关于mybatis:Mybatis16-Mybatis多对一关联查询

注：代码已托管在 GitHub 上，地址是：https://github.com/Damaer/Mybatis-Learning，我的项目是 mybatis-12-many2one，须要自取，须要配置maven 环境以及 mysql 环境 (sql 语句在 resource 下的 test.sql 中)，感觉有用能够点个小星星。

docsify文档地址在：https://damaer.github.io/Mybatis-Learning/#/

一对多关联查问：每个国家有很多大臣，领导人，当初咱们查问一个领导，心愿能将他所在国家的信息连带进去，这就是一对多关联查问。

数据表如下:
国家表：

领导人表：

# 创立数据库
CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
#创立数据表
CREATE TABLE `test`.`country` (`cid` INT(10) NOT NULL AUTO_INCREMENT ,`cname` VARCHAR(20) NOT NULL ,PRIMARY KEY(`cid`)) ENGINE = MyISAM;
CREATE TABLE `test`.`minister` (`mid` INT(10) NOT NULL AUTO_INCREMENT ,`mname` VARCHAR(20) NOT NULL ,`countryId` INT(20) NOT NULL ,PRIMARY KEY(`mid`)) ENGINE = MyISAM;

#初始化数据表
INSERT INTO `country` (`cid`, `cname`) VALUES ('1', 'USA');
INSERT INTO `country` (`cid`, `cname`) VALUES ('2', 'England');

INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('1', 'aaa', '1');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('2', 'bbb', '1');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('3', 'ccc', '2');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('4', 'ddd', '2');
INSERT INTO `minister` (`mid`, `mname`, `countryId`) VALUES ('5', 'eee', '2');

country类：

public class Country {
    private Integer cid;
    private String cname;

    public Integer getCid() {return cid;}

    public void setCid(Integer cid) {this.cid = cid;}

    public String getCname() {return cname;}

    public void setCname(String cname) {this.cname = cname;}

    @Override
    public String toString() {
        return "Country{" +
                        "cid=" + cid +
                        ", cname='" + cname + '\'' +
                        '}';
    }
}

Minister类, 在这个类外面应用 Country 当属性。

public class Minister {
    private Integer mid;
    private String mname;
    private Country country;

    public Integer getMid() {return mid;}

    public void setMid(Integer mid) {this.mid = mid;}

    public String getMname() {return mname;}

    public void setMname(String mname) {this.mname = mname;}

    public Country getCountry() {return country;}

    public void setCountry(Country country) {this.country = country;}

    @Override
    public String toString() {
        return "Minister{" +
                        "mid=" + mid +
                        ", mname='" + mname + '\'' +
                        ", country=" + country +
                        '}';
    }
}

Mybatis工具类, 获取 sqlsession 实例：

public class MyBatisUtils {/*    public SqlSession getSqlSession(){
            InputStream is;
            try {is = Resources.getResourceAsStream("mybatis.xml");
                return new SqlSessionFactoryBuilder().build(is).openSession();} catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();}
            return null;
        }*/
    static private SqlSessionFactory sqlSessionFactory;

    static public SqlSession getSqlSession() {
        InputStream is;
        try {is = Resources.getResourceAsStream("mybatis.xml");
            if (sqlSessionFactory == null) {sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
            }
            return sqlSessionFactory.openSession();} catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();}
        return null;
    }
}

在 mybatis.xml 主配置文件外面注册 mapper 文件：

<mappers>
    <mapper resource="mapper/mapper.xml"/>
</mappers>

接口的申明：

public interface IMinisterDao {Minister selectMinisterById(int mid);
    Minister selectMinisterById2(int mid);
}

mapper文件 最重要的局部 , 与一对一有点像，两种查问形式，一种是 嵌套后果 ，一种是 嵌套查问，嵌套查问能够应用懒加载模式，比拟罕用：

<?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="dao.IMinisterDao">
<!--     resultMap 能解决字段和属性不一样的问题 -->
    <!-- 当前用得比拟多，是因为能够应用提早加载 -->
    <!-- 嵌套查问 -->
    <select id="selectCountryById" resultType="Country">
    select * from country where cid=#{countryId}
    </select>
    <resultMap type="Minister" id="countryMapper">
        <id column="mid" property="mid"/>
        <result column="mname" property="mname"/>
        <!-- country 中有一个成员变量是 ministers，它的泛型是 Minister -->
        <collection property="country"
                    ofType="Country"
                    select="selectCountryById"
                    column="countryId">
        </collection>
    </resultMap>
    <select id="selectMinisterById" resultMap="countryMapper">
        select mid,mname,countryId
        from minister
        where 
        mid=#{mid}
    </select>

    <!-- 嵌套后果 -->
    <select id="selectMinisterById2" resultMap="countryMapper2">
        select mid,mname,countryId,cid,cname
        from minister,country
        where
        mid=#{mid} and countryId = cid
    </select>
    <resultMap id="countryMapper2" type="Minister">
        <id column="mid" property="mid"/>
        <result column="mname" property="mname"/>
        <collection property="country" ofType="Country">
            <id property="cid" column="cid"/>
            <result property="cname" column="cname"/>
        </collection>
    </resultMap>
</mapper>

单元测试：

public class MyTest {
  private IMinisterDao dao;
  private SqlSession sqlSession;
  @Before
  public void Before(){sqlSession=MyBatisUtils.getSqlSession();
    dao=sqlSession.getMapper(IMinisterDao.class);
  }
  @Test
  public void TestselectMinisterById(){Minister minister = dao.selectMinisterById(1);
    System.out.println(minister);
  }

  @Test
  public void TestselectMinisterById2(){Minister minister = dao.selectMinisterById2(1);
    System.out.println(minister);
  }


  @After
  public void after(){if(sqlSession!=null){sqlSession.close();
    }
  }

}

后果：

[service] 2018-07-12 20:07:23,875 - dao.IMinisterDao.selectMinisterById2 -567  [main] DEBUG dao.IMinisterDao.selectMinisterById2  - ==>  Preparing: select mid,mname,countryId,cid,cname from minister,country where mid=? and countryId = cid 
[service] 2018-07-12 20:07:23,909 - dao.IMinisterDao.selectMinisterById2 -601  [main] DEBUG dao.IMinisterDao.selectMinisterById2  - ==> Parameters: 1(Integer)
[service] 2018-07-12 20:07:23,930 - dao.IMinisterDao.selectMinisterById2 -622  [main] DEBUG dao.IMinisterDao.selectMinisterById2  - <==      Total: 1
Minister{mid=1, mname='aaa', country=Country{cid=1, cname='USA'}}

【作者简介】：
秦怀，公众号【秦怀杂货店】作者，技术之路不在一时，山高水长，纵使迟缓，驰而不息。这个世界心愿所有都很快，更快，然而我心愿本人能走好每一步，写好每一篇文章，期待和你们一起交换。

关于mybatis:Mybatis16-Mybatis多对一关联查询

创立数据库 / 表

实体类