后端技术：Mybatis中resultMap用法示例笔记

1、概念
resultMap属于mybatis返回操作结果的一个标签，可以用来映射select查询出来结果的集合，主要作用是将实体类中的字段与数据库表中的字段进行关联映射。并且支持复杂的返回结果类型。
2、使用场景
2.1 属性映射
当数据库字段和项目中的实体属性不一致时，可以使resultMap进行数据库字段和实体类属性的映射关系
比如：

 column="id" jdbcType="integer" property="id" />    
     column="user_name" jdbcType="VARCHAR" property="userName" />

说明：如果数据库字段和实体类属性一致的情况下，可以省略不写。
2.2 实现Java复杂实体类用法
a.初始化脚本

DROP TABLE IF EXISTS `t_user`;
CREATE TABLE `t_user` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `user_name` varchar(255) CHARACTER SET armscii8 DEFAULT NULL,
  `password` varchar(255) CHARACTER SET armscii8 DEFAULT NULL,
  `last_login_time` datetime DEFAULT NULL,
  `sex` tinyint(4) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;


-- ----------------------------
-- Records of t_user
-- ----------------------------
INSERT INTO `t_user` VALUES ('1', 'xiaoxin', '123', '2019-07-27 16:01:21', '1');
INSERT INTO `t_user` VALUES ('2', 'jack jo', '123', '2019-07-24 16:01:37', '1');
INSERT INTO `t_user` VALUES ('4', 'landengdeng', '123', '2019-07-24 16:01:37', '1');
INSERT INTO `t_user` VALUES ('5', 'max', '123', '2019-07-24 16:01:37', '1');
INSERT INTO `t_user` VALUES ('6', 'liua11', '123456', null, '1');
INSERT INTO `t_user` VALUES ('7', 'xiaozhang', '888888', null, '1');
DROP TABLE IF EXISTS `t_hobby`;
CREATE TABLE `t_hobby` (
  `id` int(11) NOT NULL,
  `hobbyName` varchar(50) CHARACTER SET utf8 DEFAULT NULL,
  `userId` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;


-- ----------------------------
-- Records of t_hobby
-- ----------------------------
INSERT INTO `t_hobby` VALUES ('0', '音乐', '2');
INSERT INTO `t_hobby` VALUES ('1', '篮球', '1');
INSERT INTO `t_hobby` VALUES ('2', '读书', '1');

b.定义实体类
定义实体 UserDO.java

public class UserDO {
private Integer id;
private String userName;
private String password;
private Integer sex;
private Date lastLoginTime;
public Integer getId() {
return id;
    }
public void setId(Integer id) {
this.id = id;
    }
public String getUserName() {
return userName;
    }
public void setUserName(String userName) {
this.userName = userName;
    }
public String getPassword() {
return password;
    }
public void setPassword(String password) {
this.password = password;
    }
public Integer getSex() {
return sex;
    }
public void setSex(Integer sex) {
this.sex = sex;
    }
public Date getLastLoginTime() {
return lastLoginTime;
    }
public void setLastLoginTime(Date lastLoginTime) {
this.lastLoginTime = lastLoginTime;
    }
@Override
    public String toString() {
return "UserDO{" +
"id=" + id +
", userName='" + userName + '\'' +
", password='" + password + '\'' +
", sex=" + sex +
", lastLoginTime=" + lastLoginTime +
'}';
    }​

定义实体类 HobbyDO.java

public class HobbyDO {
private Integer id;
private String hobbyName;
private Integer userId;
public Integer getId() {
return id;
    }
public void setId(Integer id) {
this.id = id;
    }
public String getHobbyName() {
return hobbyName;
    }
public void setHobbyName(String hobbyName) {
this.hobbyName = hobbyName;
    }
public Integer getUserId() {
return userId;
    }
public void setUserId(Integer userId) {
this.userId = userId;
    }
@Override
    public String toString() {
return "HobbyDO{" +
"id=" + id +
", hobbyName='" + hobbyName + '\'' +
", userId=" + userId +
'}';
    }​

c.定义model类
定义类 HobbyVO.java  用来演示一对一

public class HobbyVo extends HobbyDO {
private UserDO user ;
public UserDO getUser() {
return user;
    }
public void setUser(UserDO user) {
this.user = user;
    }
}
定义类 UserVO.java 用来演示1对多
public class UserVO extends UserDO {
private Listlist;
public ListgetList() {
return list;
    }
public void setList(Listlist) {
this.list = list;
    }
}

d.定义mapper类文件
定义UserInfoMapper.java文件

package my.springboot.mybatis.dao;
import java.util.List;
import java.util.Map;
import my.springboot.mybatis.entity.HobbyDO;
import my.springboot.mybatis.entity.UserDO;
import my.springboot.mybatis.model.UserVO;
import org.apache.ibatis.annotations.Mapper;
@Mapper
public interface UserInfoMapper {
    UserDO get(Integer id);
    List getUserVOMap(Integer id);
    List getHobbyByUserId(Integer userId);
}

定义HobbyMapper.java文件

package my.springboot.mybatis.dao;
import my.springboot.mybatis.entity.HobbyDO;
import my.springboot.mybatis.entity.UserDO;
import my.springboot.mybatis.model.HobbyVo;
import org.apache.ibatis.annotations.Mapper;
import java.util.List;
import java.util.Map;
@Mapper
public interface HobbyMapper {
    List getHobbyByUserId(Integer userId);
    List getHobbyVOMap();
}

e.定义service服务
定义IUserInfoService.java

package my.springboot.mybatis.service;
import my.springboot.mybatis.entity.UserDO;
import my.springboot.mybatis.model.HobbyVo;
import my.springboot.mybatis.model.UserVO;
import java.util.List;
public interface IUserInfoService { 
    List getUserVOMap(Integer userId);
    List getHobbyMap();
}

