利用Ajax检测用户名是否被占用的完整实例

%@ page language="java" contentType="text/html; charset=utf-8"
 pageEncoding="utf-8"%>
!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=utf-8">
title>用户注册页面/title>
script src="https://libs.baidu.com/jquery/1.8.3/jquery.min.js">/script>
script type="text/javascript">
	var xmlHttp;
	function createXMLHttpRequest(){
		if(window.XMLHttpRequest){
			xmlHttp = new XMLHttpRequest();
		}else if(window.ActiveXObject){
			xmlHttp = new ActiveXObject("Microsoft.XMLHttp");
		}
	}
	function validate(account){
		createXMLHttpRequest();
		xmlHttp.open("Get","ValidateServlet?account="+account,true);
		xmlHttp.onreadystatechange = callback;
		xmlHttp.send(null);
	}
	function callback(){
		if(xmlHttp.readyState==4){
				if(xmlHttp.status==200){
					var text = xmlHttp.responseText;
					if(text=="true"){
						//document.getElementById("msg").innerHTML = "该手机号已经被注册过";
						$("#msg").text("该手机号已经被注册");
						$("#sub").attr("disabled","true");//添加disabled属性，让按钮不可用
					}else{
						//document.getElementById("msg").innerHTML = "";
						$("#msg").text("");
						$("#sub").removeAttr("disabled");//移除disabled属性，让按钮可用
						
					}
				}else{
					alert("请求失败，错误码="+xmlHttp.status);
				}
		}
	}
	function checkInfo(){
		var account = $("#account").val();
		var pwd1 = $("#pwd1").val();
		var pwd2 = $("#pwd2").val();
		if(account==""||account==null){
			$("#msg").text("账号不能为空");
			$("#sub").attr("disabled","true");
			return false;
		}
		if(pwd1==""||pwd1==null||pwd2==""||pwd2==null||pwd1!=pwd2){
			$("#info").text("密码不能为空或者两次密码不一致");
			$("#sub").attr("disabled","true");
			return false;
		}
		$("#msg").text("");
		$("#info").text("");
		$("#sub").removeAttr("disabled");
	}
	function submit(){
		checkInfo();
		$("#reg").submit();
	}
 
/script>
/head>
body>
form id="reg" name="reg" action="RegisterServlet" method="post">
账号：input type="text" name="account" id="account" onblur="validate(this.value);">
span id="msg" style="color:red">请输入手机号/span>br>
密码：input type="password" id="pwd1" name="password1" onblur="checkInfo();">br>
确认密码：input type="password" id="pwd2" name="password2" onblur="checkInfo();">
span id="info" style="color:red">/span>br>
input type="button" id="sub" value="提交" onclick="submit();">
/form>
/body>
/html>

package com.ambow.servlet;
 
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;
 
@WebServlet("/ValidateServlet")
public class ValidateServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
 public ValidateServlet() {
  super(); 
 }
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		PrintWriter pw = response.getWriter();
		String account = request.getParameter("account");
		System.out.println("account"+account);
		if("123".equals(account)) {
			pw.print("true");
		}else {
			pw.print("false");
		}
	}
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doGet(request, response);
	}
}