Towards A Better Organization of Programs with Aspect-Oriented Programming

1
Towards A Better Organization of Programs
withAspect-Oriented Programming

• Karl Lieberherr
• Demeter Research Group

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Overview

• Aspect-Oriented Programming (AOP) Crosscuting
  concerns, controlling tangling and scattering.
• Example 3 concerns Find undefined entities.
• Tools for AOP
• AspectJ from Xerox PARC
• DJ from Northeastern
• Adaptive Programming and AOP
• Summary

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The MIT Technology Review Ten (Jan./Feb. 2001
issue)

• Ten emerging technologies that will change the
  world
• Untangling Code - Aspect-Oriented Programming
  (AOP)
• Data Mining
• Micro Fluids
• Robot Design
• ...

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Northeastern Connection

• Crista Lopes wrote the first Ph.D. Thesis on AOP
  at Northeastern supported by Xerox PARC.
• The Demeter Research Group worked with
  interesting AOP Systems long before the name AOP
  was invented.

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Quote MIT Technology Magazine

• The idea of aspects has been around for many
  years and with many different names. It is called
  adaptive programming at Northeastern University,
  and subject-oriented programming at IBM,

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AOP

• Crosscutting concerns
• Example Logging record every operation an
  application performs
• When adding a new operation to this application,
  always put a trace statement in
• Keeping track of crosscutting concerns is
  error-prone

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AOP

• Crosscutting concerns, when implemented in an
  ad-hoc way, lead to a lot of tangling and
  scattering in the program.
• Goal of AOP is to control the tangling and
  scattering in the program.

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Tangling count color changes
ordinary program
better program
structure-shy functionality
Components
structure
Aspect 1
synchronization
Aspect 2
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Scattering count number of modules to which
color goes
ordinary program
better program
structure-shy functionality
M1
Components
structure
Aspect 1
M2
M3
synchronization
Aspect 2
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Aspect-Oriented ProgrammingExample

• Separating the following cross-cutting concerns
• Object Structure
• Traversals through Objects
• Advice on Traversals
• Focus on those three concerns only. They appear
  frequently.

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Why crosscutting?
overall graph object structure green graph
traversal purple advice
r0
BusList
Route1BusRoute
buses
busStops
BusStopList
Bus15DieselPowered
passengers
CentralSquareBusStop
waiting
PersonList
PersonList
JoanPerson
PaulPerson
SeemaPerson
EricPerson
r
r
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Keeping Track

• When object structure changes at certain places,
  need to update traversal.
• Error prone.

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Why aspects Oblivious

• Object Structure
• does not have to know about traversals and advice
  on traversals
• Traversals
• dont have to know about advice on traversals
• Advice on Traversals
• has to know minimally about object structure and
  traversals

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Ad-hoc Implementationof three concerns

• Leads to lots of tangled and scattered code with
  numerous disadvantages
• The question is not how to eliminate the tangling
  but how to reduce it
• AOP is about tangling control of the
  implementation of crosscutting concerns
• Crosscutting will always lead to some tangling at
  code level

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Example

• Check whether all used entities are defined.
• Object structure, traversal, advice on traversal

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Find undefined things
definedThings

System

Thing
usedThings



UsedThingsHolder
definedThings from System bypassing
UsedThingsHolder to Thing usedThings from
System through UsedThingsHolder to Thing
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Name map
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Java Program with less tangling

• class Cd_graph
• String vi from Vertex to edu.neu.ccs.demeter.
  Ident
• void isDefined(ClassGraph cg)
• checkDefined(cg, getClasses(cg))
• HashSet getClasses(ClassGraph cg)
• String definedThings
• "from Cd_graph bypassing Neighbors to
  Vertex"
• Visitor v new Visitor()
• HashSet return_val new HashSet()
• void before(Vertex v1)
• return_val.add(cg.fetch(v1, vi) )
• public Object getReturnValue()return
  return_val
• cg.traverse(this, definedThings, v)
• return (HashSet)v.getReturnValue()

green traversal black bold structure purple
advice red parameters
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Java Program with less tangling

• void checkDefined(ClassGraph cg, final HashSet
  classHash)
• String usedThings
• from Cd_graph through Neighbors to
  Vertex"
• cg.traverse(this, usedThings, new Visitor()
• void before(Vertex v) Ident vn cg.fetch(v,
  vi)
• if (!classHash.contains(vn))
• System.out.println("The object "
  vn
• " is undefined.")
• )

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Tangling is localized

• Instead of having code spread across several
  classes, it is localized in one class.
• Java program is describing the abstract pattern
  behind the computation of interest checking
  whether used entities are defined.
• Tangling control through abstraction of patterns.
  

