Question

I am learning hibernate mapping using annotation. I have completed one section. I.e. I can insert child class automatically when I save the parent table.
see_that.

But I did n’t get the child table when I am fetching the master table. Also getting an error

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

My code is added here for to review you. Please go through it. And give me the great advice.
Student.java. ( parent class)

@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 (child class)

@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;
    }

my dao class

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;
    }

Out put is

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

Here I am using unidirectional (foreign key) one-To-many mapping (not joint table, bidirectional).

Summerizing my question

1) how to get child table when we fetch the parent table, vice versa

2) what is eager and lazy fetch.

3) unidirection, bidirection, and join table in the case of One-To-Many mapping which one is more power full.

Answer 1

1)

If you want really to do this every time that any entity of these classes is retrieved, specify FetchMode.EAGER in the @OneToMany association. @ManyToOne are eager by default. Be aware that this might be largely unefficient if you need to fetch those entities only in particular circumstances. If that’s the case, you have to do it like you’re doing, but make sure that the session that retrieved the Student object is still open. Seeing that you’re using Spring, have you tried annotating your DAO/Service with @Transactional, so that the session keeps alive during method execution? Or have you tried using Hibernate.execute(), like this:

 

   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) Take a look at this question: Difference between FetchType LAZY and EAGER in Java persistence?.

3) It depends on what you need. If you only need to navigate the association in one way, define it unidirectional in that way only. If you need both, do both. Join Table has more to do with database design. If you want to have a FK in the Phone table with the reference to the Student the phone belongs to, or you want to have a join table with the Phones of every Student.