省： <select id="province" name="province" onchange="jsSubmit()">     <option value="北京" selected="selected">北京</option>     <option value="廣東">廣東</option>     <option value="海南">海南</option>   </select> 市： <select id="city" name="city">     <option value="北京">北京</option>  </select>

function jsSubmit() {   createXMLHttpRequest();      var province = document.getElementById("province");   //解決客戶端向服務器端傳輸中文亂碼      var uri = "AjaxAction?value=" + encodeURI(encodeURI(province.value));   xmlHttp.open("POST", uri, true);   xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded;")   xmlHttp.onreadystatechange = processResponse;//回調函數?。?  xmlHttp.send(null); }

public class AjaxAction extends HttpServlet {  private static final long serialVersionUID = 1L;  private static Map<String, String[]> map = new HashMap<String, String[]>();  static {   String[] cities1 = { "海口", "瓊海", "三亞" };   String[] cities2 = { "廣州", "珠海", "佛山", "深圳" };   String[] cities3 = { "北京" };   map.put("北京", cities3);   map.put("廣東", cities2);   map.put("海南", cities1);  } st方法{   String province = request.getParameter("value");// 解決客戶端向服務器端傳輸中文亂碼   String proviceCN = URLDecoder.decode(province, "UTF-8");   String[] cities = map.get(proviceCN);//根據傳來的省，查出已經存放進map中對應的市   response.setContentType("text/xml; charset=UTF-8");   StringBuffer buff=new StringBuffer("<citylist>");///準備拼字符串......    for (String city : cities)     {      buff.append("<city>").append(city) .append("</city>");     }   buff.append("</citylist>");   response.getWriter().println(buff.toString());

function processResponse() {   if(xmlHttp.readyState == 4) {    if(xmlHttp.status == 200) {     var cities = xmlHttp.responseXML.getElementsByTagName("city");     var displaySelect = document.getElementById("city");     displaySelect.innerHTML = null;     for (var i= 0 ;i < cities.length ; i++){         if (i == 0) {            var a= xmlHttp.responseXML.getElementsByTagName("city")[i].firstChild.data;//用firstChild方法，其他方法我用text方法不管用~不知道怎么回事       var op = new Option(a, a, true, true);      } else {       var a= xmlHttp.responseXML.getElementsByTagName("city")[i].firstChild.data;       var op = new Option(a, a);       alert(a);      }      displaySelect.options[i] = op;     }        } else {     window.alert("請求的頁面有異常");    }  } }