How to dynamically load combo boxes with Ajax (code attached)-JS Tutorial-php.cn

This time I will show you how to dynamically load a combo box with Ajax (with code). What are the precautions for Ajax to dynamically load a combo box? The following is a practical case, let's take a look.

一 province.jsp

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
 <html>
  <head>
   <script type="text/javascript" language="javaScript">
    var xmlHttp = false; //全局变量，用于记录XMLHttpRequest对象
    function createXMLHttpRequest() {
     if(window.ActiveXObject) { //Internet Explorer时，创建XMLHttpRequest对象的方法
      try {
      xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
     } catch(e) {
      try {       xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
       //旧版本的Internet Explorer，创建XMLHttpRequest对象
      } catch(e) {
       window.alert("创建XMLHttpRequest对象错误"+e);
      } 
     }
    } else if(window.XMLHttpRequest) { //mozilla时，创建XMLHttpRequest对象的方法
      xmlHttp = new XMLHttpRequest();
     } 
    if(!(xmlHttp)) { //未成功创建XMLHttpRequest对象
      window.alert("创建XMLHttpRequest对象异常！");
    } 
    }
   //下拉列表项改变时的操作
   function proChange(objVal) {
     createXMLHttpRequest(); //创建XMLHttpRequest对象
     document.getElementById("city").length = 1;   //根据ID获取指定元素，并赋值
     xmlHttp.onreadystatechange = cityList; //指定onreadystatechange处理函数
     var url="CityByXMLServlet?province="+objVal; //请求的URL地址
     xmlHttp.open("POST",url,true);
     xmlHttp.send(null);
    }
    function cityList() { //onreadystatechange的处理函数
    if(xmlHttp.readyState==4) {
      if(xmlHttp.status==200) {
       parseXML(xmlHttp.responseXML);   //解析服务器返回的XML数据
     }
     }
   }
    //解析xml信息，以添加地市
    function parseXML(xmlDoc) {
    var len = xmlDoc.getElementsByTagName("city");
    //获取XML数据中所有的“city”元素对象集合
     var _citySel = document.getElementById("city");   //根据ID获取页面中的select元素
     for(var i=0;i<len.length;i++) { //遍历XML数据并给select元素添加选项
     var opt = document.createElement("OPTION");   //创建option对象
     opt.text = xmlDoc.getElementsByTagName("city")[i].firstChild.data;
     //指定新创建元素的text属性值
     opt.value = xmlDoc.getElementsByTagName("city")[i].firstChild.data;
    //指定新创建元素的value属性值
     _citySel.add(opt); //为select元素添加option
    }
    }
   </script>
   <title>动态加载组合框</title>
  </head>
  <body>
   <table align="center" border=1 width="320">
    <tr>
     <td>省份：</td>
    <td>
     <select id="province" onChange="proChange(this.value);" style="width:85">
      <option value="gd">广东</option>
       <option value="gx">广西</option>
       <option value="hn">湖南</option>
      <option value="hb">湖北</option>
      <option value="ah">安徽</option>
     </select>
    </td>
    </tr>
   <tr>
    <td>城市：</td>
    <td>
     <select id="city" style="width:85">
       <option value="">--请选择--</option>
     </select>
    </td>
    </tr>
   </table>
 </body>
 </html>

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二、CityByXMLServlet.java

package servlet;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.*;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
/**
 * Servlet implementation class CityByXMLServlet
 */
@WebServlet("/CityByXMLServlet")
public class CityByXMLServlet extends HttpServlet {
 private static final long serialVersionUID = 1L;
  private static final String CONTENT_TYPE = "text/xml; charset=UTF-8";
  /**
   * @see HttpServlet#HttpServlet()
   */
  public CityByXMLServlet() {
    super();
    // TODO Auto-generated constructor stub
  }
 /**
 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
 */
  public void doGet(HttpServletRequest request, HttpServletResponse response)
         throws ServletException, IOException {
      response.setContentType(CONTENT_TYPE); //设置服务器响应类型
       String province =request.getParameter("province");
       StringBuffer city = new StringBuffer("<citys>"); //记录返回XML串的对象
       if("gx".equals(province)){
       List list=cityInit(); //获取城市列表
        for(int i=0;i<list.size();i++){
           city.append("<city>"+list.get(i)+"</city>");
        }      
       }else if("hn".equals(province)){
       List list = cityInit1(); //获取城市列表
       for(int j=0;j<list.size();j++){
           city.append("<city>"+list.get(j)+"</city>");
       }
        }else if("hb".equals(province)){
        List list = cityInit2(); //获取城市列表
        for(int j=0;j<list.size();j++){
           city.append("<city>"+list.get(j)+"</city>");
        } 
       }
      city.append("</citys>");
      PrintWriter out = response.getWriter();
      out.println(city.toString());
      out.flush(); //输出流刷新
      out.close(); //关闭输出流
     }
     /*
     * 初始化城市
     */
  public List<String> cityInit2() {
    List<String> cityList = new ArrayList<String>();
   //添加城市列表
    cityList.add("武汉");
    cityList.add("襄阳");
    cityList.add("黄冈");
    cityList.add("荆门");
    cityList.add("十堰");
    cityList.add("黄石");
    return cityList;
  }
     public List<String> cityInit(){
       List<String> cityList = new ArrayList<String>();
     //添加城市列表
       cityList.add("南宁");
       cityList.add("桂林");
       cityList.add("北海");
       cityList.add("河池");
       cityList.add("梧州");
       cityList.add("玉林");
    return cityList;
   }
    public List<String> cityInit1() {
    List<String> cityList = new ArrayList<String>();
   //添加城市列表
    cityList.add("长沙");
    cityList.add("湘潭");
    cityList.add("岳阳");
    cityList.add("常德");
    cityList.add("衡阳");
    cityList.add("邵阳");
    return cityList;
  }
    /**
     *当前Servelt的初始化方法. <br>
     *
     * @throws ServletException发生ServletExceptio时抛出
     */
    public void init() throws ServletException {
    }
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 doGet(request, response);
 }
}

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三web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:web="http://xmlns.jcp.org/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd http://xmlns.jcp.org/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.4">
<servlet>
  <servlet-name>CityByXMLServlet</servlet-name>
  <servlet-class>servlet.CityByXMLServlet</servlet-class><!--类的位置 -->
</servlet>
<servlet-mapping>
  <servlet-name>CityByXMLServlet</servlet-name><!--你创建的类名 -->
  <url-pattern>/CityByXMLServlet</url-pattern>
</servlet-mapping>

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I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!