为什么Hibernate Session第一次可以用，第二次就不可以呢？

package common;



import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;



public class HibernateUtil {



	private static final SessionFactory sessionFactory = buildSessionFactory();



	private static SessionFactory buildSessionFactory() {



		try {

			return new Configuration().configure().buildSessionFactory();

		}



		catch (Throwable ex) {



			System.err.println("Initial SessionFactory creation failed." + ex);

			throw new ExceptionInInitializerError(ex);

		}

	}



	public static SessionFactory getSessionFactory() {

		return sessionFactory;

	}



}

package service.impl;



import java.io.UnsupportedEncodingException;

import java.util.List;



import org.hibernate.Query;

import org.hibernate.Session;



import orm.exam_info;

import service.ExamInfo;





import common.HibernateUtil;



public class ExamInfoImpl extends HibernateUtil implements ExamInfo {



	/* (non-Javadoc)

	 * @see service.ExamInfo#addExamInfo(orm.exam_info)

	 */

	

	public void addExamInfo(exam_info examinfo) {

		Session ss =HibernateUtil.getSessionFactory().openSession();

		ss.beginTransaction();

		ss.save(examinfo);

		ss.getTransaction().commit();		

	}

	public List getExamInfo(){

		Session ss =HibernateUtil.getSessionFactory().openSession();

		List list=null;

		Query q = ss.createSQLQuery("select * from SYS_EXAM_INFO").addEntity(exam_info.class);

	    list = q.list();

		return list;

	}

	public String getExamInfoById(String id){

		Session ss =HibernateUtil.getSessionFactory().openSession();

		List list=null;

		StringBuilder sb = new StringBuilder();

		Query q = ss.createSQLQuery("select * from SYS_EXAM_INFO where exam_eid="+id).addEntity(exam_info.class);

		list = q.list();

		if(list!=null){

			exam_info obj = (exam_info) list.get(0);

			try {

				sb.append("[").append("{EXAM_EID:").append(obj.getEID()).append(",EXAM_ETIME:\"").append(obj.getETIME()).append("\",EXAM_MCOUNT:").append(obj.getMCOUNT()).append(",EXAM_MONEY:").append(obj.getMONEY()).append(",EXAM_NOWCOUNT:").append(obj.getNOWCOUNT()).append(",EXAM_OTHER:\"").append(new String(obj.getOTHER().getBytes("iso-8859-1"),"gb2312")).append("\"}").append("]");

			} catch (UnsupportedEncodingException e) {

				e.printStackTrace();

			}

		}

		return sb.toString();

		

	}

	   

	

	



}