一、Spring Boot原理

用户从页面前端，也就是我们所说的 view 层进行查询访问，进入到 controller 层找到对应的接口，接 着 controller 进行对 service 层进行业务功能的调用，service 要进入 dao 层查询数据，dao 层调用 mapper.xml 文件生成 sql 语句到数据库中进行查询

二、实现过程

2.1、准备数据库user表插入四条数据

2.2、model下创建一个User类

与数据库的字段一一对应

@Getter @Setter
public class User {
    private int id;
    private String username;
    private String password;
    private int age;
}

2.3、dao下创建一个UserDao接口

@Repository注解修饰哪个类，则表明这个类具有对对象进行CRUD（增删改查）的功能

@Repository
public interface UserDao {
    public User getUserById(@Param("id") int id);
    public List<User> getUserByAge(@Param("age") int age);
    public List<User> getUserByName(@Param("username") String username);
    public int insertUser(@RequestBody User user);

}

通过UserMapping.xml配置文件实现UserDao接口

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.example.demo.dao.UserDao">
    <select id="getUserById" resultType="User">
        select * from `user` where id=#{id}
    </select>
    <select id="getUserByAge" resultType="User">
        select * from `user` where age=#{age}
    </select>
    <select id="getUserByName" resultType="User">
        select * from `user` where username like concat('%',#{username},'%')
    </select>

    <insert id="insertUser" parameterType="User">
        <selectKey keyProperty="id" order="AFTER" resultType="int">
            SELECT LAST_INSERT_ID()
        </selectKey>
        insert into user (username, password, age)
        values (#{username,jdbcType=VARCHAR}, #{password,jdbcType=VARCHAR}, #{age,jdbcType=INTEGER})
    </insert>
</mapper>

2.4、Service下创建一个UserService接口

public interface UserService {
    public User queryUserById(int id);
    public List<User> queryUserByAge(int age);
    public List<User> queryUserByName(String username);
    public int insertUser(User user);
}

再创建一个UserServiceImpl实现UserService

@Service
public class UserSeviceImpl implements UserService {
    @Autowired
    private UserDao userDao;

    @Override
    public User queryUserById(int id) {
        return userDao.getUserById(id);
    }

    @Override
    public List<User> queryUserByAge(int age) {
        return userDao.getUserByAge(age);
    }

    @Override
    public List<User> queryUserByName(String username) {
        return userDao.getUserByName(username);
    }

    @Override
    public int insertUser(User user) {
        return userDao.insertUser(user);
    }
}

2.5、controller下创建一个UserController

@GetMapping是代表该查询接口用的是get方式
@RequestMapping(value="/insert",method = RequestMethod.POST)代表该插入接口用post方式

@RestController
public class UserController {

    @Autowired
    private UserService userService;

    @GetMapping("/userid")
    public User getUserById(@RequestParam("id") int id){
        return userService.queryUserById(id);
    }

    @GetMapping("/username")
    public List<User> getUserByUsername(@RequestParam("username") String username){
        return userService.queryUserByName(username);
    }

    @RequestMapping(value="/insert",method = RequestMethod.POST)
    public User insertUser(@RequestBody User user){
        int x = userService.insertUser(user);
        return user;
    }
}

2.6、application.yml文件连接数据库

server:
  port: 8080
spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://localhost:3306/testingdev9?useUnicode=true&characterEncoding=utf-8&serverTimezone=Asia/Shanghai
    username: root
    password: 123456
mybatis:
  mapper-locations:
    - classpath:mapper/*.xml
    - classpath*:com/**/mapper/*.xml
  type-aliases-package: com.example.demo.model

2.7、启动DemoApplication文件

出现Started DemoApplication代表启动成功了

2.8、使用postman调用接口

如下图根据id查询

如下图插入数据

可以看到数据库多了一条数据