hibernate ExceptionSession is closed问题?

源码:
  1.SessionFactory 类
  package com.tangjie.hibernnate;
 
  import net.sf.hibernate.HibernateException;
  import net.sf.hibernate.Session;
  import net.sf.hibernate.cfg.Configuration;
 
  /**
  * Configures and provides access to Hibernate sessions, tied to the
  * current thread of execution. Follows the Thread Local Session
  * pattern, see {@link http://hibernate.org/42.html}.
  */
  public class SessionFactory {
 
  /**
  * Location of hibernate.cfg.xml file.
  * NOTICE: Location should be on the classpath as Hibernate uses
  * #resourceAsStream style lookup for its configuration file. That
  * is place the config file in a Java package - the default location
  * is the default Java package.<br><br>
  * Examples: <br>
  * <code>CONFIG_FILE_LOCATION = "/hibernate.conf.xml".
  * CONFIG_FILE_LOCATION = "/com/foo/bar/myhiberstuff.conf.xml".</code>
  */
  private static String CONFIG_FILE_LOCATION = "/hibernate.cfg.xml";
 
  /** Holds a single instance of Session */
  private static final ThreadLocal threadLocal = new ThreadLocal();
 
  /** The single instance of hibernate configuration */
  private static final Configuration cfg = new Configuration();
 
  /** The single instance of hibernate SessionFactory */
  private static net.sf.hibernate.SessionFactory sessionFactory;
 
  /**
  * Returns the ThreadLocal Session instance. Lazy initialize
  * the <code>SessionFactory</code> if needed.
  *
  * @return Session
  * @throws HibernateException
  */
  public static Session currentSession() throws HibernateException {
  Session session = (Session) threadLocal.get();
 
  if (session == null) {
  if (sessionFactory == null) {
    try {
    cfg.configure(CONFIG_FILE_LOCATION);