MyBatis 关联映射之多对多
我们在这里新建一个数据库并取名 mybatis 用作实验。
创建学生表 tb_student 并插入两条数据：
mysql> create table tb_student(
-> s_id int primary key auto_increment,
-> s_name varchar(20),
-> s_sex varchar(10),
-> s_age int);
insert into tb_student(s_name,s_sex,s_age) values('Tom','male',18);
insert into tb_student(s_name,s_sex,s_age) values('Jack','male',19);创建课程表 tb_course 并插入两条数据：
mysql> create table tb_course(
-> c_id int primary key auto_increment,
-> c_name varchar(20),
-> c_credit int);
insert into tb_course(c_name,c_credit) values('Math',5);
insert into tb_course(c_name,c_credit) values('Computer',4);由于学生和课程是多对多的关联关系，因此创建中间表：选课表 tb_select_course 并插入数据
mysql> create table tb_select_course(
-> sc_s_id int,
-> sc_c_id int,
-> sc_date date,
-> primary key(sc_s_id,sc_c_id),
-> foreign key(sc_s_id) references tb_student(s_id),
-> foreign key(sc_c_id) references tb_course(c_id));
insert into tb_select_course(sc_s_id,sc_c_id,sc_date) values(1,1,'2017-03-01');
insert into tb_select_course(sc_s_id,sc_c_id,sc_date) values(1,2,'2017-03-01');
insert into tb_select_course(sc_s_id,sc_c_id,sc_date) values(2,1,'2017-03-02');
insert into tb_select_course(sc_s_id,sc_c_id,sc_date) values(2,2,'2017-03-02');检测数据插入情况：
select * from tb_course;select * from tb_select_course;4.2 新建项目
按照第二节中的方法新建动态 Web 工程，命名为 ManyToMany。
4.3 导入所需 jar 包
所需 jar 包：
- MyBatis jar 包： mybatis-3.3.0.jar
- MySQL 驱动 jar 包： mysql-connector-java-5.1.35.jar
- 日志记录 jar 包： log4j-1.2.17.jar
将上述的三个 jar 包拷贝到项目的 /WebContent/WEB-INF/lib/ 目录下。
4.4 实体类
在Java Resources/src 的包 shiyanlou.mybatis.manytomany.model 下新建类 Student.java，一个学生具有 id、name、sex、age、courses（List）属性。学生和课程之间是多对多关系，一个学生可以选多门课。
Student.java 的代码如下：
package shiyanlou.mybatis.manytomany.model;
import java.util.List;
public class Student {
private Integer id;
private String name;
private String sex;
private Integer age;
private List courses;
public Student() {
}
public Student(Integer id, String name, String sex, Integer age,List courses) {
this.id = id;
this.name = name;
this.sex = sex;
this.age = age;
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 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 List getCourses() {
return courses;
}
public void setCourses(List courses) {
this.courses = courses;
}
} 再在包 shiyanlou.mybatis.manytomany.model 下新建类 Course.java，一个班级有 id，name，credit、students（List） 属性。课程和学生之间是多对多关系，一个课程可以由多个学生选。
Course.java 的代码如下：
package shiyanlou.mybatis.manytomany.model;
import java.util.List;
public class Course {
private Integer id;
private String name;
private Integer credit;
private List students;
public Course() {
}
public Course(Integer id, String name, Integer credit,
List students) {
this.id = id;
this.name = name;
this.credit = credit;
this.students = students;
}
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 Integer getCredit() {
return credit;
}
public void setCredit(Integer credit) {
this.credit = credit;
}
public List getStudents() {
return students;
}
public void setStudents(List students) {
this.students = students;
}
} 最后在包 shiyanlou.mybatis.manytomany.model 下新建类 StudentCourseLink.java，用来描述学生和课程之间的关系，其包含
student（Student）、course（Course）、date 属性。
package shiyanlou.mybatis.manytomany.model;
import java.util.Date;
public class StudentCourseLink {
private Student student;
private Course course;
private Date date;
public StudentCourseLink() {
}
public StudentCourseLink(Student student, Course course, Date date) {
this.student = student;
this.course = course;
this.date = date;
}
public Student getStudent() {
return student;
}
public void setStudent(Student student) {
this.student = student;
}
public Course getCourse() {
return course;
}
public void setCourse(Course course) {
this.course = course;
}
public Date getDate() {
return date;
}
public void setDate(Date date) {
this.date = date;
}
}4.5 创建方法接口和定义映射文件
新建包 shiyanlou.mybatis.manytomany.mapper ，并在包下新建方法接口 StudentMapper.java。
StudentMapper 接口的代码如下：
package shiyanlou.mybatis.manytomany.mapper;
import shiyanlou.mybatis.manytomany.model.Student;
import shiyanlou.mybatis.manytomany.model.StudentCourseLink;
import java.util.List;
public interface StudentMapper {
/*
