Hibernate之实现多对多关联映射关系

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

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>

在多对多关系中，可在Set里通过inverse设置只有其中一方维护。

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();
	}
}

由代码生成的表结构

执行testGet方法时控制台内容

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