Aspect-Oriented Programming with the Eclipse AspectJ plug-in

1
Aspect-Oriented Programming with the Eclipse
AspectJ plug-in

• David Gallardo

2
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.

3
Crosscutting concerns
4
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.

5
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.

6
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

7
Installing the Eclipse AspectJ plug-in

• Available for 3.1. from the update site
• http//download.eclipse.org/technology/ajdt/31/de
  v/update
• Works in conjunction with Eclipses 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.

8
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.
• Youll 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

9
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 following identifies all public getter
  methods that return an int or a long

pointcut access() call(int get(..))
call(long get(..))
10
Giving advice

• Pointcuts only identify where an aspect should
  introduce behavior. They need to be associated
  with advicea code bodyin 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
  pointexecuting it conditionally, or, in the case
  of a method call, modifying the parameters.

11
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("Good
bye!")

12
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 wont be using these here.

13
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.

14
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