休眠一对多关系

Question

I am trying to implement OnetoMany relationship for two tables student and certificates. I have two classes, Student and Certificates which have OnetoMany relationship from Student class to Certificates class.

Student Class:

@Entity
@Table(name = "student")
public class Student implements Serializable{

    @Id  
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name = "id")
    private Integer id;
    @OneToMany(cascade = CascadeType.ALL, fetch=FetchType.LAZY, mappedBy="student") 
    private List<Certificats> listCertificats;
    // Getters and Setters

Certificats class:

   @Entity
   @Table(name = "certificates")
   public class Certificates implements Serializable{

        @Id  
        @GeneratedValue(strategy=GenerationType.AUTO)
        @Column(name = "id")
        private Integer id;
        @ManyToOne
        @JoinColumn(name="student_id")
        private Student student;    
        // Getters and Setters

My DAO class has a method to fetch data from the database:

    @SuppressWarnings("unchecked")
    public List<Students> listStudents() {

        List<Students> studentList;

        studentList= sessionFactory.getCurrentSession().createQuery("from Student").list();

        return studentList;
    }

Now when I run this method, the value for the listCertificats in the Student class which is a List is always null. However, I get value for id field in the Student class. What am I doing wrong here? I checked some tutorials for Hibernate OnetoMany relationship and they described the same way I implemented. Any would be much appreciated.

Answer 1

FetchType.lazy implies that you will not be seeing data of your child entities until you access specific properties of them.

Please specify when are you seeing null? Are you seeing null even when you access them?