验证用户名是否存在的示例代码引出该思想：
<%@ 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">
<script type="text/javascript" src="${pageContext.request.contextPath}/js/myJS.js"></script>
<title>Insert title here</title>
<script type="text/javascript">
window.onload = function() {
var nameElement = document.getElementsByName("userName")[0];
nameElement.onblur = function() {
var name = this.value;
var xhr = getXMLHttpRequest();
xhr.onreadystatechange = function() {
if (xhr.readyState == 4) {
if (xhr.status == 200) {
var msg = document.getElementById("msg");
if (xhr.responseText == "true") {
msg.innerHTML = "<font color='red'>该用户名已存在</font>";
} else {
msg.innerHTML = "<font color='green'>该用户名可以使用</font>";
}
}
}
}
xhr.open("get", "${pageContext.request.contextPath}/servlet/ckNameServlet?name=" + name + "&time=" + new Date().getTime());
xhr.send(null);
}
}
</script>
</head>
<body>
用户名：<input type="text" name="userName" /><span id="msg" ></span></br>
密码：<input type="password" name="pwd" /></br>
</body>
</html>
我的GitHub地址：
https://github.com/heizemingjun
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http://www.cnblogs.com/chenmingjun
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http://blog.leanote.com/chenmingjun
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