ResultType和ResultMap（day37）

ops/2024/10/18 17:56:11/

学习目标

  1. 重点掌握ResultMap的作用

  2. 重点掌握ResultMap的使用方式

  3. 重点掌握ResultMap的级联属性

2 resultType和resultMap

2.1 resultType--自动映射

情况一 : 如果查询结果是一行数据: 例如根据id查询某个职位信息

使用POJO封装一行数据

  • 接口中的方法的返回值就是用于封装结果集的POJO类型
java">public Jobs getJobsById(String jobId);
  • select标签中的resultType属性的值是封装结果集的POJO的全限定名
<select id="getJobsById" resultType="cn.tedu.pojo.Jobs">SELECT job_id, job_title, min_salary, max_salary FROM jobs WHERE job_id = #{jobId}
</select>

情况二 : 如果查询结果为多行数据: 例如查询所有职位信息 使用List<POJO>封装多行数据

  • 接口中的方法的返回值是List<POJO>类型
java">public List<Jobs> getJobsAll();
  • select标签的resultType属性的值是POJO的全限定名
<select id="getJobsAll" resultType="cn.tedu.pojo.Jobs">SELECT job_id, job_title, min_salary, max_salary FROM jobs
</select>

2.2 resultMap--手动映射

  • 使用mybatis，有两个属性标签<resultType>、<resultMap>可以提供结果映射

  • resultMap标签: 是用于定义javaBean和数据库表的映射规则,建议只要定义resultMap,就将整表的全部列的映射全部写上

    • id属性: 给resultMap的映射关系自定义一个名称，是唯一值
    • type属性: 用于指定需要自定义规则的Java类型,其中如果设置了包扫描,就可以直接写类名即可

  • id标签: 指定主键列的封装规则,也可以使用result标签定义,但是对主键的特殊照顾,底层会有优化

    • column属性: 映射主键列
    • property属性: 映射主键属性

  • result标签: 指定普通列的封装规则

    • column属性: 映射非主键列
    • property属性: 映射非主键属性

3 入门案例

3.1 前期准备

①在JSDSecondStage项目下,创建ResultMapDemo,将版本设置为2.5.4

②在pom.xml中添加相关的依赖

<!--mysql数据库驱动依赖-->
<dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><scope>runtime</scope>
</dependency>
<!--引入相关mybatis依赖-->
<dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.2.0</version>
</dependency>

③在模块下的src/main/java/cn/tedu包中,将pojo包导入进去,该包下包含两个类

  • Locations hr库中locations表的javaBean类
  • Countries hr库中countries表的javaBean类

④在模块下的src/main/java/cn/tedu包中,将mapper包导入进去,该包下包含两个接口

  • LocationsMapper 用于定义映射locations表的接口
  • CountriesMapper 用于定义映射countries表的接口

⑤在模块下的src/main/resources目录中,将mapper文件夹导入进去,该包下包含两个SQL文件

  • LocationsMapper.xml 用于写操作locations表的SQL语句
  • CountriesMapper.xml 用于写操作countries表的SQL语句

⑥配置文件application.yml内容

#数据库链接
spring:datasource:url: jdbc:mysql://localhost:3306/hr?serverTimezone=Asia/Shanghai&characterEncoding=utf8username: rootpassword: root
#MyBatis开启驼峰映射,并且扫描xml文件
mybatis:configuration:map-underscore-to-camel-case: truemapper-locations: classpath:/mapper/*.xml#开启日志设置
logging:level:cn:tedu: debug

3.2 查询locations表中的记录

①**LocationsMapper**

java">package cn.tedu.mapper;import cn.tedu.pojo.Locations;
import org.apache.ibatis.annotations.Mapper;//指定这是一个操作数据库的mapper
@Mapper
public interface LocationsMapper {public Locations getLocationById(Integer locationId);
}

②**LocationsMapper.xml**

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tedu.mapper.LocationsMapper"><select id="getLocationById" resultType="cn.tedu.pojo.Locations">SELECT * FROM locations WHERE location_id = #{locationId}</select>
</mapper>

③**TestResultMap**

java">package cn.tedu;import cn.tedu.mapper.LocationsMapper;
import cn.tedu.pojo.Locations;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;@SpringBootTest
public class TestResultMap {@Autowiredprivate LocationsMapper locationsMapper;@Testpublic void testGetLocationById() {Locations location = locationsMapper.getLocationById(1000);System.out.println(location);}
}

3.3 使用resultMap

  • resultType配置之后,开启自动映射,我们需要在application.yml中开启驼峰映射,实现自动匹配
mybatis:configuration:map-underscore-to-camel-case: true
  • 而resultMap可以手动的将属性和表字段匹配

