四、N—N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
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public class Student {
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private Integer id;
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private String name;
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private Set<Teacher> teachers;
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public Integer getId() {
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return id;
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}
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public void setId(Integer id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public Set<Teacher> getTeachers() {
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return teachers;
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}
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public void setTeachers(Set<Teacher> teachers) {
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this.teachers = teachers;
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}
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}
Teacher
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public class Teacher {
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private Integer id;
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private String name;
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private Set<Student> students;
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public Integer getId() {
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return id;
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}
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public void setId(Integer id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public Set<Student> getStudents() {
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return students;
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}
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public void setStudents(Set<Student> students) {
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this.students = students;
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}
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}
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
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<hibernate-mapping package="com.hibernate.domain">
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<class name="Student" table="student">
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<id name="id" column="student_id">
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<generator class="native" />
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</id>
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<property name="name" column="student_name" />
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<set name="teachers" table="student_teacher">
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<key column="student_id" />
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<many-to-many class="Teacher" column="teacher_id" />
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</set>
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</class>
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</hibernate-mapping>
Teacher.hbm.xml
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<hibernate-mapping package="com.hibernate.domain">
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<class name="Teacher" table="teacher">
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<id name="id" column="teacher_id">
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<generator class="native" />
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</id>
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<property name="name" column="teacher_name" />
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<set name="students" table="student_teacher">
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<key column="teacher_id" />
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<many-to-many class="Student" column="student_id"></many-to-many>
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</set>
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</class>
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</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.../>和<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.../>和<many-to-many.../>的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
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static void add(){
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Session session = HibernateUtil.getSession();
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Transaction tx = session.beginTransaction();
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Teacher teacher1 = new Teacher();
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teacher1.setName("teacher1");
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Teacher teacher2 = new Teacher();
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teacher2.setName("teacher2");
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Student student1 = new Student();
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student1.setName("student1");
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Student student2 = new Student();
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student2.setName("student2");
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Set<Student> ss = new HashSet<Student>();
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ss.add(student1);
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ss.add(student2);
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teacher1.setStudents(ss);
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teacher2.setStudents(ss);
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session.save(teacher1);
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session.save(teacher2);
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session.save(student1);
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session.save(student2);
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tx.commit();
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session.close();
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}
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
Hibernate的关联映射之N-N关联映射 .
原文：http://blog.csdn.net/u013310119/article/details/51348264
