org.hibernate.LazyInitializationException when I pass the query result to the toJson() function

Hibernate ORM
1

I got a list of objects where a save the result of a query using hibernate. The query works fine and print me out the right result. The problem is when I try to pass this list of object to the Gson function called toJson.
Here it is my code:

List<Conversazione> conversations;

    SessionFactory factory = new Configuration()
            .configure("hibernate2.cfg.xml")
            .addAnnotatedClass(Conversazione.class)
            .buildSessionFactory();

    Session session = factory.getCurrentSession();

    try {
        session.beginTransaction();
        //HERE IT IS MY QUERY
        conversations = session.createQuery("from Conversazione ").list();
        session.getTransaction().commit();
    }
    finally {
        factory.close();
    }
    //when I print it everything is fine
    System.out.println(conversations);

    Gson gson = new Gson();
    //this is where the problem come from
    String json = gson.toJson(conversations);
    System.out.println(json);

Here it is the error that it gives me:

	org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: database.Conversazione.partecipanti, could not initialize proxy - no Session

Here it is my Conversazione class:

    @Entity
@Table(name="conversazione")
public class Conversazione {
    @Id
    @Column(name="id")
    @NotNull
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;

    @Column(name="nome", length = 128)
    private String nome;

    @Column(name="foto", length = 256, nullable = true)
    private String foto;

    @OneToMany(mappedBy = "idConversazione")
    private List<Partecipante> partecipanti = new ArrayList<>();

    @OneToMany(mappedBy = "conversazioneId")
    private List<Messaggio> messaggi = new ArrayList<>();

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="data_creazione", nullable = false)
    private Date dataCreazione;

    public Conversazione() {
        this.nome = nome;
        this.foto = foto;
        this.dataCreazione = dataCreazione;
    }

I have also tried to create a list of Conversazione objects and pass it to the Gson function toJson() and it works fine as it should. So the problem is for sure with hibernate.

2

You are not telling Hibernate to fetch the partecipanti association. After closing the session factory, no database connection is possible anymore, so Hibernate can’t lazy load the association. You either have to fetch every association that you want to use after closing the session factory or do the json serialization while the session is still open to allow lazy loading.

3

I maybe have solved adding transient to partecipanti. Like this: private transient List partecipanti = new ArrayList<>();