JSP keep ค่าใน dropdownlist เวลาหน้า page ถูก refresh

  <%
					    ArrayList eVoucherViewList = new ArrayList();
					    SearchManager searchManager = new SearchManager();
					    eVoucherViewList = searchManager.listE_Voucher_View_Beans();
					    session.setAttribute("eVoucherViewList", eVoucherViewList);
  %>
                       <font face="TECHNIC" color="#7A02E3">Office ID:</font>
                        
                        <select id="office_id" name="office_id" style="width:120px; background-color:beige;">
                        <option value="">--- Select ---</option>                    
                    <%
						ArrayList<E_Voucher_View_Bean> voucher_View_Beans = (ArrayList<E_Voucher_View_Bean>) session.getAttribute("eVoucherViewList");
							if (voucher_View_Beans != null) {
      							for (E_Voucher_View_Bean listView_Bean : voucher_View_Beans) {
      		   
                    %>
							   <option value="<%=listView_Bean.getOffice_id()%>"><%=listView_Bean.getOffice_id()%></option> 
					<% 		
								}//end for
                           	}//end if
                    %>
  						</select>