Aspect Oriented Software Design

1
Aspect Oriented Software Design

• Therapon Skotiniotis Karl Lieberherr
• Northeastern University
• Boston, MA 02115 USA

2
The talk will cover ..

• The problem
• code tangling
• The solution
• Better abstractions to provide advanced
  Separation of Concerns
• The benefits
• Higher code modularity
• Better code reuse
• Ease of program extensions/maintenance/understandi
  ng

3
The problem

• Readers Writers Protocol
• Multiple Readers
• Single Writer
• Waiting Writers get priority
• Authentication required

class Book // data Book() Message new
StringBuffer("null") public int write (String
msg, int ip) if ((ip 10) gt 0) return 0
else if (AuthClients lt MaxAuthClients)
AuthClients if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters()
if (getWWriters() 0)
Message.replace(0, Message.length(), msg)
if (getWWriters() gt
0) synchronized (WQueue)
WriteQueue.notify()
else if (getWReaders() gt
0) synchronized (ReadQueue)
ReadQueue.notify()
decrAWriters()
synchronized (AuthQueue)
AuthClients--
AuthQueue.notify()
return 1 else synchronized
(WQueue) WQueue.wait()
else synchronized
(AuthQueue) AuthQueue.wait()

• Enforcing the requirements
• Authentication
• Synchronization
• Scheduling
• Functionality

4
Concerns and their Separation

• In Readers Writers
• Functionality
• read(), write()
• Authentication
• Yes, no
• Synchronization
• Resource available or not
• Scheduling
• Policy on who has priority to go next

5
Concerns and their Separation (cont)

• In Readers Writers
• Functionality
• read(), write()
• Authentication
• Yes, no
• Synchronization
• Resource available or not
• Scheduling
• Policy on who has priority to go next

Operation
System Constraints
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Concerns and their Separation (cont)

• In OOP, functional decomposition
• has been performed along one
• dimension component.
• Crosscutting concerns are not
• random properties
• Systematically repeated at different
• points within your code
• Affect the semantics and/or
• performance of your system
• AOSD proposes the means to encapsulate
• crosscutting concerns

objects
aspects
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Concerns and their Separation (cont)

• Separation of Concerns Parnas 72 drives
  software development.
• A concern is a concept, realized as a unit of
  software.
• Each concern is addressed separately in isolation
• Each concern becomes a module keeping
  inter-module dependencies to the minimum
• Do not talk to strangers The Law Of Demeter
• There is no established methodology on how to
  achieve this principle.

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From this
class Book // data Book() Message new
StringBuffer("null") public int write (String
msg, int ip) if ((ip 10) gt 0) return 0
else if (AuthClients lt MaxAuthClients)
AuthClients if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters()
if (getWWriters() 0)
Message.replace(0, Message.length(), msg)
if (getWWriters() gt
0) synchronized (WQueue)
WriteQueue.notify()
else if (getWReaders() gt
0) synchronized (ReadQueue)
ReadQueue.notify()
decrAWriters()
synchronized (AuthQueue)
AuthClients--
AuthQueue.notify()
return 1 else synchronized
(WQueue) WQueue.wait()
else synchronized
(AuthQueue) AuthQueue.wait()

• Authentication
• Synchronization
• Scheduling
• Functionality

9
to this

• Authentication
• Synchronization
• Scheduling
• Functionality

10
Concerns and their Separation (cont)
Aspect
Base
Aspect

• Authentication
• Synchronization
• Scheduling
• Functionality

Aspect
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Putting it all together

• The program needs to maintain the overall
  problem/solution semantics
• Code in aspects is only to be executed at
  specific execution points
• Control (ordering) at this execution points
  matters
• JoinPoint Model
• Provides the means to identify points within the
  execution to which aspectual code can be executed
  
• Weaving Underlying AOSD tool performs the
  integration
• At JoinPoints as specified by your code
• Attaching code as specified by your Aspect
• The mechanics behind the JoinPoint make all the
  difference
• Static Vs Dynamic
• Expressiveness of JoinPoint definition language
• Aspect Code scope and execution capabilities

