The dispatcher has four legal values: FORWARD, REQUEST, INCLUDE,
and ERROR. A value of FORWARD means the Filter will be applied
under RequestDispatcher.forward() calls. A value of REQUEST
means the Filter will be applied under ordinary client calls to
the path or servlet. A value of INCLUDE means the Filter will be
applied under RequestDispatcher.include() calls. A value of
ERROR means the Filter will be applied under the error page
mechanism. The absence of any dispatcher elements in a
filter-mapping indicates a default of applying filters only under
ordinary client calls to the path or servlet.
<filter-mapping> 的 <dispatcher> 的作用
原文：http://www.cnblogs.com/zno2/p/4895075.html