04—mybatis的关联映射

mybatis的关联映射
一对一
一对多
多对多

一.一对一（一个人只能有一个身份证号）

1.创建表
创建表tb_card

CREATE TABLE `tb_card` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`code` varchar(20) DEFAULT NULL,
PRIMARY KEY (`id`)
)
INSERT INTO tb_card(CODE)VALUES('150421178702260614');
创建表tb_person
CREATE TABLE `tb_person` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(20) DEFAULT NULL,
`sex` VARCHAR(20) DEFAULT NULL,
`age` INT(11) DEFAULT NULL,
`card_id` INT(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `card_id` (`card_id`),
CONSTRAINT `tb_person_idfk` FOREIGN KEY (`card_id`) REFERENCES `tb_card` (`id`)
)
INSERT INTO tb_person(NAME,sex,age,card_id)VALUES('jack','男',23,1);

2.创建model

Card.java

package org.fkit.domain;
import java.io.Serializable;
public class Card implements Serializable {
    private Integer id;  // 主键id
    private String code; // 身份证编号
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    @Override
    public String toString() {
        return "Card [id=" + id + ", code=" + code + "]";
    }
}

Person.java

package org.fkit.domain;
import java.io.Serializable;
public class Person implements Serializable {
    private Integer id;  // 主键id
    private String name; // 姓名
    private String sex;  // 性别
    private Integer age; // 年龄
    
    // 人和身份证是一对一的关系，即一个人只有一个身份证
    private Card card;
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getSex() {
        return sex;
    }
    public void setSex(String sex) {
        this.sex = sex;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public Card getCard() {
        return card;
    }
    public void setCard(Card card) {
        this.card = card;
    }
    @Override
    public String toString() {
        return "Person [id=" + id + ", name=" + name + ", sex=" + sex
                + ", age=" + age + "]";
    }
}

3.Mapper类和xml文件
PersonMapper .java

package org.fkit.mapper;
import org.fkit.domain.Person;
public interface PersonMapper {
    /**
     * 根据id查询Person
     * 方法名和参数必须和XML文件中的<select.../>元素的id属性和parameterType属性一致
     * @param id
     * @return Person对象
     * */
    Person selectPersonById(Integer id);
}

PersonMapper .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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.fkit.mapper.PersonMapper">

<!-- 根据id查询Person，返回resultMap -->
  <select id="selectPersonById" parameterType="int" 
      resultMap="personMapper">
      SELECT * from tb_person where id = #{id} 
  </select>

 <!-- 映射Peson对象的resultMap -->
    <resultMap type="org.fkit.domain.Person" id="personMapper">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="sex" column="sex"/>
        <result property="age" column="age"/>
                <!-- 下边完成关联信息的映射：association -->
        <association property="card" column="card_id"
        select="org.fkit.mapper.CardMapper.selectCardById" 
        javaType="org.fkit.domain.Card"/>
    </resultMap>
</mapper>

CardMapper.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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.fkit.mapper.CardMapper">
    <!-- 根据id查询Card，返回Card对象 -->
  <select id="selectCardById" parameterType="int" resultType="org.fkit.domain.Card">
      SELECT * from tb_card where id = #{id} 
  </select>
</mapper>

4.mybatis-config.xml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">
  <!--  XML 配置文件包含对 MyBatis 系统的核心设置 -->
<configuration>
    <!-- 指定 MyBatis 所用日志的具体实现 -->
    <settings>
        <setting name="logImpl" value="LOG4J"/>
    </settings>
    <environments default="mysql">
    <!-- 环境配置，即连接的数据库。 -->
    <environment id="mysql">
    <!--  指定事务管理类型，type="JDBC"指直接简单使用了JDBC的提交和回滚设置 -->
      <transactionManager type="JDBC"/>
      <!--  dataSource指数据源配置，POOLED是JDBC连接对象的数据源连接池的实现。 -->
      <dataSource type="POOLED">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis"/>
        <property name="username" value="root"/>
        <property name="password" value="123456"/>
      </dataSource>
    </environment>
  </environments>
  <!-- mappers告诉了MyBatis去哪里找持久化类的映射文件 -->
  <mappers>
      <mapper resource="org/fkit/mapper/CardMapper.xml"/>
      <mapper resource="org/fkit/mapper/PersonMapper.xml"/>
  </mappers>
</configuration>

log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.org.fkit.mapper.PersonMapper=DEBUG
log4j.logger.org.fkit.mapper.CardMapper=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

