一、整合 MyBatisPlus

1. 导入依赖

<dependency>
   <groupId>com.baomidou</groupId>
   <artifactId>mybatis-plus-boot-starter</artifactId>
   <version>3.5.1</version>
</dependency>

2. 配置文件

spring:
  datasource:
    driver-class-name: com.mysql.cj.jdbc.Driver
    url: jdbc:mysql://127.0.0.1:3306/demo?useUnicode=true&characterEncoding=UTF-8
    username: root
    password: root

mybatis-plus:
  mapper-locations: classpath:/mapper/**/*.xml
  typeAliasesPackage: com.cnbai.*.*
  global-config:
    db-config:
      id-type: AUTO
      # 数据库字段驼峰下划线转换
      db-column-underline: true
      refresh-mapper: true
  configuration:
    # 自动驼峰命名
    map-underscore-to-camel-case: true
    # 查询结果中包含空值的列在映射的时候不会映射这个字段
    call-setters-on-nulls: true
    # 开启 sql 日志
    log-impl: org.apache.ibatis.logging.stdout.StdOutImpl
    # 关闭 sql 日志
    # log-impl: org.apache.ibatis.logging.nologging.NoLoggingImpl

3. 配置类

import com.baomidou.mybatisplus.annotation.DbType;
import com.baomidou.mybatisplus.autoconfigure.MybatisPlusProperties;
import com.baomidou.mybatisplus.extension.plugins.MybatisPlusInterceptor;
import com.baomidou.mybatisplus.extension.plugins.inner.PaginationInnerInterceptor;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;

@Configuration
public class MybatisPlusConfig {

    /**
     * 加载分页插件
     */
    @Bean
    public MybatisPlusInterceptor mybatisPlusInterceptor() {
        MybatisPlusInterceptor interceptor = new MybatisPlusInterceptor();
        interceptor.addInnerInterceptor(new PaginationInnerInterceptor(DbType.MYSQL));
        return interceptor;
    }

    @Primary
    @Bean
    @ConfigurationProperties("mybatis-plus")
    public MybatisPlusProperties mybatisPlusProperties() {
        return new MybatisPlusProperties();
    }
}

4. 业务类

public interface UserService extends Iservice<User> {}

@Service
public class UserServiceImpl extends ServiceImpl<UserDao, User> implements UserService {}

@Mapper
public interface UserDao extends BaseMapper<User> {}

二、查询

1. 普通查询

public List<User> queryUserList(String name) {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.eq(User::getName, name);
    return list(wrapper);
}

public User queryUserById(String id) {
    return getBaseMapper().selectById(id);
}

2. 拼接查询

 LambdaQueryWrapper<User> queryWrapper = new LambdaQueryWrapper<>();
 queryWrapper
        .eq(User::getName, "张三")
        .eq(User::getAge, 26);
 List<User> list = list(queryWrapper);

3. 分页查询

// offset 默认从 1 开始展示第一页
public Page<User> queryPage(Integer offset, Integer limit) {
    return userDao.selectPage(PageDTO.of(offset, limit), new QueryWrapper<>());
}

4. 查询部分字段

public List<User> queryUserList() {
    LambdaQueryWrapper<User> wrapper = new LambdaQueryWrapper<>();
    wrapper.select(User::getName, User::getAge);
    return list(wrapper);
}

三、常用条件构造器

1. 单条件

# name = 'bai'
eq("name", "bai")

2. 拼接 AND

# name = ? AND age = ?
.eq("name", "张三").eq("age", 26);

3. 拼接 OR

# id = 1 or name = 'bai'
.eq("id",1).or().eq("name","bai")

4. 嵌套

# or ( name = '李白' and status <> '活着' )
.or ( x -> x.eq("name", "李白").ne("status", "活着") )

# and (  name = '李白' or ( name = '张三' and age = 12 )  )
.and(  x -> x.eq("name", "李白").or( y -> y.eq("name", "张三").eq("age", 12) )  )

四、处理 Json 数据

1. 存储

实体类中某个字段属性是 ListMap 之类的可以转为 Json 格式其在 MySQL 中存储字段类型可以设置为 Json 类型添加注解将此类型映射为 Json 存入数据库中

@TableName(value = "t_user")
public class User {
    @TableId
    private int id;

