Jquery Ajax retun issue

I'm not getting response as json type data from server.Please look into my code here...

    jQuery( "#dialog-form" ).dialog({
        autoOpen: false,
        height: 500,
        width: 750,
        modal: true,
        buttons :{
            "Search" : function(){
                jQuery.ajax({type : 'POST',
                dataType : 'json',
                   url : '<s:url action="part" method="list" />',
               success : handledata})
            }
        }
    });
    var handledata = function(data)
    {
        alert(data);
    }

if dataType = 'json' am not getting any response, but if didn't mention anything dataType, getting html format of page.

     public String list(){
     JSONObject jo = new JSONObject();
      try {
             Iterator it = findList.iterator();
                 while(it.hasNext()){
                 SearchResult part = (SearchResult) it.next();
                 jo.put("col1",part.getcol1());
                 jo.put("col2",part.getcol2());
                 
                 }
                 log.debug--------->:"+jo.toString());
          
        } catch (Exception e) {
          log.error(e);
                              }
        return jo.toString();
        }

Output format:

    {"col1":"col1","col2":"col2"}

I'm not getting data to jQuery success.
Please correct me, whats wrong here ?