浅谈mybatis返回单一对象或对象列表的问题

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<resultMap id="BaseResultMap" type="com.ks.ssm.domain.User" >
    <id column="id" property="id" jdbcType="BIGINT" />
    <result column="username" property="username" jdbcType="VARCHAR" />
    <result column="password" property="password" jdbcType="VARCHAR" />
    <result column="email" property="email" jdbcType="VARCHAR" />
    <result column="qq" property="qq" jdbcType="VARCHAR" />
    <result column="phone" property="phone" jdbcType="VARCHAR" />
    <result column="gender" property="gender" jdbcType="BIT" />
    <result column="birthday" property="birthday" jdbcType="DATE" />
    <result column="city" property="city" jdbcType="VARCHAR" />
    <result column="mood" property="mood" jdbcType="VARCHAR" />
    <result column="single" property="single" jdbcType="BIT" />
    <result column="enrolltime" property="enrolltime" jdbcType="TIMESTAMP" />
    <result column="level" property="level" jdbcType="TINYINT" />
    <result column="status" property="status" jdbcType="BIT" />
    <result column="titlepic" property="titlepic" jdbcType="VARCHAR" />
    <result column="job" property="job" jdbcType="VARCHAR" />
    <result column="logintime" property="logintime" jdbcType="TIMESTAMP" />
    <result column="loginip" property="loginip" jdbcType="VARCHAR" />
    <result column="token" property="token" jdbcType="VARCHAR" />
    <result column="modifytime" property="modifytime" jdbcType="TIMESTAMP" />
  </resultMap>
  <sql id="Base_Column_List" >
    id, username, password, email, qq, phone, gender, birthday, city, mood, single, enrolltime, 
    level, status, titlepic, job, logintime, loginip, token, modifytime
  </sql>
  <select id="selectByPrimaryKey" resultMap="BaseResultMap" parameterType="java.lang.Long" >
    select 
    <include refid="Base_Column_List" />
    from user_info
    where id = #{id,jdbcType=BIGINT}
  </select>
  <!-- add by ks -->
    <select id="selectByUserName" resultMap="BaseResultMap" parameterType="java.lang.String" >
    select 
    <include refid="Base_Column_List" />
    from user_info
    where username = #{username,jdbcType=VARCHAR}
   </select>
   <!-- mybatis 非常的智能，返回值统一使用 resultMap="BaseResultMap"，mybatis会根据查询到的条目数量自动进行判断，如果是一条就返回对象，如果是多条就返回List对象列表-->
  <select id="selectByEmail" resultMap="BaseResultMap" parameterType="java.lang.String" >
    select 
    <include refid="Base_Column_List" />
    from user_info
    where email = #{email,jdbcType=VARCHAR}
   </select>
   <!-- end by ks -->

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public interface UserMapper {
    User selectByPrimaryKey(Long id);
    User selectByUserName(String username );
    /**关于mybatis返回单一对象或对象列表的问题：
     * 1.返回数据类型由dao中的接口和*map.xml文件共同决定。另外，不论是返回单一对象还是对象列表，*map.xml中的配置都是一样的，都是resultMap="*Map"*或resultType=“* .* .*”类型.
     * 2.每一次mybatis从数据库中select数据之后，都会检查数据条数和dao中定义的返回值是否匹配。
     * 3.若返回一条数据，dao中定义的返回值是一个对象或对象的List列表，则可以正常匹配，将查询的数据按照dao中定义的返回值存放。
     * 4.若返回多条数据，dao中定义的返回值是一个对象，则无法将多条数据映射为一个对象，此时mybatis报错。
     * */
    List<User> selectByEmail(String email );
}

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@RunWith(SpringJUnit4ClassRunner.class)     //表示继承了SpringJUnit4ClassRunner类
@ContextConfiguration(locations = {"classpath:spring-mybatis.xml"})
public class TestMyBatis {
  private static Logger logger = Logger.getLogger(TestMyBatis.class);
  @Resource
  private UserMapper userDao;
  @Test
  public void testMybatis() {
    User user = userDao.selectByUserName("ks");
    logger.info("user.........................");
    logger.info(JSON.toJSONString(user));
    List<User> users=userDao.selectByEmail("ks");
    logger.info("list.........................");
    for(User userTemp : users)
    {
        logger.info(JSON.toJSONString(userTemp));
    }
  }
}

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import com.hikari.cloud.data.entity.TbInstruction;
import lombok.Data;
import java.util.Date;
import java.util.List;
@Data
public class DeviceQuestionInstruction {//tb_instruction  使用说明表
    private String dvqsTitle;
    private List<TbInstruction> instructionList;
}

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import lombok.Data;
import java.util.Date;
@Data
public class TbInstruction {//tb_instruction  使用说明表
    private Long id;
    private Long userId;
    private String title;
    private String detail;
    private String url;
    private Integer type;
    private Integer suffix;
    private String deviceCategory;
    private String deviceTypeName;
    private String deviceTypeNum;
    private Integer views;
    private Long dvqsId;
    private Integer dvqsLevel;
    private Date gmtCreate;
}

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import com.hikari.cloud.data.bean.DeviceQuestionInstruction;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface TbDeviceQuestionMapper {
    List<DeviceQuestionInstruction> findByNo(@Param("deviceTypeNo") String deviceTypeNo);
}

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<?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.hikari.cloud.data.mapper.TbDeviceQuestionMapper">
    <resultMap id="dataMap" type="com.hikari.cloud.data.bean.DeviceQuestionInstruction">
        <result column="dvqs_title" property="dvqsTitle"/>
        <collection property="instructionList" resultMap="insResultMap"/>
    </resultMap>
    <resultMap id="insResultMap" type="com.hikari.cloud.data.entity.TbInstruction">
        <result column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="title" property="title"/>
        <result column="detail" property="detail"/>
        <result column="url" property="url"/>
        <result column="type" property="type"/>
        <result column="suffix" property="suffix"/>
        <result column="device_category" property="deviceCategory"/>
        <result column="device_type_name" property="deviceTypeName"/>
        <result column="device_type_num" property="deviceTypeNum"/>
        <result column="views" property="views"/>
        <result column="dvqs_id" property="dvqsId"/>
        <result column="dvqs_level" property="dvqsLevel"/>
        <result column="gmt_create" property="gmtCreate"/>
    </resultMap>
    <select id="findByNo" resultType="com.hikari.cloud.data.bean.DeviceQuestionInstruction" resultMap="dataMap">
        SELECT tb_device_question.title AS dvqs_title,tb_instruction.* FROM tb_device_question
        LEFT JOIN tb_instruction
        ON tb_device_question.id=tb_instruction.dvqs_id
        WHERE tb_device_question.device_type_no='HSAT-K5'
        ORDER BY tb_instruction.dvqs_level ASC
    </select>
</mapper>