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

Returns 'SessionCreateServlet' for 'sessioncreate' of this web.xml document:
&lt;servlet&gt;
  &lt;servlet-name&gt;sessioncreate&lt;/servlet-name&gt;
  &lt;servlet-class&gt;SessionCreateServlet&lt;/servlet-class&gt;
&lt;/servlet&gt;
&lt;servlet&gt;
  &lt;servlet-name&gt;sessiondestroy&lt;/servlet-name&gt;
  &lt;servlet-class&gt;SessionDestroyServlet&lt;/servlet-class&gt;
&lt;/servlet&gt;

@param webXml web.xml document, parsed as XML (cannot be null)
@param servletName The name of the servlet (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