没有懒惰地初始化角色集合:com.pojo.Student.phonenos,没有会话或会话被关闭

Question

我正在使用注释学习hibernate映射.我已经完成了一个部分.即我可以在保存父表时自动插入子类. 看到了.

但是当我获取主表时,我没有得到子表.也出错了

failed to lazily initialize a collection of role: com.pojo.one2many.unidirectional.Student.phonenos, no session or session was closed

我的代码已添加到此处以供您查看.请仔细阅读.并给我很好的建议.Student.java.(父类)

@Entity
    @Table(name="STUDENT")
    public class Student {
    private int studentid;
    private String studentName;
    private Set <Phone> phonenos;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name="studenId")
    public int getStudentid() {
        return studentid;
    }
    public void setStudentid(int studentid) {
        this.studentid = studentid;
    }
    @Column(name="studenName")
    public String getStudentName() {
        return studentName;
    }
    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }
    @OneToMany(fetch = FetchType.LAZY)
    @JoinColumn(name="studenId") 
    public Set<Phone> getPhonenos() {
        return phonenos;
    }
    public void setPhonenos(Set<Phone> phonenos) {
        this.phonenos = phonenos;
    }

Phone.java(子类)

@Entity
@Table(name = "PHONE")
public class Phone {
    private int phoneid;
    private String phoneNo;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "phoneId")
    public int getPhoneid() {
        return phoneid;
    }
    public void setPhoneid(int phoneid) {
        this.phoneid = phoneid;
    }
    @Column(name = "phoneno")
    public String getPhoneNo() {
        return phoneNo;
    }
    public void setPhoneNo(String phoneNo) {
        this.phoneNo = phoneNo;
    }

我的道教班

public List<Student> getAllStudent() {
         List<Student> studentList = null;
         try {
            DetachedCriteria criteria = DetachedCriteria.forClass(Student.class);
             studentList = (List<Student>)getHibernateTemplate().findByCriteria(criteria);
              if(studentList != null && ! studentList.isEmpty()){
                 for(Student st :studentList){
                     System.out.println(" st name : "+st.getStudentName());
                     if(st.getPhonenos() != null && ! st.getPhonenos().isEmpty()){
                         for(Phone ph : st.getPhonenos()){
                             System.out.println(" ph no : "+ ph.getPhoneNo());
                         }
                     }else{
                         System.out.println("   phone number is null");
                     }
                 }
             }else{
                 System.out.println("   student null");
             }
        } catch (DataAccessException e) {
            e.printStackTrace();
        }
        return studentList;
    }

出来就是

failed to lazily initialize a collection of role: com.pojo.one2many.unidirectional.Student.phonenos, no session or session was closed

这里我使用的是单向(外键)一对多映射(不是联合表,双向).

总结我的问题

1)当我们获取父表时如何获取子表,反之亦然

2)什么是渴望和懒惰的获取.

3)在一对多映射的情况下的单向,双向和连接表,其中一个更加充满电.

Answer 1

1)

如果您希望每次检索到这些类的任何实体时都这样做,请FetchMode.EAGER在@OneToMany关联中指定.@ManyToOne急于默认.请注意,如果您只需要在特定情况下获取这些实体,这可能在很大程度上无效.如果是这种情况,则必须按照您的方式执行此操作,但请确保检索到该Student对象的会话仍处于打开状态.看到你正在使用Spring,你是否试过用你的DAO/Service注释@Transactional,以便在方法执行期间会话保持活动状态？或者你尝试过使用Hibernate.execute(),像这样:

 

   getHibernateTemplate().execute(new HibernateCallback(){
    @SuppressWarnings("unchecked")
    public Object doInHibernate(Session s) throws HibernateException, SQLException {
        Criteria c = s.createCriteria(Student.class);
        List<Student> studentList = c.list();
        for(Student st :studentList){
            st.getPhoneNos();
        }
    }
});

2)看看这个问题:Java持久性中FetchType LAZY和EAGER之间的区别？.

3)这取决于你需要什么.如果您只需要以一种方式导航关联,那么只能以这种方式单向定义它.如果你需要两者,那就做两件事.Join Table更多地与数据库设计有关.如果你想在Phone表中有一个FK,并且 Student对手机的引用属于,或者你想要一个具有Phones每个的连接表Student.