Ajax实现二级联动菜单

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index.jsp:

<%@ 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;i<xSel.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>

SelectServlet:

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
    {
    }

}

web.xml:

<?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>

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