（九）Hibernate的多对多关联关系

一、概述

多对多关联关系在java对象中可以通过定义集合类型来实现关联关系。 在关系数据模型中，无法直接表达表和表之间的多对多关联关系，而是需要创建一个中间表包含了两边的主键，来表达两张表的多对多关联关系。

实例：我们用一个Student和Course(学生和课程)的例子来演示多对多关联关系。

（1）创建Student和Course类

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public class Student {          private Integer id;          private String name;          //用一个集合包含该学生所选的课程对象          private Set<Course> courses=new HashSet<Course>();          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<Course> getCourses() {              return courses;          }          public void setCourses(Set<Course> courses) {              this.courses = courses;          }        }

　　

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public class Course {                     private Integer id;                     private String name;                     //用一个集合包含所有选择该课程的学生                     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;                     }               }

　　（2）编写我们的映射文件 　　

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<hibernate-mapping >     <class name="com.cad.domain.Course" table="course">         <id name="id" column="id">             <generator class="native"></generator>         </id>         <property name="name" column="name"></property>         <!--table属性用来指定生成的中间表的表名称  inverse指定关联关系由Student维护-->         <set name="students" table="student_course" inverse="true">             <key column="cid"></key>             <!--<many-to-many>元素中的column属性指定本表通过中间表中的sid外键关联到Student对象-->             <many-to-many class="com.cad.domain.Student" column="sid"></many-to-many>         </set>     </class>  </hibernate-mapping>

　　

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<hibernate-mapping >     <class name="com.cad.domain.Student" table="student">         <id name="id" column="id">             <generator class="native"></generator>         </id>         <property name="name" column="name"></property>         <!-进行级联保存和更新操作-->         <set name="courses" table="student_course" cascade="save-update">             <key column="sid"></key>             <many-to-many class="com.cad.domain.Course" column="cid"></many-to-many>         </set>     </class>  </hibernate-mapping>

　　测试：

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public class Demo {             @Test             public void fun(){                 //读取配置文件                 Configuration conf=new Configuration().configure();                   //根据配置创建factory                 SessionFactory sessionfactory=conf.buildSessionFactory();                 Session session = sessionfactory.openSession();                  Transaction ts=session.beginTransaction();                 //创建两个Student                 Student s1=new Student();                 s1.setName("tom");                 Student s2=new Student();                 s2.setName("jack");                   //创建三个Course                 Course c1=new Course();                 c1.setName("语文");                 Course c2=new Course();                 c2.setName("数学");                 Course c3=new Course();                 c3.setName("英语");                   //因为设置了关联关系由Student维护，所以不需要课程再来关联Student                 s1.getCourses().add(c1);                 s1.getCourses().add(c2);                 s1.getCourses().add(c3);                   s2.getCourses().add(c1);                 s2.getCourses().add(c2);                 s2.getCourses().add(c3);                   //由于设置了级联保存，所以只保存Student即可                 session.save(s1);                 session.save(s2);                   ts.commit();                 session.close();                 sessionfactory.close();             }         }     结果，数据库中生成了三张表。 student和course 还有中间表student_course 里面的数据也相互对应。