Hibernate 一对多注解 mappedby 作用

 package oneToMany;
 import java.util.Set;
 import javax.persistence.*;
 /*
 注意导入时，是导入：import javax.persistence.*;
 非导入org.hibernate的相关类：import org.hibernate.annotations.Entity;
 */
 @Entity
 @Table(name="classes")
 public class Classes implements Serializable {
   @Id
   @GeneratedValue(strategy=GenerationType.AUTO)
   private int id;
   private String name; 

   @OneToMany(cascade=CascadeType.ALL,mappedBy="classes")
   private Set<Student> students;
 //getter,setter省略
 } 

 package oneToMany;
 import javax.persistence.*;
 @Entity
 @Table(name="student")
 public class Student implements Serializable  {
   @Id
   @GeneratedValue(strategy=GenerationType.AUTO)
   private int sid; 

   private String sname; 

   //若有多个cascade，可以是：{CascadeType.PERSIST,CascadeType.MERGE}
   @ManyToOne(cascade={CascadeType.ALL})
   @JoinColumn(name="classid")     //student类中对应外键的属性：classid
   private Classes classes;
 //getter,setter省略
 } 

 public class TestOneToMany {
 /*
 CREATE TABLE    student (    --要定义外键!!!!!!!
     `sid` double NOT NULL auto_increment,
     `classid` double NULL,
     `sname` varchar(255) NOT NULL,
     PRIMARY KEY    (sid),
     INDEX par_ind (classid),
     FOREIGN KEY (classid) REFERENCES classes(id) ON DELETE CASCADE ON UPDATE CASCADE
 ) ENGINE=InnoDB
 */
   public static void main(String[] args) throws SQLException
   {
     try
     {
       SessionFactory sf = new AnnotationConfiguration().configure().buildSessionFactory();
       Session session=sf.openSession();
       Transaction tx=session.beginTransaction();
 /*
 因为mappedBy是定义在classes中,即classes类不负责维护级联关系.即维护者是student.所以,
 1.要将clsses的数据,赋给student,即用student的setClasses()方法去捆定class数据;
 2.在进行数据插入/更新session.save()/session.update()时,最后操作的是student.
 */
       Classes classes=new Classes();
       classes.setName("access"); 

       Student st1=new Student();
       st1.setSname("jason");
       st1.setClasses(classes);
       session.save(st1); 

       Student st2=new Student();
       st2.setSname("hwj");
       st2.setClasses(classes);
       session.save(st2);
       tx.commit();
 /*
 输出如下:
 Hibernate: insert into classes (name) values (?)
 Hibernate: insert into student (classid, sname) values (?, ?)
 Hibernate: insert into student (classid, sname) values (?, ?)
 */
 /*
 因为一端维护关系另一端不维护关系的原因，我们必须注意避免在应用中用不维护关系的类(class)建立关系，因为这样建立的关系是不会在数据库中存储的。
 如上的代码倒过来,则插入时,student的外键值为空.如下:
 */
 //      Student st1=new Student();
 //      st1.setSname("jason");
 //      session.save(st1);
 //
 //      Student st2=new Student();
 //      st2.setSname("hwj");
 //      session.save(st2);
 //
 //      Set<Student> students=new HashSet<Student>();
 //      students.add(st1);
 //      students.add(st2);
 //
 //      Classes classes=new Classes();
 //      classes.setName("access");
 //      classes.setStudents(students);
 //      session.save(classes);
 /*
 输出如下:
 Hibernate: insert into student (classid, sname) values (?, ?)
 Hibernate: insert into student (classid, sname) values (?, ?)
 Hibernate: insert into classes (name) values (?)
 */
     }
     catch(HibernateException e)
     {
       e.printStackTrace();
     }
   }
 }