mybatis学习（八）——resultMap之association&&collection解析

一、resultMap的使用

resultMap 也是定义返回值类型，返回值为用户自定义的类型，可用于解决JavaBean中的属性名和数据库中的列名不一致的情况

之前对于JavaBean中属性名和数据库中的列名不一致的情况，通过有两种办法，1、通过在sql中使用别名 2、如果正好符合驼峰命名，需要在settings中配置，现在可以通过resultMap来解决

hotelMapper.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.pjf.mybatis.dao.HotelMapper">
<!-- resultMap定义
type:javaBean的全类名，
id为该resultMap的唯一标识 -->
<resultMap type="com.pjf.mybatis.po.Hotel" id="myHotel">
<!--id 指定主键的封装规则
column：数据库中列名
property：javaBean的属性名 -->
<id column="id" property="id" jdbcType="INTEGER" />
<!--result 指定非主键的封装规则
column：数据库中列名
property：javaBean的属性名 -->
<result column="hotel_name" property="hotelName" jdbcType="VARCHAR" />
<result column="hotel_address" property="hotelAddress"
jdbcType="VARCHAR" />
<result column="price" property="price" jdbcType="INTEGER" />
</resultMap>

<!-- resultMap使用 -->
<select id="getHotel" resultMap="myHotel">
select * from hotel
where
id=#{id}
</select>
</mapper> 

二、association的使用

association和collection都是用来关联另一个表的数据，区别就是用来关联对象的封装的，而collection是用来关联集合封装的，

举个例子，比如通过查询酒店，查出该酒店的城市，是一个城市对应一个酒店，用association

而查询一个城市的酒店，是一对多的，用collection，下面来具体实现下这个例子。

1、环境准备

修改hotel.java代码，增加一种类成员变量City，通过查询酒店，直接查出他所在的城市

package com.pjf.mybatis.po;

public class Hotel {
private int id;
private String hotelName;
private String hotelAddress;
private int price;
private City city;

public int getId() {
return id;
}

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

public String getHotelName() {
return hotelName;
}

public void setHotelName(String hotelName) {
this.hotelName = hotelName;
}

public String getHotelAddress() {
return hotelAddress;
}

public void setHotelAddress(String hotelAddress) {
this.hotelAddress = hotelAddress;
}

public int getPrice() {
return price;
}

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

public City getCity() {
return city;
}

public void setCity(City city) {
this.city = city;
}

@Override
public String toString() {
return "Hotel [id=" + id + ", hotelName=" + hotelName + ", hotelAddress=" + hotelAddress + ", price=" + price
+ "]";
}
}

增加城市类 City.java

package com.pjf.mybatis.po;

public class City {
private int cityCode;
private String cityName;

public int getCityCode() {
return cityCode;
}

public void setCityCode(int cityCode) {
this.cityCode = cityCode;
}

public String getCityName() {
return cityName;
}

public void setCityName(String cityName) {
this.cityName = cityName;
}

@Override
public String toString() {
return "City [cityCode=" + cityCode + ", cityName=" + cityName + "]";
}
}

还有数据库的修改，hotel表中增加一列city_code，新增一个city表，

hotel表

city表

hotelMapper接口不变

package com.pjf.mybatis.dao;

import com.pjf.mybatis.po.Hotel;

public interface HotelMapper {

    public Hotel getHotel(Integer i);
}

2、association的使用

通过association来关联city表，使用规则如下

hotelMapper.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.pjf.mybatis.dao.HotelMapper">
    <!-- resultMap定义 type:javaBean的全类名， id为该resultMap的唯一标识 -->
    <resultMap type="com.pjf.mybatis.po.Hotel" id="myHotel">
        <!--id 指定主键的封装规则 column：数据库中列名 property：javaBean的属性名 -->
        <id column="id" property="id" jdbcType="INTEGER" />
        <!--result 指定非主键的封装规则 column：数据库中列名 property：javaBean的属性名 -->
        <result column="hotel_name" property="hotelName" jdbcType="VARCHAR" />
        <result column="hotel_address" property="hotelAddress"
            jdbcType="VARCHAR" />
        <result column="price" property="price" jdbcType="INTEGER" />

        <!--association 关联的表
               property 指被关联的类成员变量
               javaType 指被关联的类成员变量的全类名   -->
        <association property="city" javaType="com.pjf.mybatis.po.City">
            <id column="city_code" property="cityCode" jdbcType="INTEGER"/>
            <result column="city_name" property="cityName" jdbcType="VARCHAR"/>
        </association>
    </resultMap>

