Design Patterns in ABAP: Factory and singleton (and the wonderous world of OO-Abap)

Introduction

Did you ever encounter that even after the statement DEFINITION DEFERRED you still need to move the definition of your class? Or have you ever wondered that if a singleton-class has several subclasses, why all those subclasses will be singletons, or that all classes in the tree will be one singleton? Those experiences were new to me, but I encountered them and I wanted to share my amazement.

This blog is a mixture of a how-to create a factory-class for singletons with inheritance, some surprises I found during coding and a request for advice. So please, please comment and ask questions. I would love to learn from my mistakes and wrong assumptions. I chose to not use test-driven approach to keep this blog more readable.

The road to a solution to test several different solutions for the same problems using inheritance and a factory class was not a straight road – not at all. It was such a bumpy ride that I thought this was worthy of its own blog. This blog first appeared at our own website.

The challenge

We want to perform an operation on something. In my case, I wanted to test several solutions for a certain operation on an object. So I need:

◉ Uniformity in calling the solution.

◉ Different reactions for the operation.

◉ Managed instantiation.

An additional “restraint” is that this is a proof of concept. I didn’t want to clog the system with dozens of artifacts, so I created it in one program. This led to one interesting issue…

Interface and inheritance – Creating the Singleton(s)

First, I want to have uniformity in calling the solution. So I define an interface that states what operation should be used:

INTERFACE: zlif_interface.

  METHODS: do_something RETURNING VALUE(zrv_text) TYPE string.

ENDINTERFACE.

This is the most important part of the whole solution, and now it’s done. Almost finished!

Let us think now about the other parts. It would be nice if we can re-use some standard code, and only have few differences. That can be solved by creating a superclass, with several subclasses. The polymorphism of the subclasses will take care of the different behaviour.

CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED.

  PUBLIC SECTION.

    INTERFACES zlif_interface.

    ALIASES: do_something FOR zlif_interface~do_something.

ENDCLASS.

CLASS zlcl_subclass_one DEFINITION INHERITING FROM zlcl_super CREATE PROTECTED.

  PUBLIC SECTION.

    METHODS: do_something   REDEFINITION.

ENDCLASS.

CLASS zlcl_subclass_two DEFINITION INHERITING FROM zlcl_super CREATE PROTECTED.

  PUBLIC SECTION.

    METHODS: do_something   REDEFINITION.

ENDCLASS.

I decided to omit the IMPLEMENTATION part here for better readability.

The UML diagram looks like this:

UML: Singleton with Inheritance

As we will use these classes for operations on other objects, I want to use them as a singleton, adding a static method get_instance( ) and define a static singleton reference in the interface. I mark the classes as CREATE PROTECTED.

INTERFACE: zlif_interface.

  METHODS: do_something RETURNING VALUE(zrv_text) TYPE string.

  CLASS-DATA:    zgr_instance TYPE REF TO zlif_interface.

  CLASS-METHODS: get_instance RETURNING VALUE(zrv_instance) TYPE REF TO zlif_interface.

ENDINTERFACE.

CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED.

  PUBLIC SECTION.

    INTERFACES zlif_interface.

ENDCLASS.

CLASS zlcl_super IMPLEMENTATION.

  METHOD do_something.

    ...

  ENDMETHOD.

  METHOD get_instance.

    IF zgr_instance IS INITIAL.

      zgr_instance = NEW #( ).

    ENDIF.

    zrv_instance = zgr_instance. 

  ENDMETHOD.

ENDCLASS.

So far so good, right? Not really!

Issue #01: I would like to implement the functionality for a singleton in my superclass, but I would like to keep it abstract as well.

What could I do? I tried creating a reference to my interface. Silly me, that’s not allowed of course. I really don’t want to give up that the superclass is abstract, so the only way to go is to define the singleton-method get_instance in every subclass.  . But wait! The get_instance( ) method should be a static (class) method. And static methods cannot be redefined. This is a Catch-22, isn’t it?

Luckily, I am not the first one who got into this trouble. “Former member” solved this issue before me by suggesting to change the parameters of the get_instance( ) from a returning to a changing parameter in an answer to this question.  Hurrah! I can keep the superclass abstract, I can even define all subclasses as CREATE PRIVATE, although I have to define all subclasses as a friend of the superclass. So the code now looks like this:

INTERFACE: zlif_interface.

  METHODS: do_something RETURNING VALUE(zrv_text) TYPE string.

  CLASS-DATA: zgr_instance TYPE REF TO zlif_interface.

  CLASS-METHODS:

    get_instance CHANGING zcv_instance TYPE any.

ENDINTERFACE.

CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED.

  PUBLIC SECTION.

    INTERFACES zlif_interface.

    ALIASES:  zgr_instance FOR zlif_interface~zgr_instance,

              do_something FOR zlif_interface~do_something,

              get_instance FOR zlif_interface~get_instance.

ENDCLASS.

CLASS zlcl_super IMPLEMENTATION.

  METHOD get_instance.

    IF  zgr_instance IS INITIAL

    AND cl_abap_refdescr=>describe_by_data( zcv_instance )->kind = cl_abap_typedescr=>kind_ref.

      DATA(lo_ref_descr) = CAST cl_abap_refdescr( cl_abap_refdescr=>describe_by_data( zcv_instance ) ).

      DATA(zlv_classname) = lo_ref_descr->get_referenced_type( )->get_relative_name( ).

      DATA: zlr_instance TYPE REF TO zlif_interface.

      CREATE OBJECT zlr_instance TYPE (zlv_classname).

      zgr_instance ?= zlr_instance.

    ENDIF.

    zcv_instance ?= zgr_instance.

  ENDMETHOD.

ENDCLASS.

Nice! Now we have a singleton with inheritance. I did encounter some frustrations though. I love chaining methods, so actually I would have liked this statement:

CREATE OBJECT DATA(zlr_instance) TYPE 

  ( CAST cl_abap_refdescr( cl_abap_refdescr=>describe_by_data( zcv_instance ) 

                          )->get_referenced_type( )->get_relative_name( ) ).

That I can’t use the CAST keyword in the type-declaration I can understand. But I find it quite annoying that I can’t use a method to retrieve the type in the type-declaration. This yields a syntax error:

CREATE OBJECT DATA(zlr_instance) TYPE "This yields an annoying syntax error

  (lo_ref_descr->get_referenced_type( )->get_relative_name( ) ).

     "This one creates an annoying syntax error as well: 

CREATE OBJECT zlr_instance TYPE (lo_abap_refdescr->get_referenced_type( )->get_relative_name( ) ).

So, would it work like this? Well, asking the question is answering it. But please, take a moment to think what could go wrong here before reading further.

Redefining methods in ABAP. (And this is where it really hurts).

I ended the last paragraph with a question, what the error is in the previous solution.

The answer is that by defining the singleton-reference-variable in the interface (or the superclass), this contains the same reference for all subclasses. And if we want to use different functionalities defined in different subclasses, I would like to be able to define a singleton-reference in each subclass (that’s possible) and redefine the “get_reference” method. In JavaScript this is possible, but in ABAP it is not. Why is this? I don’t know. If you do know, please leave a comment. Why should you (not) be able to redefine a static method?

Issue #02: How to get a singleton of the correct (sub)class; if I define the get_instance in the interface (or the abstract superclass).

My solution to solve this is to keep track of the instances per subclass. Although this is a working solution, I have the feeling there might be a better solution. Again, if you know a better, nicer, cleaner solution, please tell me.

This is the (simplified) UML diagram:

UML: Factory for a singleton with Inheritance

My implementation is displayed under here. First the definition, followed by the implementation. The subclasses don’t change, so I didn’t include them.

INTERFACE: zlif_interface.

  TYPES: BEGIN OF zlty_instance,

           classname TYPE seoclsname,

           instance  TYPE REF TO zlif_interface,

         END OF zlty_instance,

         zltty_instances TYPE SORTED TABLE OF zlty_instance 

            WITH UNIQUE KEY classname

            WITH UNIQUE SORTED KEY k2 COMPONENTS instance.

  CLASS-DATA zgt_instances TYPE zltty_instances.

  CLASS-METHODS:

    get_instance CHANGING zcv_instance TYPE any.

  METHODS: do_something RETURNING VALUE(zrv_text) TYPE string.

ENDINTERFACE.

CLASS zlcl_super DEFINITION ABSTRACT CREATE PROTECTED.

  PUBLIC SECTION.

    INTERFACES  zlif_interface.

    ALIASES:    do_something  FOR zlif_interface~do_something,

                get_instance  FOR zlif_interface~get_instance.

  PRIVATE SECTION.

    ALIASES:    zgt_instances FOR zlif_interface~zgt_instances.

ENDCLASS.

Implementation of returning the singleton per class in the inheritance tree:

CLASS zlcl_super IMPLEMENTATION.

  METHOD get_instance.

"  https://answers.sap.com/questions/6203810/inherited-static-method-to-return-instance-of-sub-.html

    "Get the class-type of the requested object to be created.

    IF cl_abap_refdescr=>describe_by_data( zcv_instance )->kind = cl_abap_typedescr=>kind_ref.