5.测试

package org.fkit.test;
import java.io.IOException;
import java.io.InputStream;
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.fkit.domain.Person;
import org.fkit.mapper.PersonMapper;

public class mytest {
    public static void main(String[] args) throws IOException {
        // 读取mybatis-config.xml文件
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        // 初始化mybatis，创建SqlSessionFactory类的实例
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        // 创建Session实例
        SqlSession session = sqlSessionFactory.openSession();
        // 获得mapper接口的代理对象
        PersonMapper pm = session.getMapper(PersonMapper.class);
        // 直接调用接口的方法，查询id为1的Peson数据
        Person p = pm.selectPersonById(1);
        // 打印Peson对象
        System.out.println(p);
        // 打印Person对象关联的Card对象
        System.out.println(p.getCard());
        // 提交事务
        session.commit();
        // 关闭Session
        session.close();
    }
}

二.一对多（一个班级有多个学生）

1.创建表

创建表tb_clazz

CREATE TABLE tb_clazz(
id INT PRIMARY KEY AUTO_INCREMENT,
CODE VARCHAR(18),
NAME VARCHAR(18)
);
INSERT INTO tb_clazz(CODE,NAME) VALUES('2017001','Java双语幼儿园中班');

创建表tb_student
CREATE TABLE tb_student(
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR(18),
sex VARCHAR(18),
age INT,
clazz_id INT,
FOREIGN KEY (clazz_id) REFERENCES tb_clazz(id)
);

INSERT INTO tb_student(NAME,sex,age,clazz_id) VALUES('aa','男',23,1);
INSERT INTO tb_student(NAME,sex,age,clazz_id) VALUES('bb','女',18,1);
INSERT INTO tb_student(NAME,sex,age,clazz_id) VALUES('cc','男',21,1);
INSERT INTO tb_student(NAME,sex,age,clazz_id) VALUES('dd','女',20,1);

2.创建model
Clazz.java

package org.fkit.domain;
import java.io.Serializable;
import java.util.List;
public class Clazz implements Serializable {
    
    private Integer id; // 班级id，主键
    private String code; // 班级编号
    private String name; // 班级名称
    
    // 班级和学生是一对多的关系，即一个班级可以有多个学生
    private List<Student> students;
    
    public Clazz() {
        super();
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getCode() {
        return code;
    }
    public void setCode(String code) {
        this.code = code;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<Student> getStudents() {
        return students;
    }
    public void setStudents(List<Student> students) {
        this.students = students;
    }
    @Override
    public String toString() {
        return "Clazz [id=" + id + ", code=" + code + ", name=" + name + "]";
    }
}

Student.java

package org.fkit.domain;
import java.io.Serializable;
public class Student implements Serializable {
    private Integer id; // 学生id，主键
    private String name; // 姓名
    private String sex;  // 性别
    private Integer age; // 年龄
    
    // 学生和班级是多对一的关系，即一个学生只属于一个班级
    private Clazz clazz;

    public Student() {
        super();
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public String getSex() {
        return sex;
    }

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

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public Clazz getClazz() {
        return clazz;
    }

    public void setClazz(Clazz clazz) {
        this.clazz = clazz;
    }

    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", sex=" + sex
                + ", age=" + age + "]";
    }

    
}

3.Mapper类和xml文件
ClazzMapper.java

package org.fkit.mapper;
import org.fkit.domain.Clazz;
public interface ClazzMapper {
    // 根据id查询班级信息
    Clazz selectClazzById(Integer id);
}

StudentMapper.java

package org.fkit.mapper;
import org.fkit.domain.Student;
public interface StudentMapper {
    // 根据id查询学生信息
    Student selectStudentById(Integer id);
}

