Try to access static private attribute via ABAP RTTI and Java Reflection

[JerryWang]

In ABAP we can define a static attribute for a class via keyword CLASS-DATA, whose validity is not associated with instances of a class but with the class itself. In order to prove this fact I use the following simple Pointer class for demonstration:

class ZCL_POINT definition

  public

  final

  create public .

public section.

  data X type I .

  methods CONSTRUCTOR

    importing

      !IV_X type I

      !IV_Y type I .

private section.

  data Y type I .

  class-data COUNT type I .

ENDCLASS.

CLASS ZCL_POINT IMPLEMENTATION.

  method CONSTRUCTOR.

    me->x = iv_x.

    me->y = iv_y.

    count = count + 1.

  endmethod.

ENDCLASS.

In this class, static attribute count is responsible to maintain the number of created Point instances.

Then create four point instances:

data(a) = new zcl_point( iv_x = 1 iv_y = 1 ).

data(b) = new zcl_point( iv_x = 1 iv_y = 2 ).

data(c) = new zcl_point( iv_x = 1 iv_y = 3 ).

data(d) = new zcl_point( iv_x = 1 iv_y = 4 ).

Via any variable of a, b, c or d, we can monitor the value of count in debugger.

Can we access the static attribute of a class without object instance in debugger?

Since in theory the static attribute belongs to class instead of any dedicated object instance, so question comes: is there approach to monitor the static attribute value in ABAP debugger directly from class instead? Yes it is possible.

1. type text "{C:ZCL_POINT} in debugger and press enter key

2. double click, and you can see the attribute value is directly maintained in class ZCL_POINT, without any object instance created on top of it.

And I try to change its visibility dynamically via class descriptor via the following code and actually it is not possible:

data(lo) = CAST cl_abap_objectdescr( cl_abap_classdescr=>describe_by_name( 'ZCL_POINT' ) ).



read TABLE lo->attributes ASSIGNING FIELD-SYMBOL(<count>) WITH KEY name = 'COUNT'.

CHECK SY-SUBRC = 0.

<count>-visibility = 'U'.

Since the structure is read-only and not editable outside cl_abap_objectdescr.

This makes sense otherwise the encapsulation will be violated. Just check many other attribute marked as read-only in Class/Object descriptor class.

Reflection in Java

Check the following code which demonstrates how to access private static attribute value in code via Reflection.

import java.lang.reflect.Field;



public class Point {

	private int x;

	private int y;

	static private int count = 0;

	public Point(int x, int y){

		this.x = x;

		this.y = y;

		count++;

	}

	private static void accessStaticPrivate(Point point){

		Class classObject = point.getClass();

		try {

			Field countField = classObject.getDeclaredField("count");

			System.out.println("count: " + countField.get(point));

		} catch (NoSuchFieldException | SecurityException | IllegalArgumentException

				| IllegalAccessException e1 ) {

			e1.printStackTrace();

		} 

	}

	public static void main(String[] arg){

		Point a = new Point(1,2);

		accessStaticPrivate(a);

		

		Point b = new Point(1,3);

		accessStaticPrivate(b);

		

		Point c = new Point(1,4);

		accessStaticPrivate(c);

		

		Point d = new Point(1,5);

		accessStaticPrivate(d);

	}

}

For ABAPer it is easy to understand the usage of Class object in Java by just comparing it with CL_ABAP_CLASSDESCR in ABAP.

When running this small program locally, you will get output in console:

count: 1

count: 2

count: 3

count: 4

Unlike RTTI in ABAP, Java reflection can sometimes lead to security issues, see one example how Java Singleton would be bypassed in blog Singleton bypass – ABAP and Java.

Further reading

I have written a series of blogs which compare the language feature among ABAP, JavaScript and Java. You can find a list of them below:

• Lazy Loading, Singleton and Bridge design pattern in JavaScript and in ABAP


• 
  
  Functional programming – Simulate Curry in ABAP


• 
  
  Functional Programming – Try Reduce in JavaScript and in ABAP


• 
  
  Simulate Mockito in ABAP


• 
  
  A simulation of Java Spring dependency injection annotation @Inject in ABAP


• 
  
  Singleton bypass – ABAP and Java


• 
  
  Weak reference in ABAP and Java


• Fibonacci Sequence in ES5, ES6 and ABAP


• Java byte code and ABAP Load


• How to write a correct program rejected by compiler: Exception handling in Java and in ABAP


• An small example to learn Garbage collection in Java and in ABAP


• String Template in ABAP, ES6, Angular and React


• Try to access static private attribute via ABAP RTTI and Java Reflection


• Local class in ABAP, Java and JavaScript


• Integer in ABAP, Java and JavaScript


• Covariance in Java and simulation in ABAP


• Various Proxy Design Pattern implementation variants in Java and ABAP


• Tag(Marker) Interface in ABAP and Java


• Bitwise operation ( OR, AND, XOR ) on ABAP Integer


• ABAP ICF handler and Java Servlet