Document Type Definition (DTD) is a set of rules for defining the structure, content, and constraints of XML documents. It serves as a blueprint that specifies how elements, attributes, and entities are used and interrelated in an XML document. Understanding DTD is essential for ensuring the integrity and consistency of XML data across different systems and applications.
DTD defines the legal building blocks of an XML document. It establishes the rules for elements and attributes in an XML file. DTD can be declared inline within an XML document (internal DTD) or as a separate file (external DTD).
Using DTD has several advantages:
There are two types of DTDs:
A DTD file typically contains declarations for:
]>
Elements are the primary building blocks of an XML document. DTD defines what elements can appear and their relationships.
#PCDATA: Parsed Character Data.
EMPTY: No content.
ANY: Any content.
Mixed Content: Combination of text and elements.
Child Elements: Specified child elements.
This defines a note
element with four child elements: to
, from
, heading
, and body
.
Attributes provide additional information about elements. They are defined using the ATTLIST
declaration.
CDATA: Character data.
ID: Unique identifier.
IDREF: Reference to an ID.
IDREFS: References to multiple IDs.
ENTITY: Reference to an unparsed entity.
ENTITIES: References to multiple unparsed entities.
NMTOKEN: Name token.
NMTOKENS: Multiple name tokens.
NOTATION: Reference to a notation.
#REQUIRED: Attribute is required.
#IMPLIED: Attribute is optional.
#FIXED value: Attribute has a fixed value.
Default value: Attribute has a default value.
This defines a date
attribute for the note
element that is required and must contain character data.
Entities are placeholders for text or special characters. They help in reusing common text and including special characters in the XML document.
This defines an entity author
with the value “John Doe”.
Internal DTD is defined within the XML document itself.
]>
Tove
Jani
Reminder
Don't forget me this weekend!
External DTD is defined in a separate file and referenced in the XML document.
Tove
Jani
Reminder
Don't forget me this weekend!
To use a DTD in an XML document, you declare it at the beginning of the document using the <!DOCTYPE>
declaration.
Tove
Jani
Reminder
Don't forget me this weekend!
Parameter entities are used to define reusable pieces of the DTD itself.
Conditional sections allow parts of the DTD to be included or ignored based on conditions.
]]>
]]>
]>
Tove
Jani
Reminder
Don't forget me this weekend!
Tove
Jani
Reminder
Don't forget me this weekend!
]>
Tove
&author;
Reminder
Don't forget me this weekend!
DTD is a crucial aspect of XML that helps in defining the structure, content, and constraints of XML documents. By understanding and using DTD, you can ensure the integrity and consistency of your XML data, making it easier to share and use across different systems. Whether using internal or external DTDs, the principles and syntax remain the same, providing a robust way to validate and standardize XML documents. Happy Coding! ❤️