Ajax验证数据唯一性
js:

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);   }}}

action:

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;}

dao:

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();}

html:

     <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>