Geppetto: Enhancing Smalltalk’s Reflective Capabilities with Unanticipated Reflection

Reflection is an important tool to extend and modify the semantics or runtime of applications. However, lot of approaches to support reflection are based on up-front fully reflective or load-time based reflection mechanisms. Using these approaches, it is not possible to apply reflective techniques on running systems without stopping them, unless the system is fully reflection which is very costly. Because many applications and systems exist that cannot be halted and stopped but have to be always on and running, such as web applications, real-time systems or mobile systems, the ability to apply reflective features at runtime is a crucial and important property. Our solution to achieve this possibility is unanticipated reflection. With unanticipated reflection, we can design the MOP required for the problem we want to solve, introduce it in the language, activate the reflective mechanisms and possibly remove them once they are not necessary anymore at runtime, without halting the system or the application in which we want to apply and use the reflective mechanisms. In this thesis we motivate first the need for unanticipated reflection, allowing us to adapt dynamically applications and the system itself. We compare already existing solutions to support reflection in different languages, such as Reflex [TANT 03] or Iguana/J [REDM 02], and learn that these proposals have several shortcomings. Hence we develop our own proposal, an approach to unanticipated reflection in the context of dynamically-typed languages. To provide unanticipated reflection in an efficient as well as expressive way we reuse the partial behavioral reflection model of Reflex. We believe that this model supports best our demands and requirements for unanticipated reflection and fits well into the concept of dynamic languages. We also propose a concrete implementation of unanticipated reflection by providing a reflective framework for Squeak/Smalltalk, called GEPPETTO, which allows us to install behavioral reflective features unanticipated into the running Squeak system ubiquitously. This reflective framework supporting unanticipated partial behavioral reflection is particulary useful to debug or profile running applications or to temporarily test extensions or adaptations of a system that cannot be halted. Another purpose is the implementation of cross-cutting behavior of an application in a causally connected but separated metalevel, similar as in aspect-oriented programming.