一、注册案例（Get方式）
1、前台
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>ajax</title>
</head>
<body>
<form>
用户名[GET]：<input id="usernameID" type="text" name="username" />
光标移出后，立即检查结果
</form>
<hr />
<span id="resultID"></span>
<script type="text/javascript">
//定位文本框，同时提供焦点失去事件
document.getElementById("usernameID").onblur = function() {
var userName=this.value;
//判断是否为空
if(userName.length==0){
document.getElementById("resultID").innerHTML="<font color='red'>用户名必填</font>";
}else{
//对汉字进行UTF-8编码
userName=encodeURI(userName);
console.log(userName);
//1、创建AJAX对象
var ajax = createAJAX();
//2、准备发送请求
var method = "GET";
//为了适应IE浏览器对其不变的地址缓存，以致其不会继续去发送请求，所以加上时间，保证唯一
var url = "${pageContext.request.contextPath}/RegisterServlet?time="+new Date().getTime()+"&userName="+userName;
ajax.open(method, url);
//3、真正发送请求的具体数据到服务器，如果球体中无数据，就用null表示
ajax.send(null);
//4、Ajax异步对象不断监听服务器
ajax.onreadystatechange = function() {
//如果状态码为4
if (ajax.readyState == 4) {
//如果响应码为200
if (ajax.status == 200) {
var tip=ajax.responseText;
document.getElementById("resultID").innerHTML = tip;
}
}
}
}
}
function createAJAX() {
var ajax = null;
try {
//如果是IE5——IE12的话
ajax = new ActiveXObject("microsoft.xmlhttp");
} catch (e1) {
//如果是非IE的话
ajax = new XMLHttpRequest();
}
return ajax;
}
</script>
</body>
</html>
2、servlet
public class RegisterServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String userName = request.getParameter("userName");
userName=new String(userName.getBytes("iso-8859-1"),"utf-8");
String tip = "<font color='green'>可以注册</font>";
if("郭庆兴".equals(userName)){
tip = "<font color='red'>该用户已存在</font>";
}
response.setContentType("text/html;charset=UTF-8");
PrintWriter pw = response.getWriter();
pw.write(tip);
pw.flush();
pw.close();
}
}
二、注册（POST）
1、前台
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>ajax</title>
</head>
<body>
<form>
用户名[POST]：<input id="usernameID" type="text" name="username" />
光标移出后，立即检查结果
</form>
<hr/>
<span id="resultID"></span>
<script type="text/javascript">
//定位文本框，同时提供焦点失去事件
document.getElementById("usernameID").onblur = function() {
var userName=this.value;
//判断是否为空
if(userName.length==0){
document.getElementById("resultID").innerHTML="<font color='red'>用户名必填</font>";
}else{
//1、创建AJAX对象
var ajax = createAJAX();
//2、准备发送请求
var method = "POST";
//为了适应IE浏览器对其不变的地址缓存，以致其不会继续去发送请求，所以加上时间，保证唯一
var url = "${pageContext.request.contextPath}/RegisterServlet?time="+new Date().getTime();
ajax.open(method, url);
//设置AJAX请求头为POST，他会将请求体中的汉字自动进行UTF-8的编码
//<form action="" enctype="application/x-www-form-urlencoded"></form>
ajax.setRequestHeader("content-type","application/x-www-form-urlencoded");
//3、真正发送请求的具体数据到服务器，
var content="userName="+userName;
ajax.send(content);
//4、Ajax异步对象不断监听服务器
ajax.onreadystatechange = function() {
//如果状态码为4
if (ajax.readyState == 4) {
//如果响应码为200
if (ajax.status == 200) {
var tip=ajax.responseText;
//创建img标签
var imgElement=document.createElement("img");
//设置imng标签的属性
imgElement.src=tip;
imgElement.style.width="12px";
imgElement.style.height="12px";
//定位span标签、
var spanElement=document.getElementById("resultID");
//清空span标签
spanElement.innerHTML = "";
//请img标签加入到span标签中去
spanElement.appendChild(imgElement);
}
}
}
}
}
function createAJAX() {
var ajax = null;
try {
//如果是IE5——IE12的话
ajax = new ActiveXObject("microsoft.xmlhttp");
} catch (e1) {
//如果是非IE的话
ajax = new XMLHttpRequest();
}
return ajax;
}
</script>
</body>
</html>
这与前面，有个地方不同，就是在open方法和send方法之间要设置一个响应头，否则后台没办法接收到数据。
2、后台servlet
public class RegisterServlet extends HttpServlet {
