An aspect weaver is a metaprogramming utility for aspect-oriented languages designed to take instructions specified by aspects (isolated representations of significant concepts in a program) and generate the final implementation code. The weaver integrates aspects into the locations specified by the software as a pre-compilation step. By merging aspects and classes (representations of the structure of entities in the program), the weaver generates a woven class.
Aspect weavers take instructions known as advice specified through the use of pointcuts and join points, special segments of code that indicate what methods should be handled by aspect code. The implementation of the aspect then specifies whether the related code should be added before, after, or throughout the related methods. By doing this, aspect weavers improve modularity, keeping code in one place that would otherwise have been interspersed throughout various, unrelated classes.
Motivation
Many programming languages are already widely accepted and understood. However, the desire to create radically different programming languages to support the aspect-oriented programming paradigm is not significant due to business-related concerns; there are risks associated with adopting new technologies. Use of an entirely new language relies on a business's ability to acquire new developers. Additionally, the existing code base of a business would need to be discarded. Finally, a business would need to acquire a new toolchain (suite of tools) for development, which is often both an expense in both money and time. Primary concerns about roadmaps for the adoption of new technologies tend to be the need to train new developers and adapt existing processes to the new technology.
To address these business concerns, an aspect weaver enables the use of widely adopted languages like Java with aspect-oriented programming through minor adaptations such as AspectJ which work with existing tools. Instead of developing an entirely new language, the aspect weaver interprets the extensions defined by AspectJ and builds "woven" Java code which can then be used by any existing Java compiler. This ensures that any existing object oriented code will still be valid aspect-oriented code and that development will feel like a natural extension of the object-oriented language. The AspectC++ programming language extends C++ through the use of an aspect weaver, offering the additional efficiency over AspectJ that is necessary for embedded systems while still retaining the benefits of aspect-oriented programming.
Implementation
Aspect weavers operate by taking instructions specified by aspects, known as advice, and distributing it throughout the various classes in the program automatically. The result of the weaving process is a set of classes with the same names as the original classes but with additional code injected into the classes' functions automatically. The advice specifies the exact location and functionality of the injected code.
Through this weaving process, aspect weavers allow for code which would have otherwise been duplicated across classes. By eliminating this duplication, aspect weavers promote modularity of cross-cutting concerns. Aspects define the implementation code which would have otherwise been duplicated and then use pointcuts and join points to define the advice. During weaving, the aspect weaver uses the pointcuts and join points, known as a pointcut designator, to identify the positions in candidate classes at which the implementation should be injected. The implementation is then injected into the classes at the points identified, thus permitting the code to be executed at the appropriate times without relying on manual duplication by the programmer.
Weaving in AspectJ
In the programming language AspectJ, pointcuts, join points, and the modularized code are defined in an aspect block similar to that of Java classes. Classes are defined using Java syntax. The weaving process consists of executing the aspect advice to produce only a set of generated classes that have the aspect implementation code woven into it.
The example at right shows a potential implementation of an aspect which logs the entry and exit of all methods. Without an aspect weaver, this feature would necessitate duplication of code in the class for every method. Instead, the entry and exit code is defined solely within the aspect.
