Mybatis的resultMap

Posted 2020-12-17 陈彦斌

Mybatis的SQL语句返回结果有两种

• resultType
  • 查询出的字段在相应的POJO中必须有和它相同的字段对应，或者基本数据类型
  • 适合简单查询
• resultMap
  • 需要自定义字段，或者多表查询，一对多等关系，比resultType更强大
  • 适合复杂查询

项目结构

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>net.cyb</groupId>
    <artifactId>cyb-mybatis</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.5.4</version>
        </dependency>
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>8.0.20</version>
        </dependency>
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>1.7.30</version>
        </dependency>
    </dependencies>
</project>

mybatis-config.xml

<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!-- 引入外部配置文件 -->
    <properties resource="config/db.properties"></properties>
    <!--下划线自动映射驼峰字段-->
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <!-- 数据库链接相关 -->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC" />
            <dataSource type="POOLED">
                <property name="driver" value="${db.driver}" />
                <property name="url" value="${db.url}" />
                <property name="username" value="${db.username}" />
                <property name="password" value="${db.password}" />
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!-- 添加映射文件 -->
        <mapper resource="mapper/VideoMapper.xml" />
    </mappers>
</configuration>

db.properties

db.driver=com.mysql.cj.jdbc.Driver
db.url=jdbc:mysql://127.0.0.1:3306/cybclass
db.username=root
db.password=root

log4j.properties

#dev env [debug] product env [info]
log4j.rootLogger=ERROR, stdout
# Console output...
# 细化到打印某个mapper
# log4j.logger.包名.方法名=TRACE
log4j.logger.net.cybclass.online=TRACE
#log4j.logger.net.cybclass.online.dao.VideoMapper.selectById=TRACE
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

VideoMapper.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">
<!--
namespace：名称空间，一般需要保持全局唯一，最好是和dao层的java接口一致。可以映射sql语句到对应的方法名称和参数、返回类型
-->
<mapper namespace="net.cybclass.online.dao.VideoMapper">
    <resultMap id="videoResultMap" type="net.cybclass.online.domain.Video">
        <id column="id" property="id" jdbcType="INTEGER"></id>
        <result column="title2" property="title" jdbcType="VARCHAR"></result>
        <result column="summary" property="summary" jdbcType="VARCHAR"></result>
    </resultMap>
    <sql id="base_video_field2">id,title title2,summary</sql>
    <select id="selectBaseFieldByIdWithResultMap" resultMap="videoResultMap">
        select <include refid="base_video_field2"></include> from video where id=#{id}
    </select>

</mapper>

VideoMapper.java

package net.cybclass.online.dao;

import net.cybclass.online.domain.Video;
import org.apache.ibatis.annotations.Param;
import org.apache.ibatis.annotations.Select;

import java.util.List;
import java.util.Map;

public interface VideoMapper {
    Video selectBaseFieldByIdWithResultMap(int id);
}

Video.java

package net.cybclass.online.domain;

import java.util.Date;

/**
 * 视频类
 */
public class Video {
    //内码
    private int id;
    //视频标题
    private String title;
    //视频详情
    private String summary;
    //视频封面
    private String coverImg;
    //加个
    private int price;
    private int c_id;
    //评分
    private double point;
    //创建日期
    private Date create_time;

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getSummary() {
        return summary;
    }

    public void setSummary(String summary) {
        this.summary = summary;
    }

    public String getCoverImg() {
        return coverImg;
    }

    public void setCoverImg(String coverImg) {
        this.coverImg = coverImg;
    }

    public int getPrice() {
        return price;
    }

    public void setPrice(int price) {
        this.price = price;
    }

    public int getC_id() {
        return c_id;
    }

    public void setC_id(int c_id) {
        this.c_id = c_id;
    }

    public double getPoint() {
        return point;
    }

    public void setPoint(double point) {
        this.point = point;
    }

    public Date getCreate_time() {
        return create_time;
    }

    public void setCreate_time(Date create_time) {
        this.create_time = create_time;
    }

    @Override
    public String toString() {
        return "Video{" +
                "id=" + id +
                ", title=\'" + title + \'\\\'\' +
                ", summary=\'" + summary + \'\\\'\' +
                ", coverImg=\'" + coverImg + \'\\\'\' +
                ", price=" + price +
                ", c_id=" + c_id +
                ", point=" + point +
                ", create_time=" + create_time +
                \'}\';
    }
}

SqlSessionDemo.java

package net.cybclass.online;

import net.cybclass.online.dao.VideoMapper;
import net.cybclass.online.domain.Video;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.IOException;
import java.io.InputStream;
import java.util.*;

public class SqlSessionDemo {
    public static void main(String[] args) throws IOException {
        String resouce="config/mybatis-config.xml";
        InputStream resourceAsStream = Resources.getResourceAsStream(resouce);
        SqlSessionFactory build = new SqlSessionFactoryBuilder().build(resourceAsStream);
        try(SqlSession session=build.openSession()){
            VideoMapper mapper = session.getMapper(VideoMapper.class);
            Video video = mapper.selectBaseFieldByIdWithResultMap(52);
            System.out.println(video.toString());
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}