Ajax动态更新上拉列表 _Ajax

Ajax动态更新下拉列表
    动态更新下拉列表是交互性Web应用的必备功能，可以为用户使用带去很大方便，希望以下示例能给学习Ajax的朋友们一些启示。

JSP页面：select-tag.jsp
<%@ page language="java" import="java.util.*" pageEncoding="GBK"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
    <base href="<%=basePath%>">

    <title>My JSP 'select-tag.jsp' starting page</title>

<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">


</head>
<script type="text/javascript">
var xmlHttp;

function createXMLHttpRequest(){
if(window.ActiveXObject){
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
else if(window.XMLHttpRequest){
xmlHttp=new XMLHttpRequest();
}
}

function updateSelect(){
var selected=document.all.slt1.value;
createXMLHttpRequest();
xmlHttp.onreadystatechange=processor;
xmlHttp.open("GET","CreateXML?selected="+selected);
xmlHttp.send(null);
}

function processor(){
var result;
if(xmlHttp.readyState==4){
if(xmlHttp.status==200){
result=xmlHttp.responseXML.getElementsByTagName("city");
while(document.all.slt2.options.length>0){
document.all.slt2.removeChild(document.all.slt2.childNodes[0]);
}

for(var i=0;i<result.length;i++){
var option=document.createElement("OPTION");
option.text=result[i].childNodes[0].childNodes[0].nodeValue;
option.value=result[i].childNodes[1].childNodes[0].nodeValue;
document.all.slt2.options.add(option);
}
}
}
}
</script>
<body>
    This is my JSP page. <br>
    <p>请选择省份：
    <select id="slt1" onChange="updateSelect()">
    <option value="1">陕西</option>
    <option value="2">湖南</option>
    </select>
    城市：
    <select id="slt2">
    <option>请选择城市</option>
    </select>
</body>
</html>

配置文件：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>
    <description>This is the description of my J2EE component</description>
    <display-name>This is the display name of my J2EE component</display-name>
    <servlet-name>CreateXML</servlet-name>
    <servlet-class>org.CreateXML</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>CreateXML</servlet-name>
    <url-pattern>/CreateXML</url-pattern>
</servlet-mapping>

<welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>

逻辑处理：CreateXML.java
package org;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CreateXML extends HttpServlet {

public CreateXML() {
super();
}

public void destroy() {
super.destroy(); // Just puts "destroy" string in log
// Put your code here
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request,response);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/xml");//注意此处的格式
response.setCharacterEncoding("UTF-8");
String selected=request.getParameter("selected");

PrintWriter out=response.getWriter();
out.println("<response>");
if(selected.equals("1")){
out.println("<city>");
out.println("<cityname>西安</cityname>");
out.println("<cityvalue>1</cityvalue>");
out.println("</city>");
out.println("<city>");
out.println("<cityname>宝鸡</cityname>");
out.println("<cityvalue>2</cityvalue>");
out.println("</city>");
out.println("<city>");
out.println("<cityname>延安</cityname>");
out.println("<cityvalue>3</cityvalue>");
out.println("</city>");
}else{
out.println("<city>");
out.println("<cityname>长沙</cityname>");
out.println("<cityvalue>1</cityvalue>");
out.println("</city>");
out.println("<city>");
out.println("<cityname>常德</cityname>");
out.println("<cityvalue>2</cityvalue>");
out.println("</city>");
out.println("<city>");
out.println("<cityname>娄底</cityname>");
out.println("<cityvalue>3</cityvalue>");
out.println("</city>");
}
out.println("</response>");
out.flush();
out.close();
}

public void init() throws ServletException {
// Put your code here
}

}