function validate(field) { 
  if (trim(field.value).length != 0) { 
    //創(chuàng)建Ajax核心對象XMLHttpRequest 
    createXMLHttpRequest(); 
     
    var url = "user_validate.jsp?userId=" + trim(field.value) + "&time=" + new Date().getTime(); 
     
    //設置請求方式為GET，設置請求的URL，設置為異步提交 
    xmlHttp.open("GET", url, true); 
     
    //將方法地址復制給onreadystatechange屬性 
    //類似于電話號碼 
    xmlHttp.onreadystatechange=callback; 
     
    //將設置信息發(fā)送到Ajax引擎 
    xmlHttp.send(null); 
  } else { 
    document.getElementById("spanUserId").innerHTML = ""; 
  } 
} 
 
function callback() { 
  //alert(xmlHttp.readyState); 
  //Ajax引擎狀態(tài)為成功 
  if (xmlHttp.readyState == 4) { 
    //HTTP協(xié)議狀態(tài)為成功 
    if (xmlHttp.status == 200) { 
      if (trim(xmlHttp.responseText) != "") { 
        //alert(xmlHttp.responseText); 
        document.getElementById("spanUserId").innerHTML = "<font color='red'>" + xmlHttp.responseText + "</font>"; 
      }else { 
        document.getElementById("spanUserId").innerHTML = ""; 
      } 
    }else { 
      alert("請求失敗，錯誤碼=" + xmlHttp.status); 
    } 
  } 
} 

<% 
  String userId = request.getParameter("userId"); 
  if(UserManager.getInstance().findUserById(userId) != null) { 
    out.println("用戶代碼已經(jīng)存在"); 
  } 
%>