Ajax完成用户名的异步校验

function checkUsername() {var username = document.getElementById("username").value;//1. 创建异步对象 var xhr = CreateXHR();//2. 设置监听xhr.onreadystatechange = function() {if (xhr.readyState == 4) {if (xhr.status == 200) {//将接收到 reponseText 输出到 span 中document.getElementById("span1").innerHTML = xhr.responseText;}}}//3. 打开链接//open 方法的三个参数 <1>发送方式。<2>请求的 URL(绝对路径)，<3>是否异步xhr.open("GET", "${pageContext.request.contextPath}/user_findByName.action?time="+new Date().getTime()+"&username="+username, true)//4. 发送xhr.send(null);}//在发送时为了避免浏览器缓存文件，在URL 添加time属性，使请求唯一。//创建异步对象function CreateXHR() {var XHR = false;try {XHR = new ActionXObject("msxml2.XMLHTTP");} catch (e1) {try {xmlHttp = new ActiveXObject("microsoft.XMLHTTP");} catch (e2) {try {XHR = new XMLHttpRequest();} catch (e3) {XHR = false;}}}return XHR;}