function subLogin(){
      
		if(trim(document.loginForm.j_username.value).length==0){
		     alert("帐户不能为空！");
		     document.loginForm.j_username.focus();
		     return;
		  }
		  if(trim(document.loginForm.j_password.value).length==0){
		     alert("密码不能为空！");
		     document.loginForm.j_password.focus();
		     return;
		  }if(trim(document.loginForm.captcha.value).length==0){
		     alert("验证码不能为空！");
		    document.loginForm.captcha.focus();
		    return;
		  }
 	var xmlHttp = false;
    var u = 'checkCaptchaServlet?captcha='+document.loginForm.captcha.value;
    xmlHttp = createXMLHttpRequest();    
    xmlHttp.open("GET", u, true);
    xmlHttp.setRequestHeader('Cache-Control', 'no-cache');
   	xmlHttp.setRequestHeader('Pragma', 'no-cache');
    xmlHttp.setRequestHeader("If-Modified-Since","0");	
    xmlHttp.onreadystatechange = function() { 		
 		if (xmlHttp.readyState != 4) {
        		return;
		}
		if (xmlHttp.status != 200) {
		        return;
		}
		  
		
   		 var result= xmlHttp.responseText;
   		if(result.indexOf("true")!=-1){
   		   document.loginForm.submit();
   		}else{
   		  alert("验证码不对！");
   		}
	}	
	 xmlHttp.send(null);

}