* 查询所有学生及他们的选择课程的信息
* @return
* @throws Exception
*/
public List selectStudentCourse() throws Exception;
/*
* 删除指定id用户的某门课（根据课程id）的选课情况
* @param StudentCourseLink
* @throws Exception
*/
public void deleteStudentCourseById(StudentCourseLink scLink) throws Exception;
} 在包 shiyanlou.mybatis.onetomany.mapper 下新建映射文件 StudentMapper.xml ，映射文件与接口名相同。
StudentMapper.xml 的配置如下：
<mapper namespace="shiyanlou.mybatis.manytomany.mapper.StudentMapper">
<select id="selectStudentCourse" resultMap="studentCourseMap">
select
s.*,c.* from
tb_student s,tb_course c,tb_select_course sc
where s.s_id=sc.sc_s_id
and c.c_id=sc.sc_c_id
select>
<delete id="deleteStudentCourseById" parameterType="StudentCourseLink">
delete from tb_select_course where sc_s_id=#{student.id} and sc_c_id=#{course.id}
delete>
<resultMap id="studentCourseMap" type="Student">
<id property="id" column="s_id" />
<result property="name" column="s_name" />
<result property="sex" column="s_sex" />
<result property="age" column="s_age" />
<collection property="courses" ofType="Course">
<id property="id" column="c_id" />
<result property="name" column="c_name" />
<result property="credit" column="c_credit" />
collection>
resultMap>
mapper>在这里，采用的是集合的嵌套结果映射的方式，使用了
<configuration>
<typeAliases>
<package name="shiyanlou.mybatis.manytomany.model" />
typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC" />
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost:3306/mybatis" />
<property name="username" value="root" />
<property name="password" value="" />
dataSource>
environment>
environments>
<mappers>
<package name="shiyanlou/mybatis/manytomany/mapper" />
mappers>
configuration>4.7 日志记录 log4j.properties
使用日志文件是为了查看控制台输出的 SQL 语句。
在项目目录 /Java Resources/src 下新建 MyBatis 日志记录文件
log4j.properties ，在里面添加如下内容：
# Global logging configuration
log4j.rootLogger=DEBUG, stdout
# 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%n4.8 测试类 Test
在包 shiyanlou.mybatis.manytomany.test 下新建测试类 Test.java ，代码如下：
package shiyanlou.mybatis.manytomany.test;
import shiyanlou.mybatis.manytomany.mapper.StudentMapper;
import shiyanlou.mybatis.manytomany.model.Course;
import shiyanlou.mybatis.manytomany.model.Student;
import shiyanlou.mybatis.manytomany.model.StudentCourseLink;
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;
public class Test {
private static SqlSessionFactory sqlSessionFactory;
public static void main(String[] args) {
// Mybatis 配置文件
String resource = "mybatis.cfg.xml";
// 得到配置文件流
InputStream inputStream = null;
try {
inputStream = Resources.getResourceAsStream(resource);
} catch (IOException e) {
e.printStackTrace();
}
// 创建会话工厂，传入 MyBatis 的配置文件信息
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
selectStudentCourse();
//deleteStudentCourseById();
}
// 查询所有学生及他们的选择课程的信息
private static void selectStudentCourse(){
// 通过工厂得到 SqlSession
SqlSession session = sqlSessionFactory.openSession();
StudentMapper mapper = session.getMapper(StudentMapper.class);
try {
List students = mapper.selectStudentCourse();
session.commit();
for(Student stu:students){
System.out.println(stu.getId()+","+stu.getName()+","+stu.getSex()+","+stu.getAge()+":");
List courses = stu.getCourses();
for(Course cou:courses){
System.out.println(cou.getId()+","+cou.getName()+","+cou.getCredit());
}
}
} catch (Exception e) {
e.printStackTrace();
session.rollback();
}
// 释放资源
session.close();
}
// 根据学生id和课程id删除该学生该门课的选课情况
private static void deleteStudentCourseById(){
SqlSession session = sqlSessionFactory.openSession();
StudentMapper mapper = session.getMapper(StudentMapper.class);
try {
Student student = new Student();
student.setId(1);
Course course = new Course();
course.setId(2);
StudentCourseLink scLink = new StudentCourseLink();
scLink.setStudent(student);
scLink.setCourse(course);
mapper.deleteStudentCourseById(scLink);
session.commit();
} catch (Exception e) {
e.printStackTrace();
session.rollback();
}
session.close();
}
} 4.9 运行测试
运行测试类 Test.java，分别调用 Test 类中的两个方法。
结果如下：
4.9.1 selectStudentCourse()
查询所有学生及他们的选择课程的信息
4.9.2 deleteStudentCourseById()
删除 id 为1的学生选择id为2的课程选课情况
再次调用 selectStudentCourse() 方法，发现 id 为1的学生已经将 id 为2的课程退选。