①**LocationsMapper.xml**

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tedu.mapper.LocationsMapper"><select id="getLocationById" resultType="cn.tedu.pojo.Locations">SELECT * FROM locations WHERE location_id = #{locationId}</select><!--resultMap自定义javaBean映射规则type: 自定义规则的Java类型id:唯一id,方便引用建议只要定义resultMap,就将整表的全部列的映射全部写上--><resultMap id="location" type="cn.tedu.pojo.Locations"><!--指定主键列的封装规则id定义主键,底层会有优化column:指定数据库表的哪一列property:指定javaBean的哪一属性--><id column="location_id" property="locationId"/><!--指定普通列的封装规则--><result column="street_address" property="streetAddress"/><result column="postal_code" property="postalCode"/><result column="city" property="city"/><result column="state_province" property="stateProvince"/><result column="country_id" property="countryId"/></resultMap>
</mapper>

②**application.yml**

spring:datasource:url: jdbc:mysql://localhost:3306/hr?serverTimezone=Asia/Shanghai&characterEncoding=utf8username: rootpassword: root
mybatis:
#  configuration:
#    map-underscore-to-camel-case: truemapper-locations: classpath:/mapper/*.xmllogging:level:cn:tedu: debug

3.4 优化

3.4.1 开启包扫描

  • resultType在定义时,每次总要写全对应的javaBean的全路径,很麻烦,所以可以在配置文件中添加如下的配置,开启包扫描
mybatis:#指定entity扫描包类让mybatis自定扫描到自定义的包路径,这样在mapper.xml中就直接写类名即可type-aliases-package: cn.tedu.pojo
  • 那么MyBatis会自动扫描该包下的javaBean,这样我们就直接写类名即可
<select id="getLocationById" resultType="Locations">SELECT * FROM locations WHERE location_id = #{locationId}
</select>

3.4.2 resultMap开启自动映射

  • resultMap是可以自动映射和手动映射兼容的
  • 在resultMap标签中使用autoMapping属性,如果为true就表示开启自动映射
  • 但是要是开启自动映射,就需要添加驼峰规则配置

①**LocationsMapper.xml**

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tedu.mapper.LocationsMapper"><select id="getLocationById" resultMap="location">SELECT * FROM locations WHERE location_id = #{locationId}</select><!--resultMap自定义javaBean映射规则type: 自定义规则的Java类型id:唯一id,方便引用建议只要定义resultMap,就将整表的全部列的映射全部写上--><resultMap id="location" type="Locations" autoMapping="true"></resultMap>
</mapper>

②**application.yml**

spring:datasource:url: jdbc:mysql://localhost:3306/hr?serverTimezone=Asia/Shanghai&characterEncoding=utf8username: rootpassword: root
mybatis:configuration:map-underscore-to-camel-case: truemapper-locations: classpath:/mapper/*.xmltype-aliases-package: cn.tedu.pojo
logging:level:cn:tedu: debug

4 resultMap的级联用法

4.1 国家表和地点表的关系

  • countries表就是国家表,用于收集国家和区域的信息
  • locations表就是地点表,用于收集地点相关信息
  • 这两张表具有以下关系:
    • 在国家表的角度: 一个国家对应多个地点,也就是一对多的关系
    • 在地点表的角度: 一个地点对应一个国家,也就是一对一的关系

4.1 一对一查询

  • 查询locations表记录的同时,将对应的countries表中的内容查询出来,SQL如下:
SELECT l.location_id,l.street_address,l.postal_code,l.city,l.state_province,l.country_id lcid,c.country_id cid,c.country_name,c.region_id
FROM locations l,countries c
WHERE l.country_id = c.country_idAND l.location_id = 2000
  • 在这条SQL中,使用了多表关联查询,并且表字段也起了别名

①Locations

java">package cn.tedu.pojo;public class Locations {private Integer locationId;private String streetAddress;private String postalCode;private String city;private String stateProvince;private String countryId;@Overridepublic String toString() {return "Locations{" +"locationId=" + locationId +", streetAddress='" + streetAddress + '\'' +", postalCode='" + postalCode + '\'' +", city='" + city + '\'' +", stateProvince='" + stateProvince + '\'' +", countryId='" + countryId + '\'' +", countries=" + countries +'}';}public Countries getCountries() {return countries;}public void setCountries(Countries countries) {this.countries = countries;}private Countries countries;public Integer getLocationId() {return locationId;}public void setLocationId(Integer locationId) {this.locationId = locationId;}public String getStreetAddress() {return streetAddress;}public void setStreetAddress(String streetAddress) {this.streetAddress = streetAddress;}public String getPostalCode() {return postalCode;}public void setPostalCode(String postalCode) {this.postalCode = postalCode;}public String getCity() {return city;}public void setCity(String city) {this.city = city;}public String getStateProvince() {return stateProvince;}public void setStateProvince(String stateProvince) {this.stateProvince = stateProvince;}public String getCountryId() {return countryId;}public void setCountryId(String countryId) {this.countryId = countryId;}}

