package net.biancheng.www.componet;
import lombok.extern.slf4j.Slf4j;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@Slf4j
public class LoginInterceptor implements HandlerInterceptor {
    /**
     * 目標方法執(zhí)行前
     *
     * @param request
     * @param response
     * @param handler
     * @return
     * @throws Exception
     */
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        Object loginUser = request.getSession().getAttribute("loginUser");
        if (loginUser == null) {
            //未登錄，返回登陸頁
            request.setAttribute("msg", "您沒有權限進行此操作，請先登陸！");
            request.getRequestDispatcher("/index.html").forward(request, response);
            return false;
        } else {
            //放行
            return true;
        }
    }
    /**
     * 目標方法執(zhí)行后
     *
     * @param request
     * @param response
     * @param handler
     * @param modelAndView
     * @throws Exception
     */
    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        log.info("postHandle執(zhí)行{}", modelAndView);
    }
    /**
     * 頁面渲染后
     *
     * @param request
     * @param response
     * @param handler
     * @param ex
     * @throws Exception
     */
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        log.info("afterCompletion執(zhí)行異常{}", ex);
    }
}

@Configuration
public class MyMvcConfig implements WebMvcConfigurer {
    ......
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new LoginInterceptor());
    }
}

@Slf4j
@Configuration
public class MyConfig implements WebMvcConfigurer {
    ......
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        log.info("注冊攔截器");
        registry.addInterceptor(new LoginInterceptor()).addPathPatterns("/**") //攔截所有請求，包括靜態(tài)資源文件
                .excludePathPatterns("/", "/login", "/index.html", "/user/login", "/css/**", "/images/**", "/js/**", "/fonts/**"); //放行登錄頁，登陸操作，靜態(tài)資源
    }
}

package net.biancheng.www.controller;
import lombok.extern.slf4j.Slf4j;
import net.biancheng.www.bean.User;
import org.springframework.stereotype.Controller;
import org.springframework.util.StringUtils;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import javax.servlet.http.HttpSession;
import java.util.Map;
@Slf4j
@Controller
public class LoginController {
    @RequestMapping("/user/login")
    public String doLogin(User user, Map<String, Object> map, HttpSession session) {
        if (user != null && StringUtils.hasText(user.getUsername()) && "123456".equals(user.getPassword())) {
            session.setAttribute("loginUser", user);
            log.info("登陸成功，用戶名：" + user.getUsername());
            //防止重復提交使用重定向
            return "redirect:/main.html";
        } else {
            map.put("msg", "用戶名或密碼錯誤");
            log.error("登陸失敗");
            return "login";
        }
    }
/*
    @RequestMapping("/main.html")
    public String mainPage(){
        return "main";
    }*/
}

@Configuration
public class MyMvcConfig implements WebMvcConfigurer {
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        //當訪問 “/” 或 “/index.html” 時，都直接跳轉到登陸頁面
        registry.addViewController("/").setViewName("login");
        registry.addViewController("/index.html").setViewName("login");
        //添加視圖映射 main.html 指向  dashboard.html
        registry.addViewController("/main.html").setViewName("main");
    }
    ......
}

<p style="color: red" th:text="${msg}" th:if="${not #strings.isEmpty(msg)}"></p>