java ssm架构怎么判断用户已经存在 java怎么判断用户是否登录

controller层：
public class Login{
public String confirmLogin(HttpServletRequest request) {
Member member = memberService.checkIdAndToken(request);
if(member==null){
return "登录失败"；}
return "登录成功"；
}
impl层
public class LoginServiceImpl{
public Member checkIdAndToken(HttpServletRequest request) {
 String id = HttpRequestUtils.getHeaderByName(request, "id");
 String token = HttpRequestUtils.getHeaderByName(request, "token");
 if (id == null) {
 id = request.getParameter("id");
 }
 if (token == null) {
 token = request.getParameter("token");
 }
 return memberRepository.findByIdAndToken(id, token);
 }}
repository 层
public interface MemberRepository{
{Member findByIdAndToken(String id, String token);
}
}