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Detailed explanation of DTD of XML constraint technology

黄舟

Mar 22, 2017 pm 04:36 PM

1.DTD official tutorial

##2.xml constraint technology:
DTD constraint: The syntax is relatively simple and the function is relatively simple.
Schema constraints appear first: the syntax is relatively complex and the functions are relatively powerful. Using a writing method similar to xml syntax, Schema constraints appear to replace DTD constraints.

3. Introduction to DTD:

Document Type Definition (DTD) can define legal XML document building blocks. It uses a series of legal elements to define the structure of the document. A DTD can be declared in an XML document as a line or as an external reference.

3.1 How to import DTD:

1. Internal import:

<code>#导入方式：
<!DOCTYPE root-element [element-declarations]>
#实例：
<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don&#39;t forget me this weekend</body>
</note>
</code>

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(第二行)定义此文档是 note（根标签） 类型的文档。<br>
(第三行)定义 note 元素有四个元素（标签）："to、from、heading,、body"<br>
(第四行)定义 to 元素为 "#PCDATA" 类型<br>
(第五行)定义 frome 元素为 "#PCDATA" 类型<br>
(第六行)定义 heading 元素为 "#PCDATA" 类型<br>
<p>(第七行)定义 body 元素为 "#PCDATA" 类型</p>
<p>外部导入方式：<br>  本地文件：</p>
<pre class="brush:xml;"><code>#导入方式：
 <!DOCTYPE note SYSTEM "note.dtd">
#note.dtd文件内容：
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)></code>

公共的外部导入：一般项目采用公共外部导入，比如ssh的xml文件基本上就是采用了这种方式

<code>#导入方式：
 <!DOCTYPE 根元素 PUBLIC "http://rlovep.com/peace.dtd">
#如hibernate.cfg.xml：
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"></code>

3.2DTD语法：

1.约束标签
语法：

<code> <!ELEMENT 元素名称 类别>  或 <!ELEMENT 元素名称 (元素内容)></code>

类别：
空标签： EMPTY。表示元素一定是空元素.例如::
普通字符串： (#PCDATA)。表示元素的内容一定是普通字符串（不能含有子标签）。例如：

任何内容： ANY。表示元素的内容可以是任意内容（包括子标签）例如：

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Element content:

<code>顺序问题：
<!ELEMENT 元素名称 (子元素名称 1,子元素名称 2,.....)>： 按顺序出现子标签
 
次数问题：
标签  ：  必须且只出现1次。
标签+ ： 至少出现1次
标签*  ： 0或n次。
标签？ ： 0 或1次。
声明"非.../既..."类型的内容</code>

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2. Constraint attributes:
Syntax:

<code><!ATTLIST 元素名称 属性名称 属性类型 默认值></code>

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Attribute type:

<code>CDATA ：表示普通字符串
(en1|en2|..)： 表示一定是任选其中的一个值
ID：表示在一个xml文档中该属性值必须唯一。值不能以数字开头</code>

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Default value:

<code>#REQUIRED 属性值是必需的
#IMPLIED   属性不是必需的
#FIXED value 属性不是必须的，但属性值是固定的</code>

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3.3 The test is as follows, please read the comments carefully:

<code><?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from+,heading*,body?,(br|b))>
<!--带有子序列的元素，需要按照先后顺序出现；
    to只能出现一次
    from最少出现一次
    heading次数随意
    body出现零次或者一次
    非出现br就出现b
-->
<!--元素约束-->
<!ELEMENT to (#PCDATA)><!--pcdata元素-->
<!ELEMENT from ANY><!--任何内容的元素-->
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
<!ELEMENT br EMPTY><!--空元素-->
<!ELEMENT b EMPTY><!--空元素-->
<!--属性约束-->
<!ATTLIST to number CDATA #REQUIRED><!--必须有属性值出现，且属性值类型为字符串-->
<!ATTLIST from length CDATA "10"><!--默认属性值，不写出属性时属性值为10-->
<!--假如您不希望强制作者包含属性，并且您没有默认值选项的话，请使用关键词 #IMPLIED。-->
<!ATTLIST heading length CDATA #IMPLIED>
<!ATTLIST body length CDATA #FIXED "123"><!--属性拥有固定的值，并不允许作者改变这个值-->
<!ATTLIST br type (check|cash) "cash"><!--属性值可以为check和cash中的一个-->
]>
<note>
<to number="1234">Tove</to>
<from>Jani</from>
<heading length="10">Reminder</heading>
<body length="123">Don&#39;t forget me this weekend</body>
<br type="check"/>
</note></code>

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