在App Engine Java Servlet中查询数据存储区统计信息时发生NullpointerException

Question

Within my app's Servlet, I want to control wether its Datastore is empty or not (the first time I run the servlet it will be empty, but that's not the case right now as I already have populated it and made it persistent) before continuing with the rest of the code. In this Datastore Docs I discovered a way of querying the datastore and fetch different kind of statistics. But while executing:

DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
Entity globalStat = datastore.prepare(new Query("__Stat_Total__")).asSingleEntity();
Long totalBytes = (Long) globalStat.getProperty("bytes");
Long totalEntities = (Long) globalStat.getProperty("count");

I get a NullPointerException when I try to store the two Long variables. Why is the Entity globalStat object null after the query?

EDIT 1 : Additional information

I'm trying to get the Datastore statistics just after populating it with a List<Entity> of Entities:

public class MyServlet extends HttpServlet {
    ArrayList<Tour> m_tours = new ArrayList<Tour>();
    Key tourKey;
    DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();

    //.... some code
    private DatastoreService populateDatastore(){
        //... some other code ...
        List<Entity> List = Arrays.asList(tour,tour1,tour2,tour3,
                                     tour4,tour5,tour6,tour7,tour8);
        datastore.put(List);
        Entity globalStat = datastore.prepare(
                             new Query("__Stat_Total__")).asSingleEntity();

        try{
            Long totalEntities = (Long) globalStat.getProperty("count");
            Long totalBytes = (Long) globalStat.getProperty("bytes");
        }catch (NullPointerException e){
            e.printStackTrace();
        }` 
    }
}

and just this instructions the Entity globalstat object still shows to be null in the debugging tool:

how come that the new Query("__Stat_Total__")).asSingleEntity(); doesn't produce any result when I just filled the datastore with several entities?

Answer 1

ANSWER : as reported here: Using App Engine Datastore Low Level API with Java , "Stat_Total" and "Stat_Kind" don't work in a local development server. They only work when deployed in App Engine Server .