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("utf-8");
String userName=request.getParameter("userName");
String tip="img/right.gif";
if ("郭庆兴".equals(userName)) {
tip="img/error.gif";
}
response.setContentType("text/html;charset=utf-8");
PrintWriter pw=response.getWriter();
pw.write(tip);
pw.flush();
pw.close();
}
}
结果如图
三、三级联动
省份-城市联动
1、前台
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>ajax</title>
</head>
<body>
省份：<select id="provinceID" style="width: 120px">
<option>选择省份</option>
<option>湖北省</option>
<option>广东省</option>
<option>湖南省</option>
</select>
城市：<select id="cityID" style="width: 120px">
<option>选择城市</option>
</select>
<script type="text/javascript">
//定位下拉框，同时添加内容改变事件
document.getElementById("provinceID").onchange = function() {
//清空城市下拉列表的内容，除了第一项
var cityElement=document.getElementById("cityID");
cityElement.length=1;
//获取选中的省份标签的索引号，从0开始
var index=this.selectedIndex;
//定位当前的option标签
var optionElement=this[index];
//获取option内容的标签
var province=optionElement.innerHTML;
//如果选中的不是提示“选择省份”,不需要向服务器去查询城市信息
if ("选择省份" != province) {
var ajax=createAJAX();
var method="post";
var url="${pageContext.request.contextPath}/ProvinceServlet?time="+new Date().getTime();
ajax.open(method, url);
//设置响应头
ajax.setRequestHeader("content-type", "application/x-www-form-urlencoded");
var content="province="+province;
ajax.send(content);
//***************等待服务器发送数据**************
ajax.onreadystatechange=function(){
if (ajax.readyState==4) {
if (ajax.status=200) {
//从ajax异步对象中获取服务器响应的xml文档
var xmlDocument = ajax.responseXML;
//按照DOM规则解析xml文档
var cityElementArray = xmlDocument.getElementsByTagName("city");
var size=cityElementArray.length;
for (var i = 0; i < size; i++) {
var city = cityElementArray[i].firstChild.nodeValue;
//开始往城市的下拉栏添加
var optionElement=document.createElement("option");
optionElement.innerHTML=city;
cityElement.appendChild(optionElement);
}
}
}
}
}
}
function createAJAX() {
var ajax = null;
try {
//如果是IE5——IE12的话
ajax = new ActiveXObject("microsoft.xmlhttp");
} catch (e1) {
//如果是非IE的话
ajax = new XMLHttpRequest();
}
return ajax;
}
</script>
</body>
2、后台servlet
package com.gqx.test1;
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;
/**
* 二级联动，基于xml方式完成
* @author 郭庆兴
*
*/
public class ProvinceServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
String province=request.getParameter("province");
//通知ajax异步对象，服务器响应的数据格式为xml
response.setContentType("text/xml;charset=UTF-8");
//获取字符流输出
PrintWriter pw=response.getWriter();
//如果是连接数据库的话，应该查出来的是一个list集合，然后将其按xml的方式封装，如下格式
pw.write("<?xml version='1.0' encoding='UTF-8'?>");
pw.write("<root>");
if ("湖北省".equals(province)) {
pw.write("<city>武汉</city>");
pw.write("<city>黄石</city>");
pw.write("<city>荆州</city>");
pw.write("<city>襄阳</city>");
pw.write("<city>宜昌</city>");
}else if ("湖南省".equals(province)) {
pw.write("<city>长沙</city>");
pw.write("<city>株洲</city>");
pw.write("<city>萍乡</city>");
pw.write("<city>湘潭</city>");
pw.write("<city>岳阳</city>");
}else if ("广东省".equals(province)) {
pw.write("<city>广州</city>");
pw.write("<city>深圳</city>");
pw.write("<city>中山</city>");
pw.write("<city>佛山</city>");
pw.write("<city>汕头</city>");
pw.write("<city>珠海</city>");
}
pw.write("</root>");
pw.flush();
pw.close();
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doGet(request, response);
}
}
这里要注意的是在后台要用xml的方式去发送数据，同时前台去接受数据的时候要去解析，先接受
//从ajax异步对象中获取服务器响应的xml文档
var xmlDocument = ajax.responseXML;
//按照DOM规则解析xml文档
var cityElementArray = xmlDocument.getElementsByTagName("city");
要获取xml最小节点内的数据可以通过如下方法去获取
var city = cityElementArray[i].firstChild.nodeValue;
最终 效果如下