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);  }}