Hibernate——关系映射之多对多

以你之姓@ 2022-05-30 10:47 143阅读 0赞

## **关系映射之多对多** ##

多对多的关系在操作和性能方面都不太理想，所以多对多的映射使用较少，实际使用中最好转换成一对多的对象模型；Hibernate会为我们创建中间表，转换成两个一对多。

经典案例：学生<–>选课，如下将学生选课关系用中间表来关联。

![多对多关系的转换][70]

这个案例在我们学习HQL语句的时候也用过，所以这里再简要介绍一下。更多的细节请参考【[Hibernate——HQL语句][Hibernate_HQL]】

三个对象的domain对象以及关系映射文件如下：

*  **Student对象以及关系映射文件：**

package com.gavin.domain;
    
    import java.util.Set;
    
    public class Student {
        
        private int id;
        private String name;
        private Set<StudentCourse> studentCourseSet;
    
        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 Set<StudentCourse> getStudentCourseSet() {
            return studentCourseSet;
        }
    
        public void setStudentCourseSet(Set<StudentCourse> studentCourseSet) {
            this.studentCourseSet = studentCourseSet;
        }
    }

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE hibernate-mapping PUBLIC
            "-//Hibernate/Hibernate Mapping DTD//EN"
            "http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    <hibernate-mapping>
        <class name="com.gavin.domain.Student" table="student" schema="hibernate">
            <id name="id" type="java.lang.Integer" column="id">
                <generator class="increment"/>
            </id>
            <property name="name" column="name" type="java.lang.String"/>
            <set name="studentCourseSet">
                <key column="sid"></key>
                <one-to-many class="com.gavin.domain.StudentCourse"/>
            </set>
        </class>
    </hibernate-mapping>

*  **Course对象以及关系映射文件：**

package com.gavin.domain;
    
    import java.util.Set;
    
    public class Course {
        
        private int id;
        private String name;
        private Set<StudentCourse> studentCourseSet;
    
        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 Set<StudentCourse> getStudentCourseSet() {
            return studentCourseSet;
        }
    
        public void setStudentCourseSet(Set<StudentCourse> studentCourseSet) {
            this.studentCourseSet = studentCourseSet;
        }
    }

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE hibernate-mapping PUBLIC
            "-//Hibernate/Hibernate Mapping DTD//EN"
            "http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    <hibernate-mapping package="com.gavin.domain">
        <class name="com.gavin.domain.Course" table="course" schema="hibernate">
            <id name="id" column="id" type="java.lang.Integer">
                <generator class="increment"/>
            </id>
            <property name="name" column="name" type="java.lang.String"/>
            <set name="studentCourseSet">
                <key column="cid"></key>
                <one-to-many class="com.gavin.domain.StudentCourse"></one-to-many>
            </set>
        </class>
    </hibernate-mapping>

*  **StudentCourse对象以及关系映射文件：**

package com.gavin.domain;
    
    public class StudentCourse {
        
        private int id;
        private Student student;
        private Course course;
        private int grade;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        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 int getGrade() {
            return grade;
        }
    
        public void setGrade(int grade) {
            this.grade = grade;
        }
    }

<?xml version="1.0" encoding="UTF-8" ?>
    <!DOCTYPE hibernate-mapping PUBLIC
            "-//Hibernate/Hibernate Mapping DTD//EN"
            "http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">
    <hibernate-mapping>
        <class name="com.gavin.domain.StudentCourse" table="studcourse" schema="hibernate">
            <id name="id" column="id" type="java.lang.Integer">
                <generator class="increment"/>
            </id>
            <property name="grade" type="java.lang.Integer" column="grade"/>
            <many-to-one name="student" column="sid"/>
            <many-to-one name="course" column="cid"/>
        </class>
    </hibernate-mapping>

*  **测试代码：**

SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
    Session session = sessionFactory.openSession();
    Transaction transaction = session.beginTransaction();
    
    Student student = new Student();
    student.setName("虾米");
    
    Course course = new Course();
    course.setName("数学");
    
    StudentCourse studentCourse = new StudentCourse();
    studentCourse.setStudent(student);
    studentCourse.setCourse(course);
    studentCourse.setGrade(89);
    
    session.save(student);
    session.save(course);
    session.save(studentCourse);
    
    transaction.commit();

*  Hibernate生成的SQL语句为：

Hibernate: drop table if exists course
    Hibernate: drop table if exists studcourse
    Hibernate: drop table if exists student
    Hibernate: create table course (id integer not null, name varchar(255), primary key (id)) engine=MyISAM
    Hibernate: create table studcourse (id integer not null, grade integer, sid integer, cid integer, primary key (id)) engine=MyISAM
    Hibernate: create table student (id integer not null, name varchar(255), primary key (id)) engine=MyISAM
    Hibernate: alter table studcourse add constraint FK3e0p086wyy1y3198jmcl1p07e foreign key (sid) references student (id)
    Hibernate: alter table studcourse add constraint FKdxogu0d4je13yayrhem591hvn foreign key (cid) references course (id)
    Hibernate: select max(id) from student
    Hibernate: select max(id) from course
    Hibernate: select max(id) from studcourse
    Hibernate: insert into student (name, id) values (?, ?)
    Hibernate: insert into course (name, id) values (?, ?)
    Hibernate: insert into studcourse (grade, sid, cid, id) values (?, ?, ?, ?)

*  **生成的表结构如下：**

![这里写图片描述][70 1]

[70]: /images/20220530/d3095ae472eb44ae99092bfe0f19b1ea.png
[Hibernate_HQL]: http://blog.csdn.net/gggavin/article/details/79478742
[70 1]: /images/20220530/54cafeeb48f74859a67fd15da484ca1b.png