To partly quote myself:
Real path revealed.
This comes from:
<form method="post" action="<%=contextPath%>/jsp/c.jsp" />
<!--needs to have action as the thing would not point back to itself without action, as browser thinks it is somewhere else as the thing was forwarded-->
So in short:
1. path a , urlrewrite to path b, forward to b
2. on b path there is c.jsp with <form method="post" which needs to point back to itself with action
How to do that without revealing real path of c.jsp, that is, preserve 'a' path in browser address bar ?