JSP高手进：The value for the useBean class XXX attribute is invalid.

类代码如下：
package com.chat;

import java.sql.*;

public class Execute_DB extends Open_DB
{
//
private Connection conn;
private Statement stmt;
private ResultSet rs;
//
private int errNum;
//
private String errDesc;

//
public Execute_DB()
{
conn = super.getConn();
stmt = null;
rs = null;
errNum = 0;
errDesc = "";
}

//
public boolean exeSql(String strSql)
{
try
{
stmt = conn.createStatement();
stmt.executeUpdate(strSql);
stmt.close();
return true;
}
catch(Exception ex)
{
this.errNum =-1;
this.errDesc = ex.toString();
return false;
}
}

//
public ResultSet exeSqlQuery(String strSql)
{
try
{
stmt=conn.createStatement();
rs=stmt.executeQuery(strSql);
}