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:
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package com.mybatis.entity;
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import java.util.List;
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public class Classes {
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private int id;
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private String name;
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private Teacher teacher;
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private List<Student> students;
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public List<Student> getStudents() {
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return students;
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}
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public void setStudents(List<Student> students) {
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this.students = students;
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}
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public int getId() {
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return id;
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}
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public void setId(int id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public Teacher getTeacher() {
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return teacher;
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}
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public void setTeacher(Teacher teacher) {
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this.teacher = teacher;
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}
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@Override
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public String toString() {
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return "Classes [id=" + id + ", name=" + name + "]";
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}
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}
Teacher.java:
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package com.mybatis.entity;
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public class Teacher {
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private int id;
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private String name;
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public int getId() {
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return id;
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}
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public void setId(int id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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}
这里以两种方式进行使用
classesMapper.xml:
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<?xml version="1.0" encoding="UTF-8" ?>
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<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
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"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
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<!--
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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
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-->
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<mapper namespace="com.mybatis.mapper.classesMapper">
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<!--
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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
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parameterType可以在MyBatis的配置文件里设置，也可以直接用包名+类名
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-->
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<select id="findById" parameterType="int" resultMap="ClassesResultMap">
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select * from classes c,teachers t where c.teacher_id=t.t_id and c.c_id=#{id}
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</select>
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<resultMap type="Classes" id="ClassesResultMap">
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<id property="id" column="c_id" />
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<result property="name" column="c_name" />
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<association property="teacher" column="teacher_id" javaType="Teacher">
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<id property="id" column="t_id" />
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<result property="name" column="t_name" />
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</association>
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</resultMap>
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<select id="findById2" parameterType="int" resultMap="ClassesResultMap2">
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select * from classes where c_id=#{id}
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</select>
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<resultMap type="Classes" id="ClassesResultMap2">
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<id property="id" column="c_id" />
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<result property="name" column="c_name" />
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<association property="teacher" column="teacher_id" javaType="Teacher" select="getTeacherInfo">
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</association>
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</resultMap>
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<select id="getTeacherInfo" parameterType="int" resultType="Teacher">
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select t_id,t_name from teachers where t_id=#{id}
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</select>
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</mapper>
然后对mybatis进行配置：
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
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"http://mybatis.org/dtd/mybatis-3-config.dtd">
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<configuration>
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<!-- 数据库配置文件 -->
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<properties resource="db.properties" />
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<!-- 实体类,简称 -设置别名 -->
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<typeAliases>
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<typeAlias alias="User" type="com.mybatis.entity.User" />
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<typeAlias alias="Classes" type="com.mybatis.entity.Classes" />
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<typeAlias alias="Student" type="com.mybatis.entity.Student" />
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<typeAlias alias="Teacher" type="com.mybatis.entity.Teacher" />
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</typeAliases>
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<!-- 读取db.properties里的数据，进行数据库配置 -->
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<environments default="development">
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<environment id="development">
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<transactionManager type="JDBC" />
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<dataSource type="POOLED">
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<property name="driver" value="${driver}"/>
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<property name="url" value="${url}" />
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<property name="username" value="${username}"/>
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<property name="password" value="${password}"/>
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</dataSource>
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</environment>
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</environments>
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<!--
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注册userMapper.xml文件
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<mappers>
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<mapper resource="com/mybatis/mapper/userMapper.xml"/>
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</mappers>
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-->
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<mappers>
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<mapper class="com.mybatis.mapper.UserMapper"/>
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<mapper resource="com/mybatis/mapper/classesMapper.xml"/>
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</mappers>
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</configuration>
db,properties:
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driver=com.mysql.jdbc.Driver
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url=jdbc\:mysql\://localhost\:3306/db_mysql?useUnicode\=true&characterEncoding\=utf-8
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username=root
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password=111
编写测试类：
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String resource = "mybatis-config.xml";
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try{
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//加载Mybatis的配置文件
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Reader reader = Resources.getResourceAsReader(resource);
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//构建sqlSession的工厂
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SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);
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//创建可以执行sql的SqlSession
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SqlSession session = sessionFactory.openSession();
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UserMapper mapper = session.getMapper(UserMapper.class);
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User user = mapper.findById(1);
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System.out.println(user);
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Classes c = session.selectOne("com.mybatis.mapper.classesMapper.findById1",1);
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System.out.println(c);
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}catch (Exception e) {
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e.printStackTrace();
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}
文章来源: smilenicky.blog.csdn.net，作者：smileNicky，版权归原作者所有，如需转载，请联系作者。
原文链接：smilenicky.blog.csdn.net/article/details/51292271