We will now consider the tags which are defined in the persistence descriptor DTD, and which are used in order to specify the persistence characteristics of persistence-capable classes.
The <extension> element occurs at various places throughout the DTD. It is a placeholder facilitating the introduction of vendor-specific extensions into persistence descriptors whilst retaining conformance to the standard DTD.
It has three attributes: vendor-name, key, and value. This element will be illustrated at the end of the chapter.
<!ELEMENT jdo ((package)+, (extension)*)>
This is the root element of the document and must be present. It has no attributes. It must ...