Hibernate does not return connection if exectuted in another thread

Question

I have the problem that I need to run multiple task in parallel.
For this purpose I am using Futures.
One of this tasks is a simple select on a postgres database via hibernate, the problem is that every time this task gets executed a new postgres connection get created an soon postgres does not accept any more connections.
The application runs on a tomcat-server and uses connection pooling.
If i execute the task not in a different thread it works fine.

This is the method that uses hibernate:

@Override
public Future<MonitoringResult> performMonitoringAction()  {

    return getExecutorService().submit(() -> {
        long milliseconds = System.currentTimeMillis();

        Session session = null;
        Transaction tx = null;
        try {
            session = HibernateUtil.getCurrentInstance().newSession();
            tx = session.beginTransaction();
            List<Entity> sle = (List<Entity>) session.createQuery("from Entity").list();

            return new MonitoringResult(System.currentTimeMillis() - milliseconds, true);
        } catch (Exception e) {
            return new ExceptionMonitoringResult(System.currentTimeMillis() - milliseconds, e);
        } finally {
            if (tx != null) {
                tx.commit();
        }
            if (session != null) {
                session.close();
            }
        }
    });
}

This is how it gets called:

public Response all() {

    List<Future<MonitoringResult>> runningMonitorTasks = new ArrayList<>(monitoredServices.length);

    // start all monitoring services
    for (MonitorableService monitoredService : monitoredServices) {
        runningMonitorTasks.add(monitoredService.performMonitoringAction());
    }

    HashMap<String, MonitoringResult> resultMap = new HashMap();

    // collect results of monitoring services
    for (int i = 0; i < monitoredServices.length; i++) {
        MonitorableService monitoredService = monitoredServices[i];
        Future<MonitoringResult> runningTask = runningMonitorTasks.get(i);

        MonitoringResult result;
        try {
            result = runningTask.get(60, TimeUnit.SECONDS); // wait till task is finished
        } catch (TimeoutException | InterruptedException | ExecutionException ex) {
            LOGGER.log(Level.SEVERE, "Monitoring task failed", ex);
            result = new ExceptionMonitoringResult(-1, ex);
        }

        logIfUnreachable(result, monitoredService);
        resultMap.put(monitoredService.getServiceName(), result);
    }

    return Response.ok(resultMap).build();
}

Called this way it works fine:

public Response all() {

    HashMap<String, MonitoringResult> resultMap = new HashMap();

    // execute monitoring services
    for (MonitorableService monitoredService : monitoredServices) {
        Future<MonitoringResult> result = monitoredService.performMonitoringAction();
        MonitoringResult get;
        try {
            get = result.get();
            logIfUnreachable(get, monitoredService);

        } catch (InterruptedException | ExecutionException ex) {
            Logger.getLogger(RestMonitorService.class.getName()).log(Level.SEVERE, null, ex);
            get = new ExceptionMonitoringResult(-1, ex);
        }
        resultMap.put(monitoredService.getServiceName(), get);

    }

    return Response.ok(resultMap).build();
}

HibernateUtil class:

public class HibernateUtil implements ServletContextListener {

    private static HibernateUtil currentInstance;
    private SessionFactory sessionFactory;
    private ServletContext servletContext;

    private final Log logger = LogFactory.getLog(LoginInfo.class);

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        // set current instance
        currentInstance = this;

        Configuration cfg = new Configuration().configure();
        StandardServiceRegistryBuilder builder = new    StandardServiceRegistryBuilder().applySettings(
                cfg.getProperties());
        sessionFactory = cfg.buildSessionFactory(builder.build());

        servletContext = sce.getServletContext();
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        // close session factory
        if(sessionFactory!=null){
            sessionFactory.close();
        }
        sessionFactory = null;

    }

    public static HibernateUtil getCurrentInstance() {
        return currentInstance;
    }

    public Session newSession() {
        return sessionFactory.openSession();
    }
}

Answer 1

The answers is real answer is that the problem occurred in a different place than I expected. But I going to share my solution as well.

Another service (not the one from my question) used the hibernate to. I a normal call in the of the web-application there is a listener that opens the connections before and on that closes the connections. But because the service got now executed on a different thread the connection was opened but never closed because the listeners did not get called.

Answer 2

Can not comment yet, may worth check the source of HibernateUtil.getCurrentInstance() to see what it is doing, it may use some threadlocal or create a new pool of connections. normally when connection got exhausted, likely due to creating a new pool instead of use existing pool to get connection.