function checkLoginName() {
var loginName = document.myform.loginName.value;
if (loginName == "" || loginName == null) {
   alert('用户名不能为空！');
   return false;
} else {
   var url = "action.do?method=checkLoginName&loginName=" + loginName;
   send(url);
   // document.myform.action = "action.do?method=checkLoginName";
   // document.myform.submit();
}
}
var xmlHttp;
function createXmlHttp() {

if (window.XMLHttpRequest) {
   xmlHttp = new XMLHttpRequest();
} else if (window.ActiveXObject) {
   xmlHttp = new ActiveXObject("MIcrosoft.XMLHttp");
}
}

function send(url) {
createXmlHttp();
xmlHttp.open("GET", url, true);
xmlHttp.onreadystatechange = processRequest;
xmlHttp.send(null);
}

function processRequest() {
if (xmlHttp.readyState == 4) {
   if (xmlHttp.status == 200) {
    var myfont = document.getElementById("name");
    while (myfont.hasChildNodes()) {
     myfont.removeChild(myfont.firstChild);
    }
    var text = document.createTextNode();
    text.nodeValue = xmlHttp.responseText;
    myfont.appendChild(text);
   }
}

}

public ActionForward checkLoginName(ActionMapping mapping, ActionForm form,
    HttpServletRequest request, HttpServletResponse response)
    throws Exception {

    LoginDao dao=new LoginDao();
   
      String loginName=request.getParameter("loginName");
      loginName=new String(loginName.getBytes("ISO-8859-1"),"UTF-8");
      System.out.println(loginName);
    int num=dao.check(loginName); 
    if(num==0){
     response.getWriter().println("可用");
     response.getWriter().flush();
    }else{
     response.getWriter().println("已经使用");
     response.getWriter().flush();
    }
    return null;
}

public int check(String loginName){
   Session ss=HibernateSessionFactory.getSession();
   ss.beginTransaction();
   Query q=ss.createQuery("select loginName from Login where loginName='"+loginName+"'");
   List list=q.list();
   ss.getTransaction().commit();
   ss.close();
   return list.size();
}

     <tr>
      <td width="100" align="right">
       用户名:
      </td>
      <td align="left">
       <input type="text" name="loginName" />&nbsp&nbsp<input type="button" value="检查用户名" onclick="checkLoginName()">
      <font color="red" size="2px" id="name"></font></td>
     </tr>