本人学习java一个多月，很多东西都不知道。今天做一个管理系统，用户能够完成注册，登陆功能，但设备信息就是无法显示，系统报空指针异常，但我仔细检查了程序，没有发现什么问题，数据库中也有数据，既然能够注册登录，肯定不是数据库连接问题，下面是代码：
Dao类中的查询设备信息的方法
public java.util.List<E_CPUInformation> getAllE_CPUInformation(){
java.util.List<E_CPUInformation> E_CPUInformationList=new java.util.ArrayList();
String sql="SELECT * FROM equipment_CPU";
try
{
Connection con=getConn();
java.sql.PreparedStatement pstm=con.prepareStatement(sql);
java.sql.ResultSet rs=pstm.executeQuery(); //执行sql语句
while(rs.next())
{
E_CPUInformation eci=new E_CPUInformation();
eci.setEquipment_CPU_id(rs.getString(1));
eci.setEquipment_CPU_model(rs.getString(2));
eci.setEquipment_CPU_CoreCode(rs.getString(3));
eci.setEquipment_CPU_SN(rs.getString(4));
eci.setEquipment_CPU_socket(rs.getString(5));
eci.setEquipment_CPU_FSB(rs.getString(6));
eci.setEquipment_CPU_Level2Cache(rs.getString(7));
eci.setEquipment_CPU_SPEED(rs.getString(8));
eci.setEquipment_CPU_version(rs.getString(9));
eci.setEquipment_CPU_status(rs.getString(10));
eci.setEquipment_CPU_custodian(rs.getString(11));
eci.setEquipment_CPU_lender(rs.getString(12));
eci.setEquipment_CPU_lenddate(rs.getString(13));
eci.setEquipment_CPU_expectreturndate(rs.getString(14));
eci.setEquipment_CPU_returndate(rs.getString(15));
eci.setEquipment_CPU_lendhonor(rs.getString(16));
eci.setEquipment_CPU_remark1(rs.getString(17));
eci.setEquipment_CPU_remark2(rs.getString(18));
eci.setEquipment_CPU_remark3(rs.getString(19));
}
rs.close();
}
catch(Exception e)
{
e.printStackTrace();
}
return E_CPUInformationList;
}
对应Action类
package advantechEMS.action;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import advantechEMS.db.DaoE_CPUInformation;
import advantechEMS.db.E_CPUInformation;
import advantechEMS.db.User;
public class SelectE_CPUAction implements ActionInterface{
public String execute(HttpServletRequest request,HttpServletResponse response)
{
DaoE_CPUInformation dbi=new DaoE_CPUInformation();
java.util.List<E_CPUInformation> E_CPUInformationList=dbi.getAllE_CPUInformation();
request.getSession().setAttribute("E_CPUInformationList",E_CPUInformationList);
return "/E_CPUInformation.jsp";
}
}
对应JSP部分文件
<tbody>
<tr>
<th>CPU_id</th>
<th>CPU_model</th>
<th>CPU_CoreCode</th>
<th>CPU_SN</th>
<th>CPU_socket</th>
<th>CPU_FSB</th>
<th>CPU_Level2Cache</th>
<th>CPU_SPEED</th>
<th>CPU_version</th>
<th>CPU_status</th>
<th>CPU_custodian</th>
<th>CPU_lender</th>
<th>CPU_lenddate</th>
<th>CPU_expectreturndate</th>
<th>CPU_returndate</th>
<th>CPU_lendhonor</th>
<th>CPU_remark1</th>
<th>CPU_remark2</th>
<th>CPU_remark3</th>
</tr>
</tbody>
<tbody>
<%
java.util.List<E_CPUInformation> E_CPUInformationList =(List)session.getAttribute("E_CPUInformationList");//从session中取出数据，并定义为list
for(int i=0;i<E_CPUInformationList.size();i++){
E_CPUInformation eci=E_CPUInformationList.get(i);
%>
<tr>
<td><%=eci.getEquipment_CPU_id()%></td>
<td><%=eci.getEquipment_CPU_model()%></td>
<td><%=eci.getEquipment_CPU_CoreCode() %></td>
<td><%=eci.getEquipment_CPU_SN() %></td>
<td><%=eci.getEquipment_CPU_socket() %></td>
<td><%=eci.getEquipment_CPU_FSB() %></td>
<td><%=eci.getEquipment_CPU_Level2Cache() %></td>
<td><%=eci.getEquipment_CPU_SPEED() %></td>
<td><%=eci.getEquipment_CPU_version() %></td>
<td><%=eci.getEquipment_CPU_status() %></td>
<td><%=eci.getEquipment_CPU_custodian() %></td>
<td><%=eci.getEquipment_CPU_lender() %></td>
<td><%=eci.getEquipment_CPU_lenddate() %></td>
<td><%=eci.getEquipment_CPU_expectreturndate() %></td>
<td><%=eci.getEquipment_CPU_returndate() %></td>
<td><%=eci.getEquipment_CPU_lendhonor() %></td>
<td><%=eci.getEquipment_CPU_remark1() %></td>
<td><%=eci.getEquipment_CPU_remark2() %></td>
<td><%=eci.getEquipment_CPU_remark3() %></td>
<td><a href="updateE_CPUInformation.AdvantechEMS?msgid=<%=eci.getEquipment_CPU_id()%>">修改</a>
</tr>
<%}%>
</tbody>
错误代码：
type Exception report
message
description The server encountered an internal error () that prevented it from fulfilling this request.
exception
org.apache.jasper.JasperException: java.lang.NullPointerException
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:522)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
root cause
java.lang.NullPointerException
org.apache.jsp.E_005fCPUInformation_jsp._jspService(E_005fCPUInformation_jsp.java:127)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.18 logs.
PS：以上是浏览器中显示信息，由于表头都没有显示，所以我估计还是程序与数据库连接出了问题，但是起码我的数据库中还是有数据的。由于我还是不很会debug，所以实在是没办法了。请教各位！谢谢！
