XML : eXtensible Markup Language :: Document Type Definition [ DTD]

• What is DTD?
• Type of DTD.
• Defining Elements
• Defining Attributes
• Defining Entities

• Document Type Definition (DTD) helps to defines the building blocks of an XML document.
• It is used to define the document structure with a list of legal elements and attributes.
• A DTD can be embedded in same XML document, or it can be mentioned as an external file and should have extension ( .dtd ).

Syntax :

<!DOCTYPE root-element [element-declarations]>

Example

Why We should Use DTD:

• Used to verify own data
• It Verifies that the data received is valid
• Defines the legal building blocks of an XML document
• Defines the document structure with a list of legal elements.

DTD can be use by two different ways

• Internal DTD
• External DTD

Internal DTD Declaration

If the DTD is declared inside the XML file,

Syntax

<!DOCTYPE dtd-name
[
<!ELEMENT element-name (element-content type)gt;
<!ATTLIST element-name attribute-name attribute-type default-value>
]>

Example : Internal DTD

External DTD Declaration

If the DTD is declared in an external file.

Syntax:

<!DOCTYPE root-element SYSTEM "filename">

Example : External DTD

In XML, an element is a logical component of the document.

An attribute, represents the characteristics of an element.

Example

HTML elements are “body” and “title”.

Empty Element
    <!ELEMENT element-name (EMPTY)>
Element with Data
    <!ELEMENT element-name (#CDATA)>
    <!ELEMENT element-name (#PCDATA)>
Element with Child element
    <!ELEMENT element-name (child-element-name)>
Element with Child element minimum occurrence of same element
    <!ELEMENT element-name (child-name+)>
Element with Child element minimum 0 or one occurrence of same element
    <!ELEMENT element-name (child-name?)>
Element with Child element minimum 0 or more occurrence of same element
    <!ELEMENT element-name (child-name*)>

An attribute, represents the characteristics of an element.

Example

HTML Element img having src as attribute . eg. <img src=”world.jpg” />

Syntax

<!ATTLIST element-name attribute-name CDATA "default-value">
Enumerated Attribute Value
    <!ATTLIST element-name attribute-name (eval|eval|..) default-value>
Fixed Attribute Value
    <!ATTLIST element-name attribute-name attribute-type #FIXED "value">
Required Attribute Value
    <!ATTLIST element-name attribute_name attribute-type #REQUIRED >
Implied Attribute Value
    <!ATTLIST element-name attribute-name attribute-type #IMPLIED >

Example

<!ELEMENT rectangle EMPTY>
<!ATTLIST rectangle width CDATA "0">

Entity Declaration in DTD

Example

HTML entity like &nbsp;

The contents of the internal entities occur within the XML document.

Syntax

<!ENTITY entity-name "entity-value">

• External entities are those whose contents are outside the XML document.
• SYSTEM keyword is used to specify any entity external to the document

Syntax

<!ENTITY entity-name SYSTEM "URI/URL">