学习mybatis的关联关系，主要注解在代码上，这里不做解释。配置文件一样的就不贴了
1.关联关系表创建（学生对应老师 多对一）
学生老师表
2.表对应的实体类
package com.home.entity; /**
* 此类是：学生
* @author hpc
* @2017年1月14日下午7:06:33
*/
public class Student {
private Integer student_id;//学生id
private String student_name;//学生名字
private Integer student_number;//学生编号
private Teacher student_teacher;//是哪个教师的学生
public Integer getStudent_id() {
return student_id;
}
public void setStudent_id(Integer student_id) {
this.student_id = student_id;
}
public String getStudent_name() {
return student_name;
}
public void setStudent_name(String student_name) {
this.student_name = student_name;
}
public Integer getStudent_number() {
return student_number;
}
public void setStudent_number(Integer student_number) {
this.student_number = student_number;
}
public Teacher getStudent_teacher() {
return student_teacher;
}
public void setStudent_teacher(Teacher student_teacher) {
this.student_teacher = student_teacher;
}
@Override
public String toString() {
return "Student [student_id=" + student_id + ", student_name=" + student_name + ", student_number="
+ student_number + ", student_teacher=" + student_teacher + "]";
}
}
package com.home.entity; import java.util.HashSet;
import java.util.Set; /**
* 此类是：老师
* @author hpc
* @2017年1月14日下午7:06:47
*/
public class Teacher {
private Integer teacher_id;// 老师id
private String teacher_name;// 老师名字
private Integer teacher_number;// 老师编号
private Set<Student> teacher_student = new HashSet<Student>();// 老师教过哪些学生 public Integer getTeacher_id() {
return teacher_id;
} public void setTeacher_id(Integer teacher_id) {
this.teacher_id = teacher_id;
} public String getTeacher_name() {
return teacher_name;
} public void setTeacher_name(String teacher_name) {
this.teacher_name = teacher_name;
} public Integer getTeacher_number() {
return teacher_number;
} public void setTeacher_number(Integer teacher_number) {
this.teacher_number = teacher_number;
} public Set<Student> getTeacher_student() {
return teacher_student;
} public void setTeacher_student(Set<Student> teacher_student) {
this.teacher_student = teacher_student;
} @Override
public String toString() {
return "Teacher [teacher_id=" + teacher_id + ", teacher_name=" + teacher_name + ", teacher_number="
+ teacher_number + ", teacher_student=" + teacher_student + "]";
} }
3.核心主要是配置文件
<?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">
<mapper namespace="com.home.entity.studentMapper"><!-- 命名空间，一定要是唯一的，只有这样才能定位到这个文件来 -->
<!-- 多对一查询（多个学生对应一个老师） -->
<!-- select的属性
id：这个查找标签的唯一标识，通过id找到这条sql语句
parameterType：传进来的参数类型
resultMap：这条语句执行完，以什么形式返回结果集
-->
<select id="loadStudent" parameterType="int" resultMap="studentMap">
select *
from students as s join teachers as t
on t.teacher_id=s.student_teacher
where s.student_id=#{id}
</select>
<!-- 学生对象映射关系 -->
<!--resultMap的属性
type：以这个类型进行封装返回
id：找到该resultMap标签的唯一标识
resultMap的内部的标签介绍
<id>专门用于映射主键的
<result>普通属性映射
<collection>集合属性映射
<association>关系映射(1-1,1-n,n-1,n-n)
<constructor>专门用于映射构造函数的标签
<discriminator>鉴别器非常容易理 解,因为它的表现很像 Java 语言中的 switch 语句。
-->
<resultMap type="student" id="studentMap" >
<!-- 主键映射 -->
<id property="student_id" column="student_id" javaType="int"/>
<!-- 普通属性映射 -->
<!--
property：映射在这个属性上
column：以这个列上的为参照
javaType：以什么类型映射
-->
<result property="student_name" column="student_name" javaType="String"/>
<result property="student_number" column="student_number" javaType="int"/>
<!-- 关联关系映射 -->
<association property="student_teacher" column="teacher_id" resultMap="teacherMap"/>
</resultMap>
<!-- 学生对象映射关系 -->
<resultMap type="teacher" id="teacherMap">
<id property="teacher_id" column="teacher_id" javaType="int"/>
<result property="teacher_name" column="teacher_name" javaType="String"/>
<result property="teacher_number" column="teacher_number" javaType="int"/>
</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="com.home.entity.teacherMapper">
<!-- teacher sql -->
<select id="loadTeacher" parameterType="int" resultMap="teacherMapper">
select *
from teachers as t join students as s
on s.student_teacher=t.teacher_id
where teacher_id=#{teacher_id}
</select>
<resultMap type="teacher" id="teacherMapper">
<id property="teacher_id" column="teacher_id" />
<result property="teacher_name" column="teacher_name" />
<result property="teacher_number" column="teacher_number" />
<collection property="teacher_student" ofType="student">
<id property="student_id" column="student_id" />
<result property="student_name" column="student_name"
javaType="String" />
<result property="student_number" column="student_number"
javaType="int" />
</collection>
</resultMap>
</mapper>
4. 测试
package com.home.mybatis; import java.io.IOException; import org.apache.ibatis.session.SqlSession; import com.home.entity.Student;
import com.home.entity.Teacher; public class TestApp {
public static void main(String[] args) throws IOException {
// 我将获取session的代码封装成了一个工具类，直接用工具类来获取session
SqlSession session = SessionUtils.getSession("mybatis.xml");
String loadTeacher="com.home.entity.teacherMapper.loadTeacher";
String loadStudent="com.home.entity.studentMapper.loadStudent";
Teacher teacher = session.selectOne(loadTeacher,1);
Student student = session.selectOne(loadStudent,1);
System.out.println(student);
System.out.println(teacher);
// 将数据属性到数据中去
session.commit();
// 关闭session
session.close();
}
}
5.结果
Student [student_id=4, student_name=hpc, student_number=1, student_teacher=Teacher [teacher_id=1, teacher_name=王老师, teacher_number=1, teacher_student=[]]]
Teacher [teacher_id=1, teacher_name=王老师, teacher_number=1, teacher_student=[Student [student_id=5, student_name=zll, student_number=2, student_teacher=null], Student [student_id=4, student_name=hpc, student_number=1, student_teacher=null]]]