1、jsp页面
<%@ 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>ajax验证用户名是否可用</title>
<script type="text/javascript">
var xmlHttp = null;
function checkUserName(){
var userName = document.getElementById("userName").value;
if(window.ActiveXObject){
// 申明XMLHttpRequest对象，针对较低版本的IE（5，6，7）
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}else{
// 申明XMLHttpRequest对象，针对较高版本的IE（8+，以及主流浏览器）
xmlHttp = new XMLHttpRequest();
}
// get方式提交，带参数，异步
xmlHttp.open("get","ajax?userName="+userName,true);
// 调用回调函数
xmlHttp.onreadystatechange = callback;
xmlHttp.send();
}
// 回调函数
function callback(){
if(xmlHttp.readyState == 4){
if(xmlHttp.status == 200){
// xmlHttp.responseText，获取后台返回信息
document.getElementById("label").innerHTML = xmlHttp.responseText;
}else{
alert("AJAX服务器返回错误！");
}
}
}
function checkForm(){
if(document.getElementById("label").innerHTML != "<font color=\"green\">用户名可用</font>"){
document.getElementById("userName").focus();
return false;
}
return true;
}
</script>
</head>
<body>
<form action="login" method="post" onsubmit="return checkForm()">
用户名：<input type="text" onblur="checkUserName()" id="userName" name="userName">
<label id="label"></label><br>
密码：<input type="password"><br>
<input type="submit">
</form>
</body>
</html>2、web.xml
Java代码
<!-- ajax获取信息 --> <servlet> <servlet-name>AjaxServlet</servlet-name> <servlet-class>com.servlet.AjaxServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>AjaxServlet</servlet-name> <url-pattern>/ajax</url-pattern> </servlet-mapping>
3、servlet
Java代码
package com.servlet;
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 AjaxServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doPost(request, response);
}
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
String userName = request.getParameter("userName");
if(userName.equals("admin")){
out.print("<font color='red'>用户名已经存在</font>");
}
else if (userName.length() == 0) {
out.print("<font color='red'>用户不能为空</font>");
}
else if (userName.indexOf(" ") > 0) {
out.print("<font color='red'>用户不能含有空格</font>");
}
else if (userName.length() > 10 || userName.length() < 4) {
out.print("<font color='red'>用户长度在4-10之间</font>");
}
else {
out.print("<font color='green'>用户名可用</font>");
}
out.flush();
out.close();
}
} 
