Singleton concept

Question

Hi
  Please anybody explain me about singleton concept in abap objects,
Thanks n Regards
vijay

Answer 1

Just to elaborate on what Sesh mentioned, A Singleton class is one that can only be instantiated once during the runtime of a program. Typical examples would be Classes that provide services(Persistance), or classes that provide resource pools(BADI Adapters, Shared Connections, etc.)
Since normal classes can be instantiated any number of times,
CLASS lcl_normal DEFINITION.
  PUBLIC SECTION.
    DATA: v type i.
    METHODS: disp_v.
ENDCLASS.
CLASS lcl_normal IMPLEMENTATION.
  METHOD disp_v.
    WRITE:/ v.
  ENDMETHOD.
ENDCLASS.
*__Report Code__*
DATA: r1 TYPE REF TO lcl_normal,
          r2 TYPE REF TO lcl_normal.
CREATE OBJECT r1.
r1->v = 10.
CREATE OBJECT r2. "Possible because class is not singleton
r2->v = 20.
r1->disp_v( ).
r2->disp_v( ). " Also possible
If we have to redesign the above class to make it singleton, in other words, irrespective of how many times the program uses the class, only a single shared instance will be reused, we have to prevent the class from being reinstantiated, we start by preventing calls to the constructor from outside of the class, ie: Only the class itself can create it's own instance!!
CLASS lcl_singleton DEFINITION CREATE PRIVATE.
Next step, now that calling programs cannot "CREATE OBJECT" our class, we provide an alternative to this...
CLASS lcl_singleton DEFINITION CREATE PRIVATE.
   PUBLIC SECTION.
      CLASS-DATA: singleton TYPE REF TO lcl_singleton READ-ONLY.
      DATA: v type i.
      CLASS-METHODS: class_constructor.
      METHODS: disp_v.
ENDCLASS.
CLASS lcl_singleton IMPLEMENTATION.
  METHOD class_constructor.
     CREATE OBJECT singleton. "Can be executed only once
  ENDMETHOD.
  METHOD disp_v.
    WRITE:/ v.
  ENDMETHOD.
ENDCLASS.
*__Report Code__*
DATA: r1 TYPE REF TO lcl_singleton,
          r2 TYPE REF TO lcl_singleton.
*Create object r1 not possible!
r1 = lcl_singleton=>singleton. " The only way to retrieve an instance
r2 = lcl_singleton=>singleton.
r1->v = 10.
r1->disp_v( ).
r2->disp_v( ). "Will also print 10
And that's how it goes !
Regards,
Dushyant Shetty
P.S. Did not get the time to syntax-check, please check before using code !