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UML class diagram ClassG
definedThings from ClassG bypassing Body to
ClassName
Entry
0..
EParse
entries
ClassG
BParse
ClassDef
Body
Part
parts
className
0..
ClassName
Concrete
Abstract
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UML class diagram ClassG
usedThings from ClassG through Body to ClassName
Entry
0..
EParse
entries
ClassG
BParse
ClassDef
Body
Part
parts
className
0..
ClassName
Concrete
Abstract
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Equation System
usedThings from EquationSystem through
Expression to Variable
Example x 1.0 . y ( x 4.0). z ( x y).
EquationSystem ltequationsgt List(Equation). Equat
ion ltlhsgt Variable ltrhsgt Expression
.. Variable Ident. Expression Simple
Compound. Simple Variable Numerical. Compound
( Op ltargsgt List(Expression) ). Op Add
Mul. Add . Mul . Numerical float.
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Equation System
definedThings from EquationSystem bypassing
Expression to Variable
Example x 1.0 . y ( x 4.0). z ( x y).
EquationSystem ltequationsgt List(Equation). Equat
ion ltlhsgt Variable ltrhsgt Expression
.. Variable Ident. Expression Simple
Compound. Simple Variable Numerical. Compound
( Op ltargsgt List(Expression) ). Op Add
Mul. Add . Mul . Numerical float.
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AspectJ (Xerox PARC)

• A general aspect-oriented programming language
  for Java.
• An aspect is like a class and may contain
  pointcut definitions defining a set of join
  points and advice saying what to do before, after
  or instead of the join points.

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DJ (Northeastern)

• Is a Java package that supports AOP for the three
  concerns class structure, traversals, and
  traversal advice.
• Connection to AspectJ both can be used
  simultaneously.
• DJ provides an implementation of Adaptive
  Programming (AP).

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Principle behind AP Polyas Inventor Paradox

• Polya observed that it is often easier to solve a
  more general problem than the one at hand and
  then to use the solution of the general problem
  to solve the specific problem. The hard work
  consists of finding the appropriate
  generalization.
• Programs become shorter and more powerful. A
  paradox. With less work we achieve more.

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Apply Polya to Programming

• Generalization Dont write the program for a
  specific data structure, write it for an abstract
  data structure.

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Concepts needed(DJ classes)

• ClassGraph
• Strategy
• ObjectGraph
• ObjectGraphSlice
• Visitor

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Adaptive Programming
Strategy
Bold names refer to DJ classes.
is use-case based abstraction of
ClassGraph
defines family of
ObjectGraph
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Adaptive Programming
Strategy
defines traversals of
ObjectGraph
plus Strategy defines
ObjectGraphSlice
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Adaptive Programming
Strategy
guides and informs
Visitor
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AspectJ (Xerox) DJ (NEU)

• Abstract pointcut
• set of execution points
• where to watch
• Advice
• what to do
• Concrete pointcut
• set notation using regular expressions

• Abstract object slice
• set of entry/exit points
• where to go
• Visitor
• what to do
• Actual object slice
• traversal strategies

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AOP AP

• Program with aspects that correspond to the
  concerns of the programmer.

• Relieve the programmer from the details of some
  concern.
• Create robustness to changes in an aspect.
• AP is about join point reduction.
• Example structure-shyness

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Technology Evolution
Object-Oriented Programming
Tangled structure/behaviors robustness to
structure changes
Other Technologies
Adaptive Programming
Tangled concerns in general (synchronization,
etc.)
Aspect-Oriented Programming
robustness to aspect changes
Aspect-Oriented Programming II
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Benefits of Adaptive Programming

• robustness to changes
• shorter programs
• design matches program,
• more understandable code
• partially automated evolution
• keep all benefits of OO technology
• improved productivity

Applicable to design and documentation of your
current systems.
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Summary

• AOP getting a lot of attention. Addresses an
  important problem how to program crosscutting
  concerns.
• AP is about relieving the programmer from the
  details of a concern traditionally from the
  structural concern.
• Want to learn more Take COM 3360 Adaptive
  Object-Oriented Software Development and attend
  the Demeter Seminar.

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Current Work

• DARPA grant with BBN (Prof. Wand, Lieberherr) on
  AOP for real-time systems.
• Doug Orleans PhD thesis, in progress Design and
  Implementation of Aspect-Oriented Languages.
• Johan Ovlinger PhD thesis, in progress The
  Collaboration/Adapter Approach to AOP. Works with
  industry SKYVA.

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Critical Mass

• Professors Lorenz, Wand, Lieberherr, Clinger
• Ph.D. students Orleans, Ovlinger, Wu
• Masters students working on JSR 31 project
  Prasenijt Adak, Huichan He
• Undergraduate student Jon Kelley
• Courses COM 3360 (graduate), COM 1205
  (undergraduate)