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Concerns and their Separation (cont)
before if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters() else
if (getWReaders() gt 0)
synchronized (ReadQueue)
ReadQueue.notify()
after decrAWriters() else synchronized
(WQueue) WQueue.wait()

before if (ip 10 0)
after authenticated --
before if (getWWriters() 0) return true
after if (getWWriters() gt 0) synchronized
(WQueue) WriteQueue.notify()

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Concerns and their Separation (cont)
pointcut call(Book.write())
before if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters() else
if (getWReaders() gt 0)
synchronized (ReadQueue)
ReadQueue.notify()
after decrAWriters() else synchronized
(WQueue) WQueue.wait()

before if (ip 10 0)
after authenticated --
before if (getWWriters() 0) return true
after if (getWWriters() gt 0) synchronized
(WQueue) WriteQueue.notify()

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Concerns and their Separation (cont)
JoinPoint
pointcut call(Book.write())
before if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters() else
if (getWReaders() gt 0)
synchronized (ReadQueue)
ReadQueue.notify()
after decrAWriters() else synchronized
(WQueue) WQueue.wait()

before if (ip 10 0)
after authenticated --
before if (getWWriters() 0) return true
after if (getWWriters() gt 0) synchronized
(WQueue) WriteQueue.notify()

15
Concerns and their Separation (cont)
pointcut call(Book.write())
1
before if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters() else
if (getWReaders() gt 0)
synchronized (ReadQueue)
ReadQueue.notify()
after decrAWriters() else synchronized
(WQueue) WQueue.wait()

before if (ip 10 0)
after authenticated --
before if (getWWriters() 0) return true
after if (getWWriters() gt 0) synchronized
(WQueue) WriteQueue.notify()

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Concerns and their Separation (cont)
pointcut call(Book.write())
2
before if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters() else
if (getWReaders() gt 0)
synchronized (ReadQueue)
ReadQueue.notify()
after decrAWriters() else synchronized
(WQueue) WQueue.wait()

before if (ip 10 0)
after authenticated --
before if (getWWriters() 0) return true
after if (getWWriters() gt 0) synchronized
(WQueue) WriteQueue.notify()

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Concerns and their Separation (cont)
pointcut call(Book.write())
3
before if ((getAReaders() 0)
(getAWriters() 0))
if(getWWriters() gt 0 )
decrWWriters() incrAWriters() else
if (getWReaders() gt 0)
synchronized (ReadQueue)
ReadQueue.notify()
after decrAWriters() else synchronized
(WQueue) WQueue.wait()

before if (ip 10 0)
after authenticated --
before if (getWWriters() 0) return true
after if (getWWriters() gt 0) synchronized
(WQueue) WriteQueue.notify()

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Terminology

• Aspect
• Encapsulation of an aspect as a software entity
• Includes code Advice to be executed at specific
  PointCuts
• Types of advice
• Before
• After
• Around
• JoinPoint Model
• A means to specify PointCuts
• PointCuts are sets JoinPoints
• JoinPoint a point within the execution of your
  base system
• Specification of PointCuts can be based on
• Dynamic program information known at runtime
• Static program information known at compile
  time

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Aspect Aspect Interaction

• Dependence of Aspectual behavior
• Orthogonal
• No dependence between aspects (Logging and
  Scheduling)
• Non-Orthogonal
• Dependence between aspects (Synchronization and
  Scheduling)
• Orthogonal aspects play nice
• Non-orthogonal aspects hinder a protocol between
  them
• Reuse requires all related dependent aspects
• Alterations in aspects should maintain the
  dependence protocol
• Alteration of the protocol may require code
  alterations throughout all affected aspects

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Benefits

• Better abstractions to capture crosscutting
  concerns
• Separate development
• Easier means for program extensibility and
  evolution
• Higher potential for reuse
• Better software quality
• Easier to reason about program behavior
• Closer to Program Design
• PointCuts can be viewed as a generalization of
  Aspect/Object interactions
• Separation between
• Functional decomposition (OOP)
• Cross-cutting Concerns ( Aspects)
• Evolution towards Plug-and-Play Software
  Components and Components of the Shelf (COTS)
• Non-invasive program reconfiguration, adaptation,
  evolution