      DATA(lo_ref_descr) = CAST cl_abap_refdescr( cl_abap_refdescr=>describe_by_data( zcv_instance ) ).

      DATA(zlv_classname) = lo_ref_descr->get_referenced_type( )->get_relative_name( ).

      DATA: zls_instance TYPE zlif_interface~zlty_instance.

      READ TABLE zgt_instances WITH KEY classname = zlv_classname INTO zls_instance .

      IF sy-subrc NE 0.

        DATA: zlr_instance TYPE REF TO zlif_interface.

        CREATE OBJECT zlr_instance TYPE (zlv_classname).

        zls_instance = VALUE zlif_interface~zlty_instance(

            classname = zlv_classname

            instance  = zlr_instance ).

        INSERT zls_instance INTO TABLE zgt_instances .

      ENDIF.

      zcv_instance ?= zls_instance-instance.

    ENDIF.

  ENDMETHOD.

ENDCLASS.

There it is! An implementation of a singleton with inheritance.

Nice to have

What I added in my productive solution is a method in the superclass that returns all possible classes for instances, by dynamically selecting all subclasses from the superclass that implements the interface (I’m looking at table SEOMETAREL). In this case you can both easily extend the amount of subclasses, and still implement some checks. It is also nice for a calling program, to be able to get a list of possible classes to create. In fact, I think it is so nice to have, I’ll support this in my factory!

And ofcourse, I added error-handling. The code above does not contain any error-handling, as currently it is already quite the listing.

And what would be also nice to have? A factory! Oh boy, wouldn’t that be exciting, just asking some class to give me an instance? You could even implement some logic that would let you select the subclass if the first nice-to-have was implemented. So, let’s start with a factory:

One factory class to rule them all

Factory

Since we have two nice inherited singletons, we’ll create a factory to manage easy instantiation for us. I would like two functionalities from this factory:

1. I would like this factory to give us a list we can use (and for the sake of simplicity, I’ll give a hardcoded solution here – but it would be possible to do this with logic).

2. And from this list I want to pick one and instantiate a singleton.

For the first part I need a table to keep track of the entries, and I need a type for that table. Furthermore, I need to fill that table. I can do that in my class-constructor. And of course I need a method to read them.

For the second part I need the factory method. My definition could look like this:

CLASS zlcl_factory DEFINITION ABSTRACT FINAL CREATE PRIVATE.

  PUBLIC SECTION.

    TYPES: BEGIN OF zlty_instances,

             instance_type TYPE string,

             classname     TYPE seoclsname,

           END OF zlty_instances,

           zltty_instances TYPE SORTED TABLE OF zlty_instances

                        WITH UNIQUE KEY  instance_type

                        WITH UNIQUE SORTED KEY k2 COMPONENTS classname.

    CLASS-METHODS:

      class_constructor,

      get_possible_instance_classes RETURNING VALUE(zrt_instances) TYPE zltty_instances,

      get_some_instance IMPORTING zip_singleton       TYPE any

                        RETURNING VALUE(zrr_instance) TYPE REF TO zlif_interface ."RAISING zlcx_error .

  PRIVATE SECTION.

    CLASS-DATA:

      zgt_instance_types TYPE zltty_instances,

      zgr_instance       TYPE REF TO zlcl_factory.

ENDCLASS.

And the implementation could look like this:

CLASS zlcl_factory IMPLEMENTATION.

  METHOD class_constructor.

    zgt_instance_types  = VALUE zltty_instances(

      ( instance_type = |one| classname = |ZLCL_SUBCLASS_ONE| )

      ( instance_type = |two| classname = |ZLCL_SUBCLASS_TWO| )

      ).

  ENDMETHOD.

  METHOD get_possible_instance_classes.

    zrt_instances = zgt_instance_types.

  ENDMETHOD.

  METHOD get_some_instance.

    DATA dref TYPE REF TO data.

    CREATE DATA dref TYPE REF TO (zip_singleton).

    ASSIGN dref->* TO FIELD-SYMBOL(<fs_ref>).

    "Create & fill the signature of the method

    DATA(ptab) = VALUE abap_parmbind_tab(

                    ( name  = 'ZCV_INSTANCE'

                      kind  = cl_abap_objectdescr=>changing

                      value = REF #( <fs_ref> ) )

                      ) .

    CALL METHOD (zip_singleton)=>('GET_INSTANCE')

      PARAMETER-TABLE ptab.

    zrr_instance ?= <fs_ref>.

  ENDMETHOD.

Now lo and behold, we have a working factory that returns us singletons in different flavours!

Source: sap.com