Aspect-Oriented Programming

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Aspect-Oriented Programming
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Coming up

• What is AOP?
• Why do we need AOP?
• How does AOP work?
• Hello World

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What is AOP?

• Part of AOSD
• An extension of OOP extracting cross-cutting
  functional units of systems
• A means of programming such functional units
  separate from other code, which are then woven
  together
• An attempt at promoting good SE practices within
  OOP

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Why do we need AOP?

• Situation in a typical O-O system
• A single requirement implemented by multiple
  components (tangling)
• A single component may include elements of
  multiple requirements (scattering)
• Change could require understanding and changing
  many components
• This impacts on software reuse

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Tangling example shared buffer
Source Sommerville, I. (2007)
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Scattering example patient records
Source Sommerville, I. (2007)
The highlighted operations implement secondary
concerns for the system, such as keeping patient
and consultant details anonymous If the policy
regarding anonymity changed, we would need to
recode each version of anonymise() throughout the
system
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The AOP approach

• Three key stages
• Aspectual Decomoposition
• Concern Implementation
• Aspectual Recomposition

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How are concerns identified?
A prism
Source Laddad, R. (2003)
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Concern Types

• Functional
• Quality of Service
• Policy
• System
• Organisational

• Functional
• related to specific functionality to be included
  in the system
• in a train control system, a specific functional
  concern is train braking

• Quality of Service
• related to the non-functional behaviour of a
  system
• performance, reliability, availability

• Policy
• related to the overall policies that govern the
  use of the system
• security, safety, concerns related to business
  rules

• System
• related to attributes of the system as a whole
• maintainability, configurability

• Organisational
• related to organisational goals and priorities
• producing a system within budget, making use of
  existing software assets, maintaining the
  reputation of an organisation

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Concern Classifications

• Core
• Functional concerns that directly relate to the
  primary purpose of a system.
• Secondary
• Functionality that shares information with the
  core concerns
• Functionality that satisfies NFRs
• Cross-cutting
• Concerns that apply to the system as a whole

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Cross-cutting example
Source Sommerville, I. (2007)
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Implementing Concerns

• Core
• Classes Operations
• Cross-cutting / Secondary
• Aspects
• Advice Join Points Pointcuts

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Aspect

• Aspects are similar to classes in that they can
• include data members and operations
• have access specifications
• declare themselves to be abstract
• extend classes and abstract aspects and implement
  interfaces
• be embedded inside classes and interfaces as
  nested aspects
• They are dissimilar in that they cannot
• be directly instantiated
• inherit from concrete aspects
• be marked as privileged

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Join Point

• Any identifiable execution point in a system
• A call to a method
• The methods execution
• Assignment to a variable
• Return statement
• Object construction
• Conditional check
• Comparison
• Exception handler
• Loops
• Not all may be exposed by each AOP language
• AspectJ does not expose loops

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Pointcut

• Pointcuts capture, or identify, one or more join
  points
• Need not be given a name
• Anonymous pointcuts must be specified as part of
  advice
• Can include wildcards

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Advice

• The code to be executed at a join point
• The join point must have been selected by a
  pointcut
• Can be defined as
• Before
• After
• Around

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O-O Hello World
gt javac MessageCommunicator.java Test.java gt java
test Wanna learn AspectJ Harry, having fun?
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A-O Hello World
pointcut
advice
gt ajc MessageCommunicator.java MannersAspect.java
Test.java gt java Test Hello! Wanna learn
AspectJ? Hello! Harry, having fun?
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A-O Hello World 2
gt ajc MessageCommunicator.java MannersAspect.java
Hindi Salutation.java Test.java gt java
Test Hello! Wanna learn AspectJ? Hello! Harry-ji,
having fun?
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Another aspect example
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How do we get a working program?
Source Laddad, R. (2003)
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Benefits of AOP

• Cleaner responsibilities of the individual module
• Higher modularisation
• Easier system evolution
• Late binding of design decisions
• More code reuse
• Improve time-to-market
• Reduced costs of feature implementation

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Realities of AOP

• Program flow is hard to follow
• Doesnt solve any new problems
• Breaks encapsulation

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Terms to remember

• Advice
• the code implementing a concern
• Aspect
• program abstraction defining a cross-cutting
  concern. Includes a definition of one or more
  pointcuts and the advice associated with that
  concern
• Join point
• an event in a program where the advice associated
  with an aspect may be executed
• Join point model
• set of events referenced in a pointcut
• Pointcut
• aspect statement defining join points where the
  associated aspect advice should be executed
• Weaving
• incorporation of advice code at specified join
  points by an aspect weaver

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Useful Sources

• Books
• Laddad, R. (2003), AspectJ in Action, Manning
  Publications Co.
• Sommerville, I. (2007), Software Engineering, 8th
  edition, Pearson Education Ltd.
• Online
• The AspectJ Project - http//www.eclipse.org/aspec
  tj/
• Video
• GoogleTechTalks (2007), Aspect Oriented
  Programming Radical Research in Modularity.
  Available at http//www.youtube.com/watch?vcq7wp
  LI0hco