在web.xml里面配置，有两种形式：
然后编写filter类：
@Override
public void doFilter(ServletRequest arg0, ServletResponse arg1, FilterChain arg2)
throws IOException, ServletException {
HttpServletRequest request = (HttpServletRequest)arg0;
HttpServletResponse response = (HttpServletResponse)arg1;
HttpSession session = request.getSession();
if(session.getAttribute("admin") == null && request.getRequestURL().indexOf("/admin/login") == -1) {
response.sendRedirect(request.getContextPath()+"/admin/login");
}else {
arg2.doFilter(arg0, arg1);
}
} @Override
public void init(FilterConfig arg0) throws ServletException {
//@Description: TODO
}