PMD provides a very handy method for creating new rules by writing an XPath query. When the XPath query finds a match, a violation is created.
    Let's take a simple example : assume we have a Factory class that must be always declared final. 
    We'd like to report a violation each time a declaration of Factory is not declared final. Consider the following class: :
<pre>
public class a {
	Factory f1;

	void myMethod() {
		Factory f2;
		int a;
	}
}
</pre>
	The following expression does the magic we need:
<pre>
//VariableDeclarator
 [../Type/ReferenceType/ClassOrInterfaceType
  [@Image = 'Factory'] and ..[@Final='false']]
</pre>
	See the <a href="http://pmd.sourceforge.net/xpathruletutorial.html">XPath rule tutorial</a> for more information.