Mybatis 一对多和多对一关联查询问题

package com.model;
import java.util.Date;
public class Student {
  private Integer sno;
  private String sname;
  private String ssex;
  private Integer sclass;
  private StudentMajor studentmajor;
  public Student() {
    super();
  }
  public Student(Integer sno, String sname, String ssex, Integer sclass, StudentMajor studentmajor) {
    super();
    this.sno = sno;
    this.sname = sname;
    this.ssex = ssex;
    this.sclass = sclass;
    this.studentmajor = studentmajor;
  }
  public StudentMajor getStudentmajor() {
    return studentmajor;
  }
  public void setStudentmajor(StudentMajor studentmajor) {
    this.studentmajor = studentmajor;
  }
  public Integer getSno() {
    return sno;
  }
  public void setSno(Integer sno) {
    this.sno = sno;
  }
  public String getSname() {
    return sname;
  }
  public void setSname(String sname) {
    this.sname = sname;
  }
  public String getSsex() {
    return ssex;
  }
  public void setSsex(String ssex) {
    this.ssex = ssex;
  }
  @Override
  public String toString() {
    return "Student [sno=" + sno + ", sname=" + sname + ", ssex=" + ssex + ", sclass=" + sclass + ", studentmajor="
        + studentmajor + "]";
  }
  public Integer getSclass() {
    return sclass;
  }
  public void setSclass(Integer sclass) {
    this.sclass = sclass;
  }
}

package com.model;
import java.util.List;
public class StudentMajor {
  private Integer id;
  private String mcode;
  private String mname;
  private List<Student> students;
  public StudentMajor() {
    super();
  }
  public StudentMajor(Integer id, String mcode, String mname, List<Student> students) {
    super();
    this.id = id;
    this.mcode = mcode;
    this.mname = mname;
    this.students = students;
  }
  @Override
  public String toString() {
    return "StudentMajor [id=" + id + ", mcode=" + mcode + ", mname=" + mname + ", students=" + students + "]";
  }
  public Integer getId() {
    return id;
  }
  public void setId(Integer id) {
    this.id = id;
  }
  public String getMcode() {
    return mcode;
  }
  public void setMcode(String mcode) {
    this.mcode = mcode;
  }
  public String getMname() {
    return mname;
  }
  public void setMname(String mname) {
    this.mname = mname;
  }
  public List<Student> getStudents() {
    return students;
  }
  public void setStudents(List<Student> students) {
    this.students = students;
  }
}

package com.dao;
import java.util.List;
import java.util.Map;
import com.model.Student;
public interface StudentMapper {
  /**
   * 全表查询
   */
  public List<Student> selectall();
　　/**
   * 根据专业查人员,给一对多用
   */
  public List<Student> selectz(Integer major);
}

package com.dao;
import java.util.List;
import com.model.StudentMajor;
public interface StudentMajorMapper {
  /**
   * 全表查询
   * @return
   */
  public List<StudentMajor> selectAll();
  /**
   * 根据主键查数据,给多对一用
   * @param id
   * @return
   */
  public StudentMajor select(Integer id);
}

<?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.dao.StudentMapper">
  <!-- 多对一查询 -->
  <resultMap type="Student" id="slist">
    <!-- 跟一对一一样用association标签,实体类定义的成员,要跟数据库字段名对应上 -->
    <association property="studentmajor" column="major" 
    select="com.dao.StudentMajorMapper.select"/> <!-- 用接口里定义的方法，根据student表中的major字段查出对应数据 -->
  </resultMap>
  <!-- 查全部 -->
  <select id="selectall" resultMap="slist" >
    select * from student
  </select>
  <!-- 根据专业查人员 -->
  <select id="selectz" parameterType="Integer" resultType="student">
    select * from student s where s.major=#{major}
  </select>
 </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="com.dao.StudentMajorMapper">
   <!-- 一对多查询关联 -->
   <resultMap type="StudentMajor" id="slist">
     <!-- 实体类属性对应数据库的主键字段，不然主键会查不到 -->
     <id property="id" column="id"/>
     <!-- 用collection标签 ,也是实体类属性要对应数据库字段-->
     <collection property="students" column="id"
     select="com.dao.StudentMapper.selectz">
     </collection>  
   </resultMap>
   <!-- 全表查询 -->
   <select id="selectAll" resultMap="slist">
     select * from studentmajor
   </select>
   <!-- 根据主键查 -->
   <select id="select" parameterType="Integer" resultType="StudentMajor">
     select * from studentmajor where id=#{id}
   </select>
 </mapper>

package com.util;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import com.dao.StudentMajorMapper;
import com.dao.StudentMapper;
import com.model.Student;
import com.model.StudentMajor;
public class JJJtest {
  private SqlSession ss;
  private StudentMapper sm;
  private StudentMajorMapper smm;
  @Before
  public void setUp() throws Exception {
    ss=SqlSessionUtil.getSqlSession();
    sm=ss.getMapper(StudentMapper.class);
    smm=ss.getMapper(StudentMajorMapper.class);
  }
  @After
  public void tearDown() throws Exception {
    ss.commit();
    ss.close();
  }
  //一对多查询
  public void test() {
    List<StudentMajor> list=smm.selectAll();
    for(StudentMajor a:list){
      System.out.println(a);
    }
  }
  //根据专业查人员，给一对多用
  public void selectz(){
    List<Student> l=sm.selectz(3);
    for(Student a:l){
      System.out.println(a);
    }
  }
  //多对一查询
  @Test
  public void selectall() {
    List<Student> st=sm.selectall();
    for(Student tt:st){
      System.out.println(tt);
    }
  }
  //根据主键查询，给多对一用
  public void select(){
    StudentMajor a=smm.select(1);
    System.out.println(a);
  }
}