    @TableField(value = "user_info", typeHandler = JacksonTypeHandler.class)
    private JSONArray userInfo;

    @TableField(value = "info", typeHandler = JacksonTypeHandler.class)
    private JSONObject info;
}

2. 取出

当没有使用到 xml 时

@TableName(value = "t_user", autoResultMap = true)
public class User {
    @TableId
    private int id;

    @TableField(value = "user_info", typeHandler = JacksonTypeHandler.class)
    private JSONArray userInfo;

    @TableField(value = "info", typeHandler = JacksonTypeHandler.class)
    private JSONObject info;
}

当使用了 xml 时

<result property="userInfo" column="user_info" typeHandler="com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler"/>
<result property="info" column="info" typeHandler="com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler"/>

五、循环遍历集合

/**
 * List : User(id=1, name=zhangsan, age=21)
 * List : User(id=2, name=lisi, age=22)
 *
 * Map : User(id=2, name=lisi, age=22)
 * Map : User(id=3, name=wangwu, age=23)
 */
public void queryTest() {
    userService.queryByList();
    userService.queryByMap();
    userService.updateByMap();
    userService.updateByList();
}

1. UserService

public class UserService {

    @Resource
    UserDao userDao;

    public void queryByList() {
        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        userDao.queryByList(list);
    }

    public void queryByMap() {
        Map<String, Object> map = new HashMap<>();
        map.put("name", username);
        userDao.queryByMap(map);
    }

    public void updateByMap() {
        Map<Integer, Integer> map = new HashMap<>();
        map.put(2, 21);
        map.put(3, 31);
        userDao.updateByMap(map);
    }

    public void updateByList() {
        List<User> list = new ArrayList<>();
        list.add(user1);
        list.add(user2);
        userDao.updateByList(list);
    }
}

2. UserDao

public interface UserDao extends BaseMapper<User> {

    // UserMapper.xml 中 collection 的值对应 @Param 里的值
    List<User> queryByList(@Param("list") List<Integer> userList);

    // 此处不能使用 @Param , 或者不用 Map 直接传参 -> queryByMap(@Param("name") String username);
    List<Map<String, Object>> queryByMap(Map<String, Object> map);

    void updateByMap(@Param("map") Map<Integer, Integer> map);
    
    void updateByList(@Param("list") List<User> userList);
}

3. UserMapper

<?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.bai.dao.UserDao">
  
  <resultMap id="userMap" type="com.bai.entity.User">
    <id column="id" jdbcType="INTEGER" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <result column="age" jdbcType="INTEGER" property="age" />
  </resultMap>

  <select id="queryByList" parameterType="java.util.List" resultMap="userMap">
    select * from user where
    <choose>
      <when test="list != null and list.size() > 0">
        id in
        <foreach collection="list" item="id" open="(" separator="," close=")">
      		#{id}
    		</foreach>
      </when>
      <otherwise>
        1 = 2
      </otherwise>
    </choose>
  </select>

  <select id="queryByMap" parameterType="java.util.Map" resultMap="java.util.Map">
    select id, name as username, age from user where
    <if test="name != null and name != ''">
      name = #{name}
    </if>
  </select>

  <update id="updateByMap" parameterType="java.util.Map">
    <foreach collection="map" index="id" item="age" separator=";">
      update user set age = #{age} where id = #{id}
    </foreach>
  </update>
  
  <update id="updateByList" parameterType="java.util.List">
    <foreach collection="list" index="index" item="user">
      update user set age = #{user.age} where id = #{user.id}
    </foreach>
  </update>
</mapper>

4. 增加配置参数

Mybatis 批量更新时需要在 url 后加上 &allowMultiQueries=true
application.yml

spring:
  datasource:
    username: root
    password: root
    url: jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=UTF-8&allowMultiQueries=true
    driver-class-name: com.mysql.jdbc.Driver

否则会报错

org.springframework.jdbc.BadSqlGrammarException: 
### Error updating database.  Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'update user set age = 1 where id = 3' at line 3
### The error may involve com.bai.faker.mapper.UserMapper.updateByMapThree-Inline
### The error occurred while setting parameters
### SQL: update user set age = ? where id = ?      ;        update user set age = ? where id = ?
### Cause: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'update user set age = 1 where id = 3' at line 3
; bad SQL grammar []; nested exception is com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'update user set age = 1 where id = 3' at line 3
... ...