新手提问：尝试第一个hibernate遇到的有关问题 http state 500

新手提问：尝试第一个hibernate遇到的问题 http state 500
几天前才开始学Java，MyEclipse＋struts＋hibernate+mysql   的构架刚刚把struts调好，这次又遇到了hibernate的问题。还请懂hibernate的大侠为我指出问题。
按照一个教程搭的构架，运行时候有数据库操作的时候就会出错。如下


HTTP   Status   500   -  

--------------------------------------------

type   Exception   report

message  

description   The   server   encountered   an   internal   error   ()   that   prevented   it   from   fulfilling   this   request.

exception  

java.lang.NullPointerException
com.Hibernate.SessionFactory.currentSession(SessionFactory.java:56)
com.yourcompany.struts.form.AddAdminForm.validate(AddAdminForm.java:52)
org.apache.struts.action.RequestProcessor.processValidate(RequestProcessor.java:942)
org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:255)
org.apache.struts.action.ActionServlet.process(ActionServlet.java:1482)
org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:507)
javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)


note   The   full   stack   trace   of   the   root   cause   is   available   in   the   Apache   Tomcat/5.5.12   logs.


--------------------------------------------

Apache   Tomcat/5.5.12


我的SessionFactory是MyEclipse自动生成的，期中的代码基本没改。
package   com.Hibernate;

import   org.hibernate.HibernateException;
import   org.hibernate.Session;
import   org.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();