一对多关联映射(单向)

public  class Student { 
   private Integer id; 
   private String name; 
   //一系列的setter.getter方法 
  @Override 
   public String toString() { 
     return  "name of student: " + name; 
  } 
}

public  class Classes { 
   private Integer id; 
   private String name; 
   private Set<Student> students; 
   //一系列的setter.getter方法 
  @Override 
   public String toString() { 
     return  "name of class: " + name; 
  } 
}

   < class  name ="com.sxt.hibernate.one2many.entity.Student"  table ="sxt_hibernate_student" > 
     < id  name ="id"  length ="4" > 
       < generator  class ="native" > </ generator > 
     </ id > 
     < property  name ="name"  length ="10" > </ property > 
   </ class >

   < class  name ="com.sxt.hibernate.one2many.entity.Classes"  table ="sxt_hibernate_class" > 
     < id  name ="id"  length ="4" > 
       < generator  class ="native" > </ generator > 
     </ id > 
     < property  name ="name"  length ="10" > </ property > 
    <!--  配置集合属性 --> 
     < set  name ="students"  cascade ="save-update" > 
      <!--  key的含义,指在另一端增加的外键指向本主键. 
        如果设置上属性not-null="true",表示该外键非空,则在由"一"的一端维护关系时, 
        可能导致插入数据异常PropertyValueException. 
        --> 
       < key  column ="class_id" > </ key > 
      <!-- one-to-many含义,指出set集合中的元素类型,以供加载时使用    --> 
       < one-to-many  class ="com.sxt.hibernate.one2many.entity.Student" /> 
     </ set > 
   </ class >

   public  static  void main(String[] args) { 
    Session session = HibernateUtils.getSession(); 
    Transaction t = session.beginTransaction(); 
     try { 
       /** 
        * 测试插入数据 
        */ 
       /* 
        * Student student1=new Student();    
        * student1.setName("奇隆"); 
        *    
        * Student student2=new Student();    
        * student2.setName("有朋"); 
        *    
        * Set<Student> students=new HashSet<Student>(); 
        * students.add(student1);    
        * students.add(student2); 
        *    
        * Classes classes=new Classes();    
        * classes.setName("不一班"); 
        * classes.setStudents(students); 
        * //存储不成功.报错：org.hibernate.TransientObjectException 
        * //因为此时student对象还没有持久化,classes引用了瞬时对象student1,student2 
        * session.save(classes); 
        */ 

        
        Student student1= new Student(); student1.setName( "奇隆"); 
         //session.save(student1);//先把student对象持久化 
         
        Student student2= new Student(); 
        student2.setName( "有朋");    
         //session.save(student2); 
         
        Set<Student> students= new HashSet<Student>(); 
        students.add(student1); 
        students.add(student2); 
         
        Classes classes= new Classes(); 
        classes.setName( "不一班"); 
        classes.setStudents(students); 
         //存储成功.sql语句如下: 
         /* 
        Hibernate: insert into sxt_hibernate_class (name, id) values (?, ?) 
        Hibernate: insert into sxt_hibernate_student (name, id) values (?, ?) 
        Hibernate: insert into sxt_hibernate_student (name, id) values (?, ?) 
        Hibernate: update sxt_hibernate_student set class_id=? where id=? 
        Hibernate: update sxt_hibernate_student set class_id=? where id=? */ 
         //可见在存储class之后,发出两个update语句,把它的两个student对象的class_id更新了. 
         //这是因为关系由"一"的一端维护(即class维护关系).这显然会降低效率. 
         //所以对于一对多,我们一般把关系交给"多"的一端维护.    
        session.save(classes); 
         
       /** 
        * 测试加载数据 
        */ 
/*      Classes classes = (Classes) session.load(Classes.class, 3); 
      System.out.println(classes); 
      Set<Student> students = classes.getStudents(); 
      for (Iterator<Student> stus = students.iterator(); stus.hasNext();) { 
        System.out.println(stus.next()); 
      }*/ 
      t.commit(); 
    }  catch (HibernateException e) { 
      e.printStackTrace(); 
      t.rollback(); 
    }  finally { 
      HibernateUtils.closeSession(session); 
    } 
  } 

} 

     本文转自NightWolves 51CTO博客，原文链接： http://blog.51cto.com/yangfei520/275656 ，如需转载请自行联系原作者