Class to fetch JSON with variable URL

Access quietly a MySQL database via webservice, I get Json perfectly, in AsyncTask's PostExecute it shows the contents of all Json entries except Json or Json text in global variables (Global.java class) but when returning to the main activity looking for either one or another global variable gives error when accessing the String variable with the Json content or checking the number of Json items.

It seems like the variable is outside the scope of the activity.

My Global.java class is like this, it looks like I need to use a variable of a different type of static for the json_str and json_rest variables, of which I will only leave one, which works first:

import org.json.JSONArray;

public class Global
{
    public static String json_str;
    public static JSONArray json_ret;
}