简单的ajax异步请求

1 用户名：<input type="text" name="username" id="username" onblur="validate();" />
2 <span id="message"></span>

 1 <script type="text/javascript">
 2     //获取不同浏览器的XMLHttp对象
 3         function getXMLHttpRequest(){
 4             var xmlHttp；
 6             try{
 7                 //firefox  opera  
 8                 xmlHttp = new XMLHttpRequest();            
 9             }catch(e){
10                 try{
11                 xmlHttp = new XMLHttpRequest("Msxml2.XMLHTTP");
12                 }catch(e){
13                     try{
14                     //低版本的ie
15                     xmlHttp = new XMLHttpRequest("Microsoft.XMLHTTP");                
16                     }catch(e){
17                         alert("此浏览器不支持AJAX!");
18                         return false;
19                     }
20                 }
21             }
22             
23             return xmlHttp;
24             
25         }

 1         function validate(){
 2             //得到xmlHttp对象
 3             var xmlHttp = getXMLHttpRequest();
 4             //获取username的值
 5             var username = document.getElementById("username").value;
 6             //发送到服务器
 7             //传送参数  请求方式  地址 是否为异步请求
 8             alert("ssss");
 9             xmlHttp.open("GET","/JavaScript0505/CheckedUsernameServlet?username="+username,true);
10             //发送请求
11             xmlHttp.send();
12             //获取服务器响应
13             xmlHttp.onreadystatechange=function(){
14                 if(xmlHttp.readyState==4){
15                     if(xmlHttp.status==200){
16                         var text = xmlHttp.responseText;
17                         var oSpan = document.getElementById("message");
18                         oSpan.innerHTML=text;
19                     }
20                 }
21             }
22         }
23     

1 xmlHttp.open("POST","/JavaScript/CheckUsernameServlet",true);
2             xmlHttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
3             xmlHttp.send("username="+username);

 1 public void doPost(HttpServletRequest request, HttpServletResponse response)
 2             throws ServletException, IOException {
 3                         //设置请求与响应的编码格式
 4             request.setCharacterEncoding("utf-8");
 5             response.setContentType("text/html;charset=utf-8");
 6             List<String> names=Arrays.asList("张三","李四","王五");
 7             String username = request.getParameter("username");
 8             if(names.contains(username)){
 9                 response.getWriter().println("<font color=‘red‘>用户名可用！</font>");
10             }else{
11                 response.getWriter().println("<font color=‘red‘>用户名不可用！</font>");
12             }
13     }