure , He from “ root ” Start , And then expand to “ Leaf ”.

One XML Document instance

XMl Documents use simple, self descriptive Syntax ：

<?xml version="1.0" encoding="UTF-8"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The first line is XML Statement . It defines XML Version of （1.0） And the code used （UTF-8： unicode , Various languages can be displayed ）. The next line describes the root element of the document （ It's like saying ：“ A text file is a note ”）

  <note>

Next 4 Line describes the root of 4 Sub elements （to, from, heading as well as body）：

<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>

The last line defines the end of the root element ：

  </note>

You can assume , From this example ,XML The document contains a Jani written Tove My note .

XML With excellent self description , Do you agree ？

XML Documents form a tree structure

XML The document must contain the root element . This element is the parent of all other elements .

XML The elements in the document form a document tree . This tree starts at the root , And extend to the bottom of the tree .

All elements can have child elements ：

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

Father 、 Terms such as children and siblings are used to describe the relationships between elements . The parent element has children . Child elements at the same level become siblings （ Brother or sister ）.

All elements can have text content and attributes （ similar HTML in ）.

example ：
Please add a picture description
The figure above shows the following XML A book in English ：

<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>

The root element in the instance is . All of the The elements are contained in in .

Elements have 4 Sub elements ：、、、.

5、XML Rule of grammar

XML The rules of grammar are very simple , And it's very logical . These rules are easy to learn , It's also easy to use .

XML The document must have a root element

XML Must contain the root element , It's the parent of all the other elements , For example, in the following example root It's the root element ：

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

In the following example note Is the root element ：

<?xml version="1.0" encoding="UTF-8"?>
<note>
  <to>Tove</to>
  <from>Jani</from>
  <heading>Reminder</heading>
  <body>Don't forget me this weekend!</body>
</note>

XML Statement

XML Optional parts of the declaration file , If it exists, you need to put it on the first line of the document , As shown below ：

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

The above examples include XML edition （

UTF-8 It's also HTML5, CSS, JavaScript, PHP, and SQL The default encoding for .

be-all XML Element must have a close tag

stay HTML in , Some elements don't have to have a close tag ：

<p>This is a paragraph.
<br>

stay XML in , It's illegal to omit the closing tag . All elements must have a close tag ：

<p>This is a paragraph.</p>
<br />

** notes ：** From the example above , You may have noticed XML The statement does not close the label . It's not a mistake . The statement is not XML Part of the document itself , It has no close tab .

XML Tags are case sensitive

XML Tags are case sensitive . label And labels Is different .

Open and close tags must be written in the same case ：

<Message> This is wrong </message>
<message> That's right </message>

** notes ：** Opening and closing tags are often referred to as start tags and end tags . Whatever term you like , They all have the same concept .

XML Must nest... Correctly

stay HTML in , You will often see elements that are not nested correctly ：

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

stay XML in , All elements must be nested correctly within each other ：

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

In the example above , Correct nesting means ： because The elements are in Open in the element , Then it must be in Close inside the element .

XML Attribute values must be quoted

And HTML similar ,XML Elements can also have attributes （ name / Pairs of values ）.

stay XML in ,XML Property values of must be quoted .

Please study the following two XML file . The first one is wrong , 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>

The error in the first document is ,note In the element date Attribute is not quoted .

Entity reference

stay XML in , Some characters have a special meaning .

If you put the character “<” Put it in XML In the elements , There will be mistakes , This is because the parser takes it as the beginning of a new element .

This will produce XML error ：

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

To avoid this mistake , Please use entity references instead of “<” character ：

<message>if salary &lt; 1000 then</message>

stay XML in , Yes 5 Predefined entity references ：

&lt ;<less than
&gt ;>greater than
&amp ;&ampersand
&apos ;’apostrophe
&quot ;"quotation mark

notes ： stay XML in , Only characters “<” and “&” It's illegal . The greater than sign is legal , But it's a good habit to replace it with entity references .

