Error org.hibernate.LazyInitializationException how to resolve?

I know this error is derived from trying to fetch some data from the database, when I do not have any open session.

But the biggest problem for me is that this error is appearing to me completely random, as I am developing a server that processes data and then sends it to clients, every 3 minutes the server repeats the same operations, but the error both appears as soon as the server is running as it may appear after 3 or 4 hours of running the server.

The error always points to class Produto more precisely for method toString , I commented the method and now it gives error in hashCode .

Database structure:

|| Lot || ---- Contain --- > || Product || ---- Contain --- > || qualityGates ||

Code - > Product.java

public class Produto  implements java.io.Serializable {


     private Integer idProduto;
     private Qualitygate qualitygateByIdQualityGate3;
     private Qualitygate qualitygateByIdQualityGate2;
     private Qualitygate qualitygateByIdQualityGate1;
     private Familiaproduto familiaproduto;
     private String noproduto;
     private Integer vt;
     private String tipo;
     private Set loteestados = new HashSet(0);

    public Produto() {
    }

//++ GET'S & SET'S

    @Override
    public int hashCode() {
        int hash = 5;
        hash = 59 * hash + Objects.hashCode(this.idProduto);
        hash = 59 * hash + Objects.hashCode(this.qualitygateByIdQualityGate3); // <-----erro
        hash = 59 * hash + Objects.hashCode(this.qualitygateByIdQualityGate2);
        hash = 59 * hash + Objects.hashCode(this.qualitygateByIdQualityGate1);
        hash = 59 * hash + Objects.hashCode(this.familiaproduto);
        hash = 59 * hash + Objects.hashCode(this.noproduto);
        hash = 59 * hash + Objects.hashCode(this.vt);
        hash = 59 * hash + Objects.hashCode(this.tipo);
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Produto other = (Produto) obj;
        if (!Objects.equals(this.idProduto, other.idProduto)) {
            return false;
        }
        if (!Objects.equals(this.qualitygateByIdQualityGate3, other.qualitygateByIdQualityGate3)) {
            return false;
        }
        if (!Objects.equals(this.qualitygateByIdQualityGate2, other.qualitygateByIdQualityGate2)) {
            return false;
        }
        if (!Objects.equals(this.qualitygateByIdQualityGate1, other.qualitygateByIdQualityGate1)) {
            return false;
        }
        if (!Objects.equals(this.familiaproduto, other.familiaproduto)) {
            return false;
        }
        if (!Objects.equals(this.noproduto, other.noproduto)) {
            return false;
        }
        if (!Objects.equals(this.vt, other.vt)) {
            return false;
        }
        if (!Objects.equals(this.tipo, other.tipo)) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return " noproduto=" + noproduto;
    }
//    @Override
//    public String toString() {
//        return "Produto{" + "idProduto=" + idProduto + ", qualitygateByIdQualityGate3=" + qualitygateByIdQualityGate3 + ", qualitygateByIdQualityGate2=" + qualitygateByIdQualityGate2 + ", qualitygateByIdQualityGate1=" + qualitygateByIdQualityGate1 + ", familiaproduto=" + familiaproduto + ", noproduto=" + noproduto + ", vt=" + vt + ", tipo=" + tipo + ", loteestados=" + loteestados + '}';
//    }

  }

Code - > Product.hbm.xml

<hibernate-mapping>
  <class catalog="mydb" name="Models.Produto" table="produto">
    <id name="idProduto" type="java.lang.Integer">
      <column name="idProduto"/>
      <generator class="identity"/>
    </id>
    <many-to-one class="Models.Qualitygate" fetch="select" name="qualitygateByIdQualityGate3">
      <column name="idQualityGate3"/>
    </many-to-one>
    <many-to-one class="Models.Qualitygate" fetch="select" name="qualitygateByIdQualityGate2">
      <column name="idQualityGate2"/>
    </many-to-one>
    <many-to-one class="Models.Qualitygate" fetch="select" name="qualitygateByIdQualityGate1">
      <column name="idQualityGate1"/>
    </many-to-one>
    <many-to-one class="Models.Familiaproduto" fetch="select" name="familiaproduto">
      <column name="idNomeProduto"/>
    </many-to-one>
    <property name="noproduto" type="string">
      <column length="45" name="noproduto" not-null="true" unique="true"/>
    </property>
    <property name="vt" type="java.lang.Integer">
      <column name="vt"/>
    </property>
    <property name="tipo" type="string">
      <column length="10" name="tipo"/>
    </property>
    <set fetch="select" inverse="true" lazy="true" name="loteestados" table="loteestado">
      <key>
        <column name="ProdutoidProduto" not-null="true"/>
      </key>
      <one-to-many class="Models.Loteestado"/>
    </set>
  </class>
</hibernate-mapping>

Code - > Generation Session creation (At first I had HibernateUtil to create me the sessions but due to the problem of creating many sessions and giving the full memory error I changed to this approach)

public class GenericDao<T extends Serializable> {

    private static Session session;
    public Class<T> persistentClass;

    public GenericDao(){
        //this.session = HibernateUtil.getSession();
        this.persistentClass = (Class<T>) ((ParameterizedType) 
            getClass().getGenericSuperclass()).getActualTypeArguments()[0];
    }

    public Session createSession(){
            Configuration cfg= new Configuration().configure();
        SessionFactory sessionfactory=cfg.buildSessionFactory();
        session=sessionfactory.openSession();

        return this.session;
    }
    public Session getSession() {
        if(this.session == null || !this.session.isOpen()){  
           return createSession();
        } else return session;
    }

    protected void save(T entity) {

         try {

            Session session = getSession();
            session.beginTransaction();
            session.save(entity);
            session.getTransaction().commit();
        } catch (Exception e) {
            session.getTransaction().rollback();

        } finally {
            session.close();
        }

   }

// ----------------------- +outros métodos 

    public T findById(Integer id) {
        return (T) getSession().createCriteria(persistentClass)
                .add(Restrictions.eq("id", id)).uniqueResult();
    }


    private void close() {
        if (getSession() != null && getSession().isOpen()) {
            getSession().close();
        }
    }
}

Any solutions to this problem?