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DTD - 介绍
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A Document Type Definition defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements.
DTD（文本类型定义）定义了XML文档的合法组件群。并且，它还定义了带有合法元素列表的文档结构。
A DTD can be declared inline in your XML document, or as an external reference.
一份DTD可以在XML文件的内部声明，也可以作为一个外部参数来声明。
Internal DOCTYPE declaration
内部DOCTYPE声明
If the DTD is included in your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:
如果DTD是包含在XML源文件里面的，那么，它应该通过下述语法预先嵌套在一份DOCTYPE定义当中：
<!DOCTYPE root-element [element-declarations]> |
Example XML document with a DTD: (Open it in IE5, and select view source):
下面列举了包含DTD的XML文档案例（在IE5中打开，选择查看源文件）：
<?xml version="1.0"?> |
The DTD above is interpreted like this:
上述DTD解释如下：
!DOCTYPE note (in line 2) defines that this is a document of the type note.
!DOCTYPE note（在第二行）定义了一份类型节点文档。
!ELEMENT note (in line 3) defines the note element as having four elements: "to,from,heading,body".
!ELEMENT note（第三行）定义了"to、from、heading、body"4个元素的note元素。
!ELEMENT to (in line 4) defines the to element to be of the type "#PCDATA".
!ELEMENT to（第4行）阐明了作为"#PCDATA"类型之一的to元素。
!ELEMENT from (in line 5) defines the from element to be of the type "#PCDATA"
and so on.....
!ELEMENT from（第5行 ）阐明了作为"#PCDATA"类型之一的from元素。
External DOCTYPE declaration
外部DOCTYPE声明
If the DTD is external to your XML source file, it should be wrapped in a DOCTYPE definition with the following syntax:
如果DTD相对于你的XML源文件而言是一个外部文件，那么，需要使用下面的语法将它嵌套到DOCTYPE声明中：
<!DOCTYPE root-element SYSTEM "filename"> |
This is the same XML document as above, but with an external DTD: (Open it in IE5, and select view source)
如上所示，下面是一份相同的 XML文件。但是，它包含了一份外部DTD文件（在IE5中打开，选择查看源文件）：
<?xml version="1.0"?> |
And this is a copy of the file "note.dtd" containing the DTD:
这是一份包含DTD的"note.dtd"文件副本。
<!ELEMENT note (to,from,heading,body)> |
Why use a DTD?
为什么要使用DTD？
With DTD, each of your XML files can carry a description of its own format with it.
通过DTD，你的每一个XML文件都可以承载一份包含其自身格式的说明。
With a DTD, independent groups of people can agree to use a common DTD for interchanging data.
通过DTD，独立的访问群体可以使用一份普通的DTD对数据交换的方式达成一致。
Your application can use a standard DTD to verify that the data you receive from the outside world is valid.
应用程序可以使用标准的DTD来验证你从外界接收的数据是否有效。
You can also use a DTD to verify your own data.
你也可以使用DTD来检验自身的数据有效性。
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