Explain DTD (Document Type Definition)

The XML has neither meaning nor context without gramer  aganist which it can be validated. The gramer is called a document type definition (DTD). While constructing XML files we follow set of rules. These rules will be returned either in the form of DTD (or) XML schema.

The XML documents that are coming based on common set of DTD (or) schema rules can be processed easily. Can be merged and compared easily. All programmers of java web application development use DTD/schema rules given by ‘Sun micro system’ while constructing web XML file. The tag names in XML file becomes pre defined tags when it is developed based on DTD rules and schema rules.

Elements:-

The XML document is composed of a number of elements.

The first node of the XML document is called the root node.

It contains all other nodes of the document on each XML document must have exactly one root node.

To link XML files with external DTD file we use < ! doctype- - - - > statement kept in web. XML of web application development.

<! Doctype webapp public: “//sunmicrosystem.Inc//DTD web application 2.3 //EN”  “http: //java. Sun.com/DTD/webapps 2.3 dtd”>

In DTD and schema rules of XML the following things will be available.

names of tags

hierarchy of tags.

data type’s of the attribute tag body values.

users defined entities.

Prepare a student. XML file based on DTD rules.

DTD Rules:-

1. ‘Students’ must be root element.

2. ‘Students’ allow one (or) more student.

3. Student must have attribute called type with one of these values “Regular/part time”.

4.Each student must have these in a sequence (<SNO>, <SNAME>, <SADD>).

5. <SNO>< <SNAME>, <SADD> tag allow #PCDATA body.

The equalent DTD code is:

Rules. Dtd

<! Element students (student) + >

<! Element student (sno. Sname, sadd)>

<! Attlist student type (regula/part time )# Required>.

<! Element  sno (# pcdata )>

<! Element  sname (#pcdata) >

< ! Element  sadd (# pcdata)>

valid XML  document:

< ! Doctype  student system  rules. dtd>

< students>

< students type= “regular”>

< sno> 101 </ sno>

<sname> Raja < / sname>

<sadd > Rjy < / sadd>

</ student>

<student type = “part time” >

< sno>102 < /sno>

<sname> Ramu </sname>

<sadd > TNK < /sadd>

</student>

</students>

Difference  between CDATA and   PCDATA: when tag body is configured C data in DTD rules the XML  parses will not  expend the entities  while reading  body of the tag.

Eg: < msg > 110& gt;15 </ msg>

Configured as CDATA – parser (o/p: 10 >15)

If tag body is configured as  PCDATA then the XML  passes will expend entity with  symbols  while reading  the body of the tag.

< msg >  10> 5 </ msg>

Configured as PCDATA – paresr (o/p:10>5)