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文章目录
  1. 1. 概述
  2. 2. E-R图
    1. 2.1. 对象模型
  3. 3. 映射文件
    1. 3.1. Teacher.hbm.xml
    2. 3.2. Student.hbm.xml
  4. 4. 测试方法

概述

  • 多对多关联映射指的是两个对象之间是多对多的关系，比如teacher-student。
  • 由于在操作和性能方面都不太理想，所以多对多的映射使用较少，实际使用中最好转换成一对多的对象模型，Hibernate会为我们创建中间关联表，转换成两个一对多关联关系。

    E-R图

    E-R图

    对象模型

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    public class Student {
    	private int id;
    	private String name;
    	private Set<Teacher> teachers;

    	//省去get、set方法
    }
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public class Teacher {
	private int id;
	private String name;
	private Set<Student> students;

	//省去get、set方法
}

映射文件

Teacher.hbm.xml

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<?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.zju.domain">

	<class name="Teacher">
		<id name="id">
			<generator class="native" />
		</id>

		<property name="name" />

		<set name="students" table="teacher_student" inverse="true">
			<key column="teacher_id"/>
			<many-to-many class="Student" column="student_id"/>
		</set>
	</class>

</hibernate-mapping>
  • table:指定中间表
  • many-to-many:指定多对多的关联关系
  • column:执行set集合中的持久化类在中间表的外键列的名称
  • 注意要在其中一端加入inverse=”true”，否则会造成主键冲突

    Student.hbm.xml

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    <?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.zju.domain">

    	<class name="Student">
    		<id name="id">
    			<generator class="native" />
    		</id>

    		<property name="name" />

    		<set name="teachers" table="teacher_student">
    			<key column="student_id"/>
    			<many-to-many class="Teacher" column="teacher_id"/>
    		</set>
    	</class>

    </hibernate-mapping>

测试方法

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public void add() {
		Session s = null;
		Transaction tx = null;
		try {
			Set<Teacher> ts = new HashSet<Teacher>();

			Teacher t1 = new Teacher();
			t1.setName("t1 name");
			ts.add(t1);

			Teacher t2 = new Teacher();
			t2.setName("t2 name");
			ts.add(t2);

			Set<Student> ss = new HashSet<Student>();
			Student s1 = new Student();
			s1.setName("s1");
			ss.add(s1);

			Student s2 = new Student();
			s2.setName("s2");
			ss.add(s2);

			t1.setStudents(ss);
			t2.setStudents(ss);
			
			s1.setTeachers(ts);
			s2.setTeachers(ts);

			s = HibernateUtil.getSession();
			tx = s.beginTransaction();
			s.save(t1);
			s.save(t2);
			s.save(s1);
			s.save(s2);
			tx.commit();
		} finally {
			if (s != null)
				s.close();
		}
	}
文章目录
  1. 1. 概述
  2. 2. E-R图
    1. 2.1. 对象模型
  3. 3. 映射文件
    1. 3.1. Teacher.hbm.xml
    2. 3.2. Student.hbm.xml
  4. 4. 测试方法