Hibernate 多对多映射关系实现

分为两种：

1，多对多单向实现；
2，多对多双向实现；

 

实例分析如下：

 

1.多对多单向实现：　　　　　　　　　　　　　　　　　　　　　　　　　　　　

单向关系：

这里通过学生可以获取这个学生选修的课程；

但是通过课程无法获取选修这门课程的学生；

 

Student.java：

 

package com.cy.model;

import java.util.HashSet;
import java.util.Set;

public class Student {
    private int id;
    private String name;
    private Set<Course> courses = new HashSet<Course>();
    
    
    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<Course> getCourses() {
        return courses;
    }
    public void setCourses(Set<Course> courses) {
        this.courses = courses;
    }
    
    
}

 

Course.java:

 

package com.cy.model;

public class Course {
    private int id;
    private String name;
    
    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;
    }
    
    
}

 

Student.hbm.xml:

 

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.cy.model">

    <class name="Student" table="t_student">
        <id name="id" column="studentId">
            <generator class="native"></generator>
        </id>
        <property name="name" column="studentName"></property>
        
        <!-- 多对多没法用两个表来实现,必须借助第三张表，中间表（student_course）
             key column="student_id"表示学生课程表中的一个外键student_id关联学生表的主键
         -->
        <set name="courses" table="student_course" cascade="save-update">
            <key column="student_id"></key>
            <!-- many to many映射的另外一张表
                 student_course的外键course_id关联course表的主键
             -->
            <many-to-many class="com.cy.model.Course" column="course_id"></many-to-many>
        </set>
    </class>

</hibernate-mapping>

 

Course.hbm.xml:

 

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.cy.model">

    <class name="Course" table="t_course">
        <id name="id" column="courseId">
            <generator class="native"></generator>
        </id>
        <property name="name" column="courseName"></property>
    </class>

</hibernate-mapping>

 

测试代码：

StudentTest.java:

 

package com.cy.service;

import java.util.Set;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import com.cy.model.Course;
import com.cy.model.Student;
import com.cy.util.HibernateUtil;

public class StudentTest {

    private SessionFactory sessionFactory=HibernateUtil.getSessionFactory();
    private Session session;
    
    @Before
    public void setUp() throws Exception {
        session=sessionFactory.openSession(); 
        session.beginTransaction();
    }

    @After
    public void tearDown() throws Exception {
         session.getTransaction().commit();
         session.close();
    }

    @Test
    public void testSave1(){
        Course course1=new Course();
        course1.setName("语文");
        
        Course course2=new Course();
        course2.setName("数学");
        
        Student student1=new Student();
        student1.setName("张三");
        student1.getCourses().add(course1);
        student1.getCourses().add(course2);
        
        Student student2=new Student();
        student2.setName("李四");
        student2.getCourses().add(course1);
        student2.getCourses().add(course2);
        
        session.save(student1);
        session.save(student2);
    }
    
    /**
     * 获取张三选修的课程
     */
    @Test
    public void testLoad1(){
        Student s = (Student) session.get(Student.class, 1);
        Set<Course> courses = s.getCourses();
        for(Course course : courses) {
            System.out.println(course.getName());
        }
    }
}

 

生成的表结构：

 

 

插入数据后：

t_student：

t_course：

student_course：

 

 

2. 学生和课程是多对多的关系；

一个学生可以选多个课程；

一个课程可以被多个学生选中，所以是多对多的关系；

 

Student2.java:

 

package com.cy.model;

import java.util.HashSet;
import java.util.Set;

public class Student2 {
    private int id;
    private String name;
    private Set<Course2> courses = new HashSet<Course2>();
    
    
    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<Course2> getCourses() {
        return courses;
    }
    public void setCourses(Set<Course2> courses) {
        this.courses = courses;
    }
    
    
}

 

Course2.java:

 

package com.cy.model;

import java.util.HashSet;
import java.util.Set;

public class Course2 {
    private int id;
    private String name;
    private Set<Student2> students = new HashSet<Student2>();
    
    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<Student2> getStudents() {
        return students;
    }
    public void setStudents(Set<Student2> students) {
        this.students = students;
    }
    
    
}

 

Student2.hbm.xml:

 

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.cy.model">

    <class name="Student2" table="t_student2">
        <id name="id" column="studentId">
            <generator class="native"></generator>
        </id>
        <property name="name" column="studentName"></property>
        
        <set name="courses" table="student_course2" cascade="save-update">
            <key column="student_id"></key>
            <many-to-many class="com.cy.model.Course2" column="course_id"></many-to-many>
        </set>
    </class>

</hibernate-mapping>

 

Course2.hbm.xml:

 

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.cy.model">

    <class name="Course2" table="t_course2">
        <id name="id" column="courseId">
            <generator class="native"></generator>
        </id>
        <property name="name" column="courseName"></property>
        
        <!-- inverse=true: 在双向关系中，由学生端来维护关联关系 -->
        <set name="students" table="student_course2" inverse="true">
            <key column="course_id"></key>
            <many-to-many class="com.cy.model.Student2" column="student_id"></many-to-many>
        </set>
    </class>

</hibernate-mapping>

 

测试代码：

 

@Test
    public void testSave2(){
        Course2 course1=new Course2();
        course1.setName("语文");
        
        Course2 course2=new Course2();
        course2.setName("数学");
        
        Student2 student1=new Student2();
        student1.setName("张三");
        student1.getCourses().add(course1);
        student1.getCourses().add(course2);
        
        Student2 student2=new Student2();
        student2.setName("李四");
        student2.getCourses().add(course1);
        student2.getCourses().add(course2);
        
        session.save(student1);
        session.save(student2);
    }
    
    /**
     * 获取选修 语文 这门课的学生
     */
    @Test
    public void testLoad2(){
        Course2 course = (Course2) session.get(Course2.class, 1);
        Iterator<Student2> students = course.getStudents().iterator();
        while(students.hasNext()){
            Student2 s = students.next();
            System.out.println(s.getName());
        }
    }

 

生成的表结构：

表的结构、插入后的数据，都和单向关系中创建的一模一样；

区别就是，现在可以从课程获取到选修这门课程的学生了。