Tuesday, 25 April 2017

Aspect Oriented Programming with Spring

Aspect Oriented Programming with Spring

Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects. 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. Aspect-Oriented Programming requires breaking down program logic into distinct parts called concerns. 

The functions that span multiple points of an application are called cross-cutting concerns and these cross-cutting concerns are conceptually separate from the application's business logic. Examples of aspects are logging, auditing, declarative transactions, security, caching, etc. Spring AOP module provides interceptors to intercept an application. For example, when a method is executed, you can add extra functionality before or after the method execution.

AOP is used in the Spring Framework to

Provide declarative enterprise services, especially as a replacement for EJB declarative services. 
Allow users to implement custom aspects, complementing their use of OOP with AOP.

AOP Terminologies

Before we start working with AOP, let us become familiar with the AOP concepts and terminology. These terms are not specific to Spring, rather they are related to AOP.
Aspect: a modularization of a concern that cuts across multiple classes. Transaction management is a good example of a crosscutting concern in Java EE 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
Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut.
Introduction: declaring additional methods or fields on behalf of a type. Spring AOP allows you to introduce new interfaces to any advised object. 
Target object: object being advised by one or more aspects. Also referred to as the advised object. 
AOP proxy: an object created by the AOP framework in order to implement the aspect contracts. In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.
Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time, load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.


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