Hibernate读书笔记Hibernate的关联映射之NN关联映射

          四、NN关联关系

         1.1单向N-N的关联

         单向的N-N关联和1-N关联的持久化类完全一样,控制关系的一端需要增加一个set集合属性,被关联的持久化实例以集合的形式存在。

           N-N关联必须使用连接表,N-N关联与有连接表的1-N关联非常相似,只需要去掉<many-to-many.../>元素的unique="true"即可。其他的配置和1-N关联一样。

          由于与1-N关联非常相似,这里就不演示了。

         1.2双向N-N的关联

          对于双向的N-N关联,我们只需要转换为两个1-N关联模型即可。双向N-N关联两端都需要使用set集合属性,两端都增加对集合属性的访问。双向N-N同样必须使用连接表来建立两个实体之间的关联关系。

          以学生、老师为例:下面为两个实体的持久化类:

          Student

 1 public class Student {
 2     private Integer id;
 3     private String name;
 4     private Set<Teacher> teachers;
 5 
 6     public Integer getId() {
 7         return id;
 8     }
 9 
10     public void setId(Integer id) {
11         this.id = id;
12     }
13 
14     public String getName() {
15         return name;
16     }
17 
18     public void setName(String name) {
19         this.name = name;
20     }
21 
22     public Set<Teacher> getTeachers() {
23         return teachers;
24     }
25 
26     public void setTeachers(Set<Teacher> teachers) {
27         this.teachers = teachers;
28     }
29 }

         Teacher

 1 public class Teacher {
 2     private Integer id;
 3     private String name;
 4     private Set<Student> students;
 5 
 6     public Integer getId() {
 7         return id;
 8     }
 9 
10     public void setId(Integer id) {
11         this.id = id;
12     }
13 
14     public String getName() {
15         return name;
16     }
17 
18     public void setName(String name) {
19         this.name = name;
20     }
21 
22     public Set<Student> getStudents() {
23         return students;
24     }
25 
26     public void setStudents(Set<Student> students) {
27         this.students = students;
28     }
29 
30 }

          双向N-N关联的映射文件需要使用<set.../>元素,用以映射集合属性。<set.../>属性还需要增加<key.../>子元素来映射外键列,同时还应该增加<many-to-many.../>子元素来映射关联实体类。两个映射文件如下:

          Student.hbm.xml

 1 <hibernate-mapping package="com.hibernate.domain">
 2     <class name="Student" table="student">
 3         <id name="id" column="student_id">
 4             <generator class="native" />
 5         </id>
 6         
 7         <property name="name" column="student_name" />
 8         
 9         <!-- 映射N-N关联实体,两边的table应该以样 -->
10         <set name="teachers" table="student_teacher">
11             <!-- 映射关联的外键 列-->
12             <key column="student_id" />
13             <!-- 映射关联类属性 -->
14             <many-to-many class="Teacher" column="teacher_id" />
15         </set>
16     </class>
17 </hibernate-mapping>

         Teacher.hbm.xml

<hibernate-mapping package="com.hibernate.domain">
    <class name="Teacher" table="teacher">
        <id name="id" column="teacher_id">
            <generator class="native" />
        </id>
        
        <property name="name" column="teacher_name" />
        
        <!-- 映射N-N关联实体,两边的table应该以样 -->
        <set name="students" table="student_teacher">
            <!-- 映射关联的外键 列-->
            <key column="teacher_id" />
            <!-- 映射关联类属性 -->
            <many-to-many class="Student" column="student_id"></many-to-many>
        </set>
    </class>
</hibernate-mapping>
          双向N-N关联的双边都需要指定连接表的表名,外键列的列名,所以两个<set.../>元素的table属性的值必须指定且一样。<set.../>元素的两个子元素:<key../><many-to-many.../>都必须指定column属性。<key.../><many-to-many.../>分别是指定本持久化类、关联类在连接表中的外键列。因此两边的<key.../><many-to-many.../>column属性的值应该是交叉相等。

          通过下面的操作类来添加两个Student对象和两个Teacher对象

 1     static void add(){
 2         Session session = HibernateUtil.getSession();
 3         Transaction tx = session.beginTransaction();
 4         
 5         Teacher teacher1 = new Teacher();
 6         teacher1.setName("teacher1");
 7         
 8         Teacher teacher2 = new Teacher();
 9         teacher2.setName("teacher2");
10         
11         Student student1 = new Student();
12         student1.setName("student1");
13         
14         Student student2 = new Student();
15         student2.setName("student2");
16         
17         //建立两者之间的关系
18         Set<Student> ss = new HashSet<Student>();
19         ss.add(student1);
20         ss.add(student2);
21         
22         teacher1.setStudents(ss);
23         teacher2.setStudents(ss);
24         
25         session.save(teacher1);
26         session.save(teacher2);
27         session.save(student1);
28         session.save(student2);
29         
30         tx.commit();
31         session.close();
32     }


          注意:这里只能由一边建立关联关系,就是说只能由Student对象建立与Teacher对象的关联或者由Teachert对象建立与Student对象的关联。否则将会出现主键重复错误

原文地址:https://www.cnblogs.com/chenssy/p/2594874.html