    <!-- resultMap使用 -->
    <select id="getHotel" resultMap="myHotel">
        select
        h.id,h.hotel_name,h.hotel_address,h.price,c.city_code,c.city_name from
        hotel h ,city c
        where
        h.city_code=c.city_code and h.id=#{id}
    </select>
</mapper>

测试类：

package com.pjf.mybatis;

import java.io.IOException;
import java.io.InputStream;
import java.util.Map;

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.junit.Test;

import com.pjf.mybatis.dao.HotelMapper;
import com.pjf.mybatis.po.Hotel;

public class TestHotel {

    public SqlSessionFactory sqlSessionFactory() throws IOException {
        // mybatis的配置文件
        String resource = "mybatis_config.xml";
        // 使用类加载器加载mybatis的配置文件（它也加载关联的映射文件）TestHotel.class.getClassLoader()
        InputStream is = Resources.getResourceAsStream(resource);
        // 构建sqlSession的工厂
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
        return sessionFactory;
    }

    // 查
    @Test
    public void getHotel() throws IOException {

        SqlSessionFactory sessionFactory = sqlSessionFactory();
        SqlSession session = sessionFactory.openSession();
        HotelMapper hotelMapper = session.getMapper(HotelMapper.class);
        System.out.println(hotelMapper.getClass());
        Hotel hotel = hotelMapper.getHotel(1004);
        // 打印酒店
        System.out.println(hotel);
        // 打印城市
        System.out.println(hotel.getCity());
        session.close();
    }
}

这时候就可以看到结果了

3、association的分步查询的使用

三、collection的使用

实例：查询某个城市的全部酒店

修改city类

package com.pjf.mybatis.po;

import java.util.List;

public class City {
    private int cityCode;
    private String cityName;
    private List<Hotel> hotel;

    public List<Hotel> getHotel() {
        return hotel;
    }

    public void setHotel(List<Hotel> hotel) {
        this.hotel = hotel;
    }

    public int getCityCode() {
        return cityCode;
    }

    public void setCityCode(int cityCode) {
        this.cityCode = cityCode;
    }

    public String getCityName() {
        return cityName;
    }

    public void setCityName(String cityName) {
        this.cityName = cityName;
    }

    @Override
    public String toString() {
        return "City [cityCode=" + cityCode + ", cityName=" + cityName + "]";
    }

}

修改hotelMapper接口（可以重新定义一个接口和mapper.xml文件）

package com.pjf.mybatis.dao;

import com.pjf.mybatis.po.City;

public interface HotelMapper {

    public City  getCityHotel(Integer i);
}

hotelMapper.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="com.pjf.mybatis.dao.HotelMapper">
    <resultMap type="com.pjf.mybatis.po.City" id="cityHotel">
     <id column="city_code" property="cityCode"/>
     <result column="city_name" property="cityName"/>
     <!--collection被关联的集合
           ofType被关联集合元素的全类名
     -->
     <collection property="hotel" ofType="com.pjf.mybatis.po.Hotel">
        <id column="id" property="id"/>
        <result column="hotel_name" property="hotelName"/>
        <result column="hotel_address" property="hotelAddress"/>
        <result column="price" property="price"/>
     </collection>
    </resultMap>

    <select id="getCityHotel" resultMap="cityHotel">
       SELECT c.city_code,c.city_name ,h.id,h.hotel_name,h.hotel_address,h.price
       FROM city c LEFT JOIN hotel h ON c.city_code=h.city_code
       WHERE c.city_code=#{cityCode}
    </select>
</mapper>    

测试类

package com.pjf.mybatis;

import java.io.IOException;
import java.io.InputStream;
import java.util.Map;

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.junit.Test;

import com.pjf.mybatis.dao.HotelMapper;
import com.pjf.mybatis.po.City;
import com.pjf.mybatis.po.Hotel;

public class TestHotel {

    public SqlSessionFactory sqlSessionFactory() throws IOException {
        // mybatis的配置文件
        String resource = "mybatis_config.xml";
        // 使用类加载器加载mybatis的配置文件（它也加载关联的映射文件）TestHotel.class.getClassLoader()
        InputStream is = Resources.getResourceAsStream(resource);
        // 构建sqlSession的工厂
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(is);
        return sessionFactory;
    }

    // 查
    @Test
    public void getHotel() throws IOException {

        SqlSessionFactory sessionFactory = sqlSessionFactory();
        SqlSession session = sessionFactory.openSession();
        HotelMapper hotelMapper = session.getMapper(HotelMapper.class);
        System.out.println(hotelMapper.getClass());
        City city = hotelMapper.getCityHotel(1);
        // 打印城市
        System.out.println(city);
        // 打印酒店
        for (Hotel  hotel : city.getHotel()) {
            System.out.println(hotel);
        }
        session.close();
    }
}

查看结果