This method is useful for extensions that do not have controllers (such extensions usually return path to JSP as their include URL).
method is called before processing is passed to JSP. It allows page extension to modify model passed to JSP.
Note that since main page controller also uses this model for storing its parameters in case of parameter name conflict
extension parameter will be removed and warning message will be logged into the server log.
Returns true if this extension is available (should be included on the page) for the specified request.
If extension needs to be shown for some pages only, it can obtain original page URL via request attribute called "pageUrl":