使用ABAP的RTTI和Java反射机制访问static private属性

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.

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