ClazzMapper.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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.fkit.mapper.ClazzMapper">
    <!-- 根据id查询班级信息，返回resultMap -->
      <select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
          SELECT * FROM tb_clazz  WHERE id = #{id}
      </select>
       <!-- 映射Clazz对象的resultMap -->
    <resultMap type="org.fkit.domain.Clazz" id="clazzResultMap">
        <id property="id" column="id"/>
        <result property="code" column="code"/>
        <result property="name" column="name"/>
        <!-- 一对多关联映射:collection fetchType="lazy"表示懒加载  -->
        <collection property="students" javaType="ArrayList"
      column="id" ofType="org.fkit.domain.Student"
      select="org.fkit.mapper.StudentMapper.selectStudentByClazzId"
      fetchType="lazy">
          <id property="id" column="id"/>
          <result property="name" column="name"/>
          <result property="sex" column="sex"/>
          <result property="age" column="age"/>
        </collection>
    </resultMap>
</mapper>

StudentMapper.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">
<!-- namespace指用户自定义的命名空间。 -->
<mapper namespace="org.fkit.mapper.StudentMapper">
  <!-- 根据id查询学生信息，多表连接，返回resultMap -->
  <select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
      SELECT * FROM tb_clazz c,tb_student s
      WHERE c.id = s.clazz_id
       AND s.id = #{id}
  </select>
  
  <!-- 根据班级id查询学生信息，返回resultMap -->
  <select id="selectStudentByClazzId" parameterType="int" 
  resultMap="studentResultMap">
      SELECT * FROM tb_student WHERE clazz_id = #{id}
  </select>
   <!-- 映射Student对象的resultMap -->
    <resultMap type="org.fkit.domain.Student" id="studentResultMap">
        <id property="id" column="id"/>
          <result property="name" column="name"/>
          <result property="sex" column="sex"/>
          <result property="age" column="age"/>
        <!-- 多对一关联映射:association   -->
        <association property="clazz" javaType="org.fkit.domain.Clazz">
            <id property="id" column="id"/>
            <result property="code" column="code"/>
            <result property="name" column="name"/>
        </association>
    </resultMap>
</mapper>

4.mybatis-config.xml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">
  <!--  XML 配置文件包含对 MyBatis 系统的核心设置 -->
<configuration>
    <!-- 指定 MyBatis 所用日志的具体实现 -->
    <settings>
        <setting name="logImpl" value="LOG4J"/>
        <!-- 要使延迟加载生效必须配置下面两个属性 -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>
    <environments default="mysql">
    <!-- 环境配置，即连接的数据库。 -->
    <environment id="mysql">
    <!--  指定事务管理类型，type="JDBC"指直接简单使用了JDBC的提交和回滚设置 -->
      <transactionManager type="JDBC"/>
      <!--  dataSource指数据源配置，POOLED是JDBC连接对象的数据源连接池的实现。 -->
      <dataSource type="POOLED">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://127.0.0.1:3306/mytestdb"/>
        <property name="username" value="root"/>
        <property name="password" value="root"/>
      </dataSource>
    </environment>
  </environments>
  <!-- mappers告诉了MyBatis去哪里找持久化类的映射文件 -->
  <mappers>
      <mapper resource="org/fkit/mapper/ClazzMapper.xml"/>
      <mapper resource="org/fkit/mapper/StudentMapper.xml"/>
  </mappers>
</configuration>

log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.org.fkit.mapper.ClazzMapper=DEBUG
log4j.logger.org.fkit.mapper.StudentMapper=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

5.测试

package org.fkit.test;
import java.io.InputStream;
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.fkit.domain.Clazz;
import org.fkit.domain.Student;
import org.fkit.mapper.ClazzMapper;
import org.fkit.mapper.StudentMapper;
public class OneToManyTest {

    public static void main(String[] args) throws Exception {
        // 读取mybatis-config.xml文件
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        // 初始化mybatis，创建SqlSessionFactory类的实例
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(inputStream);
        // 创建Session实例
        SqlSession session = sqlSessionFactory.openSession();
        OneToManyTest t = new OneToManyTest();
        t.testSelectClazzById(session);
//        t.testSelectStudentById(session);
        
