tomacat连接中出现的java.lang.NullPointerException

我在调试一个系统中，把在别的机器上演示好的系统直接放到tomcat的webapps下，把数据库导入mysql，在登录的时候出现java.lang.NullPointerException错误，提醒是DBOperation.java文件的64行出错，可是64行是this.rs.close();this.stmt.close();，这个没错啊。
附错误代码
type Exception report

message

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

exception

org.apache.jasper.JasperException
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:460)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:373)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

root cause

java.lang.NullPointerException
hospital.db.dboperation.DBOperation.closeConnection(dboperation.java:64)
org.apache.jsp.patient.register_jsp._jspService(register_jsp.java:114)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:331)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)

note The full stack trace of the root cause is available in the Apache Tomcat/5.5.26 logs.

还有DBOperation文件
package hospital.db.dboperation;

import java.sql.*;
import hospital.db.dboperation.*;
import hospital.db.*;

/**
* 抽象类，用以封装子类需要用到的数据库操作对象和方法
*

*/
abstract class DBOperation{
protected Statement stmt;
protected ResultSet rs;
protected ResultSet result;
protected String strSQL;
protected String id;
protected Connection conn;

/**
* 构造器
*
* 参数：
* id－用户ID
*/
protected DBOperation(String id)
throws InvalidUserException{
this.id=id;
this.result=null;

try{
this.checkUser(id);
}
catch(InvalidUserException iue){
throw iue;
}
}

/**
* 返回数据集
*
* 返回值－数据集对象
*/
public ResultSet getResultSet(){
return result;
}

/**
* 用户登录，需要子类实现
*
* 参数：
* password－密码
*
* 返回值－操作结果代码
*/
protected abstract int login(String password);

/**
* 关闭数据库连接
*/
public void closeConnection(){
try{
this.rs.close();
this.stmt.close();
}
catch(SQLException sqle){
Debug.log(Debug.getExceptionMsg(sqle));
}
}

/**
* 查询用户名合法性，需要子类实现
*
* 参数：
* id－用户标识符
*/
protected abstract void checkUser(String id)
throws InvalidUserException;
}