求助！！请高手指点！（有时提示“无效的游标错误！”）

/*
 *下边是jsp页面文件
 */
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*,bean.DBClass" errorPage="" %>
<table width="75%" align="right" border="0">
<%
String query="SELECT * FORM message";
String userId="";
String message="";
String date="";
DBClass db=new DBClass();
db.connect();
ResultSet rt=db.executeQuery(query);
while(rt.next()){
userId=rt.getString("userId");
message=rt.getString("message");
date=rt.getString("date");
%>
<tr><td>
留言者:<%=userId %>&nbsp;&nbsp;&nbsp;&nbsp;<%=date %></td></tr>
<tr><%=message %></tr>
<%
}
db.closeConnection();
%>
<tr><td>
<form name="form3" method="post" action="/MsgBoard/UserComment">
<div align="center">我要发言:(不超过50字)<font  color="#FF0000" size="-1"><% if (request.getAttribute("errInf2")!=null){ %>
    <%=(String)request.getAttribute("errInf2") %> <% } %></font>
<p align="center">
<textarea name="comment" cols="30" rows="9" ></textarea></p>
<p align="center">
<input name="submit" type="submit" value="发布"/>&nbsp;&nbsp;
<input name="reset" type="reset" value="重写"/></p>
</div>
</form></td></tr>
</table>/* DBClass 原代码，编译过 没有错误
 * Designed by *****
 */
 package bean;
 
 import java.io.*;
 import java.util.*;
 import java.sql.*;
 
 public class DBClass
 {
  private String driver;
  private String url;
  private String username;
  private String password;
  private Connection connection;
  private Statement statement;
  private String message="";
 public DBClass(){
  driver="sun.jdbc.odbc.JdbcOdbcDriver";
  url="jdbc:odbc:msgboard";
  username="";
  password="";
  connection=null;
  statement=null;
  message="";
 }
 public DBClass(String driver,String url,String username,String password){
  this.driver=driver;
  this.url=url;
  this.username=username;
  this.password=password;
  this.connection=null;
  this.statement=null;
  this.message="";
 }
 public String getDriver(){
  return driver;
 }
 public void setDriver(String driver){
  this.driver=driver;
 }
  public String getUrl(){
  return url;
 }
 public void setUrl(String url){
  this.url=url;
 }
  public String getUsername(){
  return username;
 }
 public void setUsername(String username){
  this.username=username;
 }
  public String getPassword(){
  return password;
 }
 public void setPassword(String password){
  this.password=password;
 }
  public Connection getConnection(){
  return connection;
 }
 public void setConnection(Connection connection){
  this.connection=connection;
 }
  public Statement getStatement(){
  return statement;
 }
 public void setStatement(Statement statement){
  this.statement=statement;
 }
  public String getMessage(){
  return message;
 }
 public void setMessage(String message){
  this.message=message;
 }
 /*link to database*/
 public void connect(){
  try{
  Class.forName(driver);
  connection=DriverManager.getConnection(url,username,password);
  statement=connection.createStatement();
  }catch(ClassNotFoundException cnfe){
  message="connection:"+cnfe;
  }catch(SQLException sqle){
  message="executeQuery:"+sqle;
  }
 }
 /*Seclet and return result*/
 public ResultSet executeQuery(String query){
  ResultSet resultset=null;
  try{
  resultset=statement.executeQuery(query);
  }catch(SQLException sqle){
  message="executeQuery:"+sqle;
  }
  return resultset;
 }
 /*Update the database*/
 public void executeUpdate(String command){
  try{
            statement.executeUpdate(command);
  }catch(SQLException sqle){
  message="executeUpdate:"+sqle;
  }
 }
 /*close the database*/
 public void closeConnection(){
  try{
  connection.close();
  }catch(SQLException sqle){
  message="closeConnection:"+sqle;
  }
 }
} 
/*
 *jsp页面调试错误，调试环境tomcat6.0+jdk1.5      ,   Dreamweaver8.0
 */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: An exception occurred processing JSP page /utility/comment.jsp at line 118: DBClass db=new DBClass();
9: db.connect();
10: ResultSet rt=db.executeQuery(query);
11: while(rt.next()){
12: userId=rt.getString("userId");
13: message=rt.getString("message");
14: date=rt.getString("date");
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:515)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:426)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
root cause java.lang.NullPointerException
org.apache.jsp.utility.comment_jsp._jspService(comment_jsp.java:65)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:384)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:320)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.10 logs.
--------------------------------------------------------------------------------Apache Tomcat/6.0.10