Ajax实现二级联动菜单

%@ page language="java" pageEncoding="UTF-8"%>
html>
 head>
  title>二级菜单联动演示/title>
  script type="text/javascript">
 var req;
 window.οnlοad=function()
 {//页面加载时的函数
 }
 
 function Change_Select(){//当第一个下拉框的选项发生改变时调用该函数
  var province = document.getElementById('province').value;
  var url = "select?id="+ escape(province);
  if(window.XMLHttpRequest){
  req = new XMLHttpRequest();
  }else if(window.ActiveXObject){
  req = new ActiveXObject("Microsoft.XMLHTTP");
  }
  if(req){
  req.open("GET",url,true);
   //指定回调函数为callback
  req.onreadystatechange = callback;
  req.send(null);
  }
 }
 //回调函数
 function callback(){
  if(req.readyState ==4){
  if(req.status ==200){
   parseMessage();//解析XML文档
  }else{
   alert("不能得到描述信息:" + req.statusText);
  }
  }
 }
 //解析返回xml的方法
 function parseMessage(){
  var xmlDoc = req.responseXML.documentElement;//获得返回的XML文档
  var xSel = xmlDoc.getElementsByTagName('select');
  //获得XML文档中的所有select>标记
  var select_root = document.getElementById('city');
  //获得网页中的第二个下拉框
  select_root.options.length=0;
  //每次获得新的数据的时候先把每二个下拉框架的长度清0
  
  for(var i=0;ixSel.length;i++){
  var xValue = xSel[i].childNodes[0].firstChild.nodeValue;
  //获得每个select>标记中的第一个标记的值,也就是value>标记的值
  var xText = xSel[i].childNodes[1].firstChild.nodeValue;
  //获得每个select>标记中的第二个标记的值,也就是text>标记的值
  
  var option = new Option(xText, xValue);
  //根据每组value和text标记的值创建一个option对象
  
  try{
   select_root.add(option);//将option对象添加到第二个下拉框中
  }catch(e){
  }
  }
 }  
 /script>
 /head>
 
 body>
  div align="center">
   form name="form1" method="post" action="">
    table width="70%" border="0" cellspacing="0" cellpadding="0">
     tr>
      td align="center">
       二级联动示例
      /td>
     /tr>
     tr>
      td>
       select name="province" id="province" onChange="Change_Select()">
        !--第一个下拉菜单-->
        option value="0">
         请选择
        /option>
        option value="1">
         北京
        /option>
        option value="2">
         天津
        /option>
        option value="3">
         山东
        /option>
       /select>
       select name="city" id="city">
        !--第二个下拉菜单-->
        option value="0">
         请选择
        /option>
       /select>
      /td>
     /tr>
     tr>
      td>
      /td>
     tr>
    /table>
   /form>
  /div>
 /body>
/html>

package com;
 
import java.io.IOException;
 
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/***
 * 
 * @author zdw
 *
 */
public class SelectServlet extends HttpServlet
{
 
 private static final long serialVersionUID = 1L;
 
 public SelectServlet()
 {
  super();
 }
 
 public void destroy()
 {
  super.destroy();
 }
 
 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException
 {
//  response.setCharacterEncoding("GBK");
  response.setContentType("text/xml");
  response.setHeader("Cache-Control", "no-cache");
  request.setCharacterEncoding("GBK");
  response.setCharacterEncoding("UTF-8");
  String targetId = request.getParameter("id").toString();
  System.out.println(targetId);
  // 获得请求中参数为id的值
  String xml_start = "selects>";
  String xml_end = "/selects>";
  String xml = "";
 
  if (targetId.equalsIgnoreCase("0"))
  {
   xml = "select>value>0/value>text>请选择/text>/select>";
  } else if (targetId.equalsIgnoreCase("1"))
  {
   xml = "select>value>1/value>text>昌平/text>/select>";
   xml += "select>value>2/value>text>丰台/text>/select>";
   xml += "select>value>3/value>text>海淀/text>/select>";
   xml += "select>value>4/value>text>朝阳/text>/select>";
  } else if (targetId.equalsIgnoreCase("2"))
  {
   xml = "select>value>1/value>text>塘沽区/text>/select>";
   xml += "select>value>2/value>text>汉沽区/text>/select>";
   xml += "select>value>3/value>text>大港区/text>/select>";
   xml += "select>value>4/value>text>东丽区/text>/select>";
  } else
  {// 如果是3,则返回下面的字符
   xml = "select>value>1/value>text>济南/text>/select>";
   xml += "select>value>2/value>text>青岛/text>/select>";
   xml += "select>value>3/value>text>淄博/text>/select>";
   xml += "select>value>4/value>text>枣庄/text>/select>";
  }
 
  String last_xml = xml_start + xml + xml_end;
  response.getWriter().write(last_xml);
 
 }
 
 public void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException
 {
  doGet(request, response);
 }
 
 public void init() throws ServletException
 {
 }
 
}

?xml version="1.0" encoding="UTF-8"?>
web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
 http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
 servlet>
  servlet-name>SelectServlet/servlet-name>
  servlet-class>com.SelectServlet/servlet-class>
 /servlet>
 
 servlet-mapping>
  servlet-name>SelectServlet/servlet-name>
  url-pattern>/select/url-pattern>
 /servlet-mapping>
 welcome-file-list>
  welcome-file>index.jsp/welcome-file>
 /welcome-file-list>
/web-app>