        // 提交事务
        session.commit();
        // 关闭Session
        session.close();
    }
    
    // 测试一对多，查询班级Clazz（一）的时候级联查询学生Student（多）  
    public void testSelectClazzById(SqlSession session){
        // 获得ClazzMapper接口的代理对象
        ClazzMapper cm = session.getMapper(ClazzMapper.class);
        // 调用selectClazzById方法
        Clazz clazz = cm.selectClazzById(1);
        // 查看查询到的clazz对象信息
        System.out.println(clazz.getId() + " "+ clazz.getCode() + " "+clazz.getName());
        // 查看clazz对象关联的学生信息
        List<Student> students = clazz.getStudents();
        for(Student stu : students){
            System.out.println(stu);
        }
    }
    
    // 测试多对一，查询学生Student（多）的时候级联查询 班级Clazz（一）
    public void testSelectStudentById(SqlSession session){
        // 获得StudentMapper接口的代理对象
        StudentMapper sm = session.getMapper(StudentMapper.class);
        // 调用selectStudentById方法
        Student stu = sm.selectStudentById(1);
        // 查看查询到的Student对象信息
        System.out.println(stu);
        // 查看Student对象关联的班级信息
        System.out.println(stu.getClazz());
    }
}

三.多对多（学生与课程的关系，学生可以学多门课程，课程同样也可以有多个学生）

1.创建表

CREATE TABLE `t_student` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `student_name` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
)
INSERT INTO t_student (student_name)VALUES ('小米');
INSERT INTO t_student (student_name)VALUES ('小王');
INSERT INTO t_student (student_name)VALUES ('小李');

CREATE TABLE `t_courses` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `courses_name` varchar(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
)
INSERT INTO t_courses (courses_name)VALUES ('java');
INSERT INTO t_courses (courses_name)VALUES ('c#');
INSERT INTO t_courses (courses_name)VALUES ('php');
INSERT INTO t_courses (courses_name)VALUES ('vb');
INSERT INTO t_courses (courses_name)VALUES ('javascript');

CREATE TABLE `t_stu_cou` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `stu_id` int(11) DEFAULT NULL,
  `cou_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `stu_id` (`stu_id`),
  KEY `cou_id` (`cou_id`),
  CONSTRAINT `t_stu_cou_ibfk_1` FOREIGN KEY (`stu_id`) REFERENCES `t_student` (`id`),
  CONSTRAINT `t_stu_cou_ibfk_2` FOREIGN KEY (`cou_id`) REFERENCES `t_courses` (`id`)
)  

INSERT INTO t_stu_cou (stu_id,cou_id)VALUES (1,1);
INSERT INTO t_stu_cou (stu_id,cou_id)VALUES (1,2);
INSERT INTO t_stu_cou (stu_id,cou_id)VALUES (1,3);

INSERT INTO t_stu_cou (stu_id,cou_id)VALUES (2,2);
INSERT INTO t_stu_cou (stu_id,cou_id)VALUES (2,3);
INSERT INTO t_stu_cou (stu_id,cou_id)VALUES (2,4);

2.创建model
CoursesBean.java

package org.fkit.domain;
import java.io.Serializable;
import java.util.List;
public class CoursesBean implements Serializable{
    private static final long serialVersionUID = 1L;
    private Integer id;
    private String name;
   // 使用 List<StudentBean>集合，是说明学习这门课程的所有学生
    private List<StudentBean> student;
    public CoursesBean() {
        super();
    }
    public CoursesBean(Integer id, String name, List<StudentBean> student) {
        super();
        this.id = id;
        this.name = name;
        this.student = student;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<StudentBean> getStudent() {
        return student;
    }
    public void setStudent(List<StudentBean> student) {
        this.student = student;
    }
    @Override
    public String toString() {
        return "CoursesBean [id=" + id + ", name=" + name + ", student="
                + student + "]";
    }
}

