Aspect-Oriented Programming with the Eclipse AspectJ plug-in David Gallardo

The problem Object-oriented programming provides a solution for business logic, but other concerns complicate a clean OO design and implementation Common services are often required: Logging Authentication Persistence Instrumentation Etc. These requirements lead to code unrelated to the business being scattered throughout the application and are called crosscutting concerns. Problems in more detail: Code tangling (classes deal with issues besides business logic) Code scattering (service code needs to be included in multiple classes) Lower productivity (code duplicated instead of re-used) Limited re-usability (business logic is polluted with service code that is potentially system or platform specific)

Crosscutting concerns Crosscutting concerns, are also called orthogonal concerns. Removing these concerns (removing the code that does logging, for example) doesn’t affect the behavior of the base program.

The solution: Aspect-Oriented Programming Researchers at Xerox PARC start examining limitations of OOP in early 1990s Separation of concerns is identified as the key Crosscutting concerns are implemented separately as Aspects. Aspects are the fundamental unit in AOP and are equivalent to a class in OOP. Aspects interact with an application at interesting and pre-defined points in its execution such as method calls and field assignments. These are called join points. AOP has a small amount of jargon to learn. Here we introduce Aspects, crosscutting concerns and join points. We’ll see a few more terms when AspectJ, a specific AOP language is introduced.

Programming in AOP Decompose application into separate business and crosscutting concerns Implement concerns as classes and aspects Recompose business and crosscutting logic using an AOP compiler. This process is called weaving and the compiler that performs this is called a weaver. The details of this process can vary. The weaver could use source-to-source translation (with an additional compilation step), source to bytecode translation, or just-in-time execution. AspectJ does the second, source to bytecode translation, since it is basically a standard Java compiler that’s been extended to support AspectJ.

AspectJ: The first AOP language Xerox PARC researchers develop AspectJ, the first AOP language. AspectJ is a simple, well-integrated extension of Java It was given to the Eclipse Consortium in early 2002 Available on the Eclipse.org website

Installing the Eclipse AspectJ plug-in Available for 3.1. from the update site: http://download.eclipse.org/technology/ajdt/31/dev/update Works in conjunction with Eclipse’s Java development tools. This provides a source-to-bytecode weaver, based on the Eclipse Java compiler. Plus views and perspectives that support for AOP programming. Eclipse projects have one or more "natures" associated with them, so tools can identify what they can work with. Aspect projects have both Java and Aspect natures, so you can use both the Java tools and AspectJ tools to work with them.

Creating an aspect You create an aspect in Eclipse pretty much like you do a class: Create an AspectJ project for it to live Use the New wizard and select Aspect. You’ll get something that looks like this: public aspect Example { } If you named your aspect Example, it will be in a file named (by default) Example.aj

Identifying join points with a pointcut A joint point or a set of joint points is identified with a pointcut expression. A pointcut uses patterns and wildcards that match type, name and method parameters. The following identifies calls to public methods of type void that begin with the letters “say”: pointcut talk() : call(public void say*(..)); The pointcut expression language is fairly extensive & isn’t covered here. The following identifies all public getter methods that return an int or a long: pointcut access() : call(int get*(..)) || call(long get*(..));

Giving advice Pointcuts only identify where an aspect should introduce behavior. They need to be associated with advice—a code body—in order to do anything. Advice can run before or after a join point. Advice can also run around a join point. Advice of this type can affect the execution of the join point—executing it conditionally, or, in the case of a method call, modifying the parameters.

Examples of advice Here are some examples of advice for the pointcut talk from two slides ago: before() : talk() { System.out.println("Hello!"); } after() : talk() { System.out.println("Goodbye!"); }; These two pieces of advice will print out “Hello!” before, and “Goodbye!” after, every join point identified by the pointcut talk—that is, every public void method that begins with the letters “say”.

Introductions One more AspectJ feature can be included in an aspect, an introduction, or inter-type declaration. This allows the aspect to alter classes by adding fields or methods, altering class heirarchy, etc. We won’t be using these here.

Eclipse AspectJ support Decorators in the editor view identify which methods and fields in a class have advice associated with them. Cross References view provides details about advice are associated with each method or field, or methods or fields associate with each advice. Aspect Visualization perspectives shows graphically which class is affected by each aspect. Debugging support is extended to advice. We’ll see these in the demo that follows…

Eclipse AspectJ demo Demonstrate how to install using Update Create an Aspect Project Create a class and an aspect and run Demonstrate editor support, outline, cross-reference. Demonstrate Aspect Visualization perspective with Eclipse-provided example