m_path = request.getContextPath() + request.getPathInfo();

getContextPath never return null (according to javadoc) and return "" for ROOT context. On tomcat, getPathInfo return null and getServletPath should be used (see https://mail-archives.apache.org/mod_mbox/tomcat-users/200103.mbox/%3C20010305171525.13786.qmail@web4901.mail.yahoo.com%3E for explanation)

this result in "" + null = "null" (by Java spec, arguably stupid)
then trying to substract root path fails.