Servlet：package com.servlet.ajax;import java.io.IOException;import java.io.PrintWriter;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 AjaxServlet */@WebServlet("/ajax.do")public class AjaxServlet extends HttpServlet {private static final long serialVersionUID = 1L; /** * Default constructor. */ public AjaxServlet() { // TODO Auto-generated constructor stub }/** * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response) */protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {this.doPost(request, response);}/** * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response) */protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType("text/html;charset=utf-8");String type = request.getParameter("type");String value = request.getParameter("value");PrintWriter pw = response.getWriter();System.out.println(type);if ("1".equals(type) && value != null && !"".equals(value)) { // 根据省获取市// 这里可以从数据库获取if ("hubei".equals(value)) {StringBuilder citys = new StringBuilder();citys.append("[");citys.append("{name:'武汉',value:'hubei001'},");citys.append("{name:'襄阳',value:'hubei002'}");citys.append("]");System.out.println(citys.toString());pw.print(citys.toString());}}else if ("2".equals(type) && value != null && !"".equals(value)) { // 根据省获取县// 这里可以从数据库获取StringBuilder countries = new StringBuilder();if ("hubei001".equals(value)) {countries.append("[");countries.append("{name:'AAA县',value:'wuhan001'},");countries.append("{name:'BBB县',value:'wuhan002'},");countries.append("{name:'CCC县',value:'wuhan003'}");countries.append("]");}else if ("hubei002".equals(value)) {countries.append("[");countries.append("{name:'南漳县',value:'xiangyang001'},");countries.append("{name:'古城县',value:'xiangyang002'}");countries.append("]");}System.out.println(countries.toString());pw.print(countries.toString());}else if ("3".equals(type) && value != null && !"".equals(value)) {StringBuilder towns = new StringBuilder();if ("wuhan001".equals(value)) {towns.append("[");towns.append("{name:'AAA镇',value:'wuhan001'},");towns.append("{name:'BBB镇',value:'wuhan002'},");towns.append("{name:'CCC镇',value:'wuhan003'}");towns.append("]");}else if ("wuhan002".equals(value)) {towns.append("[");towns.append("{name:'CCC镇',value:'wuhan001'},");towns.append("{name:'DDD镇',value:'wuhan002'},");towns.append("{name:'EEE镇',value:'wuhan003'}");towns.append("]");}else if ("wuhan003".equals(value)) {towns.append("[");towns.append("{name:'FFF镇',value:'wuhan001'},");towns.append("{name:'GGG镇',value:'wuhan002'}");towns.append("]");}else if ("xiangyang001".equals(value)) {towns.append("[");towns.append("{name:'WWW镇',value:'wuhan001'},");towns.append("{name:'RRR镇',value:'wuhan002'}");towns.append("]");}else if ("xiangyang002".equals(value)) {towns.append("[");towns.append("{name:'XXX镇',value:'wuhan001'},");towns.append("{name:'YYY镇',value:'wuhan002'}");towns.append("]");}System.out.println(towns.toString());pw.print(towns.toString());}}}JSP：<%@ page language="java" contentType="text/html; charset=GB18030" pageEncoding="GB18030"%><!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><html><head><meta http-equiv="Content-Type" content="text/html; charset=GB18030"><title>四级联动下拉列表选中值保持不变</title><style type="text/css">div #tip {font:consolas normal 20px;color:blue;}</style><script type="text/javascript" src="js/jquery-1.4.4.min.js"></script><script type="text/javascript">$(function() {$("#province").change(function() {if ($("#province").val() == '0') {$("#city").html('');$("#county").html('');$("#town").html('');return;}$("#tip").append("<span>请求Ajax获取城市...<br></span>");$.get("ajax.do",{type:'1', value:$("#province").val(), time:new Date()},function(data) {$("#tip").append("<span>"+$("#province").val()+"下城市信息获取成功<br></span>");$("#city").html('<option value="0" selected="selected">请选择一个市</option>');var pros = eval(data);for (var i=0;i<pros.length;i++) {var cityName = pros[i].name;var cityValue = pros[i].value;var opt = $("<option value='" + cityValue + "'>"+cityName+"</option>");$("#city").append(opt);}});});$("#city").change(function() {$("#tip").append("<span>请求Ajax获取县...<br></span>");if ($("#city").val() == '0') {$("#county").html('');$("#town").html('');return;}$.get("ajax.do",{type:'2', value:$("#city").val(), time:new Date() },function(data) {$("#tip").append("<span>"+$("#city").val()+"下县的信息获取成功<br></span>");$("#county").html('<option value="0" selected="selected">请选择一个县</option>');var pros = eval(data);for (var i=0;i<pros.length;i++) {var countyName = pros[i].name;var countyValue = pros[i].value;var opt = $("<option value='" + countyValue + "'>"+countyName+"</option>");$("#county").append(opt);}});});$("#county").change(function() {$("#tip").append("<span>请求Ajax获取镇...<br></span>");if ($("#county").val() == '0') {$("#town").html('');return;}$.get("ajax.do",{type:'3', value:$("#county").val(), time:new Date() },function(data) {$("#tip").append("<span>"+$("#county").val()+"下镇的信息获取成功<br></span>");$("#town").html('');var pros = eval(data);for (var i=0;i<pros.length;i++) {var townName = pros[i].name;var townValue = pros[i].value;var opt = $("<option value='" + townValue + "'>"+townName+"</option>");$("#town").append(opt);}});});});</script></head><body>省：<select id="province"><option value="0" selected="selected">请选择一个省</option><option value="hubei">湖北</option></select>市：<select id="city"></select>县：<select id="county"></select>镇：<select id="town"></select><div id="tip"></div></body></html>注意：需引入servlet3.0的jar
