1、什么是拦截器？

Spring MVC中的拦截器（Interceptor）类似于Servlet中的过滤器（Filter），它主要用于拦截用户请求并作相应的处理。例如通过拦截器可以进行权限验证、判断用户是否登录等。

拦截器依赖于web框架，在实现上基于Java的反射机制，属于面向切面编程（AOP）的一种运用。

 1.1创建拦截器

package com.by.interceptor;

import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CustomHandlerInterceptor implements HandlerInterceptor {
    /**
     * 在控制器方法调用前执行
     * 返回值为是否中断
     *      true：表示继续执行（下一个拦截器或处理器）
     *      false：则会中断后续的所有操作，所以我们需要使用response来继续响应后续请求
     */
    @Override
    public boolean preHandle(HttpServletRequest request,
                      HttpServletResponse response, Object object) throws Exception {
        System.out.println("HandlerInterceptor preHandle ....");
        return true;
    }

    /**
     * 在控制器方法调用后，解析视图前调用，我们可以对视图和模型做进一步渲染或修改
     * 可在modelAndView中加入数据，比如当前时间
     */
    @Override
    public void postHandle(HttpServletRequest request,HttpServletResponse response, 
                         Object object, ModelAndView modelAndView) throws Exception {
        System.out.println("HandlerInterceptor postHandle ....");
    }

    /**
     * 整个请求完成，即视图渲染结束后调用，这个时候可以做些资源清理工作，或日志记录
     */
    @Override
    public void afterCompletion(HttpServletRequest request,
        HttpServletResponse response,Object object, Exception e) throws Exception {
        System.out.println("HandlerInterceptor afterCompletion ....");
    }
}

    /**
     * handler执行后，返回jsp后
     */
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        HandlerInterceptor.super.afterCompletion(request, response, handler, ex);
    }
}

 1.2配置拦截器

<!--配置拦截器 -->
    <mvc:interceptors>
        <mvc:interceptor>
            <!-- 匹配的是url路径 -->
            <mvc:mapping path="/**"></mvc:mapping>
            <bean class="com.by.interceptor.CustomHandlerInterceptor"></bean>
        </mvc:interceptor>
    </mvc:interceptors>

1.3测试

• 编写controller

@Controller
@RequestMapping("/account")
public class AccountController {

    @RequestMapping("/findAccount12")
    public String findAccount12(Model model) {
        model.addAttribute("msg", "欢迎你 springmvc");
        System.out.println("controller的方法执行了......");
        return "success";
    }
}

• 在index.jsp里面定义超链接

<a href="/account/findAccount12">拦截器</a>

 2、案例，配置登录拦截器

2.1创建拦截器

package com.zhy.interceptor;

import com.zhy.pojo.User;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class LoginInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        System.out.println("-----------------------------------------------------------");
        //判断是否登录
       User loginUser = (User) request.getSession().getAttribute("loginUser");
       if (loginUser!=null){
           return true;//如果登录，则进行放行
       }
       response.sendRedirect("/login.jsp");
       return false;//不放行
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        HandlerInterceptor.super.postHandle(request, response, handler, modelAndView);
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        HandlerInterceptor.super.afterCompletion(request, response, handler, ex);
    }
}

2.2配置拦截器

<!--    配置拦截器-->

    <mvc:interceptors>
        <mvc:interceptor>
            <!-- 匹配的是url路径 -->

            <mvc:mapping path="/**"></mvc:mapping>
<!--            放行以下请求-->
            <mvc:exclude-mapping path="/user/login"/>
            <mvc:exclude-mapping path="/user/register"/>
            <mvc:exclude-mapping path="/user/toRegister"/>

            <bean class="com.zhy.interceptor.LoginInterceptor"></bean>

        </mvc:interceptor>
    </mvc:interceptors>

 2.3测试

@Controller
@RequestMapping("/user")
public class UserController {

    @Autowired
    private UserService userService;
//    注册功能
    @RequestMapping("/register")
    public String register(){
        return "register";
    }
    @RequestMapping("/toRegister")
    public String toRegister(User user){
        userService.toRegister(user);
        return "redirect:/login.jsp";
    }
//登录
    @RequestMapping("/login")
    public String login(User user, HttpSession session){
       User loginUser =userService.login(user);
       if(loginUser!=null){
           session.setAttribute("loginUser",loginUser);
           return "redirect:/account/selectAccount";
       }
        return "redirect:/login.jsp";
    }
}