Better Separation of Crosscutting Concerns with Aspectual Components

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Better Separation of Crosscutting Concerns
withAspectual Components

• Karl Lieberherr
• College of Computer Science
• Demeter Team
• Northeastern University, Boston
• www.ccs.neu.edu/research/demeter

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Overview

• Aspect-Oriented Programming (AOP) Crosscutting
  concerns, controlling tangling and scattering
• Example Collect
• Aspectual Components DJ
• Comparison to AspectJ
• Summary

3
Messages

• AOP is useful
• Aspectual component, an AOP mechanism, is useful
• Adaptive Programming mechanisms must be used to
  properly express traversal-related aspects. Can
  be done in plain Java using DJ library

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The MIT Technology Review (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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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 not every concern fits into a component
crosscutting
Goal find new component structures that
encapsulate rich concerns
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AOP

• 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.

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Scattering count number of components to which
color goes
ordinary program
better program
structure-shy functionality
CM1
Concern 1
structure
CM2
Concern 2
CM3
synchronization
Concern 3
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Aspect-Oriented ProgrammingExample

• Separating the following crosscutting concerns
• Object Structure concern (JAXB, optional package
  for Java 2 platform, XML schema UML class graph)
• Traversal-related concerns traverse a group of
  connected objects and execute code attached to
  nodes and edges of object graph (advice).
• separate traversals and advice
• Those two kinds of concerns appear frequently.

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Crosscutting in Equation System
overall graph object structure green graph
traversal purple advice
usedThings from EquationSystem through
Expression to Variable
equations
esEquationSystem
elsEquation_List
new HashSet
i1Ident
lhs
e1Equation
v1Variable
Object graph
rhs
elsExpression_List
c1Compound
i2Ident
v2Variable
a1Add
add
i3Ident
v3Variable
add
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What is a component?

• any identifiable slice of behaviour that delivers
  a meaningful service, involving in general
  several participants,
• used via well-defined interfaces,
• formulated for an ideal ontology
• can be deployed into a concrete ontology,
• is subject to composition by 3rd parties
  (black-box reuse)
• is subject to refinement by 3rd parties
  (white-box reuse)

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Aspectual Component
Class Graph
P1
P3
P2
Behavior Definition
P
P1
before / around
provided everything public

after
written to the CG similar to an OO
program is written to a concrete class graph

...
P3
enhancements before/around/after provided
...
before / around
after
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M1 Equation System
EquationSystem
equations
Equation_List
Ident

Variable
lhs
Equation
Numerical
rhs
Expression_List
Simple
args
Expression

op
Add
Compound
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Collect Things
System EquationSystem Definition
Equation Body Expression Thing
Variable
definedThings
System

Thing

usedThings


Definition
Body
definedThings from System bypassing Body to
Thing usedThings from System through Body
to Thing from System
constraint to Thing
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M1 Equation System
definedThings from EquationSystem bypassing
Expression to Variable
EquationSystem
equations
Equation_List
Ident

lhs
Equation
Variable
Numerical
rhs
Simple
args
Expression_List
Expression
S
T

op
Add
Compound
D
B
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M1 Equation System
usedThings from EquationSystem through
Expression to Variable
EquationSystem
equations
Equation_List
Ident

lhs
Equation
Variable
Numerical
rhs
Simple
args
Expression_List
Expression
S
T

op
Add
Compound
D
B
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Aspectual Component

• component COLLECT
• participant Source
• expected static ClassGraph cg
• public HashSet collect(String constraint)
• String id from Target to
  edu.neu.ccs.demeter.Ident
• Visitor v new Visitor()
• HashSet return_val new HashSet()
• void before(Target v1)
• return_val.add(cg.fetch(v1, id) )
• public Object getReturnValue()return
  return_val
• Source.cg.traverse(this,
• from Source constraint to Target,
  v
• participant Target

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Aspectual Component

• Find appropriate class graph and traversals for
  task at hand
• Simplify from graph with 11 nodes to graph with 4
  nodes to graph with two nodes.

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Adapter part

• // EquationSystem class graph
• attach COLLECT
• EquationSystem Source with
• provide EquationSystemcg to
• Sourcecg
• Variable Target

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Related Work

• MzSchemes units Flatt/Felleisen, PLDI 1998.
  Serves as foundation for a part of AC.
• unit/lang Linguistic Reuse, Krishnamurthi.
• Each AC defines its own domain specific language
  for describing its inputs. Adapters describe
  composition of those languages.

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simulate in pure Java using DJ

• class System
• String id from Thing to edu.neu.ccs.demeter.I
  dent
• HashSet collect(ClassGraph cg, String
  constraint)
• Visitor v new Visitor()
• HashSet return_val new HashSet()
• void before(Thing v1)
• return_val.add(cg.fetch(v1, id) )
• public Object getReturnValue()return
  return_val
• cg.traverse(this,from Systemconstraintto
  Thing, v)
• return (HashSet)v.getReturnValue()
• HashSet defined(ClassGraph cg)
• return (HashSet) this.collect(cg, bypassing
  Body )
• HashSet used(ClassGraph cg)
• return (HashSet) this.collect(cg, through
  Body )

green traversal black bold structure purple
advice red parameters
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Ad-hoc Implementation oftraversal-related
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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Need more than localizationof crosscutting
concerns

• If we localize a crosscutting traversal-related
  concern in the standard way, we get a method that
  violates the Law of Demeter it duplicates much
  class graph information
• In addition Use traversal strategies to
  eliminate accidental noise in class graph
• Need AP to improve AOP

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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 for 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
  traversal-related concerns.
• Integration with AspectJ both can be used
  simultaneously.
• DJ provides an implementation of Adaptive
  Programming (AP).

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AspectJ (Xerox) DJ (NEU)
refers to existing join points
refers to new join points in traversal

• aspect SimpleTracing
• pointcut traced()
• call(void D.update()
• call(void D.repaint()
• before()traced()
• println(Entering
• thisJoinPoint)

• class Source
• HashSet collect(ClassGraph cg,
• String constraint)
• String s from Source
• constraint to Target
• HashSet result (HashSet)
• /weave/ cg.traverse(this, s,
• new Visitor()
• public void before
• (Target h)
• public void start() )
• return result

green pointcut purple advice
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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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Concepts needed(DJ classes)

• ClassGraph
• Strategy ( Java String traversal strategy)
• Visitor

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Adaptive Programming
Strategy
Bold names refer to DJ classes.
is use-case based abstraction of
ClassGraph
defines family of
Object
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Adaptive Programming
Strategy
defines traversals of
Object
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Adaptive Programming
Strategy
guides and informs
Visitor
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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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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 and
implementation of your current Java 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.
• DJ supports Adaptive and Aspect-Oriented
  Programming for traversal concerns in Java.

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Summary

• We view components as slices of behavior
• Aspectual components
• reconcile between functional and object
  decomposition
• add new behavior and modify existing behavior
• are a good model for AOP