//String contextPath = req.getServletContext().getContextPath(); //

        /**

         * ServletRequest的getServletContext方法是Servlet3.0添加的，这个可以看一下官方文档

            http://docs.oracle.com/javaee/6/api/javax/servlet/ServletRequest.html#getServletContext()

            而Tomcat6只支持到Servlet2.5看它的官方文档可以知道，要用J2EE6的话得换成Tomcat7

         *

         * */

        String contextPath = null;

        if(req instanceof HttpServletRequest){

            HttpServletRequest hreq = (HttpServletRequest) req;

            contextPath = hreq.getSession().getServletContext().getContextPath();

        }