Hibernate @OneToMany 一对多注解

配置一

   @OneToMany(cascade=CascadeType.ALL)
    @JoinColumn(name="fatherID") //对应儿子类的哪个字段  子类中配置ManyToOne的字段名称
    private Set<Child> children;  

配置二

@OneToMany(mappedBy = "qeTopic", cascade = CascadeType.ALL, fetch = FetchType.LAZY) //qeTopic 对应儿子类的哪个字段  子类中配置ManyToOne的字段名称
private Set<QeTopicDiscussion> listQeTopicDiscussion ;

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();
    }
  }
}