Ajax二级联动菜单实现原理及代码

%@ 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.onload=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>