以學生和老師為例的來講解多對多映射。

實體類：

Student

package cn.itcast.g_hbm_manyToMany;import java.util.HashSet;import java.util.Set;public class Student {    PRivate Long id;    private String name;    private Set<Teacher> teachers = new HashSet<Teacher>();    public Long getId() {        return id;    }    public void setId(Long id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Set<Teacher> getTeachers() {        return teachers;    }    public void setTeachers(Set<Teacher> teachers) {        this.teachers = teachers;    }    @Override    public String toString() {        return "[Student: id=" + id + ", name=" + name + "]";    }}

Teacher 

package cn.itcast.g_hbm_manyToMany;import java.util.HashSet;import java.util.Set;public class Teacher {    private Long id;    private String name;    private Set<Student> students = new HashSet<Student>();         public Long getId() {        return id;    }    public void setId(Long id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public Set<Student> getStudents() {        return students;    }    public void setStudents(Set<Student> students) {        this.students = students;    }    @Override    public String toString() {        return "[Teacher: id=" + id + ", name=" + name + "]";    }}

映射文件：

Student.hbm.xml

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.itcast.g_hbm_manyToMany">        <class name="Student" table="student">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name"/>                <!-- teachers屬性，Set集合。            表達的是本類與Teacher的多對多。                        table屬性：中間表（集合表）            key子元素：集合外鍵（引用當前表主鍵的那個外鍵）         -->        <set name="teachers" table="teacher_student" inverse="false">            <key column="studentId"></key>            <many-to-many class="Teacher" column="teacherId"></many-to-many>        </set>                </class>    </hibernate-mapping>

Teacher.hbm.xml

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping package="cn.itcast.g_hbm_manyToMany">        <class name="Teacher" table="teacher">        <id name="id">            <generator class="native"></generator>        </id>        <property name="name" type="string" column="name"/>                <!-- students屬性，Set集合。            表達的是本類與Student的多對多。         -->        <set name="students" table="teacher_student" inverse="true">            <key column="teacherId"></key>            <many-to-many class="Student" column="studentId"></many-to-many>        </set>            </class>    </hibernate-mapping>

測試

App.java

package cn.itcast.g_hbm_manyToMany;import org.hibernate.session;import org.hibernate.SessionFactory;import org.hibernate.cfg.Configuration;import org.junit.Test;import com.java1234.util.HibernateSessionFactory;public class App {    private static SessionFactory sessionFactory =HibernateSessionFactory.getSessionFactory();    // 保存，有關聯關系    @Test    public void testSave() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // 新建對象        Student student1 = new Student();        student1.setName("王同學1");        Student student2 = new Student();        student2.setName("李同學2");        Teacher teacher1 = new Teacher();        teacher1.setName("趙老師3");        Teacher teacher2 = new Teacher();        teacher2.setName("蔡老師4");        // 關聯起來        student1.getTeachers().add(teacher1);        student1.getTeachers().add(teacher2);        student2.getTeachers().add(teacher1);        student2.getTeachers().add(teacher2);        teacher1.getStudents().add(student1);        teacher1.getStudents().add(student2);        teacher2.getStudents().add(student1);        teacher2.getStudents().add(student2);        // 保存        session.save(student1);        session.save(student2);        session.save(teacher1);        session.save(teacher2);        // --------------------------------------------        session.getTransaction().commit();        session.close();    }    // 獲取，可以獲取到關聯的對方    @Test    public void testGet() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // 獲取一方，并顯示另一方信息        Teacher teacher = (Teacher) session.get(Teacher.class, 3L);        System.out.println(teacher);        System.out.println(teacher.getStudents());        // --------------------------------------------        session.getTransaction().commit();        session.close();    }    // 解除關聯關系    @Test    public void testRemoveRelation() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // 如果inverse=false就可以解除，如果為true就不可以解除        Teacher teacher = (Teacher) session.get(Teacher.class, 3L);        teacher.getStudents().clear();        // --------------------------------------------        session.getTransaction().commit();        session.close();    }    // 刪除對象，對關聯對象的影響    @Test    public void testDelete() throws Exception {        Session session = sessionFactory.openSession();        session.beginTransaction();        // --------------------------------------------        // a, 如果沒有關聯的對方：能刪除。        // b, 如果有關聯的對方且inverse=false，由于可以維護關聯關系，他就會先刪除關聯關系，再刪除自己。        // c, 如果有關聯的對方且inverse=true，由于不能維護關聯關系，所以會直接執行刪除自己，就會有異常。        Teacher teacher = (Teacher) session.get(Teacher.class, 9L);        session.delete(teacher);        // --------------------------------------------        session.getTransaction().commit();        session.close();    }}

兩張表的多對多關系，在數據庫中通常是通過第三張中間表來實現的，第三張中間表放的是兩張表各自的主鍵值，通過主鍵與主鍵的對應來體現表直接的關系。比如在權限系統中，一個用戶可以擁有多種權限，而一種權限也可以授予多個用戶。