②**LocationsMapper接口**

java">package cn.tedu.mapper;import cn.tedu.pojo.Locations;
import org.apache.ibatis.annotations.Mapper;//指定这是一个操作数据库的mapper
@Mapper
public interface LocationsMapper {public Locations getLocationById(Integer locationId);
}

③**LocationsMapper.xml**

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tedu.mapper.LocationsMapper"><select id="getLocationById" resultMap="location">SELECT * FROM locations WHERE location_id = #{locationId}</select><select id="getLocationCountryById" resultMap="location2">SELECT l.location_id,l.street_address,l.postal_code,l.city,l.state_province,l.country_id lcid,c.country_id cid,c.country_name,c.region_idFROM locations l,countries cWHERE l.country_id = c.country_idAND l.location_id = #{locationId}</select><!--resultMap自定义javaBean映射规则type: 自定义规则的Java类型id:唯一id,方便引用建议只要定义resultMap,就将整表的全部列的映射全部写上--><resultMap id="location" type="Locations" autoMapping="true"><!--指定主键列的封装规则id定义主键,底层会有优化column:指定数据库表的哪一列property:指定javaBean的哪一属性--><id column="location_id" property="locationId"/><!--指定普通列的封装规则--><result column="street_address" property="streetAddress"/><result column="postal_code" property="postalCode"/><result column="city" property="city"/><result column="state_province" property="stateProvince"/><result column="country_id" property="countryId"/></resultMap><!--联合查询: 使用级联属性封装结果集--><resultMap id="location2" type="Locations"><id column="location_id" property="locationId"/><!--指定普通列的封装规则--><result column="street_address" property="streetAddress"/><result column="postal_code" property="postalCode"/><result column="city" property="city"/><result column="state_province" property="stateProvince"/><!--此处对应的是locations表中的country_id,但是由于SQL中使用了别名,所以使用别名作为column的值--><result column="lcid" property="countryId"/><!--此处对应的是countries表中的country_id,但是由于SQL中使用了别名,所以使用别名作为column的值,封装到countries属性中的countryId中--><result column="cid" property="countries.countryId"/><result column="country_name" property="countries.countryName"/><result column="region_id" property="countries.regionId"/></resultMap>
</mapper>

④**TestResultMap**

java">@SpringBootTest
public class TestResultMap {@Autowiredprivate LocationsMapper locationsMapper;@Autowiredprivate CountriesMapper countriesMapper;@Testpublic void testGetLocationById() {Locations location = locationsMapper.getLocationById(1000);System.out.println(location);}@Testpublic void testGetLocationCountryById() {Locations location = locationsMapper.getLocationCountryById(2000);System.out.println(location);System.out.println(location.getCountries());}
}

4.2 association定义一对一关系映射

  • 上述的案例中,如果像类似于<result column="cid" property="countries.countryId"/>,这样的字段比较多的情况,每次关联属性都要写countries的级联属性的方式封装结果集,会比较繁琐,重复字段较多,其次,看不出关联关系,所以可以使用association标签进行封装

  • association标签: 可以指定联合的javaBean对象

    • property属性: 指定哪个属性是联合的对象
    • javaType属性: 指定这个属性对象的类型[不能省略]

