当光标移开文本框时，基于HTML，以GET或POST方式，检查注册用户名是否存在，使用DOM对象。效果如下图：
Jsp和javascript代码如下：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP 'checkUser.jsp' starting page</title>
<script type="text/javascript" src="js/createAjax.js"></script>
</head>
<body>
用户名：<input type="text" name="user" id="userId">
<br>
<span></span>
<script type="text/javascript">
//创建ajax对象
var ajax = createAjax();
//创建鼠标失去焦点事件
document.getElementById("userId").onblur = function(){
//准备发送请求
var method = "post";
var url = "${pageContext.request.contextPath}/CheckUserServlet?id="+new Date().getTime();
ajax.open(method,url,true);
/*
*设置AJAX自动将请求体的中文进行编码，
*在默认情况下，AJAX不会自动对请求体中文编码的
*AJAX将会以表单POST形式发送到服务器
*如果该没该句代码，AJAX不会以表单形式发送请求体数据
*/
ajax.setRequestHeader("content-type","application/x-www-form-urlencoded");
//真正发送请求
username = this.value;
var content = "username="+ username;
ajax.send(content);
//监听服务器响应
ajax.onreadystatechange = function(){
//判断响应状态和响应码
if(ajax.readyState==4){
if(ajax.status){
//接受服务器响应的字符串
var text = ajax.responseText;
//将text添加到span标签内
document.getElementsByTagName("span")[0].innerHTML = text;
}
}
};
};
</script>
</body>
</html>
servlet代码如下：
package kerwin.ajax;
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 CheckUserServlet extends HttpServlet {
public void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException,IOException {
//设置解码方式
request.setCharacterEncoding("utf-8");
response.setContentType("text/html; charset=utf-8");
//获取用户名
String username = request.getParameter("username");
//模拟调用业务层
String msg = "<font color='green'>可以注册！！</font>";
if("杰克".equals(username)){
msg = "<font color='red'>该用户名已被注册！！</font>";
}
//已流IO流的方式，将普通字符串，输出到ajax引擎
response.getWriter().write(msg);
}
public void doPost(HttpServletRequest request,IOException {
doGet(request,response);
}
}
