Aspects in Web Application Architectures

What I Do After 20 years in IT, with clients in three continents and a U.S. visa for “extraordinary abilities in Sciences”, I now help IT organizations to explore new ways to develop better software. I also support teams and individuals who want to improve technically and relationally. Full bio at: http:// andreaprovaglio.com

What We’ll Be Talking About • Aspects in a Nutshell • Complexities in Web Applications • Aspect-oriented Web Architectures

Aspects in a Nutshell Primary separation Cross-cutting (orthogonal) concerns of concerns (business logic) Security Transactions Auditing Add Item Remove Item Buy Join Point (pointcut)

Classic AspectJ Example aspect Logger { pointcut log(): call(public * BizObj.*(..)); before(): log() { System.out.println("before method call"); } after(): log() { System.out.println("after method call"); } } Note the se paration of oss-cutting concerns Advices cr

More About the Example • Uses a declarative language, which is better suited for describing facts rather than processes • Isolates common code, following the DRY principle • Easily conﬁgurable for added ﬂexibility Think of Aspec ts as a way to formalize “WHENEVER”

Content vs. Presentation Separatin g Conten from Pre t sentation the basic is s, but o not enou ften gh.

(Semantic Content) ⊥ (Language)

(Content) ⊥ (Layout)

(Presentation) ⊥ (Accessibility 1)

(Presentation) ⊥ (Accessibility 2)

(Structural Content) ⊥ (Context)

(Generated Output) ⊥ (Client Agent)

When Aspects Fit In • Lots of “Whenever” in Web applications, but you can rarely use an AOP language for your Web development • So we need to start thinking in Aspects and ﬁnd ways to achieve the beneﬁts of Aspects without an AOP language

An Architectural Approach to Aspects • Domain-Speciﬁc Languages (well, almost) • Interception (Proxy, Chain of Responsibility, Template Method) • Event Handling (Observer)

A Common DSL: CSS #context-menu-container * ul { padding: 0; margin: 0 12px 0 0; white-space: normal; } hink about it.. . Isn’t this T almos t an Aspect?

Interception by Proxy • Proxy •“Provide a surrogate or placeholder for another object to control access to it.” [GoF]

Interception by C.o.R • Chain of Responsibility • “Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. [...]” [GoF]

Interception by Template Method • Template Method •“Deﬁne the skeleton of an algorithm in an operation, deferring some steps to subclasses. [...]” [GoF]

Front Controller and Filters • Front Controller • Use a contro%er as the initial point of contact for handling a request. • Filters • Create plu&able ﬁlters to process common services in a standard manner without requiring changes to core request processing code

Event Handling • Your application server of choice is likely to provide an application event facility (based on Observer) • Each event becomes a potential injection point for advices

Glue It All with Dependency Injection • An OOP technique for supplying an external dependency to a software component, indicating to a part of a program which other parts it can use • Using dependency injection, the life-cycle of a service is handled by a dependency provider (typically a container) rather than the consumer • Such a pattern involves at least three elements: a dependent, its dependencies and an injector (sometimes referred to as a provider or container) [excerpts from Wikipedia]

Wrap-up WHENEVER

Andrea Provaglio http: // andreaprovaglio.com me! act ont ?C info ore .: M P.S

Aspects in Web Application Architectures

by Andrea Provaglio on Jul 06, 2010

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Aspects in Web Application Architectures — Presentation Transcript

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