ComboBox Dinamico Jsp+servlet

Script

<script language="javascript">
function popapartamento(){   
    document.forms[0].acaoo.value="Pesquisar";   
    document.forms[0].action="ControlApartamento";   
    document.forms[0].submit();
}
</script>

Meu form
<form action="ControlApartamento" method="get" enctype="text/plain"> 
<table width="550" border="0">
  <tr>
  <td><font color="#000033"><b>Apartamentos cadastrados:</b></font></td>
  <td>
	<select name="selnumbloco" onchange="popapartamento();">
	    <% List<Apartamento> listapto = (List<Apartamento>)request.getSession().getAttribute("Aptonum"); %>
	    <% for (int i=0; i<listapto.size(); i++) { %>
	         <option><%= ((Apartamento)listapto.get(i)).getNumerobloco()%></option>
	    <% } %>
	</select>
  </td>
  </tr>
  <tr>
    <td><font color="#000033"><b>Numero do apartamento:</b></font></td>
    <td><input type="text" name="txtnumero" size="30" maxlength="5" value="" ></td>
  </tr>
    <tr>
    <td><font color="#000033"><b>Bloco do apartamento:</b></font></td>
    <td><input type="text" name="txtbloco" size="30" maxlength="5"></td>
  </tr>
</table>
<br /><br />
  <input type="submit" value="Incluir" name="btn">
  <input type="submit" value="Excluir" name="btn">
  <input type="Reset" value="Limpar"> 
</form>

Servlet
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		try{
			//parte do apartamento
			String acaoo = request.getParameter("acaoo");
			HttpSession session = request.getSession(true);
				if ("Pesquisar".equals(acaoo)){
					String popapto = request.getParameter("selnumbloco");
					session.setAttribute("Numeroblocos", popapto);
					ApartamentoDAO aptodao = new ApartamentoDAO();
					List<Apartamento> apartamento = aptodao.Pesquisar();
					request.getSession().setAttribute("Apartamentos", apartamento);
					RequestDispatcher rd = request.getRequestDispatcher("aptapartamento.jsp");
					rd.forward(request,response);
				}
				return;
		    }
		    catch(SQLException e){
		    	e.printStackTrace();
		    }

	}