public static String getFilterClassName( Document webXml, String filterName )
    {
        return getClassName( "filter", webXml, filterName );
    }
Retrieves the class name for a particular filter from a web.xml document.

Returns 'com.acme.filter.MessageFilter' for 'message' of this web.xml document:
&lt;filter&gt;
  &lt;filter-name&gt;message&lt;/filter-name&gt;
  &lt;filter-class&gt;com.acme.filter.MessageFilter&lt;/filter-class&gt;
&lt;/filter&gt;

@param webXml web.xml document, parsed as XML (cannot be null)
@param filterName The name of the filter (cannot be null or empty).
@return The name of the filter class, or null when no such class was defined.
xmppserver/src/main/java/org/jivesoftware/util/WebXmlUtils.java