Spring AOP

Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. In addition to classes, AOP gives you aspects. Aspects enable modularization of concerns such as transaction management that cut across multiple types and objects. (Such concerns are often termed crosscutting concerns.)

One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP, meaning you don't 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. These terms are not Spring-specific. Unfortunately, AOP terminology is not particularly intuitive; however, it would be even more confusing if Spring used its own terminology.
. Aspect: A modularization of a concern that cuts across multiple objects. Transaction management is a good example of a crosscutting concern in J2EE applications. In Spring AOP, aspects are implemented using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (@AspectJ style).
. 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. Join point information isavailable in advice bodies by declaring a parameter of type org.aspectj.lang.JoinPoint.
. 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: Spring uses the AspectJ pointcut language by default.
. Introduction: (Also known as an inter-type declaration). Declaring additional methods or fields on behalf of a type. Spring AOP allows you to introduce new interfaces (and a corresponding implementation) to any proxied object. For example, you could use an introduction to make a bean implement an IsModified interface, to simplify caching.
. 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). In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy. Proxy creation is transparent to users of the schema-based and @AspectJ styles of aspect declaration introduced in Spring 2.0.
. 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.

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