注意：一对多时，数据库外键设置在一对多的 多 表中，这样设计数据库比较简洁（本例为一个班级对应多个学生）

1.class的javabean（此时设置变量students来表示多个学生）

package com.bean;

import java.util.List;

public class Classes {

 private Integer id;
 private Integer code;
 private List<Student> students;
 public Integer getId() {
  return id;
 }
 public void setId(Integer id) {
  this.id = id;
 }
 public Integer getCode() {
  return code;
 }
 public void setCode(Integer code) {
  this.code = code;
 }
 public List<Student> getStudents() {
  return students;
 }
 public void setStudents(List<Student> students) {
  this.students = students;
 }
 @Override
 public String toString() {
  return "Classes [id=" + id + ", code=" + code +"]";
 }
}

2.

package com.bean;

public class Student {

 private Integer id;
 private String name;
 private Integer age;
 private String sex;
 public Integer getId() {
  return id;
 }
 public void setId(Integer id) {
  this.id = id;
 }
 public String getName() {
  return name;
 }
 public void setName(String name) {
  this.name = name;
 }
 public Integer getAge() {
  return age;
 }
 public void setAge(Integer age) {
  this.age = age;
 }
 public String getSex() {
  return sex;
 }
 public void setSex(String sex) {
  this.sex = sex;
 }
 @Override
 public String toString() {
  return "Student [id=" + id + ", name=" + name + ", age=" + age + ", sex=" + sex + "]";
 }
}

3.log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.com.mapper=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

4.jdbc.properties（对一些变量的设置）

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/mybatis
username=root
password=123456

5.mybatis配置文件

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
    "http://mybatis.org/dtd/mybatis-3-config.dtd">
   
    <!--1.配置环境 ，默认的环境id为mysql-->
 <configuration>
 
  <!-- 导入 jdbc.properties 引用里面的值-->
  <properties resource="jdbc.properties"/>
  
  <!-- 1.指定mybatis所用日志的具体实现 -->
  <settings>
   <setting name="logImpl" value="LOG4j"/>
  </settings>
  <!--2.配置环境 ，默认的环境id为mysql-->
  <environments default="mysql">
   <!--2.2.配置id为mysql的数据库环境 -->
   <environment id="mysql">
    <!-- 2.2.1使用JDBC的事务管理   使用了JDBC的提交和回滚设置-->
    <transactionManager type="JDBC"></transactionManager>
    <!--2.2.2数据库连接池   POOLED是JDBC连接对象的数据源连接池的实现 -->
    <dataSource type="POOLED">
     <property name="driver" value="${driver}"/>
     <property name="url" value="${url}"/>
     <property name="username" value="${username}"/>
     <property name="password" value="${password}"/>
    </dataSource>
   </environment>
  </environments>
  
  <!--3.配置Mapper的位置 -->
  <mappers>
   <mapper resource="com/mapper/UserMapper.XML"/>
   <!-- 使用本地文件 -->
   <!-- <mapper url="file:///D:/java/workspaces/mybatis0/src/com/mapper/UserMapper.xml"/> -->
  </mappers>
 </configuration>

6.数据库操作的配置文件

<?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">
<!-- namespace表示命名空间
 程序代码访问文件是通过namespace完成的，包名+文件名命名
-->
<mapper namespace="com.mapper.UserMapper">

 <!-- 1.1   分表查询 -->
 <resultMap type="com.bean.Classes" id="classResultMap">
  <id property="id" column="id"/>
  <result property="code" column="code"/>
  <collection property="students" column="id" ofType="com.bean.Student" select="selectStudent" fetchType="lazy"/>
 </resultMap>
 <select id="selectClass" parameterType="Integer" resultMap="classResultMap">
  select * from class where id=#{id}
 </select>
 <select id="selectStudent" resultType="com.bean.Student">
  select * from student where class_id=#{id}
 </select>
 
 <!-- 1.2 分表查询的第二种写法 -->
 <resultMap type="com.bean.Classes" id="classResultMap2">
  <id property="id" column="id"/>
  <result property="code" column="code"/>
  <collection property="students" column="id" ofType="com.bean.Student" select="selectStudent2" fetchType="lazy">
   <id property="id" column="id"/>
   <result property="name" column="name"/>
   <result property="age" column="age"/>
   <result property="sex" column="sex"/>
  </collection>
 </resultMap>
 <select id="selectClass2" parameterType="Integer" resultMap="classResultMap2">
  select * from class where id=#{id}
 </select>
 <select id="selectStudent2" resultType="com.bean.Student">
  select * from student where class_id=#{id}
 </select>
 
 <!-- 2. 多表联合查询 -->
 <resultMap type="com.bean.Classes" id="Class">
  <id property="id" column="id"/>
  <result property="code" column="code"/>
  <collection property="students" column="id" ofType="com.bean.Student">
   <id property="id" column="bid"/>
   <result property="name" column="name"/>
   <result property="age" column="age"/>
   <result property="sex" column="sex"/>
  </collection>
 </resultMap>
 <select id="select" parameterType="Integer" resultMap="Class">
  select a.id as bid,a.name,a.age,a.sex,b.*
  from student a,class b
  where a.class_id=b.id and b.id=#{id}
 </select>
</mapper>

7.查询

package com.Test;

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

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.apache.tools.ant.types.CommandlineJava.SysProperties;
import org.junit.Test;

import com.bean.Classes;
import com.bean.Student;

public class test {

 @Test
 public void selectById() throws IOException
 {
  InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
  SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream); 
  SqlSession session = sessionFactory.openSession();
  Classes classes=session.selectOne("com.mapper.UserMapper.selectClass",2);
  List<Student> list=classes.getStudents();
  System.out.println(classes);
  list.forEach(student->System.out.println(student));
  /*for(Student student:list)
  {
   System.out.println(student);
  }*/
 }
 @Test
 public void selectById2() throws IOException
 {
  InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
  SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream); 
  SqlSession session = sessionFactory.openSession();
  Classes classes=session.selectOne("com.mapper.UserMapper.selectClass2",2);
  List<Student> list=classes.getStudents();
  System.out.println(classes);
  list.forEach(student->System.out.println(student));//这句话等同于下面注释的for循环
  /*for(Student student:list)
  {
   System.out.println(student);
  }*/
 }
 @Test
 public void select() throws IOException
 {
  InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
  SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(inputStream); 
  SqlSession session = sessionFactory.openSession();
  Classes classes=session.selectOne("com.mapper.UserMapper.select", 2);
  List<Student> list=classes.getStudents();
  System.out.println(classes);
  list.forEach(student->System.out.println(student));
 }
}