1、配置文件

application.yml

server:
  port: 8081
spring:
  mvc:
    path match:
      matching-strategy: ant_path_matcher
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    username: root
    password: 123456
    url: jdbc:mysql://localhost:3306/management?useUnicode=true&useSSL=false&characterEncoding=utf8&serverTimezone=GMT%2b8
mybatis:
  type-aliases-package: com.xqh.entity
  mapper-locations: classpath:/mapper/*.xml

mybatis的配置：一个是扫描实体类的位置；一个是扫描mapper的位置
出现BUG：Invalid bound statement (not found): com.xqh.demo.mapper.UserMapper.update] with root cause启动报错，原因就是没有设置扫描mapper包，加上 mapper-locations: classpath:/mapper/*.xml就可以了！

2、mapper

UserMapper.java

package com.xqh.mapper;
import com.xqh.entity.User;
import org.apache.ibatis.annotations.*;
import org.springframework.stereotype.Repository;

import java.util.List;
@Mapper
@Repository
public interface UserMapper {
    //查出所有数据
    List<User>findAll();

    int insert(User user);

    int delete(@Param("id") Integer id);

    int update(User user);
}

实现增删改查的接口

3、UserMapper.xml

UserMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xqh.mapper.UserMapper">
    <insert id="insert" parameterType="User">
        INSERT into user(username,password,nickname,email,phone,address) VALUES (#{username},#{password},#{nickname},#{email},#{phone},#{address})
    </insert>

    <delete id="delete" parameterType="int">
        delete from user where id=#{id}
    </delete>

<!--    要写动态sql，因为很多情况是只修改其中几项，而不是全部修改。如果传过来的参数不为空才修改，为空就保持原样-->

    <update id="update" parameterType="User">

        update user
        <set>
            <if test="username!=null">
                username=#{username}
            </if>
            <if test="password!=null">
                password=#{password}
            </if>
            <if test="nickname!=null">
                nickname=#{nickname}
            </if>
            <if test="email!=null">
                email=#{email}
            </if>
            <if test="phone!=null">
                phone=#{phone}
            </if>
            <if test="address!=null">
                address=#{address}
            </if>

        </set>
        <where>
            id=#{id}
        </where>

    </update>
    <select id="findAll" resultType="User">
        select * from user
    </select>

</mapper>

将sql语句写在mapper里（resource下新建mapper包），UserMapper.xml
对于更新操作update，因为我们不是每次都要改所有的参数，有时候只会改其中几个，而另外的则传的是null，所以要实现没改的地方还用原来的，就要用动态sql语句

4、Service层

UserService.java

package com.xqh.service;
import com.xqh.entity.User;
import com.xqh.mapper.UserMapper;
import org.apache.ibatis.annotations.Param;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.List;

@Service
public class UserService {
    @Autowired
    private UserMapper userMapper;

    public List<User>findAll(){
        return userMapper.findAll();
    };

    public int delete(@Param("id") int id){
        return userMapper.delete(id);
    };


    public int save(User user){
        if (user.getId()==null){  //user没有id，则表示是新增，否则为更新
            return userMapper.insert(user);
        }else {
            return userMapper.update(user);
        }
    }
}

更新和添加操作可以在同一个方法里实现，因为都是更新一个user对象，所以只需要用一个if语句，来判断传入的user对象的id是否存在，有id则是原本就有的数据—执行修改操作；如果id为null，则是新的数据，那么执行添加

5、Controller层

UserController.java

package com.xqh.controller;
import com.xqh.entity.User;
import com.xqh.service.UserService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.*;
import java.util.List;

@RestController  //返回json
@RequestMapping("/user")   //加前缀，这下面的所有接口都要加/user前缀
public class UserController {
    @Autowired
    private UserService userService;

    //查询所有
    @GetMapping
    public List<User> index(){
        return userService.findAll();
    }

    //插入和修改操作  ， 需要外界发送数据，用post
    @PostMapping
    public Integer save(@RequestBody User user){
        return userService.save(user);
    }

    //删除
    @DeleteMapping("/{id}")
    public Integer delete(@PathVariable Integer id){
        return userService.delete(id);
    }


}