Lodewijk Bergmans

Aspect-Oriented Programming

We have found many programming problems for which neither procedural nor object-oriented programming techniques are sufficient to clearly capture some of the important design decisions the program must implement. This forces the implementation of those design decisions to be scattered throughout the code, resulting in “tangled” code that is excessively difficult to develop and maintain. We present an analysis of why certain design decisions have been so difficult to clearly capture in actual code. We call the properties these decisions address aspects, and show that the reason they have been hard to capture is that they cross-cut the systems basic functionality. We present the basis for a new programming technique, called aspect-oriented programming, that makes it possible to clearly express programs involving such aspects, including appropriate isolation, composition and reuse of the aspect code. The discussion is rooted in systems we have built using aspect-oriented programming.

Composing crosscutting concerns using composition filters

It has been demonstrated that certain design concerns, such as access control, synchronization, and object interactions cannot be expressed in current OO languages as a separate software module [4, 7]. These so-called crosscutting concerns generally result in implementations scattered over multiple operations. If a crosscutting concern cannot be treated as a single module, its adaptability and reusability are likely to be reduced. A number of programming techniques have been proposed to express crosscutting concerns, for example, adaptive programming [9], AspectJ [8], Hyperspaces [10], and Composition Filters [1]. Here, we present the Composition Filters (CF) model and illustrate how it addresses evolving crosscutting concerns.

Abstracting Object Interactions Using Composition Filters

It is generally claimed that object-based models are very suitable for building distributed system architectures since object interactions follow the client-server model. To cope with the complexity of todays distributed systems, however, we think that high-level linguistic mechanisms are needed to effectively structure, abstract and reuse object interactions. For example, the conventional object-oriented model does not provide high-level language mechanisms to model layered system architectures. Moreover, we consider the message passing model of the conventional object-oriented model as being too low-level because it can only specify object interactions that involve two partner objects at a time and its semantics cannot be extended easily. This paper introduces Abstract Communication Types (ACTs), which are objects that abstract interactions among objects. ACTs make it easier to model layered communication architectures, to enforce the invariant behavior among objects, to reduce the complexity of programs by hiding the interaction details in separate modules and to improve reusability through the application of object-oriented principles to ACT classes. We illustrate the concept of ACTs using the composition filters model.

Real-Time Specification Inheritance Anomalies and Real-Time Filters

Real-time programs are, in general, difficult to design and verify. The inheritance mechanism can be useful in reusing well-defined and verified real-time programs. In applications developed by current real-time object-oriented languages, however, changing application requirements or changing real-time specifications in sub-classes may require excessive redefinitions although this seems to be intuitively unnecessary. We refer to this as the real-time specification inheritance anomaly. This paper introduces three kinds of real-time specification inheritance anomalies that one may experience while constructing object-oriented programs. As a solution to these anomalies, the concept of real-time composition filters is introduced. Filters affect the real-time characteristics of messages that are received or sent by an object. Through proper configuration of filters, one can specify real-time constraints, and reuse of these constraints without causing inheritance anomalies.

Obstacles in object-oriented software development

Recently, a considerable number of object-oriented software development methods have been introduced to produce extensible, reusable, and robust software. We have been involved in the development of a large number of pilot applications to form our own view on object-oriented methods. Although our experiences confirmed the claims about the benefits of object-oriented methods, we identified a number of important obstacles that are not addressed by current methods. This paper summarizes these obstacles and evaluates them with respect to our pilot applications. The aim of this paper is to make software engineers aware of problems they may encounter during object-oriented development, and to inspire researchers to initiate new research activities.

Composing Synchronization and Real-Time Constraints

There have been a number of publications illustrating the successes of object-oriented techniques in creating highly reusable software systems. Several concurrent languages have been proposed for specifying reusable synchronization specifications. Recently, a number of real-time object-oriented languages have been introduced for building object-oriented programs with real-time behavior. Composing and reusing object-oriented programs with both synchronization and real-time constraints has not been addressed adequately, although most real-time systems are concurrent. This paper analyzes the origins of the problems in composing and reusing synchronization and real-time specifications, first as separate concerns, and later as composed behavior. To overcome the so-called inheritance anomaly problems, this paper proposes modular and composable synchronization and real-time specification extensions to the object-oriented model. The applicability of the proposed mechanisms is illustrated through a number of examples.

A QoS-Control Architecture for Object Middleware

This paper presents an architecture for QoS-aware middleware platforms. We present a general framework for control, and specialise this framework for QoS provisioning in the middleware context. We identify different alternatives for control, and we elaborate the technical issues related to controlling the internal characteristics of object middleware. We illustrate our QoS control approach by means of a scenario based on CORBA.

Prototyping and Composing Aspect Languages

Domain specific aspect languages (DSALs) are becoming more popular because they can be designed to represent recurring concerns in a way that is optimized for a specific domain. However, the design and implementation of even a limited domain-specific aspect language can be a tedious job. To address this, we propose a framework that offers a fast way to prototype implementations of domain specific aspect languages. A particular goal of the framework is to be general enough to support a wide range of aspect language concepts, such that existing language concepts can be easily used, and new language concepts can be quickly created. We briefly introduce the framework and its underlying model, as well as the workflow used when implementing DSALs. Subsequently, we show mappings of several domain specific aspect languages to demonstrate the framework. Since in our approach the DSALs are mapped to a common model, the framework provides an integrating platform allowing us to compose programs that use aspects written in multiple DSALs. The framework also provides explicit mechanisms to specify composition of advices written in multiple DSALs.

A design method for modular energy-aware software

Nowadays reducing the overall energy consumption of software is important. A well-known solution is extending the functionality of software with energy optimizers, which monitor the energy consumption of software and adapt it accordingly. To make such extensions manageable and to cope with the complexity of the software, modular design of energy-aware software is necessary. Therefore, this paper proposes a dedicated design method for energy-aware software.

Aspect Composition Using Composition Filters

This chapter first discusses a number of software reuse and extension problems in current object-oriented languages. For this purpose, a change case for a simplified mail system is presented. Each evolution step in the change case consists of the addition or refinement of certain aspects to existing classes. These examples illustrate that both inheritance and aggregation mechanisms cannot adequately express certain aspects of evolving software. This deficiency manifests itself in the number of superfluous (method) definitions that are required to realize the change case. As a solution to these problems, the composition filters model is introduced. We evaluate the effectiveness of various language mechanisms in coping with evolving software as in the presented change case.