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I have recently created a web service that uses a static method in Java to obtain a list of items from the database.

The web service works perfectly and returns JSON back to the caller. However, it works only once. If you try to refresh or make a new request, I get a EntityManagerFactory is closed error.

Here's how the Web Service class looks like:

public class WebService extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
    //obtain the list of vehicles from the database
        List<Vehicle> vehicles = ExecuteVehicle.getVehicleList();
        //create the Gson object and generate the Json
        Gson gson = new Gson();
        JsonElement element = gson.toJsonTree(vehicles, new TypeToken<List<Vehicle>>(){}.getType());
        //send the list of vehicles
        JsonArray jsonArray = element.getAsJsonArray();
        resp.setContentType("application/json");
        resp.getWriter().print(jsonArray);
As you can see, the list of vehicles is being populated using the ExecuteVehicle.getVehicleList() method.
Here's how that method looks like:
public static List<Vehicle> getVehicleList(){
    //open a Session
    Session session = HibernateUtilities.getSessionFactory().openSession();
    //start a transaction
    session.beginTransaction();
    //SELECT STATEMENT for the entire list of Vehicles
    Query<Vehicle> query = session.getNamedQuery("SelectAllVehicles"); //query name is declared in the mapping file
    List<Vehicle> vehicles = query.list();
    //commit the changes and end the transaction
    session.getTransaction().commit();
    //close the Session
    session.close();
    //close the SessionFactory
    HibernateUtilities.getSessionFactory().close();
    return vehicles;
Here's the HibernateUtilities class that takes care of the Session and so on:
public class HibernateUtilities {
    private static SessionFactory sessionFactory;
    private static StandardServiceRegistry standardServiceRegistry;
    static{
        try {
            //configure and build the service registry
            standardServiceRegistry = new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
            //create the metadata
            Metadata metadata = new MetadataSources(standardServiceRegistry).getMetadataBuilder().build();
            //build the SessionFactory
            sessionFactory = metadata.getSessionFactoryBuilder().build();
        } catch (HibernateException e) {
            //in case the SessionFactory cannot be built, then the stackTrace is displayed
            e.printStackTrace();        
    //method that returns the Hibernate session factory
    public static SessionFactory getSessionFactory(){
        return sessionFactory;
The question I have is - how can I avoid the EntityManagerFactory is closed error. Furthermore, I will have to obtain this list again and again, in a real time manner. Is that feasible with Hibernate? To obtain a list of items from a database in a real-time manner (say, every 2 seconds or so)? I know this depends on the number of items and so on, but I'm asking from a technical standpoint - from what I understand, opening and closing the Session takes a long time - could I do this over and over again in the same Session and if so, how?
I would say that you are doing too much there.
You have to flush/commit the transaction and close the session as you are using the openSession() method of the factory.
But i dont think you need to close the SessionFactory itself 
//close the SessionFactory
HibernateUtilities.getSessionFactory().close();
remove this line and you can would be able to use the factory many times.
                It seems you're absolutely right. The interesting thing is that I tried this in the past and it didn't work. There's a possibility I've also removed (commented out) the session.close(); line of code.  I'm pretty much a complete beginner in Hibernate and don't know exactly what all these things do, the only thing I can do is to "guess" or search in tutorials what exactly they do.  Anyway, it seems to work now! Thanks a lot!
– Robert Ruxandrescu
                Feb 6, 2017 at 11:08
                By the way, is there any "danger" to leave the SessionFactory open? Should I worry about that in any way?
– Robert Ruxandrescu
                Feb 6, 2017 at 11:09
                In all the application i have worked with, the SessionFActory has been created once during the startup and was destroyed only when the application has been closed.. Creating the Factory is a very intenside process and once it has been created, i would leave it alone. You have to close your sessions as they are stateful but the factory is stateless so it does not need to recreated per-request.
– Maciej Kowalski
                Feb 6, 2017 at 11:12
                You open the session.. but you close the factory at the end.. you cannot create any more sessions when the factory is closed
– Maciej Kowalski
                Feb 6, 2017 at 11:21
        
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