Question

I am trying to implement a type of client-side paging, where I only display X records at a time, then when the client wants to see more data I display the next X records and so forth. To do this I am trying to use a session variable, but every time I examine its value it is empty. I don’t really know all that much about Spring MVC so any help would be appreciated:

@Controller
@RequestMapping(value = "/api/rest/da")
@SessionAttributes({"sessionRowKey"})
public class DAController {
    /**
     * Default constructor for DAController
     */
    public DAController() {
    }

    /**
     *  Initialize the SessionAttribute 'sessionRowKey' if it does not exist
     */
    @ModelAttribute("sessionRowKey")
    public String createSessionRowKey()
    {
        return "";
    }

Here I check if the value is empty, which is what I initialize it to, then set the value:

@RequestMapping(value = "/getModelData/{namespace}/{type}/{rowkey:[^\\.]*\\.[^\\.]*}", method = RequestMethod.GET)
public
@ResponseBody
Map<String, Map<String, String>> getModelData(String namespace,
                                              String type,
                                              String rowkey,
                                              @ModelAttribute("sessionRowKey") String sessionRowKey,
                                              HttpServletRequest request)
{
    try 
    {
        ModelType modelType = ModelType.fromString(type);
        Model model;
        if(modelType == ModelType.STATISTICAL)  //page the data
        {
            //code abstracted
            List<KeyValue> records = results_arr[30].list();

            if(sessionRowKey.equals(""))
            {
                model = modelReader.readModel(namespace, modelType, rowkey);
                request.getSession().setAttribute("sessionRowKey", records.get(0).toString());
            }
            else model = modelReader.readModel(namespace, modelType, sessionRowKey);
        }
        else
        {
            model = modelReader.readModel(namespace, modelType, rowkey);
        }
    } 
    catch (Exception e) 
    {
        logger.log(Level.ERROR, "Error in DAController.getModelData: " + e.getMessage());
    }
}

Every time that I examine the session variable it is always “”, How long does the session variable live?

Answer 1

Instead of the @ModelAttribute annotated parameter (sessionRowKey), use an HttpSession parameter and get the sessionRowKey using this parameter. For example:

... HttpSession httpSession, ...

...

String sessionRowKey = (String)httpSession.getAttribute("sessionRowKey");

...

Note: the above is for Java EE 5 and above. For J2EE 1.4 and before use the HttpSession.getValue and HttpSession.setValue methods.