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();
}
}
}