Introduction to DTD In XML

Document Type Definition (DTD), defined to some extent differently within the XML and SGMLspecifications, is one of several SGML and XML schema languages, and is the term used to describe a document or portion thereof that is authored in the DTD language. A DTD is primarily used for the expression of a schema via a set of declarations that conform to a particular markup syntax and that describe a class, or type, of SGML or XML documents, in terms of constraints on the structure of those documents.

A Document Type Definition (DTD) defines the legal building blocks of an XML document. It defines the document structure with a list of legal elements and attributes. A DTD can be declared inline inside an XML document, or as an external reference.

Types of DTD

Internal DTD

This is an XML document with a Document Type Definition:

<?xml version=”1.0″ encoding=”ISO-8859-1″?>
     <!DOCTYPE email [
    <!ELEMENT email (sender,recipient,subject,message)>
     <!ELEMENT sender (sender_name,sender_email_id)>
    <!ELEMENT recipient (recipient_name,recipient_email_id)>
    <!ELEMENT subject (#PCDATA)>
    <!ELEMENT message (message_header,message_body_text,message_footer)>
]>

    <email>
        <sender>
            <sender_name>XYZ Kumar</sender_name>
            <sender_email_id>[email protected]</sender_email_id>
        </sender>
        <recipient>
            <recipient_name>ABC</recipient_name>
            <recipient_email_id>[email protected]</recipient_email_id>
        </recipient>
        <subject>Hello! how r U</subject>
        <message>
            <message_header>Hello dear</message_header>
            <message_body_text>Hello this is xyz, how are you dear</message_body_text>
            <message_footer>bye!, take care</message_footer>
        </message>
</email>

Click here to view the XML file.

The DTD is interpreted like this:

!ELEMENT email (in line 2) defines the element “email” as having four elements: ” sender, recipient, subject, message” and so on.

External DTD

This is the same XML document with an external DTD:

<?xml version=”1.0″ encoding=”ISO-8859-1″?>
  <!DOCTYPE email SYSTEM “emailStructure.dtd”>
  <email>
    <sender>
    <sender_name>XYZ Kumar</sender_name>
    <sender_email_id>[email protected]</sender_email_id>
  </sender>
  <recipient>
    <recipient_name>ABC</recipient_name>
    <recipient_email_id>[email protected]</recipient_email_id>
  </recipient>
  <subject>Hello! how r U</subject>
  <message>
    <message_header>Hello dear</message_header>
    <message_body_text>Hello this is xyz, how are you dear</message_body_text>
    <message_footer>bye!, take care</message_footer>
  </message>
  </email>

Click here to view the XML file.

Content of the file “emailStructure.dtd” containing the Document Type Definition:

<?xml version=”1.0″ encoding=”ISO-8859-1″?>
<!ELEMENT email (sender,recipient,subject,message)>
<!ELEMENT sender (sender_name,sender_email_id)>
<!ELEMENT recipient (recipient_name,recipient_email_id)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT message (message_header,message_body_text,message_footer)>