请教一个用ThreadLocal管理session的问题

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.cfg.Configuration;

public class NetshopSessionFactory {	

    private static String CONFIG_FILE_LOCATION = "/hibernate-netshop.cfg.xml";
	private static final ThreadLocal<Session> threadLocal = new ThreadLocal<Session>();
    private  static Configuration configuration = new Configuration();
    private static org.hibernate.SessionFactory sessionFactory;
    private static String configFile = CONFIG_FILE_LOCATION;

    private NetshopSessionFactory() {
    }
	
	/**
     * Returns the ThreadLocal Session instance.  Lazy initialize
     * the <code>SessionFactory</code> if needed.
     *
     *  @return Session
     *  @throws HibernateException
     */
    public static Session getSession() throws HibernateException {
        Session session = (Session) threadLocal.get();
        
		if (session == null || !session.isOpen()) {
			if (sessionFactory == null) {
				rebuildSessionFactory();
			}
			session = (sessionFactory != null) ? sessionFactory.openSession()
					: null;
			
			threadLocal.set(session);
		}
		
		
        return session;
    }
    
    /**
     * Returns the ThreadLocal Session instance.  Lazy initialize
     * the <code>SessionFactory</code> if needed.
     *
     *  @return Session
     *  @throws HibernateException
     */
    public static Session getNewSession() throws HibernateException {
        return sessionFactory.openSession();
    }

	/**
     *  Rebuild hibernate session factory
     *
     */
	public static void rebuildSessionFactory() {
		try {
			configuration.configure(configFile);
			sessionFactory = configuration.buildSessionFactory();
		} catch (Exception e) {
			System.err
					.println("%%%% Error Creating SessionFactory %%%%");
			e.printStackTrace();
		}
	}

	/**
     *  Close the single hibernate session instance.
     *
     *  @throws HibernateException
     */
    public static void closeSession() throws HibernateException {
        Session session = (Session) threadLocal.get();
        threadLocal.set(null);

        if (session != null) {
            session.close();
        }
    }

	/**
     *  return session factory
     *
     */
	public static org.hibernate.SessionFactory getSessionFactory() {
		return sessionFactory;
	}

	/**
     *  return session factory
     *
     *	session factory will be rebuilded in the next call
     */
	public static void setConfigFile(String configFile) {
		NetshopSessionFactory.configFile = configFile;
		sessionFactory = null;
	}

	/**
     *  return hibernate configuration
     *
     */
	public static Configuration getConfiguration() {
		return configuration;
	}


}

	public static void saveOrUpdateItem(Item item){
		Session session = NetshopSessionFactory.getSession();
		Transaction tx = session.beginTransaction();
		session.saveOrUpdate(item);
		session.flush();
		tx.commit();
		item = null;		
		session.close();
	}


	public static Item getItemByid(Integer id){		
		Session session = NetshopSessionFactory.getSession();
		session.clear();
		Item item = (Item)session.load("netshop.dolchn.hibernate.Item", id);
		//System.out.println(item.getItname());
		session.close();
		return item;
	}