1. 防止用户直接访问url的权限控制
   通过判断父类URL是否存在，判断是通过系统跳转的链接还是通过浏览器直接输入的跳转链接 参考如下
   UsersFilter
   public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
   HttpServletRequest req = (HttpServletRequest) request;
   // 获取Session
   HttpSession session = req.getSession();
   if (session != null) {
   Map user = (Map) session.getAttribute("user");
   if (user != null) {
   //new 只允许通过应用访问，不允许用户修改URL访问
   HttpServletResponse resp = (HttpServletResponse) response;
   String conString = req.getHeader("REFERER");//获取父url
   if("".equals(conString) || null==conString){ //判断如果上一个(父)url为空的话，说明是用户直接输入url访问的
   resp.sendRedirect(req.getContextPath() + "/index.jsp");//跳回首页
   return;
   }else{
   //应用正常跳转URL
   }
   } else {

String url = req.getRequestURI();
boolean match = false;
for (String key : resources)
{
if (url.indexOf(key) >= 0)
{
match = true;
break;
}
}
if (!match)
{
HttpServletResponse res = (HttpServletResponse) response;
res.sendRedirect(req.getContextPath() + "/index.jsp");
return;
}
}
}
chain.doFilter(request, response);
}