StudentBean.java

package org.fkit.domain;
import java.io.Serializable;
import java.util.List;
public class StudentBean implements Serializable{
    private static final long serialVersionUID = 1L;
    private Integer id;
    private String name;    
    private List<CoursesBean> courses;
    public StudentBean() {
        super();
        // TODO Auto-generated constructor stub
    }
    public StudentBean(Integer id, String name, List<CoursesBean> courses) {
        super();
        this.id = id;
        this.name = name;
        this.courses = courses;
    }
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List<CoursesBean> getCourses() {
        return courses;
    }
    public void setCourses(List<CoursesBean> courses) {
        this.courses = courses;
    }
    @Override
    public String toString() {
        return "StudentBean [id=" + id + ", name=" + name + ", courses="
                + courses + "]";
    }
}

3.Mapper类和xml文件

CoursesMapper.java

package org.fkit.mapper;
import org.fkit.domain.CoursesBean;
public interface CoursesMapper {
    
    /**
     * 根据id查询课程
     * @param id
     * @return
     */
    public CoursesBean findCouById(int id);
    
    /**
     * 要求查课时，将选课的学生一并查出
     * @param id
     * @return
     */
    public CoursesBean findCouAndStu(int id);
}

CoursesMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org/DTD Mapper 3.0" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.fkit.mapper.CoursesMapper">
 <resultMap type="CoursesBean" id="coursesMap">
   <!--   在默认情况下，mybatis会自动在TypeAliasRegistry初始化的时候挂在很多jdk常用类，
                          所以javaType="java.lang.Integer"可以写成javaType="Integer"-->  
    <id property="id" column="id" javaType="java.lang.Integer"/>
    <result property="name" column="courses_name" javaType="java.lang.String"/>
 </resultMap>
 <resultMap type="CoursesBean" id="couAndStu">
    <id property="id" column="id" javaType="java.lang.Integer"/>
    <result property="name" column="courses_name" javaType="java.lang.String"/>
    <!-- 对于一个属性的类型是一个集合，就使用collection
                      对于一个属性的类型是一个类，就使用association   -->
    <collection property="student" column="id" select="findStudentByCourses"></collection>
 </resultMap>
 <select id="findCouById" resultMap="coursesMap">
  select * from t_courses where id=#{id}
 </select>
 <!-- 有学生表，课程表这两张表都没有外键，我们就要使用第三张关联表。我们就要根据课程表的fk_cou_id，把学生的id值得到。
               对于多对多那么这个学生的id值就不可能是一个值 。在数据库里就要使用in-->
 <select id="findStudentByCourses" resultMap="org.fkit.mapper.StudentMapper.studentMap">
    select * from t_student where id in (select stu_id from t_stu_cou where cou_id=#{id})
 </select>
 <select id="findCouAndStu" resultMap="couAndStu">
 select * from t_courses where id=#{id}
 </select>
</mapper>

StudentMapper.java

package org.fkit.mapper;
import org.fkit.domain.StudentBean;
public interface StudentMapper {
    /**
     * 根据id值查询学生信息
     * @param id
     * @return
     */
    public StudentBean findStuById(int id);
    
    /**
     * 要求查询学生时，将学生选择的课程查出
     * @param id
     * @return
     */
    public StudentBean findStuAndCou(int id);
}

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org/DTD Mapper 3.0" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="org.fkit.mapper.StudentMapper">
<resultMap type="StudentBean" id="studentMap">
    <id property="id" column="id" javaType="java.lang.Integer"/>
    <result property="name" column="student_name" javaType="java.lang.String"/>
</resultMap>
<resultMap type="StudentBean" id="studentAndCourses">
     <id property="id" column="id" javaType="java.lang.Integer"/>
     <result property="name" column="student_name" javaType="java.lang.String"/>
     <collection property="courses" column="id" select="findCoursesByStudent"></collection>
</resultMap>
    <select id="findStuById" resultMap="studentMap">
        select * from t_student where id = #{id}
    </select>
    <select id="findStuAndCou" resultMap="studentAndCourses">
        select * from t_student where id = #{id}
    </select>
    <select id="findCoursesByStudent" resultMap="org.fkit.mapper.CoursesMapper.coursesMap">
        select * from t_courses where id in (select cou_id from t_stu_cou where stu_id = #{id})
    </select>
</mapper>

