Why we are getting currentsession twice in the application?

Recently I did some work around to check, where to give the session related code in our application, ie .Getting current session (sessionFactory.getCurrentSession());

I have seen this code in place twice in the application, one in HibernateSessionRequestFileter Class

package com.persistence;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.hibernate.SessionFactory;
import org.hibernate.StaleObjectStateException;
public class HibernateSessionRequestFilter implements Filter {

    private static Log log = LogFactory.getLog(HibernateSessionRequestFilter.class);

    private SessionFactory sf;

    public void doFilter(ServletRequest request,
                         ServletResponse response,
                         FilterChain chain)
            throws IOException, ServletException {

        try {
            log.debug("Starting a database transaction");
            sf.getCurrentSession().beginTransaction();

            // Call the next filter (continue request processing)
            chain.doFilter(request, response);

            // Commit and cleanup
            log.debug("Committing the database transaction");
            sf.getCurrentSession().getTransaction().commit();

        } catch (StaleObjectStateException staleEx) {
            log.error("This interceptor does not implement optimistic concurrency control!");
            log.error("Your application will not work until you add compensation actions!");
            // Rollback, close everything, possibly compensate for any permanent changes
            // during the conversation, and finally restart business conversation. Maybe
            // give the user of the application a chance to merge some of his work with
            // fresh data... what you do here depends on your applications design.
            throw staleEx;
        } catch (Throwable ex) {
            // Rollback only
            ex.printStackTrace();
            try {
                if (sf.getCurrentSession().getTransaction().isActive()) {
                    log.debug("Trying to rollback database transaction after exception");
                    //sf.getCurrentSession().getTransaction().rollback();
                }
            } catch (Throwable rbEx) {
                log.error("Could not rollback transaction after exception!", rbEx);
            }

    开发者_Go百科        // Let others handle it... maybe another interceptor for exceptions?
            throw new ServletException(ex);
        }
    }

    public void init(FilterConfig filterConfig) throws ServletException {
        log.debug("Initializing filter...");
        log.debug("Obtaining SessionFactory from static HibernateUtil singleton");
        sf = HibernateUtil.getSessionFactory();
    }

    public void destroy() {}


}

Another one in GenericHibernateDAO Class Like below,

protected Session getSession() {

        if (session == null) {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
        } else if (!session.isConnected()) {
            session = HibernateUtil.getSessionFactory().getCurrentSession();
        }
        return session;
    }

Can anybody help me to understand, why we have to get currentsession in two places? While we begin the transaction, we are getting the currentsession , same as while we persist or getting object from database, again we are getting the currentsession, why is it so?

This looks like a version of an OpenSessionInView pattern, where a Hibernate Session is opened when a request is received and closed after the response is rendered.

In the filter a session is opened and a transaction is started.

Then the request is processed and in the Dao the call to getCurrentSession() only get's the current open session, it isn't creating a new Session.

The dao does its work. Then the filter commits the transaction and the session is closed.