Behavioral reflection is crucial to support for example functional upgrades, on-the-fly debugging, or monitoring critical applications. However the use of reflective features can lead to severe problems due to infinite metacall recursion even in simple cases. This is especially a problem when reflecting on core language features since there is a high chance that such features are used to implement the reflective behavior itself. In this paper we analyze the problem of infinite meta-object call recursion and solve it by providing a first class representation of meta-level execution: at any point in the execution of a system it can be determined if we are operating on a meta-level or base level so that we can prevent infinite recursion. We present how meta-level execution can be represented by a meta-context and how reflection becomes context-aware. Our solution makes it possible to freely apply behavioral reflection even on system classes: the meta-context brings stability to behavioral reflection. We validate the concept with a robust implementation and we present benchmarks. Slides from presentation at TOOLS 2008 Zuerich, 30.06.2008

1. The Meta in Meta-Object
Architectures
Marcus Denker, University of Bern
Mathieu Suen, INRIA
Stéphane Ducasse, INRIA
© Marcus Denker

2. Partial Behavioral Reflection
meta-object
links
activation
condition
source code
(AST)
Tanter, OOPSLA 2003
© Marcus Denker 2

3. Partial Behavioral Reflection
meta-object
links
activation
condition
source code
(AST)
David Röthlisberger, Marcus Denker and Éric Tanter:
Dynamic Unanticipated Partial Behavioral Reflection:
(Smalltalk) Adapting Applications at Runtime
Journal of Computer Languages, Systems and Structures, vol.
34, no. 2-3, July 2008, pp. 46-65.
© Marcus Denker 3

4. Partial Behavioral Reflection
meta-object
links
activation
condition
source code
(AST)
AST
Marcus Denker, Stéphane Ducasse, Adrian Lienhard
Philippe Marschall: Sub-Method Reflection
Journal of Object Technology, vol. 6, no. 9,
© Marcus Denker 4

5. Behavioral Reflection: Flexible
meta-object
links
activation
condition
source code
(AST)
> Very Flexible
© Marcus Denker 5

6. Behavioral Reflection: CLOS
meta-object
links
activation
condition
source code
(AST)
> Meta-class MOP (CLOS)
© Marcus Denker 6

7. Behavioral Reflection: AOP
meta-object
links
activation
condition
source code
(AST)
> Aspects
© Marcus Denker 7

8. Behavioral Reflection: Tracer
tracer metaobject
link
source code
(AST)
> Tracer
© Marcus Denker 8

9. Problem: Recursion
> Behavioral reflection cannot be applied to the whole
system
— System classes
— Meta-objects
© Marcus Denker 9

10. Example: Beeper
> Call the Beeper from OrderedCollection>>#add
beepLink := Link new metaObject: Beeper.
beepLink selector: #beep.
(OrderedCollection>>#add:) methodNode link: beepLink.
© Marcus Denker 10

11. Meta-object Call Recursion
Base Level Meta Object Meta Object
#add: send
#beep send
#add: send
#beep send
#add: send
Infinite recursion
© Marcus Denker 11

12. Ad-hoc Solutions
> Code duplication
> Adding special tests
© Marcus Denker 12

13. Tower of Interpreters
> Smith, 1982
© Marcus Denker 13

14. The Real Problem
Representing Meta-Level Execution
© Marcus Denker 14

15. The Meta-Context
Base Level Meta Level
MetaContext activation
> Link enables
MetaContext
MetaContext deactivation
© Marcus Denker 15

16. Context-aware Links
Base Level Meta Level
Stop meta-level call
> Disable call when
already on the
meta-level
© Marcus Denker 16

17. MetaContext
> Recursion problem solved meta-object
meta
links
level 0
operation
base
© Marcus Denker 17

18. MetaContext
metameta-
object
meta-2
link
level 1
meta-object
> Meta-level analysis:
meta
— Trace the tracer links
level 0
operation
base
© Marcus Denker 18

19. Future Work
> Generalize context model:
— Beyond the meta-context
> Virtual machine support
> Context-aware language kernel
© Marcus Denker 19

20. Conclusion
> Recursion problem
> Missing representation of meta-level execution
> Meta-context
— Solves the recursion problem
— Enables meta-level analysis
© Marcus Denker 20

21. Conclusion
> Recursion problem
> Missing representation of meta-level execution
> Meta-context
— Solves the recursion problem
— Enables meta-level analysis
Questions?
© Marcus Denker 21