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mybatis-config.xml
<?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>
    <settings>
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>
    <environments default="dev">
        <environment id="dev">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/imooc?useUnicode=true&amp;characterEncoding=UTF-8"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mappers/student.xml"/>
        <mapper resource="mappers/classes.xml"/>
    </mappers>
</configuration>
==========================================
classes.xml
<?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="classes">
<select id="selectAll" resultType="com.imooc.mybatis.entity.Classes">
        select * from classes
    </select>
    <select id="selectById" parameterType="String" resultType="com.imooc.mybatis.entity.Classes">
        select * from classes where classno = #{value}
    </select>
    <resultMap id="rmClasses" type="com.imooc.mybatis.entity.Classes">
        <id column="classno" property="classNo"></id>
        <collection property="students" select="student.selectById"
                    column="classno"/>
    </resultMap>
    <select id="selectOneToMany" resultMap="rmClasses">
        select * from classes
    </select>
</mapper>
===============================
student.xml
<?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="student">
    <select id="selectAll" resultType="com.imooc.mybatis.entity.Student">
        select * from student order by stuno desc limit 10
    </select>
    <select id="selectById" parameterType="String" resultType="com.imooc.mybatis.entity.Student">
        select * from student where classno = #{value}
    </select>

    <resultMap id="rmStudent" type="com.imooc.mybatis.entity.Student">
        <id column="classno" property="classNo"/>
        <association property="classes" select="classes.selectById" column="classno"></association>
    </resultMap>
    <select id="selectManyToOne" resultMap="rmStudent">
        select *from student
    </select>
</mapper>
==========================
classes.java
​package com.imooc.mybatis.entity;

import java.util.List;

public class Classes {
    private Integer id;
    private String classNo;
    private String name;
    private String major;
    private List<Student> students;

    public List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getClassNo() {
        return classNo;
    }

    public void setClassNo(String classNo) {
        this.classNo = classNo;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }
}
=========================
student.java
​package com.imooc.mybatis.entity;

public class Student {
    private Integer id;
    private String stuNo;
    private String stuName;
    private String sex;
    private String classNo;
    private Classes classes;

    public Classes getClasses() {
        return classes;
    }

    public void setClasses(Classes classes) {
        this.classes = classes;
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getStuNo() {
        return stuNo;
    }

    public void setStuNo(String stuNo) {
        this.stuNo = stuNo;
    }

    public String getStuName() {
        return stuName;
    }

    public void setStuName(String stuName) {
        this.stuName = stuName;
    }

    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public String getClassNo() {
        return classNo;
    }

    public void setClassNo(String classNo) {
        this.classNo = classNo;
    }
}