hibernate 怎么实现多对多联系
直接以老师与学生的关系为例。在多对多关联关系中，其中一方都可通过Set保留另一方的所有信息，这样的关联是双向关联。在多对多关联关系中，也只能是双向关联。老师和学生分别对应一张表，通过一张有双方id的中间表来维护多对多的关联。
实体类
[java] view plain copy
package test.hibernate.hbmManyToMany;
import java.util.HashSet;
import java.util.Set;
public class Teacher {
private Integer id;
private String name;
private Set<Student> students = new HashSet<Student>();
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 Set<Student> getStudents() {
return students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}
@Override
public String toString() {
// TODO Auto-generated method stub
return "[Teacher:id=" + id + ",name=" + name + "]";
}
}
package test.hibernate.hbmManyToMany;
import java.util.HashSet;
import java.util.Set;
public class Student {
private Integer id;
private String name;
private Set<Teacher> teachers = new HashSet<Teacher>();
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 Set<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
@Override
public String toString() {
// TODO Auto-generated method stub
return "[Student:id=" + id + ",name=" + name + "]";
}
}
- 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="test.hibernate.hbmManyToMany">
<class name="Student" table="student">
<id name="id" type="integer" column="id">
<generator class="native" />
</id>
<property name="name" />
<!-- inverse属性：默认为false,表示本方维护关联关系; 如果设为true,表示本方不维护关联关系 只是影响是否能设置外键列的值(设成有效值或是null值),对获取信息没有影响 -->
<set name="teachers" table="teacher_student">
<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="test.hibernate.hbmManyToMany">
<class name="Teacher" table="teacher">
<id name="id" type="integer" column="id">
<generator class="native" />
</id>
<property name="name" />
<set name="students" table="teacher_student">
<key column="teacherId"></key>
<many-to-many class="Student" column="studentId"></many-to-many>
</set>
</class>
</hibernate-mapping>
- 测试类
package test.hibernate.hbmManyToMany;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import org.junit.Test;
public class App {
private static SessionFactory sessionFactory = new Configuration()//
.configure()//
.addClass(Teacher.class)// 添加Hibernate实体类（加载对应的映射文件）
.addClass(Student.class)//
.buildSessionFactory();
@Test
public void testSave() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// --------------------------------------------
// 构建对象
Teacher teacher1 = new Teacher();
teacher1.setName("林老师");
Teacher teacher2 = new Teacher();
teacher2.setName("张老师");
Student student1 = new Student();
student1.setName("李明");
Student student2 = new Student();
student2.setName("刘备");
teacher1.getStudents().add(student1);
teacher1.getStudents().add(student2);
teacher2.getStudents().add(student1);
teacher2.getStudents().add(student2);
// 下面部分不能与上面的代码块同时进行,只能选择其中一部分,不然会抛出在中间表中重复插入相同记录的异常
// 如果两边都要写，注意设置其中一方的inverse为true
// student1.getTeachers().add(teacher1);
// student1.getTeachers().add(teacher2);
// student2.getTeachers().add(teacher1);
// student2.getTeachers().add(teacher2);
// 保存
session.save(teacher1);
session.save(teacher2);
session.save(student1);
session.save(student2);
// --------------------------------------------
session.getTransaction().commit();
session.close();
}
// 获取到部门关联的员工
@Test
public void testGet() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// 获取数据
Teacher teacher = (Teacher) session.get(Teacher.class, 4);
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Teacher.hbm.xml中inverse=false就可以删除，否则不可以删除
Teacher teacher = (Teacher) session.get(Teacher.class, 4);
teacher.getStudents().clear();
// --------------------------------------------
session.getTransaction().commit();
session.close();
}
// 删除部门及对员工的影响
@Test
public void testDelete() throws Exception {
Session session = sessionFactory.openSession();
session.beginTransaction();
// --------------------------------------------
// 会同时删除teacher_student和teacher中教师id=4的记录
Teacher teacher = (Teacher) session.get(Teacher.class, 4);
session.delete(teacher);
// --------------------------------------------
session.getTransaction().commit();
session.close();
}
}
[java] view plain copy
[html] view plain copy
在多对多关系中，可在Set里通过inverse设置只有其中一方维护。
[html] view plain copy
[java] view plain copy
执行testGet方法时控制台内容
原文出处：http://blog.csdn.net/lindonglian/article/details/46916891
