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

Question

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.