MyBatisPlus中JSON处理器的实现

CREATE TABLE orders (
    id INT PRIMARY KEY AUTO_INCREMENT,
    user_id INT,
    extra JSON  -- JSON 类型字段
);

INSERT INTO orders (user_id, extra) VALUES (1, '{"color": "red", "size": "XL"}');

-- -> 返回 JSON 格式，->> 返回纯文本
SELECT extra->'$.color' FROM orders;      -- 返回 "red"（带引号）
SELECT extra->>'$.color' FROM orders;     -- 返回 red（不带引号）

UPDATE orders SET extra = JSON_SET(extra, '$.color', 'blue') WHERE id = 1;

SELECT * FROM orders WHERE extra->>'$.color' = 'red';

select * from order where JSON_CONTAINS(extra, '"red"', '$.color');

JSON_SET(json字段, 路径, 新值, 路径, 新值, ...)

{"color": "red", "size": "XL"}

UPDATE orders SET extra = JSON_SET(extra, '$.color', 'blue');
-- 结果：{"color": "blue", "size": "XL"}

UPDATE orders SET extra = JSON_SET(extra, '$.price', 99);
-- 结果：{"color": "red", "size": "XL", "price": 99}

UPDATE orders SET extra = JSON_SET(extra, '$.color', 'blue', '$.size', 'M');
-- 结果：{"color": "blue", "size": "M"}

@Data
@NoArgsConstructor
@AllArgsConstructor(staticName = "of")
public class JsonInfo {

    private String color;

    private String size;

}

数据库查询结果 (ResultSet)
         ↓
    ResultMap (映射规则)
         ↓
   Java 对象 (POJO/Entity)

// 实体类
@TableName(value = "users", autoResultMap = true)
public class User {
    @TableId
    private Long id;
    
    // JSON 字段，需要类型转换
    @TableField(typeHandler = JacksonTypeHandler.class)
    private UserInfo info;
    
    @TableField(typeHandler = JacksonTypeHandler.class)
    private List<String> tags;
}

# application.yml 配置
mybatis-plus:
  type-handlers-package: com.example.demo.handler
  type-aliases-package: com.example.demo.entity

@Data
@TableName(value = "tb_user", autoResultMap = true)  // 必须开启！
public class User {
    private Long id;
    private String name;

    // JSON 字段 → Java List
    @TableField(typeHandler = JacksonTypeHandler.class)
    private List<Role> roles;

    // JSON 字段 → Java Map
    @TableField(typeHandler = JacksonTypeHandler.class)
    private Map<String, Object> extra;
}

// 自定义类型处理器
public class EncryptTypeHandler extends BaseTypeHandler<String> {
    @Override
    public void setParameter(PreparedStatement ps, int i, String parameter, JdbcType jdbcType) {
        ps.setString(i, encrypt(parameter));
    }
    
    @Override
    public String getNullableResult(ResultSet rs, String columnName) {
        return decrypt(rs.getString(columnName));
    }
    // ... 其他方法
}

// 实体类
@TableName(value = "users", autoResultMap = true)
public class User {
    @TableField(typeHandler = EncryptTypeHandler.class)
    private String phone;  // 数据库中加密存储
}

// 实体类
@TableName(value = "articles", autoResultMap = true)
@Data
public class Article {
    @TableId
    private Long id;
    
    private String title;
    
    // JSON 字段存储为对象
    @TableField(typeHandler = JacksonTypeHandler.class)
    private ArticleContent content;
    
    // JSON 字段存储为列表
    @TableField(typeHandler = JacksonTypeHandler.class)
    private List<String> tags;
}

// JSON 对象
@Data
public class ArticleContent {
    private String text;
    private String coverImage;
    private Integer wordCount;
}

// Mapper 接口
@Mapper
public interface ArticleMapper extends BaseMapper<Article> {
    // 自定义查询也需要 ResultMap 支持
    @Select("SELECT * FROM articles WHERE id = #{id}")
    Article selectDetail(Long id);
}

@Mapper
public interface UserMapper extends BaseMapper<User> {

    @ResultMap("mybatis-plus_User")  // 引用自动生成的 ResultMap
    @Select("SELECT * FROM tb_user WHERE age > #{age}")
    List<User> selectByAge(@Param("age") Integer age);
}

autoResultMap = true
       ↓
让 MyBatis-Plus 自动生成 ResultMap
       ↓
使 @TableField(typeHandler = ...) 生效
       ↓
实现 数据库特殊类型 ↔ Java 对象 的转换