关于 HTTP Status 500 内部错误

我一周前买了一本求是科技的<<Java信息管理系统开发实例导航>>,配置好环境后,运行第一个图书馆管理系统时出现了HTTP Status 500 内部错误:HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception root cause java.lang.NullPointerException
kjb.main.DataBase.getOnePage(DataBase.java:677)
kjb.main.DataBase.getOnePage(DataBase.java:653)
kjb.main.ParentBean.getOnePage(ParentBean.java:475)
kjb2.library.bean.BookBean.getData(BookBean.java:50)
org.apache.jsp._1_1.list_jsp._jspService(list_jsp.java:116)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:966)
org.apache.jsp._1_1.index_jsp._jspService(index_jsp.java:146)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:966)
org.apache.jsp.frame_jsp._jspService(frame_jsp.java:215)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.runtime.JspRuntimeLibrary.include(JspRuntimeLibrary.java:966)
org.apache.jsp.index_jsp._jspService(index_jsp.java:116)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:332)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:314)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:264)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.17 logs.
--------------------------------------------------------------------------------Apache Tomcat/5.5.17//------------------------------------------------------------------------------------
我的运行环境是 jdk1.5 + tomcat 5 + mysql + jdbc for mysql
我把问题的源代码附上:
DataBase.java://查询总页数
// pstm.clearBatch();
pstm = conn.prepareStatement(sql,ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_UPDATABLE); //这就是677行
rs = pstm.executeQuery();
int rows = 0;
while(rs.next())
{
rows++;
}
int sum = rows/zdrecords;
if(rows%zdrecords!=0||rows==0)sum++;
vect.add(""+sum); rs.close();pstm.close();
请哪位大哥帮忙指点一下,小弟在此感激不尽!!