Java Cookbook

Verifying Structure with a DTD

Problem

Up to now, I have simply provided XML and asserted that it is valid. Now you want to verify the structure using a Document Type Definition (DTD).

Solution

Write the DTD and refer to it in one or more XML documents.

Discussion

This is not the place for a full dissertation on creating a Document Type Definition. Briefly, a DTD consists of a header and a list of the elements and any attributes. The DTD is written in a special language that allows you to specify the elements and attributes. Example 21-7 is people.dtd , a DTD for the people.xml file used earlier in this chapter.

Example 21-7. people.dtd

<!ELEMENT people (person)*>
<!ELEMENT person (name, email, country)>

<!ELEMENT name (#PCDATA)>
<!ATTLIST email type CDATA #IMPLIED>
<!ELEMENT email (#PCDATA)>
<!ELEMENT country (#PCDATA)>

To verify that a file conforms to a DTD, you do two things:

1. Refer to the DTD from within the XML file, as is sometimes seen in HTML documents. The <!DOCTYPE> line should follow the <?xml> line but precede any actual data.

   <?xml version="1.0"?>
   <!DOCTYPE people SYSTEM "people.dtd">
   
   <people>
   <person>
   <name>Ian Darwin</name>
   <email>[e-mail]</email>
   <country>Canada</country>
   </person>

2. Pass true as a second argument to the createXMLDocument( ) method; true means “enforce document validity.”

   XmlDocument doc = XmlDocument.createXmlDocument(uri);

Now any elements found in the document that are not valid according to the DTD will result in an exception being thrown.