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XML Document Type Declaration (DTD)

The XML Document Type Declaration (DTD) is used to formally describe the grammar of XML documents. In this way, an XML document can be a valid XML document.

What is a DTD and what is it used for?​

DTD stands for Document Type Definition and it defines the structure, the elements and attributes of an XML document.

An application can use a DTD to verify if a given XML data is valid.

Should I use a DTD?​

It depends on what you want to do:

  • YES, if you want to verify that the data is valid against the DTD. For example, you can verify that the data you received from a server is valid.

  • NO, if you are experimenting with XML or you have small XML files and you don't have time to create DTDs.

Well-formed and Valid XML Documents​

  • An XML document is called well-formed if its syntax is correct.
  • An XML document is called valid if it is well-formed and it is validated against a DTD.

Example of Internal DTD Declaration​

<?xml version="1.0"?>  
<!DOCTYPE book [  
	<!ELEMENT book (title,author,publisher)>
	<!ELEMENT title(#PCDATA)>
	<!ELEMENT author (#PCDATA)>
	<!ELEMENT publisher(#PCDATA)>
]>

<book>  
	<title>A Great Book</title>  
	<author>Tom Nolan</author>  
	<publisher>Tutorial Reference</publisher>  
</book>

The DTD above is interpreted in this way:

  • !DOCTYPE book defines that the root element of the document is book
  • !ELEMENT book defines that the note element must contain the elements: title, author, publisher
  • !ELEMENT title defines the title element to be of type #PCDATA
  • !ELEMENT author defines the author element to be of type #PCDATA
  • !ELEMENT publisher defines the publisher element to be of type #PCDATA

Example of External DTD Declaration​

The following example is a well-formed and valid XML document with book.dtd DTD reference defined in <!DOCTYPE>

book.xml
<?xml version="1.0"?>  
<!DOCTYPE book SYSTEM "book.dtd">  
<book>  
	<title>A Great Book</title>  
	<author>Tom Nolan</author>  
	<publisher>Tutorial Reference</publisher>  
</book>

The book.dtd is the following:

book.dtd
<!ELEMENT book (title,author,publisher)>
<!ELEMENT title(#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT publisher(#PCDATA)>

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