MyBatis实体关系映射
MyBatis既然是一个ORM框架，则它也有像Hibernate那样的一对多，多对多，多对一的实体关系映射功能。下面我们就来介绍一下如何使用MyBatis的实体关系映射
1.MyBatis实体关系映射，对于我个人来讲常用的有下面两种
多对一：在子表的映射文件中添加association
一对多：在父表的映射文件中添加collection
2.MyBatis中多对一的案例
先创建两张表
CREATE TABLE `student` (
`sid` int(11) default NULL,
`sname` varchar(10) default NULL,
`t_id` int(11) default NULL
) ;
CREATE TABLE `teacher` (
`t_id` int(11) NOT NULL,
`t_name` varchar(20) default NULL,
PRIMARY KEY (`t_id`)
) ;
创建实体类
package com.zlt.pojo;
public class Teacher {
private intt_id;
private String t_name;
public intgetT_id() {
return t_id;
}
public void setT_id(int t_id) {
this.t_id = t_id;
}
public String getT_name() {
return t_name;
}
public void setT_name(String t_name) {
this.t_name = t_name;
}
}
package com.zlt.pojo;
public class Student {
private int sid;
private String sname;
private Teacher teacher;
public int getSid() {
return sid;
}
public void setSid(intsid) {
this.sid = sid;
}
public String getSname() {
return sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
}
创建StudentMapper.xml文件，在此XML文件中配置多对一的关系
<?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.zlt.mapper.StudentMapper">
<resultMap type="com.zlt.pojo.Student" id="Student">
<id property="sid" column="sid"/>
<result property="sname" column="sname"/>
<association property="teacher" javaType="com.zlt.pojo.Teacher">
<id property="t_id" column="t_id"/>
<result property="t_name" column="t_name"/>
</association>
</resultMap>
<select id="getStudent" resultMap="Student">
select * from student a, teacher b where a.t_id = b.t_id and sid = 123
</select>
</mapper>
完成多对一关系的代码测试
package com.zlt.test;
import java.io.IOException;
import java.io.Reader;
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.junit.Test;
import com.zlt.pojo.Student;
public class Test03 {
private static SqlSessionFactorysqlSessionFactory;
private static Reader reader;
static {
try {
reader = Resources.getResourceAsReader("config.xml");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
} catch (IOException e) {
e.printStackTrace();
}
}
@Test
public void m01() {
SqlSessionsqlSession = sqlSessionFactory.openSession();
String sql = "com.zlt.mapper.StudentMapper.getStudent";
Student student = sqlSession.selectOne(sql);
System.out.println(student.getSname() + " === " + student.getTeacher().getT_name());
sqlSession.close();
}
}
3.MyBatis一对多的案例
修改Student和Teacher这两个实体类
package com.zlt.pojo;
public class Student {
private intsid;
private String sname;
private Teacher teacher;
public intgetSid() {
return sid;
}
public void setSid(intsid) {
this.sid = sid;
}
public String getSname() {
return sname;
}
public void setSname(String sname) {
this.sname = sname;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
}
package com.zlt.pojo;
import java.util.List;
public class Teacher {
private intt_id;
private String t_name;
private List<Student> student;
public List<Student>getStudent() {
return student;
}
public void setStudent(List<Student> student) {
this.student = student;
}
public intgetT_id() {
return t_id;
}
public void setT_id(intt_id) {
this.t_id = t_id;
}
public String getT_name() {
return t_name;
}
public void setT_name(String t_name) {
this.t_name = t_name;
}
}
创建TeacherMapper的映射文件，在此文件的<resultMap>标签中加入<collection>
<?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.zlt.mapper.TeacherMapper">
<resultMap type="com.zlt.pojo.Teacher" id="Teacher">
<id property="t_id" column="t_id"/>
<result property="t_name" column="t_name"/>
<collection property="student" ofType="com.zlt.pojo.Student">
<id property="sid" column="sid"/>
<result property="sname" column="sname"/>
</collection>
</resultMap>
<select id="getTeacher" resultMap="Teacher">
select * from teacher a,student b where a.t_id = b.t_id and a.t_id = 1
</select>
</mapper>
测试
package com.zlt.test;
import java.io.IOException;
import java.io.Reader;
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.junit.Test;
import com.zlt.pojo.Student;
import com.zlt.pojo.Teacher;
public class Test03 {
private static SqlSessionFactorysqlSessionFactory;
private static Reader reader;
static {
try {
reader = Resources.getResourceAsReader("config.xml");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
} catch (IOException e) {
e.printStackTrace();
}
}
@Test
public void m01() {
SqlSessionsqlSession = sqlSessionFactory.openSession();
String sql = "com.zlt.mapper.StudentMapper.getStudent";
Student student = sqlSession.selectOne(sql);
System.out.println(student.getSname() + " === " + student.getTeacher().getT_name());
sqlSession.close();
}
@Test
public void m02() {
SqlSessionsqlSession = sqlSessionFactory.openSession();
String sql = "com.zlt.mapper.TeacherMapper.getTeacher";
Teacher teacher = sqlSession.selectOne(sql);
List<Student> student = teacher.getStudent();
for (Student s : student) {
System.out.println(teacher.getT_name() + "====" + s.getSname());
}
sqlSession.close();
}
}
原创：知了堂Java培训讲师——子墨
