Hibernate 一对多注解 mappedby 作用

Hibernate 一对多注解 mappedby 作用

http://blog.csdn.net/z69183787/article/details/8197106

2012-11-18 20:12 4992人阅读 评论(1) 收藏 举报

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();

}

}

}