Mybatis框架中实现一对多关系映射

<?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">

<configuration>
  <environments default="development">
    <environment id="development">
      <transactionManager type="JDBC">
        <property name="" value=""/>
      </transactionManager>
      <dataSource type="UNPOOLED">
        <property name="driver" value="oracle.jdbc.OracleDriver"/>
        <property name="url" value="jdbc:oracle:thin:@localhost:1521:orcl"/>
        <property name="username" value="practice"/>
        <property name="password" value="123"/>
      </dataSource>
    </environment>
  </environments>
  
   <mappers>
    <mapper resource="config/Student.xml"/>
    <mapper resource="config/Grade.xml"/>
  </mappers> 

</configuration>

package entity;
/*
 * 学生类
 * */
public class Student {
    //学生编号
    private Integer sid;
    //学生名称
    private String sname;
    //学生性别
    private String sex;
    
    public Student() {
    }
    public Student(String sname, String sex) {
        this.sname = sname;
        this.sex = sex;
    }
    public Integer getSid() {
        return sid;
    }
    public void setSid(Integer sid) {
        this.sid = sid;
    }
    public String getSname() {
        return sname;
    }
    public void setSname(String sname) {
        this.sname = sname;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    
}

package entity;

import java.util.HashSet;
import java.util.Set;

/*
 * 班级类
 * */
public class Grade {
    //班级编号
    private Integer gid;
    //班级名称
    private String gname;
    //班级描述
    private String gdesc;
    //班级下的学生信息
    private Set<Student> stus=new HashSet<Student>();
    
    public Set<Student> getStus() {
        return stus;
    }
    public void setStus(Set<Student> stus) {
        this.stus = stus;
    }
    public Grade() {
    }
    public Grade(Integer gid, String gname, String gdesc) {
        this.gid = gid;
        this.gname = gname;
        this.gdesc = gdesc;
    }
    public Integer getGid() {
        return gid;
    }
    public void setGid(Integer gid) {
        this.gid = gid;
    }
    public String getGname() {
        return gname;
    }
    public void setGname(String gname) {
        this.gname = gname;
    }
    public String getGdesc() {
        return gdesc;
    }
    public void setGdesc(String gdesc) {
        this.gdesc = gdesc;
    }
    
}

<?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="Student">

  <resultMap type="entity.Student" id="StudentResult">
    <id column="sid" jdbcType="INTEGER" property="sid"/>
    <result column="sname" jdbcType="VARCHAR" property="sname"/>
    <result column="sex" jdbcType="VARCHAR" property="sex"/>
  </resultMap>

</mapper>

<?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="Grade">

  <resultMap type="entity.Grade" id="GradeResult">
    <id column="gid" jdbcType="INTEGER" property="gid"/>
    <result column="gname" jdbcType="VARCHAR" property="gname"/>
    <result column="gdesc" jdbcType="VARCHAR" property="gdesc"/>
    <!-- 一对多关系 -->
    <collection property="stus" resultMap="Student.StudentResult"></collection>
  </resultMap>
     
    <!-- 查询所有信息 -->
    <select id="selectAllInfo" resultMap="GradeResult">
        <!-- select sid,sname,sex,g.gid,gname,gdesc from Student s,Grade g where s.gid=g.gid -->
        select sid,sname,sex,g.gid,gname,gdesc from Student s left join Grade g on s.gid=g.gid
    </select>
</mapper>

/*
     * 1.1 查询所有的班级和班级下的所有学生(一对多)
     * */
    @Test
    public void selectAllStu() throws Exception{
        //通过配置文件获取到数据库连接信息
        Reader reader = Resources.getResourceAsReader("config/Configuration.xml");
        //通过配置信息构建一个SessionFactory工厂
        SqlSessionFactory sqlsessionfactory=new SqlSessionFactoryBuilder().build(reader);
        //通过SessionFaction打开一个回话通道
        SqlSession session = sqlsessionfactory.openSession();
        /*SqlSession session =MybatisUtil.getSession();*/
        //调用配置文件中的sql语句
        List<Grade> list = session.selectList("Grade.selectAllInfo");
        //遍历查询出来的结果
        for (Grade grade : list) {
            System.out.println("班级："+grade.getGname());
            for (Student stu : grade.getStus()) {
                System.out.println("学生："+stu.getSname());
            }
        }
        
        session.close();
    }