XML document types

A document type definition (DTD) defines the structure of a class of XML documents, making it possible to distinguish between classes. A DTD is a list of element and attribute definitions unique to a class. Once you have set up a DTD, you can reference that DTD in another document, or embed it in the current XML document.

The DTD for XML Order documents, discussed in “A sample XML document” looks like this:

<!ELEMENT Order (Date, CustomerId, CustomerName,Item+)>
 <!ELEMENT CustomerId (#PCDATA)>
 <!ELEMENT CustomerName (#PCDATA)>
 <!ELEMENT Item (ItemId, ItemName, Quantity)>
 <!ELEMENT ItemName (#PCDATA)>
 <!ELEMENT Quantity (#PCDATA)>
 <!ATTLIST Quantity units CDATA #IMPLIED>

Read line by line, this DTD specifies that:

The character text of XML documents is not constrained. For example, there is no way to specify that the text of a quantity element should be numeric, and thus the following display of data would be valid:

<Quantity unit=”Baker’s dozen”>three</Quantity>
<Quantity unit=”six packs”>plenty</Quantity>

Restrictions on the text of elements must be handled by the applications that process XML data.

An XML’s DTD must follow the <?xml version="1.0"?> instruction. You can either include the DTD within your XML document, or you can reference an external DTD.

DTDs are not required for XML documents. However, a valid XML document has a DTD and conforms to that DTD.