Aspect-Oriented Programming (AOP) is a new paradigm that allows you to organize and layer your software applications in ways that are impossible with traditional object-oriented approaches. Aspects allow you to transparently glue functionality together so that you can have a more layered design. AOP allows you to intercept any event in a Java program and trigger functionality based on those events. Combined with JDK 5.0 annotations, it allows you to extend the Java language with new syntax.

An aspect is a common feature that's typically scattered across methods, classes, object hierarchies, or even entire object models. It is behavior that looks and smells like it should have structure, but you can't find a way to express this structure in code with traditional object-oriented techniques.

For example, metrics is one common aspect that is orthogonal to the business logic. In AOP, a feature like metrics is called a crosscutting concern, as it's a behavior that "cuts" across multiple points in your object models, yet is distinctly different. As a development methodology, AOP recommends that you abstract and encapsulate crosscutting concerns.

For example, let's say you wanted to add code to an application to measure the amount of time it would take to invoke a particular method. In plain Java, the code would look something like the following.

   public class BankAccountDAO
   {
     public void withdraw(double amount)
     {
       long startTime = System.currentTimeMillis();
       try
       {
         // Actual method body...
       }
       finally
       {
         long endTime = System.currentTimeMillis() - startTime;
         System.out.println("withdraw took: " + endTime);
       }
     }
   }

While this code works, it is difficult to turn it on and off and we have a code bloat if we want to obtain the metrics for all our methods. This approach to metrics is very difficult to maintain, expand, and extend, because it's dispersed throughout your entire code base. And this is just a tiny example! In many cases, OOP may not always be the best way to add metrics to a class.

Aspect-oriented programming gives you a way to encapsulate this type of behavior functionality. It allows you to add behavior such as metrics "around" your code. For example, AOP provides you with programmatic control to specify that you want calls to BankAccountDAO to go through a metrics aspect before executing the actual body of that code.