hibernate 一对多，多对一 mappedBy用法详解

来源：互联网发布：c语言getchar的用法编辑：程序博客网时间：2024/05/13 00:39

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

@Column(name = "CID")
  @GeneratedValue(strategy=GenerationType.AUTO)
  private int id;
  private String name;

  @OneToMany(cascade=CascadeType.ALL,mappedBy="classes")   //mappedBy内的意思是不维护级联关系则由student 一方维护
  private Set<Student> students;
//getter,setter省略
}

package oneToMany;
import javax.persistence.*;
@Entity
@Table(name="student")
public class Student implements Serializable  {
  @Id

@Column(name = "SID")
  @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",referencedColumnName="CID")     //设置student表中对应Class表中的主键的外键属性：classid 默认为主键相当于student表有一个外键为：classid 指向Classes表的主键
  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();
    }
  }
}

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