Singleton for database connection: availability problem

Trying to avoid global variables I had the idea to introduce a singleton class for my database connection. The aim is to create one database connection object which is available in functions called by other functions e.g. if I create an instance of myConnection and call a function adjust() in main.m then the instance should be recognized within the function adjust().

Thanks a lot!

classdef (Sealed) MyConnection < handle

      properties 
          db          = 'test';
          user        = 'test';
          passwd      = 'test'; 
      end

      properties (SetAccess = protected, GetAccess = public) 
          conn        = '';        
          isNewInstance = 1;        
          error       = ''; 
      end

      properties (Dependent) 
          isConn      = 0;
      end

     methods (Access = private)
        function obj = MyConnection
        end      
     end

     methods (Static)     

        function singleObj = getInstance
              persistent localObj 
              if isempty(localObj) 
                  localObj = SdwConnection;
                  disp('New instance of MyConnection created.');  
                  localObj.isNewInstance = 1;                
                  try           
                      localObj.conn = database(localObj.db,localObj.user,localObj.passwd);
                  catch ME
                      localObj.error = ME.message ; 
                  end                
              else                                                
                  disp('Old instance of MyConnection is used.');
                  localObj.isNewInstance = 0;
              end  
              singleObj = localObj;   
        end

     end

     methods

         function isConn = get.isConn(obj)
              isConn =  isconnection(obj.conn);
         end

     end   
  end

[Nobel Mondal]

It is a great approach to use singletons for DB connections. Although, you might want to reduce the complexity of the above code a little bit. And, there is one sure mistake in there, your localObj has to be an object of the singleton class.

localObj = MyConnection; % not SdwConnection

[Michaela]

Thanks Nobel!

This was not really the issue, as it is only a leftover from the original version of my code which I modified for this post. Seems as I have to check my code again to solve the problem.

Answer 1

[Nobel Mondal]

I became intrigued and scribbled something on my own. I guess you already know this stuff:

classdef (Sealed) MyDBConnection < handle
  % Usage:
  %
  % To create an instance of the class -
  %       obj = MyDBConnection.getInstance
  %
  % To establish database connection -
  % obj.DBname, obj.userName, obj.passWd should
  % be assigned before this operation
  %       obj.getConnection

    properties (SetAccess = private)
        connObj;        
    end

    properties
        DBname;
        userName;
        passWd;        
    end

    methods (Access = private)
      function obj = MyDBConnection
      end
   end

   methods (Static)
      function singleObj = getInstance
         persistent localObj
         if isempty(localObj) || ~isvalid(localObj)
            localObj = SingleInstance;
         end
         singleObj = localObj;
      end

   end

   methods

      % This would ensure that you don't need to establish...
      % the connection as soon as the object is instantiated.
      % Also, I would keep this method outside the static attribute...
      % so that it can be overridden later for playing around.
      % Like for, multiple connection for different users still ...
      % with the flexibility to control from a single object.
      function getConnection(obj)
          if isempty(obj.connObj)
              try           
                  localObj.connObj = database(obj.DBname, obj.userName, obj.passWd);
                  disp('Database connection established.')
              catch ME
                  disp(ME.message); 
              end 
          else
              disp('Connection exists.');
          end
      end

       function myConn = validConnExists(obj)
           myConn = isconnection(obj.connObj);           
       end

       % Create methods here for querying database...
       % and pre-post-process data and query string.

   end

end