Introduction To Aspect Oriented Programming

This is a normal introduction to Aspect Oriented Programming (AOP) and related Terms.

Aspect Oriented Programming

 

                 Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects. (Such concerns are often termed crosscutting concerns in AOP literature.)

              One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP, meaning you do not need to use AOP if you don't want to, AOP complements Spring IoC to provide a very capable middleware solution.

AOP concepts

Let us begin by defining some central AOP concepts and terminology. These terms are not Spring-specific...

• Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in enterprise Java applications. In Spring AOP, aspects are implemented using regular classes or regular classes annotated with the @Aspect annotation 

• Join point: a point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.
   
• Advice: action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. (Advice types are discussed below.) Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors around the join point.

• Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name).
  The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.
  
  
• Target object: object being advised by one or more aspects. Also referred to as the advised object. Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.
  
  
• AOP proxy: an object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on).
  
  
• Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.
  
  
  
  
  Types of advice:
   
  
  
• Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception)

• After returning advice: Advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception. If the method throws any exception this advice will not get called. In case of methods return void, the advice will be invoked as soon as the method execution is finished.

• After throwing advice: Advice to be executed if a method exits by throwing an exception. 
  
  
• After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return). This mean irrespective of method returns normally or with method throws exception this advice is going to be executed.
  
  
• Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.
  
  Note: This is a very brief introduction about the AOP. I will post another few article about each and every advice. If you have any questions, doubts or suggestions please let me know by your comments so I can improve.

 
Click here for OOP Introduction :  Check More about Object Oriented Programming