AspectJ, Aspect Oriented Programming in Java

Aspect J is a Java extension that implements aspect-oriented programming, a technique that modularize crosscutting concerns.

Using AspectJ can reduce dramatically the size of a Java Program without a lost in performance, and simplify the design in the same degree.
It improves modularity and reusability of code.
And it is especially useful for debugging large projects.

AspectJ was made available in 2001 and AOP is more than fifteen years old and developed at Palo Alto Research Center since 1994.
Here the unit is not the class, but a concern, that spans multiple classes. Concern may be for example properties, area of interest, of a system and AOP describes their relationship, and compose them together into a program. Aspects encapsulates behavior that concern multiple classes.
Aspects of a system can be inserted, changed, removed at compile time.

Features of AspectJ

• Add-on to Java: may be added to an existing program (and replace lot of code or add some controls or debug processing to it).
• Join points represent defined point in a program's execution as method call, exception catch, and so one.
• A join point may contain other join points.
• Pointcut is a construct that picks out a set of join point, according to defined criteria.
  For example, the set of all methods execution in a package.
• Advice is a code that executes before, after or around a designed join point.
• The ajc compiler generates from AspectJ source either .java sources or directly .class bytecode files.
• Aspect code may be reused throught programs.

Sample code

This aspect concerns the execution of any method in the Eclipse program.
An advice executes some code before each method is called, and an other one executes some code after the call.

public aspect xxxx
{
      pointcut anyMethod() : execution (* org.eclipse.. *(..));
      before(): anyMethod()
      {
            ... some code...
      }
      after() : anyMethod()
      {
           ... some code...
       } 
}

Tools and documentation

• Aspect
  Official site of aspect-oriented programming.
• Eclipse AspectJ
  The AspectJ project as part of Eclipse.
• DeveloperWorks
  Read the article of N. Lesiecki.
• gwScript.
  Aspect-oriented scripting language.