The method to handle a cancel button is "cancelled". (see the
javadoc). You need to overrride that method and include whatever logic you want executed when the cancel button is pressed.
It also appears that you may have to send some value in the "operation" parameter since that is the declared handler parameter. Adding an onclick event to the button might do the trick. Something like this:
onklick="this.form.action += '?operation=dummy';"