4.5 MyBatis_映射文件_select_resultMap之association&&collection解析

悠悠 2022-05-31 00:05 203阅读 0赞

### 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

### 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

![1050601-20171019141841521-745511432.png][]

city表

![1050601-20171019141823834-967439008.png][]

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 type="com.pjf.mybatis.po.Hotel" id="myHotel">
            
            <id column="id" property="id" jdbcType="INTEGER" />
            
            <result column="hotel_name" property="hotelName" jdbcType="VARCHAR" />
            <result column="hotel_address" property="hotelAddress"
                jdbcType="VARCHAR" />
            <result column="price" property="price" jdbcType="INTEGER" />
            
            
            <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>
    
        
        <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

![1050601-20171019142354021-49843924.png][]

###  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 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

![1050601-20171019145555802-565510747.png][]

来源于：http://www.cnblogs.com/pjfmeng/p/7692520.html

[1050601-20171019141841521-745511432.png]: /images/20220531/07ed6cdb55e041cdbce7844549709db1.png
[1050601-20171019141823834-967439008.png]: /images/20220531/afb040733e4a4ad6a792a458eade6808.png
[1050601-20171019142354021-49843924.png]: /images/20220531/1c069fdf6ee74d569501b030b9462204.png
[1050601-20171019145555802-565510747.png]: /images/20220531/ee16e3cdafbd4169bf1724755cc792d7.png