mybatis的一对多双向关联映射

这里我们一对多中的一为班级，多为学生，一个班级有多个学生，而一个学生只能属于一个班级，采用mapper动态代理方式实现。

我们主要有以下几个文件：Student.java(学生类)，Grade.java(班级类)，StudentMapper.xml(学生类的映射文件)，GradeMapper.xml(班级类的映射文件)，StudentMapper.java(StudentMapper.xml对应的接口)，GradeMapper.java(GradeMapper.xml对应的接口)，测试类Test01.java，获得sqlSession类Access.java。pojo类对应的两个数据库表，Mybatis的核心配置文件SqlMapConfig,xml。

这是我的目录结构。



Student.java

package com.liukun.po;

public class Student {
private Integer studentId;
private String studentName;
private Grade studentGrade;//多的一方保存为一的一方的单个对象

public Student(){

}

public Integer getStudentId() {
return studentId;
}

public void setStudentId(Integer studentId) {
this.studentId = studentId;
}

public String getStudentName() {
return studentName;
}

public void setStudentName(String studentName) {
this.studentName = studentName;
}

public Grade getStudentGrade() {
return studentGrade;
}

public void setStudentGrade(Grade studentGrade) {
this.studentGrade = studentGrade;
}
}

Grade.java
package com.liukun.po;

import java.util.ArrayList;
import java.util.List;

public class Grade {

private Integer gradeId;
private String gradeName;
//一的一方保存为多的一方的集合
private List<Student> studentIdList = new ArrayList<Student>();

public Grade(){

}

public Integer getGradeId() {
return gradeId;
}

public void setGradeId(Integer gradeId) {
this.gradeId = gradeId;
}

public String getGradeName() {
return gradeName;
}

public void setGradeName(String gradeName) {
this.gradeName = gradeName;
}

public List<Student> getStudentIdList() {
return studentIdList;
}

public void setStudentIdList(List<Student> studentIdList) {
this.studentIdList = studentIdList;
}

}

StudentMapper.xml
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<!-- 与自己相对应的mapper接口绑定（映射） -->
<mapper namespace="com.liukun.mapper.StudentMapper">

<!--
type:返回的类型
id:和下面的resultMap对应
-->
<resultMap type="student" id="result">
<!--
id:主键（唯一性）
column:列的名字,与你下面的sql语句查询结果的列名（你写的而不是数据库的列名，如果你下面为列定义别名，这里也要别名）相对应
property:你所定义的pojo类（Student或Grade）的getxxx()方法去掉get，首字母小写的属性
-->
<id column="studentId" property="studentId"/>
<result column="studentName" property="studentName"/>
<!--
单一映射用 association
javaType:属性对应的java类型
-->
<association property="studentGrade" javaType="Grade">
<id column="gradeId" property="gradeId"/>
<result column="gradeName" property="gradeName"/>
</association>
</resultMap>

<!--
id:定义一个唯一标识符，（StudentMapper.java中的方法必须与id名字一样）
resultMap:定义结果的映射（和上面的resultMap相对应）
-->
<select id="findStudent" resultMap="result">
select s.studentId, s.studentName, g.gradeId, g.gradeName from student s, grade g where s.grade_id = g.gradeId
</select>

</mapper>
GradeMapper.xml

<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.liukun.mapper.GradeMapper">

<resultMap type="Grade" id="result">
<id column="gradeId" property="gradeId"/>
<result column="gradeName" property="gradeName"/>
<!--
集合用collection
ofType:集合的类型（泛型类型）
-->
<collection property="studentIdList" ofType="Student">
<id column="studentId" property="studentId"/>
<result column="studentName" property="studentName"/>
</collection>

</resultMap>

<select id="findList" resultMap="result">
select s.studentId, s.studentName, g.gradeId, g.gradeName from student s, grade g where s.grade_id = g.gradeId
</select>

</mapper>
StudentMapper.java

package com.liukun.mapper;

import java.io.IOException;
import java.util.List;

import com.liukun.po.Student;

public interface StudentMapper {

public List<Student> findStudent() throws IOException;

}
GradeMapper.java

package com.liukun.mapper;

import java.io.IOException;
import java.util.List;

import com.liukun.po.Grade;
import com.liukun.po.Student;

public interface GradeMapper {

public List<Grade> findList() throws IOException;
}
Access.java

package com.liukun.access;

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;

public class Access {
public static SqlSession getSqlSession() throws IOException{
//通过配置文件获取数据库连接信息
Reader reader = Resources.getResourceAsReader("SqlMapConfig.xml");
//通过配置信息构建一个SqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
//通过sqlSessionFactory打开一个数据库会话
SqlSession sqlSession = sqlSessionFactory.openSession();
return sqlSession;
}

}
Test01.java

package com.liukun.test;

import java.io.IOException;
import java.util.List;

import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import com.liukun.access.Access;
import com.liukun.mapper.GradeMapper;
import com.liukun.mapper.StudentMapper;
import com.liukun.po.Grade;
import com.liukun.po.Student;
public class Test01{

//通过班级找学生（多对一）
@Test //单元测试
public void fun() throws IOException{
SqlSession sqlSession = Access.getSqlSession();
GradeMapper gm = sqlSession.getMapper(GradeMapper.class);
List<Grade> list = gm.findList();
for (Grade s : list) {
System.out.print(s.getGradeId()+" "+s.getGradeName()+":");
for (Student g : s.getStudentIdList()) {
System.out.print(g.getStudentId()+" "+g.getStudentName()+" ");
}
System.out.println();
}
sqlSession.close();
}
//通过学生找班级(一对多)
@Test //单元测试
public void fun1() throws IOException{
SqlSession sqlSession = Access.getSqlSession();
StudentMapper sm = sqlSession.getMapper(StudentMapper.class);
List<Student> list = sm.findStudent();
for (Student s : list) {
System.out.println(s.getStudentId()+" "+s.getStudentName()+" "+s.getStudentGrade().getGradeId()+" "+s.getStudentGrade().getGradeName());
}
sqlSession.close();
}
}


SqlMapConfig.xml(全局配置文件)

<?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>

<!-- 使得整个包下的类可以直接通过类名访问，大小写不敏感 -->
<typeAliases>
<package name="com.liukun.po"/>
</typeAliases>
<!-- 连接数据相关的配置 -->
<environments default="development">
<environment id="development">
<transactionManager type="JDBC">
<property name="" value=""/>
</transactionManager>
<dataSource type="UNPOOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://127.0.0.1:3306/t5"/>
<property name="username" value="root"/>
<property name="password" value="123"/>
</dataSource>
</environment>
</environments>

<!-- 使得整个包下的.xml文件都可以找到 -->
<mappers>
<package name="com.liukun.mapper"/>

<!-- 否则要这样写，一个一个加载
<mapper resource="com.liukun.mapper.StudentMapper.xml"/>
<mapper resource="com.liukun.mapper.GradeMapper.xml"/>
-->
</mappers>
</configuration>
Student.java对应的表student
create table student(
studentId int(11) primary key,
studentName varchar(20) not null,
grade_id int(11) not null,
constraint student_fk_grade FOREIGN KEY (grade_id) REFERENCES grade (gradeId) //为student表添加了一个外键关联grade表的主键
Grade.java对应的表grdae

create table grade(
gradeId int(11) primary key,
gradeName varchar(20) not null
);

整个对于mybatis的一对多双向关联mapper动态代理开发就完成了，上面的StudentMapper.java和GradeMapper.java与其相对应的.xml文件要放在同一个文件夹中（包)，接口中的方法要与相对应的mapper,xml中的select的id一样，返回值，参数（这里没用到参数）都一样。