我们以一个例子demo的形式，说明Mybatis的一对一关联查询
设计teachers表
classes表设置外键
classes表
classes表是课程表，teachers表是教师信息表
这里是一对一的关联关系，当然实际中一般不是这样的，一般可以是多对多，不过这里只是举例子
项目的结构，如图：
Classes.java:
代码语言：javascript复制package com.mybatis.entity;
import java.util.List;
public class Classes {
private int id;
private String name;
private Teacher teacher;
private List<Student> students;
public List<Student> getStudents() {
return students;
}
public void setStudents(List<Student> students) {
this.students = students;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Teacher getTeacher() {
return teacher;
}
public void setTeacher(Teacher teacher) {
this.teacher = teacher;
}
@Override
public String toString() {
return "Classes [id=" id ", name=" name "]";
}
}Teacher.java:
代码语言：javascript复制package com.mybatis.entity;
public class Teacher {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}这里以两种方式进行使用
classesMapper.xml:
代码语言：javascript复制<?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">
<!--
为这个mapper指定一个唯一的namespace，namespace的值习惯上设置成包名 sql映射文件名，这样就能够保证namespace的值是唯一的
-->
<mapper namespace="com.mybatis.mapper.classesMapper">
<!--
在select标签中编写查询的SQL语句， 设置select标签的id属性为findById，id属性值必须是唯一的，不能够重复
parameterType可以在MyBatis的配置文件里设置，也可以直接用包名 类名
-->
<select id="findById" parameterType="int" resultMap="ClassesResultMap">
select * from classes c,teachers t where c.teacher_id=t.t_id and c.c_id=#{id}
</select>
<resultMap type="Classes" id="ClassesResultMap">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" column="teacher_id" javaType="Teacher">
<id property="id" column="t_id" />
<result property="name" column="t_name" />
</association>
</resultMap>
<select id="findById2" parameterType="int" resultMap="ClassesResultMap2">
select * from classes where c_id=#{id}
</select>
<resultMap type="Classes" id="ClassesResultMap2">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacherInfo">
</association>
</resultMap>
<select id="getTeacherInfo" parameterType="int" resultType="Teacher">
select t_id,t_name from teachers where t_id=#{id}
</select>
</mapper>然后对mybatis进行配置：
代码语言：javascript复制<?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>
<!-- 数据库配置文件 -->
<properties resource="db.properties" />
<!-- 实体类,简称 -设置别名 -->
<typeAliases>
<typeAlias alias="User" type="com.mybatis.entity.User" />
<typeAlias alias="Classes" type="com.mybatis.entity.Classes" />
<typeAlias alias="Student" type="com.mybatis.entity.Student" />
<typeAlias alias="Teacher" type="com.mybatis.entity.Teacher" />
</typeAliases>
<!-- 读取db.properties里的数据，进行数据库配置 -->
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}" />
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<!--
注册userMapper.xml文件
<mappers>
<mapper resource="com/mybatis/mapper/userMapper.xml"/>
</mappers>
-->
<mappers>
<mapper class="com.mybatis.mapper.UserMapper"/>
<mapper resource="com/mybatis/mapper/classesMapper.xml"/>
</mappers>
</configuration>db,properties:
代码语言：javascript复制driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/db_mysql?useUnicode=true&characterEncoding=utf-8
username=root
password=111编写测试类：
代码语言：javascript复制String resource = "mybatis-config.xml";
try{
//加载Mybatis的配置文件
Reader reader = Resources.getResourceAsReader(resource);
//构建sqlSession的工厂
SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);
//创建可以执行sql的SqlSession
SqlSession session = sessionFactory.openSession();
UserMapper mapper = session.getMapper(UserMapper.class);
User user = mapper.findById(1);
System.out.println(user);
Classes c = session.selectOne("com.mybatis.mapper.classesMapper.findById1",1);
System.out.println(c);
}catch (Exception e) {
e.printStackTrace();
}