①**LocationsMapper.xml**

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tedu.mapper.LocationsMapper"><select id="getLocationById" resultMap="location">SELECT * FROM locations WHERE location_id = #{locationId}</select><select id="getLocationCountryById" resultMap="location3">SELECT l.location_id,l.street_address,l.postal_code,l.city,l.state_province,l.country_id lcid,c.country_id cid,c.country_name,c.region_idFROM locations l,countries cWHERE l.country_id = c.country_idAND l.location_id = #{locationId}</select><!--resultMap自定义javaBean映射规则type: 自定义规则的Java类型id:唯一id,方便引用建议只要定义resultMap,就将整表的全部列的映射全部写上--><resultMap id="location" type="Locations" autoMapping="true"><!--指定主键列的封装规则id定义主键,底层会有优化column:指定数据库表的哪一列property:指定javaBean的哪一属性--><id column="location_id" property="locationId"/><!--指定普通列的封装规则--><result column="street_address" property="streetAddress"/><result column="postal_code" property="postalCode"/><result column="city" property="city"/><result column="state_province" property="stateProvince"/><result column="country_id" property="countryId"/></resultMap><!--联合查询: 使用级联属性封装结果集--><resultMap id="location2" type="Locations" autoMapping="true"><id column="location_id" property="locationId"/><!--指定普通列的封装规则--><result column="street_address" property="streetAddress"/><result column="postal_code" property="postalCode"/><result column="city" property="city"/><result column="state_province" property="stateProvince"/><!--此处对应的是locations表中的country_id,但是由于SQL中使用了别名,所以使用别名作为column的值--><result column="lcid" property="countryId"/><!--此处对应的是countries表中的country_id,但是由于SQL中使用了别名,所以使用别名作为column的值,封装到countries属性中的countryId中--><result column="cid" property="countries.countryId"/><result column="country_name" property="countries.countryName"/><result column="region_id" property="countries.regionId"/></resultMap><!--association属性定义单个对象封装规则--><resultMap id="location3" type="Locations"><id column="location_id" property="locationId"/><result column="street_address" property="streetAddress"/><result column="postal_code" property="postalCode"/><result column="city" property="city"/><result column="state_province" property="stateProvince"/><result column="lcid" property="countryId"/><!--association可以指定联合的javaBean对象property="countries" 指定哪个属性是联合的对象javaType="Countries" 指定这个属性对象的类型[不能省略]--><association property="countries" javaType="Countries" autoMapping="true"><id column="cid" property="countryId"/><result column="country_name" property="countryName"/><result column="region_id" property="regionId"/></association></resultMap>
</mapper>

4.3 collection定义一对多关系映射

  • 查询指定的国家中包含的地点信息,SQL如下:
SELECT c.country_id,c.country_name,l.location_id,l.street_address,l.city
FROM countries c LEFT JOIN locations lON c.country_id = l.country_id
WHERE c.country_id = 'US';
  • collection标签: 可以指定联合的集合类型的属性
    • property属性: 指定哪个属性是联合的对象
    • ofType属性: 指定集合的元素类型[不能省略]

①**Countries**

java">package cn.tedu.pojo;import java.util.List;public class Countries {private String countryId;private String countryName;private Double regionId;private List<Locations> locations;public List<Locations> getLocations() {return locations;}public void setLocations(List<Locations> locations) {this.locations = locations;}public String getCountryId() {return countryId;}public void setCountryId(String countryId) {this.countryId = countryId;}public String getCountryName() {return countryName;}public void setCountryName(String countryName) {this.countryName = countryName;}public Double getRegionId() {return regionId;}public void setRegionId(Double regionId) {this.regionId = regionId;}@Overridepublic String toString() {return "Countries{" +"countryId='" + countryId + '\'' +", countryName='" + countryName + '\'' +", regionId=" + regionId +", locations=" + locations +'}';}
}

②**CountriesMapper接口**

java">package cn.tedu.mapper;import cn.tedu.pojo.Countries;
import org.apache.ibatis.annotations.Mapper;//指定这是一个操作数据库的mapper
@Mapper
public interface CountriesMapper {public Countries getCountryByIdPlus(String countryId);
}

③**CountriesMapper.xml**

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapperPUBLIC "-//mybatis.org//DTD Mapper 3.0//EN""http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.tedu.mapper.CountriesMapper"><select id="getCountryByIdPlus" resultMap="country">SELECT c.country_id,c.country_name,l.location_id,l.street_address,l.cityFROM countries cLEFT JOIN locations lON c.country_id = l.country_idWHERE c.country_id = #{countryId}</select><resultMap id="country" type="Countries"><id column="country_id" property="countryId"/><result column="country_name" property="countryName"/><!--定义关联集合类型的属性的封装规则ofType 指定集合的元素类型--><collection property="locations" ofType="Locations"><!--定义集合中的元素的封装规则--><id column="location_id" property="locationId"/><result column="street_address" property="streetAddress"/><result column="city" property="city"/></collection></resultMap>
</mapper>

④**TestResultMap**

java">@SpringBootTest
public class TestResultMap {@Autowiredprivate LocationsMapper locationsMapper;@Autowiredprivate CountriesMapper countriesMapper;@Testpublic void testGetLocationById() {Locations location = locationsMapper.getLocationById(1000);System.out.println(location);}@Testpublic void testGetLocationCountryById() {Locations location = locationsMapper.getLocationCountryById(2000);System.out.println(location);System.out.println(location.getCountries());}@Testpublic void testGetCountryByIdPlus() {Countries country = countriesMapper.getCountryByIdPlus("US");System.out.println(country);List<Locations> locations = country.getLocations();for (Locations location : locations) {System.out.println(location);}}
}

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