很多程序员在实体类中看到mybatis-plus的注解（@Data @TableName(resultMap = "c_r") //不配合 typeHandler 或 numericScale 使用无意义）不知道是什么意思，不会使用了，本篇文章，就告诉大家这些注解使用的方法。

在使用这个注解时，主要是实体类的配置和.xml文件的配置相合。如如果指定了@TableName(resultMap = "c_r")这里的resultMap="c_r"如果配置了它，第一需要在实体类的字段中含有typeHandler的使用。

如实体类：

@Data@TableName(resultMap = "c_r") //不配合 typeHandler 或 numericScale 使用无意义,演示而已public class Child {    private Long id;    private String name;    private Long laoHanId;    private Long laoMaId;    @TableField(exist = false,typeHandler = JacksonTypeHandler.class)    private Man laoHan;    @TableField(exist = false)    private Woman laoMa;}

上面实体类配置了@TableName(resultMap = "c_r")，并且字段private Man laoHan;指定了@TableField(exist = false,typeHandler = JacksonTypeHandler.class)，这样resultMap = "c_r"才有效果。

第二，resultMap = "c_r"指的是xml文件的ID值，那么这个xml文件中，首先要有这个结果值，再次就得有对应着的关于@TableField(exist = false,typeHandler = JacksonTypeHandler.class)的配置如示例：

XML文件配置

<mapper namespace="cn.test.mapper.ChildMapper"><!--    </resultMap>-->    <resultMap id="c_r" type="cn.test.entity.Child">        <id property="id" column="id"/>        <result property="name" column="name"/>        <result property="laoHanId" column="lao_han_id"/>        <result property="laoMaId" column="lao_ma_id"/>        <!--laoHan和实体类里面的laoHan对应，lao_han_id传递的参数，          就是child的对应的这一列的参数-->        <association property="laoHan" column="lao_han_id"                     select="cn.test.mapper.ManMapper.selectById"typeHandler="com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler"/>        <!--laoHan和实体类里面的laoMa对应，lao_ma_id传递的参数，                       就是child的对应的这一列的参数-->        <association property="laoMa" column="lao_ma_id"                     select="cn.test.mapper.WomanMapper.selectById"/>    </resultMap>    <select id="selectLinkById" resultMap="c_r">        select *        from child        where id = #{id}    </select></mapper>

综合上面两个联合起来配置实体类上面的@TableName(resultMap = "c_r")才有真正的意义。

下面是全部代码包括了多表查询的配置：

mapper:

public interface ChildMapper extends BaseMapper<Child> {//    @Select("select * from child where id = #{id}")    Child selectLinkById(Long id);    @Select("select * from child where lao_han_id = #{id}")    List<Child> selectByLaoHanId(Long id);    @Select("select * from child where lao_ma_id = #{id}")    List<Child> selectByLaoMaId(Long id);    @Select({            "<script>" +                    "select * from child " +                    "where 1=1" +                    "<if test=\"id != null\">" +                    "and id = #{id} " +                    "</if>" +                    "</script>"})    Child selectOneChile(Long id);    @Select("select * from child where id = #{id}")    Child getChildOne(Long id);}

package cn.test.entity;import com.baomidou.mybatisplus.annotation.TableField;import com.baomidou.mybatisplus.annotation.TableName;import com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler;import lombok.Data;/** * @author miemie * @since 2019-11-27 */@Data@TableName(resultMap = "c_r") //不配合 typeHandler 或 numericScale 使用无意义,演示而已public class Child {    private Long id;    private String name;    private Long laoHanId;    private Long laoMaId;    @TableField(exist = false,typeHandler = JacksonTypeHandler.class)    private Man laoHan;    @TableField(exist = false)    private Woman laoMa;}

<mapper namespace="cn.test.mapper.ChildMapper"><!--    </resultMap>-->    <resultMap id="c_r" type="cn.test.entity.Child">        <id property="id" column="id"/>        <result property="name" column="name"/>        <result property="laoHanId" column="lao_han_id"/>        <result property="laoMaId" column="lao_ma_id"/>        <!--laoHan和实体类里面的laoHan对应，lao_han_id传递的参数，          就是child的对应的这一列的参数-->        <association property="laoHan" column="lao_han_id"                     select="cn.test.mapper.ManMapper.selectById"  typeHandler="com.baomidou.mybatisplus.extension.handlers.JacksonTypeHandler"/>        <!--laoHan和实体类里面的laoMa对应，lao_ma_id传递的参数，就是child的对应的这一列的参数-->        <association property="laoMa" column="lao_ma_id"                     select="cn.test.mapper.WomanMapper.selectById"/>    </resultMap>    <select id="selectLinkById" resultMap="c_r">        select *        from child        where id = #{id}    </select></mapper>

public interface ChildService {List<Child> selectByLaoHanId(Long id);Child selectLinkById();Child getChildOne();Child selectOneChile();}

@Service@Transactionalpublic class ChildServiceImp extends ServiceImpl‘<ChildMapper, Child>  implements ChildService {    @Override    public List<Child> selectByLaoHanId(Long id) {        return baseMapper.selectByLaoHanId(id);    }    @Override    public Child selectLinkById() {        return baseMapper.selectLinkById(1l);    }    @Override    public Child getChildOne() {        return baseMapper.getChildOne(1l);    }    @Override    public Child selectOneChile() {        return baseMapper.selectOneChile(6L);    }}

@RestController@RequestMapping("child")public class ChildController {    @Autowired    private ChildService childService;    @GetMapping("/selectByLaoHanId")    public List<Child> selectByLaoHanId(){       return childService.selectByLaoHanId(1l);    }    @GetMapping("/selectLinkById")    public Child selectLinkById(){        return childService.selectLinkById();    }    @GetMapping("/getChildOne")    public Child getChildOne(){        return childService.getChildOne();    }    @GetMapping("/selectOneChile")    public Child selectOneChile(){        return childService.selectOneChile();    }}

@MapperScan("cn.test.mapper")@SpringBootApplicationpublic class MyApplicationStart {    public static void main(String[] args){        SpringApplication.run(MyApplicationStart.class,args);    }}

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