On the design of software composition : mechanisms and the analysis of composition conflicts

A large variety of programming languages exists, and these languages differ\ud substantially with regard to the modularization and composition mechanisms they support. Such differences may have an impact on important software engineering characteristics, such as reusability, exibility, analyzability and stability. The design of software languages often entails making trade-offs between such characteristics. In this thesis, we study several state-of-the-art programming languages, in particular focusing on aspect-oriented languages. Aspects have been proposed as a means to improve software modularization in the presence of crosscutting concerns; the aim is that such improved modularization leads to better software maintainability. In this thesis, we strive to improve several engineering characteristics, in particular those that are influenced by the use of aspect-oriented languages. Specically, to improve the analyzability of programs written in aspect-oriented languages, we propose several analysis techniques. First, we introduce an algorithm that ensures the declarative application of introductions by aspects. However, the use of introductions can also cause new kinds of composition conflicts. We propose automated tools to detect such conflicts. In addition, we employ graph-based formalisms to detect several other categories of issues related to introductions by aspects. In addition, we attempt to improve changeability, both from the perspective of software engineers who write programs using aspects, as well as from the perspective of language engineers who prototype new (aspect) languages. To\ud this end, we define an aspect metamodel interpreter framework, which can be used to define many aspect-oriented languages, including domain-specific aspect languages, of which we give several examples. Using this framework, it is also possible to write programs that use aspects written in multiple aspect languages, expressed in terms of the interpreter framework. Finally, we present a powerful composition infrastructure that supports the definition of a range of composition mechanisms, such as different styles of inheritance, but also aspect-based or domain-specific composition mechanisms. The infrastructure is defined in terms of a language that defines no fixed composition semantics, but provides a generic mechanism for constructing composition mechanisms from primitives that are expressed as rst-class objects

A Case for Custom, Composable Composition Operators

Programming languages typically support a fixed set of com- position operators, with fixed semantics. This may impose limits on software designers, in case a desired operator or semantics are not supported by a language, resulting in suboptimal quality characteristics of the designed software system. We demonstrate this using the well-known State design pattern, and propose the use of a composition infrastructure that allows the designer to define custom, composable composition operators. We demonstrate how this approach improves several quality factors of the State design pattern, such as reusability and modularity, while taking a reason- able amount of effort to define the necessary pattern-related code

An Analysis of Aspect Composition Problems

The composition of multiple software units does not always yield the desired results. In particular, aspect-oriented composition mechanisms introduce new kinds of composition problems. These are caused by different characteristics as compared to object-oriented composition, such as inverse dependencies. The aim of this paper is to contribute to the understanding of aspect-oriented composition problems, and eventually composition problems in a more general context. To this extent we propose and illustrate a systematic approach to analyze composition problems in a precise and concrete manner. In this approach we represent aspect-based composition mechanisms as transformation rules on program graphs. We explicitly model and show where composition problems occur, in a way that can easily be fully automated. In this paper we focus on structural superimposition (cf. intertype declarations) to illustrate our approach; this results in the identification of three categories of causes of composition problems. \u

First-class Compositions - Defining and composing object and aspect compositions with first-class operators

A considerable amount of research, especially within the OO and AOSD communities, has focused on understanding the potential and limitations of various composition techniques. This has led to a large number of proposals for alternative composition techniques, including many variations of message dispatch, inheritance, and aspect mechanisms. This paper makes the case that there is no single perfect composition technique that suits every situation, since different techniques incur different trade-offs. The proper composition technique to use depends on the particular design problem and its requirements (e.g., with respect to adaptability, reusability, understandability, robustness, etc. of the various elements of the design). However, most programming languages limit the available composition techniques to a very few. To address this, we propose a novel composition model, called Co-op. The model provides dedicated abstractions that can be used to express a wide variety of object composition techniques ("composition operators''). Examples include various forms of inheritance, delegation, and aspects. The proposed model unifies objects (with encapsulated state and a message interface) and composition operators; composition operators are specified as first-class citizens. Multiple composition operators can be combined within the same application, and composition operators can even be used to compose new composition operators from existing ones. This opens new possibilities for developing domain-specific composition operators, taxonomies of composition operators, and for reuse and refinement of composition operators. To validate and experiment with the proposed model, we have designed and implemented a simple language, Co-op/I, that we also use in this paper to show concrete examples

A graph-based aspect interference detection approach for UML-based aspect-oriented models

Aspect Oriented Modeling (AOM) techniques facilitate separate modeling of concerns and allow for a more flexible composition of these than traditional modeling technique. While this improves the understandability of each submodel, in order to reason about the behavior of the composed system and to detect conflicts among submodels, automated tool support is required. Current techniques for conflict detection among aspects generally have at least one of the following weaknesses. They require to manually model the abstract semantics for each system; or they derive the system semantics from code assuming one specific aspect-oriented language. Defining an extra semantics model for verification bears the risk of inconsistencies between the actual and the verified design; verifying only at implementation level hinders fixng errors in earlier phases. We propose a technique for fully automatic detection of conflicts between aspects at the model level; more specifically, our approach works on UML models with an extension for modeling pointcuts and advice. As back-end we use a graph-based model checker, for which we have defined an operational semantics of UML diagrams, pointcuts and advice. In order to simulate the system, we automatically derive a graph model from the diagrams. The result is another graph, which represents all possible program executions, and which can be verified against a declarative specification of invariants.\ud To demonstrate our approach, we discuss a UML-based AOM model of the "Crisis Management System" and a possible design and evolution scenario. The complexity of the system makes con°icts among composed aspects hard to detect: already in the case of two simulated aspects, the state space contains 623 di®erent states and 9 different execution paths. Nevertheless, in case the right pruning methods are used, the state-space only grows linearly with the number of aspects; therefore, the automatic analysis scales

An Abstract Metamodel for Aspect Languages

AOP is widely accepted as a language concept to improve separation of concerns, but it has often been pointed out that an encompassing theory of aspect composition is as yet missing. We define an abstract metamodel in which we capture the elements that we believe are the typical aspect oriented programming concepts. Also, these elements are crucial to the understanding of the semantics of AOP language constructs. We intend the metamodel to be a first step towards modeling and comparing AOP languages, as well as a foundation to define the semantics of AOP languages