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/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');实体类
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: 1Minister{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。