Hibernate关联关系映射之多对多关联关系

例如，在教师表中有id,name两个属性，学生表中同样有id,name两个属性。教师表中有两条记录分别是（1，董老师），（2，李老师）；学生表中同样有两条（1，张三），（2，李四）。在中间表中有两个字段（teacherId,studentId)，这两个字段构成一个联合主键，同时这两个字段又是教师表和学生表的外键，老师和学生进行关联的时候直接插入关联的老师和学生的id在中间表内即可，例如在中间表中插入（1,1）表示董老师与张三进行关联，插入（1,2）表示董老师与李四关联。

下面我们看一下Teacher类和Student类

package entity;

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;
	}
}

package entity;

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;
	}	
}

在POJO类中仍然使用集合表示有多条记录。

下面在看一个他们的映射配置文件

首先看Teacher.hbm.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="entity.Teacher" table="Teacher" schema="MYHR">
        <id name="id" type="long">
            <column name="ID"/>
            <generator class="assigned" />
        </id>
        <property name="name" type="string">
            <column name="NAME" not-null="true" />
        </property>
        <!-- students属性，Set集合，表示本类与Student类的多对多关系 -->
        <set name="students" table="teacher_student" inverse="false">
            <key column="teacherId" />
            <many-to-many class="entity.Student" column="studentId"/>
        </set>
    </class>
</hibernate-mapping>

类中的Set类型的属性在映射配置文件中仍然使用<set>标签进行映射，在<set>标签中使用<many-to-many>表示他们之间的关系是多对多。

然后看Student.hbm.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="entity.Student" table="Student" schema="MYHR">
        <id name="id" type="long">
            <column name="ID" />
            <generator class="assigned" />
        </id>
        <property name="name" type="string">
            <column name="NAME" not-null="true" />
        </property>
        <!-- teachers属性， Set集合，表示本类与Teacher类的多对多关联关系-->
        <!-- table:是实现多对多映射的中间表 -->
        <!-- key:集合外键（引用当前表主键的那个外键） -->
        <set name="teachers" table="teacher_student">
            <key column="studentId" />
            <many-to-many class="entity.Teacher" column="teacherId"/>
        </set>
    </class>
</hibernate-mapping>

这样，教师和学生之间的多对多关联映射就配置完了，下面看一下测试类。这里使用JUtil进行测试

package test;

import static org.junit.Assert.*;

import java.util.Iterator;
import java.util.Set;

import org.hibernate.Session;
import org.hibernate.Transaction;

import entity.Student;
import entity.Teacher;
import factory.HibernateSessionFactory;

public class Test {

	private Session session = null;
	private Transaction tran = null;

	// 存储对象
	@org.junit.Test
	public void save() {
		session = HibernateSessionFactory.getSession();
		tran = session.beginTransaction();
		try {
			Teacher teacher = new Teacher();
			teacher.setId(1L);
			teacher.setName("董老师");

			Teacher t2 = new Teacher();
			t2.setId(2l);
			t2.setName("李老师");

			Student s1 = new Student();
			s1.setId(1L);
			s1.setName("李四");

			Student s2 = new Student();
			s2.setId(2l);
			s2.setName("张三");

			teacher.getStudents().add(s1);
			teacher.getStudents().add(s2);
			t2.getStudents().add(s1);
			t2.getStudents().add(s2);
			s1.getTeachers().add(teacher);
			s1.getTeachers().add(t2);
			s2.getTeachers().add(teacher);
			s2.getTeachers().add(t2);

			session.save(teacher);
			session.save(t2);
			session.save(s1);
			session.save(s2);

			tran.commit();
		} catch (Exception e) {
			tran.rollback();
		}
	}

	// 获取老师信息
	@org.junit.Test
	public void GetTeacher() {
		session = HibernateSessionFactory.getSession();
		tran = session.beginTransaction();
		try {
			Teacher teacher = (Teacher) session.get(Teacher.class, 1l);
			Set<Student> set = teacher.getStudents();
			System.out.println(teacher.getName() + "的学生是：");
			Iterator<Student> it = set.iterator();
			while (it.hasNext()) {
				Student s = it.next();
				System.out.println(s.getName());
			}
			tran.commit();
		} catch (Exception e) {
			tran.rollback();
		}
	}

	// 解除关联关系
	@org.junit.Test
	public void RemoveRelation() {
		session = HibernateSessionFactory.getSession();
		tran = session.beginTransaction();
		try {
			Student s = (Student) session.get(Student.class, 1l);
			Teacher teacher = (Teacher) session.get(Teacher.class, 1l);
			// 如果Teacher的inverse属性为false可以解除，如果为true不可以解除
			teacher.getStudents().remove(s);
			tran.commit();
		} catch (Exception e) {
			tran.rollback();
		}
	}

	// 删除关联关系
	@org.junit.Test
	public void DeleteRelation() {
		session = HibernateSessionFactory.getSession();
		tran = session.beginTransaction();
		try {
			Teacher teacher = (Teacher) session.get(Teacher.class, 2l);
			// 当teacher的inverse属性为false时，可以将教师信息删除，并且将中间表中相关记录删除
			// 当inverse属性为true时将教师信息删除时抛出异常
			session.delete(teacher);
			tran.commit();
		} catch (Exception e) {
			tran.rollback();
		}
	}
}