What is a DTD?
A DTD is a Document Type Definition.
A DTD defines the structure and the legal elements and attributes of an XML document.
Why Use a DTD?
With a DTD, independent groups of people can agree on a standard DTD for interchanging data.
An application can use a DTD to verify that XML data is valid.
An Internal DTD Declaration
If the DTD is declared inside the XML file, it must be wrapped inside the <!DOCTYPE> definition:
XML document with an internal DTD
<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend</body>
</note>
View XML file »
In the XML file, select "view source" to view the DTD.
The DTD above is interpreted like this:
- !DOCTYPE note defines that the root element of this document is note
- !ELEMENT note defines that the note element must contain four elements: "to,from,heading,body"
- !ELEMENT to defines the to element to be of type "#PCDATA"
- !ELEMENT from defines the from element to be of type "#PCDATA"
- !ELEMENT heading defines the heading element to be of type "#PCDATA"
- !ELEMENT body defines the body element to be of type "#PCDATA"
An External DTD Declaration
If the DTD is declared in an external file, the <!DOCTYPE> definition must contain a reference to the DTD file:
XML document with a reference to an external DTD
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
View XML file »
And here is the file "note.dtd", which contains the DTD:
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>