以学生和老师为例的来讲解多对多映射。
实体类：
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属性，Set集合。
表达的是本类与Teacher的多对多。 table属性：中间表（集合表）
key子元素：集合外键（引用当前表主键的那个外键）
-->
<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属性，Set集合。
表达的是本类与Student的多对多。
-->
<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>
测试
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，有关联关系
@Test
public void testSave() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// -------------------------------------------- // 新建对象
Student student1 = new Student();
student1.setName("王同学1"); Student student2 = new Student();
student2.setName("李同学2"); Teacher teacher1 = new Teacher();
teacher1.setName("赵老师3"); Teacher teacher2 = new Teacher();
teacher2.setName("蔡老师4"); // 关联起来
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();
} // 获取，可以获取到关联的对方
@Test
public void testGet() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// -------------------------------------------- // 获取一方，并显示另一方信息
Teacher teacher = (Teacher) session.get(Teacher.class, 3L);
System.out.println(teacher);
System.out.println(teacher.getStudents()); // --------------------------------------------
session.getTransaction().commit();
session.close(); } // 解除关联关系
@Test
public void testRemoveRelation() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// -------------------------------------------- // 如果inverse=false就可以解除，如果为true就不可以解除
Teacher teacher = (Teacher) session.get(Teacher.class, 3L);
teacher.getStudents().clear(); // --------------------------------------------
session.getTransaction().commit();
session.close();
} // 删除对象，对关联对象的影响
@Test
public void testDelete() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// -------------------------------------------- // a, 如果没有关联的对方：能删除。
// b, 如果有关联的对方且inverse=false，由于可以维护关联关系，他就会先删除关联关系，再删除自己。
// c, 如果有关联的对方且inverse=true，由于不能维护关联关系，所以会直接执行删除自己，就会有异常。
Teacher teacher = (Teacher) session.get(Teacher.class, 9L);
session.delete(teacher); // --------------------------------------------
session.getTransaction().commit();
session.close();
} }
两张表的多对多关系，在数据库中通常是通过第三张中间表来实现的，第三张中间表放的是两张表各自的主键值，通过主键与主键的对应来体现表直接的关系。比如在权限系统中，一个用户可以拥有多种权限，而一种权限也可以授予多个用户。