高手请进！！！！！！！！！


运行该代码：
<%@ page contentType="text/html;charset=GBK" language="java" %>
<%@ page import = "BookStore.*" %>
<%@ page import = "java.util.*" %>
<%
Vector catList = (Vector)session.getAttribute(Constants.BOOK_CAT_LIST_KEY);
Vector bookList = (Vector)session.getAttribute(Constants.BOOK_LIST_KEY);
Book book = null;
BookCat bookCat = null;

SearchStatus schSta = (SearchStatus)session.getAttribute(Constants.BOOK_SEARCH_STATUS_KEY);
%><HTML>
<HEAD><TITLE>书籍查询</TITLE>
<META http-equiv=Content-Type content="text/html; charset=gb2312">
<LINK href="images/new.css" type="text/css" rel=stylesheet>
</HEAD>
<BODY bgcolor="#7387B8"> 
<div align="center">
  <img border="0" src="images/ruanjian.jpg" width="770" height="154" ></div>
<TABLE cellSpacing=0 cellPadding=0 width="788" border=0>
  <TR vAlign=top bgColor=#cbd7f3>
    <TD align=middle width=170 bgColor=#7387B8>
      <br><br>
      <TABLE cellSpacing=0 cellPadding=0 width="90%" align=center border=0>
       <TR> <TD bgcolor="#FFFFFF">
       
    <%if(catList != null ){       for(int i = 0;i < catList.size();i++)
       {  
        bookCat = (BookCat)catList.get(i);
       %>
       
       <a href="bookSearch.do?bookCatId=<%=bookCat.getId()%>&mode=1"><font color="#000000"><%=bookCat.getName()%></font></a><br>       
          <%
         }
      }
      %>
       
         
</TD> </TR>
  </TABLE>
</TD>
    <TD bgColor=#7387B8>
<br>
 <TABLE cellSpacing=0 cellPadding=0 width="90%" align=center border=0 bgColor=#ffffff>
      <TR><TD>
  <FORM action="bookSearch.do" method=post>
  <DIV align=center><font size=2>请输入关键字：<font>
  <INPUT type=hidden name=mode value="2"> 
  <INPUT class=button2 size=12 name=key> 
   <SELECT class=button1 name=field> 
   <OPTION value=1 selected>书名</OPTION> 
   <OPTION  value=2>作者</OPTION> </SELECT> 
  <INPUT type=submit value=搜索 name=Submit> 
<BR><BR></DIV></FORM>
  </TD></TR>
  <TR><TD width="50%" height=20>&nbsp;&nbsp;课程信息总目录：</TD></TR>
  <TR><TD align=right width="48%" height=10>共查到 <%=schSta.getBookCount()%>条记录，显示第<%=schSta.getPageId()+1%>页/总<%=schSta.getPageCount()%>页</TD></TR>
  <TR><TD>
  <table class="txt" cellSpacing="1" cellPadding="1" width="515" align="center" bgColor="#005b4b" border="0">
        <tr bgColor="#bad4a5">
          <td vAlign="top" width="197" bgcolor="#0099CC" align="center"><b>
          <font color="#FFFFFF">书名</font></b></td>
          <td vAlign="top" width="39" bgcolor="#0099CC" align="center"><b>
          <font color="#FFFFFF">定价</font></b></td>
          <td vAlign="top" width="41" bgcolor="#0099CC" align="center"><b>
          <font color="#FFFFFF">会员价</font></b></td>
          <td vAlign="top" width="112" bgcolor="#0099CC" align="center"><b>
          <font color="#FFFFFF">作者</font></b></td>
          <td vAlign="top" width="42" bgcolor="#0099CC" align="center"><b>
          <font color="#FFFFFF">详细</font></b></td>
        </tr>
        
   <%if(bookList != null ){       for(int i = 0;i < bookList.size();i++)
       {  
        book = (Book)bookList.get(i);
      %> <tr bgColor="#ffffff">
          <td width="197" bgColor="#ffffff"><%=book.getName()%></td>
          <td width="39" bgColor="#ffffff">￥<%=book.getPrice()%></td>
          <td width="41" bgColor="#ffffff">￥<%=book.getSalePrice()%></td>
          <td width="112"><%=book.getAuthor()%></td>
          <td vAlign="center" width="42">
          <a href="bookDetail.do?bookId=<%=book.getId()%>">
          <img src="images/view.gif" border="0" width="25" height="25"></a></td>
      <%
         } %>
      
         
          
        </tr> 
<%}
      %>
      </table>
      <p>　<TR height=24>
<TD align=right>| <font color="#999999">
            <a href="bookPage.do?pageId=<%=0%>">首页</a></font> | <font color="#999999">
            <a href="bookPage.do?pageId=<%=schSta.getPageId()-1%>">上页</a></font> | <a href="bookPage.do?pageId=<%=schSta.getPageId()+1%>">下页</a> </TD>
</TR>
<br></TD></TR>
 </TABLE>                              
 <br>
    </TD>
  </TR>
 </TABLE>
<div align="center"><font size=2></font></div>
</body>
</html>
抱错为：
HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:372)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause java.lang.NullPointerException
org.apache.jsp.bookMain_jsp._jspService(bookMain_jsp.java:113)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.28 logs.
--------------------------------------------------------------------------------Apache Tomcat/5.0.28
为什么？？？？？？