一、Interception(拦截器)是什么？

        拦截器（Interceptor）是一种用于在请求到达目标方法之前或之后执行特定逻辑的机制。它是基于Java反射机制，属于面向切面编程（AOP）的一种应用。拦截器可以用于多种应用场景，例如验证用户登录状态、权限验证、记录系统日志、通用处理等。

二、任务要求

        1.在登录界面对用户名和密码进行验证

        2.未登录状态下，直接访问main页面，拦截回login页面

        3.在登录状态下，直接跳转到main页面，在页面显示用户名

三、实现代码

目录结构

1.创建Spring项目

2.在pom文件中引入相关依赖

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.ljl</groupId>
  <artifactId>SpringMVC_class_interceptor</artifactId>
  <packaging>war</packaging>
  <version>1.0-SNAPSHOT</version>
  <name>SpringMVC_class_interceptor Maven Webapp</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>

    <!--    Spring 基础配置-->
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-context</artifactId>
      <version>5.3.10</version>
    </dependency>
    <!--    SpringMVC工具插件-->
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.3.10</version>
    </dependency>
    <!--    servlet插件-->
    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>javax.servlet-api</artifactId>
      <version>3.1.0</version>
    </dependency>
    <dependency>
      <groupId>javax.servlet.jsp</groupId>
      <artifactId>jsp-api</artifactId>
      <version>2.1</version>
    </dependency>

    <dependency>
      <groupId>mysql</groupId>
      <artifactId>mysql-connector-java</artifactId>
      <version>8.0.32</version>
    </dependency>
    <dependency>
      <groupId>org.mybatis</groupId>
      <artifactId>mybatis</artifactId>
      <version>3.5.11</version>
    </dependency>
    <dependency>
      <groupId>org.junit.jupiter</groupId>
      <artifactId>junit-jupiter</artifactId>
      <version>RELEASE</version>
      <scope>compile</scope>
    </dependency>

    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-core</artifactId>
      <version>2.15.4</version>
    </dependency>

    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>2.15.4</version>
    </dependency>

    <dependency>
      <groupId>commons-fileupload</groupId>
      <artifactId>commons-fileupload</artifactId>
      <version>1.4</version>
    </dependency>
  </dependencies>
  <build>
    <finalName>SpringMVC_class_interceptor</finalName>
  </build>
</project>

3.在Web.xml文件中配置相关依赖

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee
                      http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"
         version="3.1">


  <!--允许访问jpg。 并且必须加在springmvc的servlet之前-->
  <servlet-mapping>
    <servlet-name>default</servlet-name>
    <url-pattern>*.jpg</url-pattern>
  </servlet-mapping>
  <servlet>
    <servlet-name>DispatcherServlet</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>classpath:spring-mvc.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>DispatcherServlet</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>


  <filter>
    <filter-name>chinese_encoder</filter-name>
    <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
    <init-param>
      <param-name>encoding</param-name>
      <param-value>UTF-8</param-value>
    </init-param>
  </filter>
  <filter-mapping>
    <filter-name>chinese_encoder</filter-name>
    <url-pattern>/*</url-pattern>
  </filter-mapping>
</web-app>

4.创建spring-mvc.xml文件，对springmvc进行配置

<?xml version="1.0" encoding="UTF-8" ?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:mvc="http://www.springframework.org/schema/mvc"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/mvc
        https://www.springframework.org/schema/mvc/spring-mvc.xsd">
    <mvc:annotation-driven/>
    <context:component-scan base-package="com.ljl.controller"/>
    <bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping" />
    <bean class="org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter" />
<!--    配置视图解释器-->
    <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/pages/"/>
        <property name="suffix" value=".jsp"/>
    </bean>

    <bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
        <property name="defaultEncoding" value="UTF-8"/>
        <property name="maxUploadSize" value="2097152"/>
    </bean>
    <mvc:default-servlet-handler/>

    <!--  配置拦截器-->
    <mvc:interceptors>
        <bean class="com.ljl.interceptor.LoginInterceptor"/>
    </mvc:interceptors>


</beans>

5.创建pages/login.jsp 和 pages/main.jsp前端界面

login.jsp

<%--
  Created by IntelliJ IDEA.
  User: ljl
  Date: 2024/12/9
  Time: 下午4:24
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>login</title>
</head>
<body>
<form action="login" method="post">
    <div>${msg}</div>
    用户名：<input type="text" name="username"><br>
    密 码:<input type="password" name="password"><br>
    <input type="submit" value="登录">
</form>
</body>
</html>

main.jsp

<%--
  Created by IntelliJ IDEA.
  User: ljl
  Date: 2024/12/9
  Time: 下午4:23
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>main</title>
</head>
<body>
<ul>
    <li>您好:${USER_SESSION.username}</li>
    <li><a href="${pageContext.request.contextPath}/logout">退出</a> </li>
</ul>
</body>
</html>

6.创建Entity/UserEntity实体类

package com.ljl.Entity;

public class User {

    private String username;
    private String password;

    public User() {
    }

    public User(String username, String password) {
        this.username = username;
        this.password = password;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String toString() {
        return "User{" +
                "username='" + username + '\'' +
                ", password='" + password + '\'' +
                '}';
    }
}

7.创建Controller/UserController控制器

package com.ljl.controller;


import com.ljl.Entity.User;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;

import javax.servlet.http.HttpSession;

@Controller
public class UserController {

    @RequestMapping("/main")
    public String toMainPage(){
        return "main";
    }

//    检查是否存在cookies，存在的话直接跳转主页
    @RequestMapping("/tologin")
    public String toLoginPage(HttpSession session){
        if (session.getAttribute("USER_SESSION") != null){
            return "main";
        }
        return "login";
    }

    @RequestMapping("/login")
    public String login(User user, Model model, HttpSession session){
        String username = user.getUsername();
        String password = user.getPassword();
        if (username != null && username.equals("pjl") && password != null && password.equals("123")){
           session.setAttribute("USER_SESSION", user);
           return "main";
        }
        model.addAttribute("msg", "用户名或密码错误！");
        return "login";
    }

    @RequestMapping("/logout")
    public String logout(HttpSession session){
        session.invalidate();
        return "redirect:tologin";
    }
}

8.创建interceptor/LoginInterceptor拦截控制器*******

package com.ljl.interceptor;

import org.springframework.web.servlet.HandlerInterceptor;

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

public class LoginInterceptor implements HandlerInterceptor {

    public boolean preHandle(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Object o) throws Exception {
        String uri = httpServletRequest.getRequestURI();
        if (uri.indexOf("/login") >= 0){
            return true;
        }
        HttpSession hs = httpServletRequest.getSession();
        if (hs.getAttribute("USER_SESSION") != null){
            return true;
        }
        httpServletRequest.setAttribute("msg", "没有登录!");
        httpServletRequest.getRequestDispatcher("/WEB-INF/pages/login.jsp").forward(httpServletRequest, httpServletResponse);
        return false;
    }
}

四、运行测试

1.部署项目，对tomcat进行配置

2.进行账户验证

(http://localhost:8090/SpringMVC_class_interceptor_war_exploded/tologin)

错误账号：ljl 错误密码：123

正确账户：pjl 正确密码：123

3.成功登录后，退出登录，直接访问main页面，拦截回login页面

4.成功登录后，重新访问login页面，系统判断到已登录状态，直接跳转到main页面，显示用户名

五、本Java EE基础完结