定义UserInfoService.java

package my.springboot.mybatis.service.impl;
import my.springboot.mybatis.dao.HobbyMapper;
import my.springboot.mybatis.dao.UserInfoMapper;
import my.springboot.mybatis.entity.UserDO;
import my.springboot.mybatis.model.HobbyVo;
import my.springboot.mybatis.model.UserVO;
import my.springboot.mybatis.service.IUserInfoService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.List;
@Service
public class UserInfoService implements IUserInfoService {
@Autowired
    private UserInfoMapper mapper;
@Autowired
    private HobbyMapper hobbyMapper;
@Override
    public  List getUserVOMap(Integer id)
    {
return this.mapper.getUserVOMap(id);
    }
@Override
    public ListgetHobbyMap() {
return hobbyMapper.getHobbyVOMap();
    }
}

f.定义测试控制器
HomeController.java

package my.springboot.mybatis.controller;
import my.springboot.mybatis.entity.UserDO;
import my.springboot.mybatis.model.HobbyVo;
import my.springboot.mybatis.model.UserVO;
import my.springboot.mybatis.service.IUserInfoService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
import javax.jws.soap.SOAPBinding;
import java.util.Date;
import java.util.List;
@Controller
public class HomeController {
@Autowired
    private IUserInfoService userInfoService;
@RequestMapping("index") //注解映射请求路径
    @ResponseBody //可以将java对象转为json格式的数据
    public String index()
    {
        List vo=userInfoService.getUserVOMap(1);
        List list=userInfoService.getHobbyMap();
        return "Hello World !";
    }
}

g.定义xml文件
定义HobbyMapper.xml文件

<?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="my.springboot.mybatis.dao.HobbyMapper">
<select id="getHobbyByUserId" resultType="my.springboot.mybatis.entity.HobbyDO" parameterType="integer">
       select  * from t_hobby where userId= #{id}
</select>
<resultMap id="hobbyVOMap" type="my.springboot.mybatis.model.HobbyVo">
<id property="id" column="id"/>
<association property="user" column="userId" javaType="my.springboot.mybatis.entity.HobbyDO"
                     select="my.springboot.mybatis.dao.UserInfoMapper.get">
</association>
</resultMap>
<select id="getHobbyVOMap" resultMap="hobbyVOMap" parameterType="integer">
       select  * from t_hobby
</select>
</mapper>​

定义UserInfoMapper.xml

<?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="my.springboot.mybatis.dao.UserInfoMapper">
<select id="get" resultType="my.springboot.mybatis.entity.UserDO">
   select `id`,`user_name`,`password`,`last_login_time`,`sex` from t_user where id = #{value}
</select>
<!--跨xml文件写法 my.springboot.mybatis.dao.HobbyMapper.getHobbyByUserId-->
    <resultMap id="userVOMap" type="my.springboot.mybatis.model.UserVO">
<id property="id" column="id"/>
<collection property="list" column="id" ofType="my.springboot.mybatis.entity.HobbyDO" select="my.springboot.mybatis.dao.HobbyMapper.getHobbyByUserId">
</collection>
</resultMap>
<select id="getUserVOMap" resultMap="userVOMap" parameterType="integer">
       select  * from t_user
</select>
<select id="getHobbyByUserId" resultType="my.springboot.mybatis.entity.HobbyDO" parameterType="integer">
       select  * from t_hobby where userId= #{id}
</select>

</mapper>

访问地址：http://localhost:8090/index
项目结构：
3、总结
resultMap用途主要有一下两点：

1、数据库字段和Java类属性映射

2、实现复杂的model类的查询

model中包含实体类使用关键字：association

<select id="getHobbyByUserId" resultType="my.springboot.mybatis.entity.HobbyDO" parameterType="integer">
       select  * from t_hobby where userId= #{id}
</select>
<resultMap id="hobbyVOMap" type="my.springboot.mybatis.model.HobbyVo">
<id property="id" column="id"/>
<association property="user" column="userId" javaType="my.springboot.mybatis.entity.HobbyDO"
                     select="my.springboot.mybatis.dao.UserInfoMapper.get">
<!-- 说明：如果调用的方法不在当前xml里面，需要指明完整的mapper路径 my.springboot.mybatis.dao.UserInfoMapper.get -->           </association>
</resultMap>
<select id="getHobbyVOMap" resultMap="hobbyVOMap" parameterType="integer">
       select  * from t_hobby
</select>

model中包含集合使用：collection 关键代码

<resultMap id="userVOMap" type="my.springboot.mybatis.model.UserVO">
<id property="id" column="id"/>
<collection property="list" column="id" ofType="my.springboot.mybatis.entity.HobbyDO" select="my.springboot.mybatis.dao.HobbyMapper.getHobbyByUserId">
</collection>
</resultMap>
<select id="getUserVOMap" resultMap="userVOMap" parameterType="integer">
   select  * from t_user
</select>
<select id="getHobbyByUserId" resultType="my.springboot.mybatis.entity.HobbyDO" parameterType="integer">
   select  * from t_hobby where userId= #{id}
</select>

注意：实际的查询过程当中，并不是必须要保证数据库字段名称和实体的属性名称保持一致，如果查询语句查询字段指定别名，也是可以正常做映射的，要会灵活使用。

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