【MyBatis】一对一配置

【意义】

这种方式取Car下的Engine，实际还是走了两次DB，一次是拿id取Car的属性，一次是用car的eid去取Engine的属性，没有节省时间，效率不比在代码中配置Engine和Car的关联高，意义不大。

 例程：https://files.cnblogs.com/files/heyang78/MybatisCli-CarEnginCompanyEmp-220528.rar?t=1653685453

【目标】

完成汽车Car中对应引擎Engine的配置。

【两个类】

Car类：

package com.hy.entity;

public class Car {
    private long id;
    private String name;
    private Engine engine;

    public String toString(){
        return String.format("Car id=%d name=%s engine=%s",id,name,engine.getName());
    }

    public long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Engine getEngine() {
        return engine;
    }

    public void setEngine(Engine engine) {
        this.engine = engine;
    }
}

Engine类：

package com.hy.entity;

public class Engine {
    private long id;
    private String name;

    public long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

注意：以下Mapper接口和xml可以合并成一个，不是非两个不可。

【两个Mapper接口】

Car接口：

package com.hy.dao;

import com.hy.entity.Car;
import com.hy.entity.Company;
import org.apache.ibatis.annotations.Mapper;

@Mapper
public interface CarMapper {
    Car fetchOne(long id);
}

Engine接口：

package com.hy.dao;

import com.hy.entity.Car;
import com.hy.entity.Engine;
import org.apache.ibatis.annotations.Mapper;

@Mapper
public interface EngineMapper {
    Engine fetchOne(long id);
}

【两个Mapper.xml】

CarMapper.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="com.hy.dao.CarMapper">
    <select id="fetchOne" resultMap="map2">
        select id,name,eid from car where id=#{id}
    </select>

    <resultMap id="map2" type="com.hy.entity.Car">
        <id column="id" property="id"/>
        <result column="name" property="name"/>

        <association column="eid" property="engine" select="com.hy.dao.EngineMapper.fetchOne" javaType="com.hy.entity.Engine"/>
    </resultMap>
</mapper>

EngineMapper.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="com.hy.dao.EngineMapper">
    <select id="fetchOne" resultType="com.hy.entity.Engine">
        select id,name from engine where id=#{id}
    </select>
</mapper>

【执行程序】

package com.hy;

import com.hy.dao.CarMapper;
import com.hy.dao.CompanyMapper;
import com.hy.entity.Car;
import com.hy.entity.Company;
import com.hy.entity.Emp;
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 org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

import java.io.InputStream;
import java.util.List;

@SpringBootApplication
public class MyApplication  implements CommandLineRunner {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }

    @Override
    public void run(String... args) throws Exception {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory =
                new SqlSessionFactoryBuilder().build(inputStream);

        try (SqlSession session = sqlSessionFactory.openSession()) {
            CarMapper mapper = session.getMapper(CarMapper.class);

            Car honda= mapper.fetchOne(1);
            System.out.println(honda);

            Car benz= mapper.fetchOne(3);
            System.out.println(benz);
        }
    }
}

【输出】

Car id=1 name=honda engine=v1
Car id=3 name=benz engine=v3

【参考资料】

《Spring+MyBatis企业应用实战》（第2版）疯狂软件编著 P211 

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