XML syntax, constraints

Dom4J analysis XML、Xpath retrieval XML

XML summary

XML Is an extensible markup language （eXtensible Markup Language） Abbreviation , It is a data representation format , Can describe very complex data structures , It is often used to transmit and store data .

<?xml version="1.0" encoding="UTF-8"?>
<data>
    <sender> Zhang San </sender>
    <receiver> Li Si </receiver>
    <src>
    	 <addr> Beijing </addr>
    	 <date>2022-11-11 11:11:11</date>
    </src>
    <current> wuhan </current>
    <dest> Guangzhou </dest>
</data>

XML Characteristics
One is plain text , By default UTF-8 code ; Second, it can be nested ;

XML Usage scenarios of
XML Content is often transmitted over the network as a message , Or use it as a configuration file to store system information ;

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XML The grammar of

XML The suffix of the file is ：xml;

The first line is the document declaration

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

version：XML Default version number 、 The attribute must exist 
encoding： Ben XML Coding of documents 

XML The label of ( Elements ) The rules

XML You can customize the label .

1. The tag consists of a pair of angle brackets and legal identifiers : <name></name>, There must be a root tag , There is one and only one ;
2. Labels must appear in pairs , There is a beginning , There is an end : <name></name>;
3. Special labels can not be paired , But there must be an end tag , Such as :<br/>;
4. Attributes can be defined in tags , Property and tag name are separated by spaces , Property values must be enclosed in quotation marks <student id = “1”></name>;
5. Tags need to be nested properly ;

XML Notes
XML Annotation information can be defined in the file ：<!– The comment -->;

XML Escape character

&lt;    <   Less than 
&gt;    >   Greater than 
&amp;   &   And no. 
&apos;  '   Single quotation marks 
&quot;  "   quotes 

XML There can be CDATA District

<![CDATA[ … Content … ]]>

Strictly speaking , stay XML There are only ”<” and ”&” It's illegal. , The other three are legal , however , It's a good habit to escape them all , You can also use <![CDATA[]]> To include not being xml What the parser parses .

But we need to pay attention ：
1. This part can no longer contain ”]]>”;
2. Nesting is not allowed ;
3.]]> This part cannot contain spaces or line breaks ;

Code example

<?xml version="1.0" encoding="UTF-8" ?>
<!--  notes ： There can only be one root tag  -->
<student>
    <name> Daughter King </name>
    <sex> Woman </sex>
    <hobby> Tang's monk , Chasing Tang Monk </hobby>
    <info>
        <age>30</age>
        <addr> Women's country </addr>
    </info>
    <sql>
        select * from user where age &lt; 18;
        select * from user where age &lt; 18 &amp;&amp; age > 10
        <![CDATA[ select * from user where age < 18 ]]>
    </sql>
</student>

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XML constraint

What are document constraints
because XML The file can customize the label , Lead to XML Files can be defined at will , The program may have problems when parsing , So it's OK to XML Carry out writing standard restrictions .

Classification of document constraints
1.DTD
2.schema

DTD constraint

To write DTD Constraint document , The suffix must be .dtd, In the need to write XML Import the file DTD Just constrain the document , Then write according to the provisions of the constraint XML The content of the document .

shortcoming
Cannot constrain specific data types .

Code example
data.dtd

<!ELEMENT  bookshelf  ( book +)>
<!ELEMENT  book  ( Title , author , The price is )>
<!ELEMENT  Title  (#PCDATA)>
<!ELEMENT  author  (#PCDATA)>
<!ELEMENT  The price is  (#PCDATA)>

test.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE  bookshelf  SYSTEM "data.dtd">
< bookshelf >
    < book >
        < Title > Master JavaSE To strengthen </ Title >
        < author >dlei</ author >
        <!-- DTD Constraints cannot constrain data types  -->
        < The price is > Very expensive </ The price is >
    </ book >
    < book >
        < Title ></ Title >
        < author ></ author >
        < The price is ></ The price is >
    </ book >
    < book >
        < Title ></ Title >
        < author ></ author >
        < The price is ></ The price is >
    </ book >
</ bookshelf >

schema constraint

schema You can constrain specific data types , Stronger constraints ,schema It's also a xml file , It is also subject to the requirements of other binding documents , So the writing is more rigorous .

schema Use
1. To write schema Constraint document , The suffix must be .xsd;
2. In the need to write XML Import the file schema Constraint document ;
3. Write according to the constraint content XML The label of the document ;

Code example
data.xsd

<?xml version="1.0" encoding="UTF-8" ?>
<schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.itcast.cn" elementFormDefault="qualified" >
    <!-- targetNamespace: Declare the address of the constraint document （ Namespace ）-->
    <element name=' bookshelf '>
        <!--  Write child element  -->
        <complexType>
            <!-- maxOccurs='unbounded':  There can be any number of sub elements under the bookshelf ！-->
            <sequence maxOccurs='unbounded'>
                <element name=' book '>
                    <!--  Write child element  -->
                    <complexType>
                        <sequence>
                            <element name=' Title ' type='string'/>
                            <element name=' author ' type='string'/>
                            <element name=' The price is ' type='double'/>
                        </sequence>
                    </complexType>
                </element>
            </sequence>
        </complexType>
    </element>
</schema>

test.xml

<?xml version="1.0" encoding="UTF-8" ?>
< bookshelf  xmlns="http://www.itcast.cn" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.itcast.cn data.xsd">
    <!-- xmlns="http://www.itcast.cn"  Basic position  xsi:schemaLocation="http://www.itcast.cn books02.xsd"  Specific location  -->
    < book >
        < Title > The Brave Archer and His Mate </ Title >
        < author > Jin yong </ author >
        < The price is >399.9</ The price is >
    </ book >
    < book >
        < Title > The Brave Archer and His Mate </ Title >
        < author > Jin yong </ author >
        < The price is >19.5</ The price is >
    </ book >

</ bookshelf >

Dom4J analysis XML、Xpath retrieval XML