XML The comments in

stay XML The syntax and syntax of writing comments in HTML The grammar of English is very similar .

<!-- This is a comment -->

stay XML in , Spaces will be reserved

HTML Will cut multiple consecutive space characters （ Merge ） For one ：
stay XML in , Spaces in the document will not be deleted .

XML With LF Store newlines

  • stay Windows In the application , Newlines are usually stored as a pair of characters ： A carriage return （CR） And line breaks （LF）.

  • stay Unix and Mac OSX in , Use LF To store new lines .

  • In the old Mac In the system , Use CR To store new lines .

  • XML With LF Store newlines .

6、XML Elements

XML Document contains XML Elements .

What is? XML Elements ？

XML Element means from （ And includes ） Start tagging until （ And includes ） End the part of the tag .

An element can contain ：

  • Other elements
  • Text
  • attribute
  • Or mix all of the above …
<bookstore>
    <book category="CHILDREN">
        <title>Harry Potter</title>
        <author>J K. Rowling</author>
        <year>2005</year>
        <price>29.99</price>
    </book>
    <book category="WEB">
        <title>Learning XML</title>
        <author>Erik T. Ray</author>
        <year>2003</year>
        <price>39.95</price>
    </book>
</bookstore>

In the example above , and There are Element content , Because they contain other elements . Elements also have attributes （category=“CHILDREN”）.、、 and There is text content , Because they contain text .

XML Naming rules

XML Elements must follow the following naming rules ：

  • Names can contain letters 、 Numbers and other characters
  • Names cannot begin with numbers or punctuation
  • The name cannot be in letters xml（ perhaps XML、Xml wait ） Start
  • The name cannot contain spaces
  • You can use any name , There are no reserved words .

Best naming habits

Make the name descriptive . It's also nice to use underlined names ：<first_name>、<last_name>.

The name should be short and simple , such as ：<book_title>, instead of ：<the_title_of_the_book>.

avoid “-” character . If you name it this way ：“first-name”, Some software will think you want to go from first Subtract... From it name.

avoid “.” character . If you name it this way ：“first.name”, Some software will think “name” It's the object “first” Properties of .

avoid “:” character . Colons are converted to namespaces to use （ Later on ）.

XML Documents often have a corresponding database , The fields will correspond to XML Elements in the document . Have a practical experience , That is, use the database naming rules to name XML Elements in the document .

stay XML in ,éòá It's perfectly legal to wait for non English letters , But we need to pay attention to , Problems that may arise when your software vendor does not support these characters .

XML Elements are extensible

XML Elements are extensible , To carry more information .

Please look at the following XML example ：

Tove Jani Don't forget me this weekend! Let's imagine , We created an application , Can be 、 as well as Elements from XML Extracted from the document , And produce the following output ：

MESSAGE
To: Tove
From: Jani

Don’t forget me this weekend!

Imagine ,XML Some additional information added by the author of the document ：

<note>
    <date>2008-01-10</date>
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!</body>
</note>

So will the application break or crash ？

Can't . This application can still find XML In document 、 as well as Elements , And produce the same output .

XML One of the advantages of , It can be extended without interrupting the application .

7、XML attribute

XML Elements have attributes , similar HTML.

attribute （Attribute） Provide additional information about the element .

XML attribute

stay HTML in , Property provides additional information about the element ：

<img src="computer.gif">
<a href="demo.html">

Attributes usually provide information that is not part of the data . In the following example , File types have nothing to do with data , But it's important for the software that needs to deal with this element ：

<file type="gif">computer.gif</file>

XML Attribute must be quoted

Property values must be enclosed in quotation marks , However, single and double quotation marks can be used . For example, a person's gender ,person Elements can be written like this ：

<person sex="female">

Or it could be ：

<person sex='female'>

If the property value itself contains double quotes , You can use single quotes , Like this example ：

<gangster name='George "Shotgun" Ziegler'>

Or you can use character entities ：

<gangster name="George &quot;Shotgun&quot; Ziegler">

XML Elements vs. attribute

Look at these examples ：

<person sex="female">
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>

<person>
<sex>female</sex>
<firstname>Anna</firstname>
<lastname>Smith</lastname>
</person>

In the first example ,sex Is an attribute . In the second example ,sex Is an element . Both instances provide the same information .

