请教jsp中下拉列表的动态查询 - Java / Web 开发

下拉列表中的值是从数据库中得到的，怎样实现取到下拉列表的值并查询数据库

sql语句：String sql="select * from xiaofei where regname like '%"+userid+"%' and xfdate=从下拉列表中取得的值";


select的语句：
<form name="form" method="post">
  <div align="center">
  <table width="959" height="524" border="0">
  <tr>
  <td height="37" colspan="6" align="right"><div align="center">请选择
  </div>
  <label>
  <div align="center">
  <select name="aaa" id="aaa" onChange="disSel();">
  <%
  request.setCharacterEncoding("gb2312");
  String strError = null;
  String userid=(String)session.getAttribute("user");
 
   
   
  try{
  Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
  Connection con = DriverManager.getConnection("jdbc:odbc:goodschaginfo");
  Statement stmt = con.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE,ResultSet.CONCUR_READ_ONLY);
  String sql="select distinct xfdate from xiaofei where regname like '%"+userid+"%' order by xfdate