4.mybatis-config.xml文件

log4j.properties

# Global logging configuration
log4j.rootLogger=ERROR, stdout
# MyBatis logging configuration...
log4j.logger.org.fkit.mapper.PersonMapper=DEBUG
log4j.logger.org.fkit.mapper.CardMapper=DEBUG
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

db.properties

driver=com.mysql.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/mybatis
username=root
password=123456

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">
<!-- XML 配置文件包含对 MyBatis 系统的核心设置 -->
<configuration>
    <properties resource="db.properties" />
    <!-- 指定 MyBatis 所用日志的具体实现 -->
    <settings>
        <setting name="logImpl" value="LOG4J" />
    </settings>
    <!-- 为JAVA Bean起类别名 -->
    <typeAliases>
        <package name="org.fkit.domain" />
    </typeAliases>
    <environments default="mysql">
        <!-- 环境配置，即连接的数据库。 -->
        <environment id="mysql">
            <!-- 指定事务管理类型，type="JDBC"指直接简单使用了JDBC的提交和回滚设置 -->
            <transactionManager type="JDBC" />
            <!-- dataSource指数据源配置，POOLED是JDBC连接对象的数据源连接池的实现。 -->
            <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>
    <!-- mappers告诉了MyBatis去哪里找持久化类的映射文件 -->
    <mappers>
        <mapper resource="org/fkit/mapper/CoursesMapper.xml" />
        <mapper resource="org/fkit/mapper/StudentMapper.xml" />
    </mappers>
</configuration>

5.测试

package org.fkit.test;
import java.io.IOException;
import java.io.InputStream;
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.fkit.domain.CoursesBean;
import org.fkit.domain.StudentBean;
import org.fkit.mapper.CoursesMapper;
import org.fkit.mapper.StudentMapper;
public class ManyToManyTest {
    public static void main(String[] args) throws IOException {
        // 读取mybatis-config.xml文件
        InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
        // 初始化mybatis，创建SqlSessionFactory类的实例
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder()
                .build(inputStream);
        // 创建Session实例
        SqlSession session = sqlSessionFactory.openSession();
        ManyToManyTest t = new ManyToManyTest();
        // 根据用户id查询用户，测试一对多
        //t.testSelectCoursesById(session);
        // 根据订单id查询订单，测试多对多
        t.testSelectStudentById(session);
        // 提交事务
        session.commit();
        // 关闭Session
        session.close();

    }
    public void testSelectStudentById(SqlSession session){
        // 获得UserMapper接口的代理对象
        StudentMapper sm = session.getMapper(StudentMapper.class);
        // 调用selectUserById方法
        StudentBean stu = sm.findStuAndCou(1);
        System.out.println(stu);
        // 查看查询到的user对象信息
        System.out.println(stu.getId() + " " + stu.getName());
        // 查看user对象关联的订单信息
        List<CoursesBean> CoursesList = stu.getCourses();
        for(CoursesBean item : CoursesList){
            System.out.println(item);
        }
    }
    
    // 测试多对多，查询订单Order（多）的时候级联查询订单的商品Article（多）  
    public void testSelectCoursesById(SqlSession session){
        // 获得OrderMapper接口的代理对象
        CoursesMapper cm = session.getMapper(CoursesMapper.class);
        // 调用selectOrderById方法
        CoursesBean course = cm.findCouAndStu(2);
        System.out.println(course);
        // 查看查询到的order对象信息
        System.out.println(course.getId() + " " + course.getName());
        // 查看order对象关联的商品信息
        List<StudentBean> studentlist = course.getStudent();
        for(StudentBean stu : studentlist){
            System.out.println(stu);
        }
    }
}