Ajax如何检查用户名是否已经存在？

Question

<script type="text/javascript"> 
function createXMLHttpRequest() { 
    try { 
     return new XMLHttpRequest(); 
    } catch (e) { 
     try { 
      return ActiveXObject("Msxm12.XMLHTTP"); 
     } catch (e) { 
      try { 
       return ActiveXObject("Microsoft.XMLHTTP"); 
      } catch (e) { 
       alert("dude,What kind of browser you have?"); 
       throw e; 
      } 
     } 
    } 
} 

window.onload = function() { 
    var username = document.getElementById("loginname"); 
    username.onblur = function() { 
     var xmlHttp = createXMLHttpRequest(); 

     xmlHttp.open("POST", "<c:url value='/ajaxValidateLoginname'/>", true); 

     xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded"); 

     xmlhttp.send("loginname=" + username.value); 

     xmlhttp.onreadystatechange = function() { 
      if (xmlhttp.readyState === 4 && xmlhttp.status === 200) { 
       var text = xmlhttp.responseText; 
       var label = document.getElementById("loginnameError"); 
       if (text === false) { 
        label.innerHTML = "The user name has been registered!"; 
       }else{ 
        label.innerHTML = ""; 
       } 
      } 
     }; 
    }; 
}; 

这是servlet来验证登录名是否已经存在。

public void doPost ajaxValidateLoginname(HttpServletRequest request, HttpServletResponse response) throws IOException { 
     String loginname = request.getParameter("loginname"); 
     boolean flag = us.ajaxValidateLoginname(loginname); 
     response.getWriter().print(flag); 
     return null; 
    } 

Answer 1

什么问题？ 我认为代码是正确的。只删除返回null;在servlet中。