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In an electronic computer , A marker is a symbol of information that a computer can understand , By such a mark , Computers can process various kinds of information, such as articles, etc . for example ：table label , After the browser parses , Generate a table .

2.XML Role of documents

To hold data , And the data are self descriptive
It can be used as the configuration file of the project or template
Data that can be used to transmit data over the network （ But not much is used now , At present, they are all using JSON Mainly ）
XML Inside DTD Specifications can be defined , The function is equivalent to interface , That is, only by using this specification can we realize XML The function of
In order to facilitate different applications 、 Data sharing and communication between different platforms .

3.XML Format

There is a root element （ The root element is like HTML Medium <html></html> The label is the same , All other labels are written in this label , All such tags can only have one ）
XML Labels are strictly case sensitive
Use the end tag correctly , Have a beginning and an end （<html></html> It's like this, from beginning to end ）
Nesting labels correctly
Use legal tag name
Define valid properties

Two . Element definition

1. What is? DTD

DTD The full name is Document Type Definition, Is a file definition format .
DTD Specifies the XML The file structure is XML The file provides syntax and rules .
stay DTD In the definition of XML File structure , And then according to DTD To write XML file . It's like function definition in programming language , When using a function, you should reference it according to the format of the function declaration .
PS： In short ,DTD It's used to constrain XML Document , Make it used under certain specifications , except DTD Outside technology , also Schema technology , Also for constraints XML Document .

2.DTD Statement statement

stay XML Add DTD Statement <!DOCTYPE root[]>,root----> Root element

for example ：

The root element in this code is <persons></persons>, Then let's change the sentence to <!DOCTYPE persons[]>.

Then the code when we define the specification is written in DTD In a declaration statement .

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE persons[]>
<persons>
	<person pid="p1" sex=" male " qq="aaa" parent="p2">
		<name> Zhang Xiaoming </name>
		<age>10</age>
		<contact>
			<phone>1234567</phone>
		</contact>
		<br/>
	</person>
	<person pid="p2">
		<name> Zhang Daming </name>
		<age>35</age>
		<contact>
			<email>[email protected]</email>
		</contact>
	</person>
</persons>

3. The classification of elements

3.1 <ELEMENT The element name EMPTY> Empty elements

What is an empty element ？
Tags without content are called empty elements .
because HTML The content of the element is the content between the start tag and the end tag . And some HTML Element has empty content .(empty content), Those with empty content HTML Elements , Is an empty element . Empty elements are closed in the start tag .
ps: I define a tag casually <abc/> This is an empty element , Our common html Some tags of are also empty elements , for example <br><hr><input> These are empty elements .

3.2 <ELEMENT The element name (#PCDATA)> Text elements

The so-called text element , That is, text data can be put into the label body
for example ：
<name> Zhang Xiaoming </name>

3.3 <ELEMENT The element name （e1,e2）> Mixed elements

A mixed element means that there are other child elements under the element .

for example ：

<persons><contact> There are many sub elements under the element

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE persons[
]>
<persons>
	<person pid="p1" sex=" male " qq="aaa" parent="p2">
		<name> Zhang Xiaoming </name>
		<age>10</age>
		<contact>
			<phone>1234567</phone>
		</contact>
		<br/>
	</person>
	<person pid="p2">
		<name> Zhang Daming </name>
		<age>35</age>
		<contact>
			<email>[email protected]</email>
		</contact>
	</person>
</persons>

3.4. Element limits

Element symbols
1. And ( ,)
2. Not ( | )
The number of occurrences of the element
1. 0 or 1：？
2. 0-N： *
3. 1-N：+

Element defines code practice ：

Have made detailed remarks , These are the contents just mentioned above

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE persons[
<!ELEMENT persons (person+)>   <!--persons Yes 1-N individual person Elements -->
<!ELEMENT person (name,age,contact,br*)>  <!--person There is... Under the element name,age,contact,br These sub elements -->
<!ELEMENT name (#PCDATA)>   <!--name Is a text element -->
<!ELEMENT age (#PCDATA)>    <!--age Is a text element -->
<!ELEMENT contact (phone|email*)>  <!--contact There can be phone Element or email Elements -->
<!ELEMENT phone (#PCDATA)>  <!--phone Is a text element -->
<!ELEMENT email (#PCDATA)>  <!--email Is a text element -->
<!ELEMENT br EMPTY>   <!--br Is an empty element -->

]>
<persons>
	<person>
		<name> Zhang Xiaoming </name>
		<age>10</age>
		<contact>
			<phone>1234567</phone>
		</contact>
		<br/>
	</person>
	<person>
		<name> Zhang Daming </name>
		<age>35</age>
		<contact>
			<email>[email protected]</email>
		</contact>
	</person>
</persons>

3、 ... and . Attribute definitions

1. grammar

<!ATTLIST Element name The attribute name type describe >

2. Attribute types

ID : Assign a id Number ,id No. cannot be repeated
CDATA : The text type
IDREF: Reference type , Enter the ID
（ male | Woman ）： Either male or female , You can also set other , such as 1|2, Either for 1 Either for 2

3. Property description

#REQUIRED: Required
#IMPLED: Not required
" The default value is ", Only like ( male | Woman ） Type , Only by default can the description be used .

Attribute definition code practice

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE persons[
<!ELEMENT persons (person+)>   <!--persons Yes 1-N individual person Elements -->
<!ELEMENT person (name,age,contact,br*)>  <!--person There is... Under the element name,age,contact,br These sub elements -->
<!ELEMENT name (#PCDATA)>   <!--name Is a text element -->
<!ELEMENT age (#PCDATA)>    <!--age Is a text element -->
<!ELEMENT contact (phone|email*)>  <!--contact There can be phone Element or email Elements -->
<!ELEMENT phone (#PCDATA)>  <!--phone Is a text element -->
<!ELEMENT email (#PCDATA)>  <!--email Is a text element -->
<!ELEMENT br EMPTY>   <!--br Is an empty element -->

<!-- Attribute describes code practice -->

<!ATTLIST person
 pid ID #REQUIRED 
 sex ( male | Woman ) " male "  
 qq  CDATA #IMPLIED
 parent IDREF #IMPLIED
 >
]>
<persons>
	<person  pid="p1" sex=" male " qq="193968" parent="p2">
		<name> Zhang Xiaoming </name>
		<age>10</age>
		<contact>
			<phone>1234567</phone>
		</contact>
		<br/>
	</person>
	<person pid="p2">
		<name> Zhang Daming </name>
		<age>35</age>
		<contact>
			<email>[email protected]</email>
		</contact>
	</person>
</persons>

Four .XML Existing problems

We set the specification so that when we don't write according to the specification , Immediately remind us that there is a problem , But if there is no problem with how to write , Please follow the following steps to set .

To open the first Window ==> Preferences ==>Validation, Then check the selected . Click Apply , And then restart eclipse.