XML (2) Document Type Definitions (DTD)

XML提供了和显示无关的内容数据，可是数据的结构怎么确定呢？对于复杂问题，肯定是先有一个提纲，在给出具体内容，所以我们需要一种方式来提供XML的数据结构定义，这个就是DTD。

Element Declarations

<!ELEMENTSAMPLE(#PCDATA)>

Document Type Declarations

<!DOCTYPEname_of_root_elementSYSTEM"URL"[internalDTDsubset]>

external DTD subsets: Link to an outside file( often *.dtd)
internal DTD subsets: declaration in the XML file

Command-line validation
libxml2

Wed-based validation
www.cogsci.ed.ac.uk/~richard/xml-check.html
www.stg.brown.edu/service/xmlvalid

Elements Declarations

1. Define elements, like
   <!ELEMENTSAMPLE1ANY>
   
   
2. Find out the #PCDATA, change all necessary elements to #PCDATA, like
   <!ELEMENTSAMPLE2(#PCDATA)>
   
   
3. Find out the child elements, add to necessary elements, like
   <!ELEMENTSAMPLE3(CHILD_SAMPLE1+,CHILD_SAMPLE2?,(CHILD_SAMPLE3*|CHILD_SAMPLE4|CHILD_SAMPLE5)?,(CHILD_SAMPLE6,CHILD_SAMPLE7)*,#PCDATA)>
   

Empty elements, like <!ELEMENT SAMPLE4 EMPTY>
Comments in DTD, like <!-- comments -->

Tips:
Case sensitive
Sequence sensitive

Attribute Declarations
<!ATTLIST Element_name Attribute_name Type Default_value>
Attribute Types: CDATA, Emumerated, ID, IDREF, IDREFS, ENTITY, ENTINITYS, NMTOKEN, NMTOKENS, NOTION

<!ELEMENTAUTHOREMPTY>
<!ATTLISTAUTHORCOMPANYCDATA#FIXED"TIC">
<!ATTLISTEXTENSIONCDATA#IMPLIED>
<!ATTLISTAUTHORNAMECDATA#REQUIRED>

Namespaces
Everyone can define elements, so name collision can be avoided. Like java, to avoid the class name collision, import Name Space.

All from 《XML 1.1 Bible 3rd Edition》