There are no rules to tell us when to use properties , And when to use elements . My experience is that HTML in , Attributes are easy to use , But in XML in , You should try to avoid using properties . If information feels like data , So use elements .

My favorite way

The next three XML The document contains exactly the same information ：

The first example uses date attribute ：

<note date="10/01/2008">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

In the second example date Elements ：

<note>
<date>10/01/2008</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

The third example uses the extended date Elements （ This is my favorite ）：

<note>
<date>
<day>10</day>
<month>01</month>
<year>2008</year>
</date>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>

avoid XML attribute ？

Some problems caused by using attributes ：

Property cannot contain more than one value （ Elements can ）
Property cannot contain tree structure （ Elements can ）
Properties are not easy to extend （ For future changes ）
Properties are hard to read and maintain . Try to use elements to describe the data . Only attributes are used to provide data independent information .

Don't do such a stupid thing （ This is not XML The way it should be used ）：

<note day="10" month="01" year="2008" to="Tove" from="Jani" heading="Reminder" body="Don't forget me this weekend!">
</note>

For metadata XML attribute

Sometimes elements are assigned ID quote . these ID An index can be used to identify XML Elements , The way it works and HTML in id The attributes are the same . This example shows us this situation ：

<messages>
<note id="501">
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
<note id="502">
<to>Jani</to>
<from>Tove</from>
<heading>Re: Reminder</heading>
<body>I will not</body>
</note>
</messages>

above id An attribute is just an identifier , Used to identify different notes . It's not part of the memo data .

The idea we are trying to convey to you here is ： Metadata （ Data about data ） Should be stored as an attribute , And the data itself should be stored as elements .

8、XML verification

Having the right grammar XML go by the name of " In good form " Of XML.

adopt DTD Verified XML yes " legal " Of XML.

In good form XML file

" In good form " Of XML The document has the right syntax .

The grammar rules described in the previous chapter ：

  • XML The document must have a root element
  • XML Element must have a close tag
  • XML Tags are case sensitive
  • XML Elements must be nested correctly
  • XML Attribute values must be quoted 
<?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>

verification XML file

legal XML The document is " In good form " Of XML file , This also fits the document type definition （DTD） The rules of ：

<?xml version="1.0" encoding="ISO-8859-1"?> Tove Jani Reminder Don't forget me this weekend! In the example above ,DOCTYPE The declaration is external DTD References to documents . The following paragraphs show the contents of this file .

XML DTD

DTD The purpose of this is to define XML Document structure . It uses a series of legal elements to define the document structure ：

<!DOCTYPE note [ <!ELEMENT note (to,from,heading,body)> <!ELEMENT to (#PCDATA)> <!ELEMENT from (#PCDATA)> <!ELEMENT heading (#PCDATA)> <!ELEMENT body (#PCDATA)> ]>

If you want to learn DTD, Please find it on our homepage DTD course .

XML Schema

W3C Support a method based on XML Of DTD Substitute , It's called XML Schema：

<xs:element name="note">

<xs:complexType>
<xs:sequence>
<xs:element name="to" type="xs:string"/>
<xs:element name="from" type="xs:string"/>
<xs:element name="heading" type="xs:string"/>
<xs:element name="body" type="xs:string"/>
</xs:sequence>
</xs:complexType>

</xs:element>

If you want to learn XML Schema, Please find it on our homepage Schema course .

A generic XML Validator

To help you check XML The syntax of the document , We created XML Validator , So that you can understand any XML File syntax check .

原网站

版权声明
本文为[Champion_ me]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/184/202207022324337401.html

边栏推荐

猜你喜欢

随机推荐