Unplugging Components using Aspects

Normally, Aspect-Oriented Programming (AOP) is used for plugging and not for unplugging. Aspects (concerns) are weaved into a system, thus providing a modularized way to add crosscutting features to a base program. For example, given a base program without logging and given a logging aspect, the AOP weaver produces from the untangled code an executable with the desired crosscutting logging behaviors. The benefit is that the logging code is modularized, e.g., you can easily activate or deactivate logging. However, what if a legacy system written without AOP technology already contains logging functionality hard-coded in the base program? How do you deactivate logging then? In this paper, we explore the use of AOP for writing aspects that transform exiting hard-coded calls into plugs that can be used to retarget the client to use other subcomponents. Applying AOP for unplugging has potential usage in connection with components. You are given a monolithic system. You apply AOP to non-intrusively decouple the system’s components. You may then replace some of the legacy components with alternative third-party components. 1. AOP FOR UNPLUGGING Consider a client that is strongly coupled with a component , for which third-party alternatives, , exist. Presumably, present a spectrum of trade-offs for the client to choose from, e.g., on certain methods, or may be more efficient than . Suppose that the client calls methods , , and directly on , and that and were developed independently by third-parties and provided without their source code [13]. AOP [4] can help to selectively unplug the client from certain calls to , and plug those calls back into or (Figure 1). Normally, one would need to change the code of the client or the code of the components. Changing the client code is intrusive and the result is not retargetable. Changing the component is also undesired, because it will affect other clients too. Moreover, it may be impossible to change a third-party component. Instead one can employ AOP to achieve pluggability. We first motivate this approach using the standard logging example in AspectJ [6, 3]. In Section 2 we illustrate this approach for the complex case of retargeting a client of reflection in Java [1]. 1.1 Logging Unplugged The components of the system may already be coupled and their code tangled. Consider a legacy system where the client code (class Supported in part by the National Science Foundation (NSF) under Grants No. CCR-0098643 and CCR-0204432, and by the Institute for Complex Scientific Software at Northeastern University. Legacy code Third-party Component Retarget Aspect call call