public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
throws IOException, ServletException {
// session属于http范畴，所以要将ServletRequest转换成httpServletRequest
try {
HttpServletRequest req = (HttpServletRequest) request;
HttpSession session = req.getSession();
if (session.getAttribute("username") != null) {
chain.doFilter(request, response);
} else {
request.getRequestDispatcher("error.jsp").forward(request, response);
}
} catch (RuntimeException e) {
e.printStackTrace();
}
}
