1. AOSD and the Law of Demeter: Shyness in Programming
Karl Lieberherr
Demeter Research Group
Northeastern University
Boston
DEMETER
DHMHTRA

2. PhD Visitation Weekend 2003
Coupling Aspect-Oriented and Adaptive Programming: Shyness in Programming
PhD Visitation Weekend 2003

3. Our Intuition behind AOP
Our Aspect-Oriented Programming (AOP) intuition has been: "adaptiveness". It comes in two flavors: adaptiveness
to the class graph ("painting the class graph in broad strokes with code") and
to the call graph of the traversal ("picking points in the graph where additional code gets called").

4. MIT Technology Review 2001 DEMETER DHMHTRA
Aspect-oriented programming is called “adaptive programming” at Northeastern University.
DEMETER
DHMHTRA

5. Definitions y is x-shy if:
(1) y relies only on minimal information of x
(2) y can adapt to small changes in x
(3) y is loosely coupled with x
(4) y can work with x1, x2, ... which are close or similar to x.
What is a concern? A concern is something that the programmer cares about.

6. Examples of concerns the programmer has to deal with
Production concerns
How do I compute the price allowing for multiple pricing schemes?
Non-Production concerns
What do I have to print to understand why this program does not work?
Are all objects of class A created in class Afactory?

7. Scattering and Tangling: Static
aspecti is scattered across many classes (i = 1,2,3)
class X tangles aspects 1, 2 and 3
class A consisting
of three aspects
class
diagram
aspect1
aspect2
aspect3
Class X
classes for
aspect1
classes for
aspect3
classes for
aspect2
Adding to classes

8. Scattering and Tangling: Dynamic
Each aspect (colors) is scattered across many classes (shapes)
Class tangles all three aspects
program execution
involving three
aspects (colors r b g)
this(s)
f(..)
target(t)
program call tree
(classes
executing
method calls)
Enhancing calls
t.f(..);
classes
At those calls the aspect enhances the behavior

9. AP-Concern-shy DEMETER DHMHTRA
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-Concern-shy
Program with multiple building blocks, say b1:B1 and b2:B2, that are woven together: b1*b2. b1 relies only on partial information about b2 which makes b1 more robust and reusable. Goal: b1 should be loosely coupled to b2.
DEMETER
DHMHTRA

10. Law of Demeter (LoD) for Concerns
A concern implementation should not rely on too much information about other concern implementations.
(Classic LoD: A method should not rely on too much information about other classes/objects.)
Ian Holland PhD 1992:
Vice President of Architecture and Systems Engineering
at Kronos Incorporated in Chelmsford, MA empls.

11. Adaptive Programming. AP-Concern-shy: concerns shy of other concerns
AP-Structure-shy: concerns shy of graph structure
AP-WildCard: aspects shy through wildcards
AspectJ: *, .., this, target, args, call, execution, … (call graph)
AP-Strategy: three level model using strategies
AP-Call: aspects shy of call graph using strategies
AspectJ: cflow
AP-Demeter: behavior shy of class graph using strategies; advice on traversal
AP-DJ: ClassGraph, Strategy, Visitor (in Java)
AP-DAJ: Strategy enhances ClassGraph; Visitor enhances traversal defined by Strategy (in AspectJ)
AP-DemeterJ (new programming language)
50 pages
of theory
Best of
both worlds

12. Adaptive Programming. AP-Concern-shy AP-Structure-shy AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
Alternative
organization:
AP-Concern-shy
AP-Graph-shy
AP-CallGraph-shy
AP-ClassGraph-shy
AP-WildCard
AP-Strategy
X-shy
subX-shy
Mechanism-to-achieve-shyness

13. AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-Structure-shy
Program with multiple building blocks including b1and b2:Graph, that are woven together. b1 relies only on partial information about b2 which makes b1 more robust and reusable. Goal: b1 should be loosely coupled to b2. b1 enhances b2 at nodes and edges.

14. AP-WildCard DEMETER DHMHTRA
AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-WildCard
Program with multiple building blocks including b1 and b2:Graph, that are woven together. b1 uses wildcard techniques which makes b1 more robust and reusable. Goal: b1 should be loosely coupled to b2.
DEMETER
DHMHTRA

15. One contributor: Ignacio Silva-Lepe PhD 1994
Currently at IBM Watson Research Lab
AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-Strategy
Program with multiple building blocks including b1:Strategy and b2:Graph, that are woven together. b1 reveals only partial information about b2 which makes b1 more robust and reusable. b1 is written against an abstraction of b2 so that the application to b2 is well defined. Goal: b1 should be loosely coupled to b2.

16. A General Strategy-based Adaptive Mechanism
From TOPLAS 2003 paper (Lieberherr, Patt-Shamir, Orleans)
A General Strategy-based Adaptive Mechanism
Three layers of graphs: Bottom, Middle, Top
Bottom layer: trees to select subtrees guided by top layer. Each bottom layer tree has a graph from the
Middle layer associated with it that contains meta-information about the bottom layer tree. Acts as an abstraction barrier between the top and bottom layers. Used to reduce search space.
Top layer graph is basically a subgraph of the transitive closure of the middle layer graph, decorated with additional information attached to the edges.

17. Top graph: subgraph of transitive closure of middle layer
Middle graph: Abstraction barrier B A C
Bottom tree: select subtrees B
c1:C
c2:C
c3:C A

18. Strategy-based adaptiveness
The call graph application (AspectJ):
Top: computational pattern,
Middle: static call graph,
Bottom: call tree.
The standard application (Demeter):
Top: strategy graph,
Middle: class graph,
Bottom: object trees.
DEMETER
DHMHTRA

19. AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-Call
Program with multiple building blocks including b1:CrossCut and b2:CallGraph, that are woven together. b1 reveals only partial information about b2 which makes b1 more robust and reusable. Advice on b2 at b1.

20. AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-AspectJ
Program with multiple building blocks including b1:PointCut and b2:CallGraph, that are woven together. b1 reveals only partial information about b2 which makes b1 more robust and reusable. Advice on b2 at b1.

21. AP-AspectJ
Many AspectJ programs are adaptive (designed for a family of Java programs)
Context: Java program or its execution tree (lexical joinpoints or dynamic join points)
Features enabling adaptiveness:
*, .. (wildcards)
cflow, + (graph transitivity)
this(s), target(s), args(a), call (…), … (inheritance as wild card)
pc(Object s, Object t):
this(s) && target(t) && call(… f …)

22. AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-COOL
Program with multiple building blocks including b1:Coordinator and b2:CallGraph, that are woven together. b1 reveals only partial information about b2 which makes b1 more robust and reusable. Advice on b2 at b1.
Crista Lopes, PhD 1997
Assistant Professor at UC Irvine, first PhD thesis on AOP.

23. AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-Demeter
Program with multiple building blocks including b1:Strategy and b2:ClassGraph, that are woven together. b1 reveals only partial information about b2 which makes b1 more robust and reusable. Advice on traversal defined by b1 and b2.

24. AP-DJ Program in terms of ClassGraph-, Strategy- and Visitor-objects.
AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-DJ
Program in terms of ClassGraph-, Strategy- and Visitor-objects.
Example:
in Java: cg.traverse(o, s, v);
The easiest tool to learn good structure-shy
programming

25. AP-Concern-shy
AP-Structure-shy
AP-WildCard
AP-Strategy
AP-Call
AP-Demeter
AP-DAJ
Program in terms of Strategy-objects that introduce traversal methods into ClassGraph-objects (adaptiveness to class graph).
Enhance the execution of the traversal methods with Visitor-objects that may modify the run-time traversal (adaptiveness to the traversal execution).
Example: In AspectJ: declare traversal f(): s V; o.f();

26. crosscutting
base
Adaptive to
family of
bases
Connected join points
Isolated join points

27. Demeter crosscutting I
Class graph or
Object graph
From Company to Salary
Adaptive to
family of
class graphs
From BusRoute via BusStop to Person
Connected join points

28. Demeter crosscutting II
Static call graph or
Dynamic call graph
From Company to Salary
Adaptive to
family of
traversal call
graphs
…_salary(Employee, Object)
Connected join points
Isolated join points

29. AspectJ crosscutting I
Class graph or
Object graph
Company.cache(){}
Vector BusRoute.busses;
Isolated join points

30. Static call graph or
Dynamic call graph
AspectJ crosscutting II
Adaptive to
family of
call graphs
cflow
target(Employee)
Connected join points
Isolated join points

31. Demeter crosscutting
Graph
From BusRoute via Bus to Person
Adaptive to
family of
graphs
…_salary(Employee, Object)
Connected join points
Isolated join points

32. How are AP and AOP coupled?
AOP: module-shy programming
Modularize programs that cut across modules (with minimal reliance on information in modules).
Programming is module-shy if the modular structure of the program does not prevent concerns that cut across other concerns to be modularized.
AP: concern-shy programming
Can we view concern implementations as modules?

33. Many open questions DEMETER DHMHTRA
Doug Orleans: Simple model of AOP: Fred
Johan Ovlinger: Modules and Aspects
Pengcheng Wu: Statically Executable Advice
Theo Skotiniotis: Contracts for Aspects
DEMETER
DHMHTRA

34. The End

35. Crosscutting in Demeter
generated
Java program
Demeter program
structure-shy
functionality
structure
replicated!
aspect descriptions are not just copied into programs, they may be replicated several times.
synchronization

36. range of AOP languages means of … join points JPM join points
identifying
specifying semantics at
AspectJ dynamic JPM
points in execution call, get, set…
signatures w/ wildcards & other properties of JPs
advice
static JPM
class members
signatures
add members
DemeterJ, DAJ
dynamic JPM
static JPM 1
static JPM 2
static JPM 3
when traversal reaches object or edge
class members
visitor method signatures
traversal spec. s
class graph g
class names
class graph
visitor method bodies
s + g (result = traversal implementation)
add members
class graph with tokens=grammar (result = parsing and printing implementation)

37. Adaptiveness
The next 7 viewgraphs show how two traversals (parts of an adaptive program) adapt to two different class graphs.

38. Class graph: Find undefined things
Ident
System *
Thing *
usedThings * *
def S T
Definition
body
Body D B
definedThings= from System bypassing Body to Thing
usedThings = from System through Body to Thing

39. M1: Equation System EquationSystem equations Equation_List Ident *
Variable
lhs
Equation
Numerical
rhs
Expression_List
Simple
args
Expression * op
Add
Compound

40. M1: Equation System definedThings = from EquationSystem
bypassing Expression to Variable
M1: Equation System
EquationSystem
equations
Equation_List
Ident *
lhs
Equation
Variable
Numerical
rhs
Simple
args
Expression
Expression_List S T * op
Add
Compound D B

41. M1: Equation System usedThings = from EquationSystem
through Expression to Variable
M1: Equation System
EquationSystem
equations
Equation_List
Ident *
lhs
Equation
Variable
Numerical
rhs
Simple
args
Expression
Expression_List S T * op
Add
Compound D B

42. CS1: Grammar 0..* Entry EParse entries Grammar BParse Production Body
rhs
parts
Part
lhs
0..*
NonTerm
Concrete
Abstract

43. CS1: Grammar definedThings = from Grammar bypassing Body to NonTerm S
0..*
Entry
EParse
entries
Grammar
BParse
Production
Body
rhs
parts
Part
lhs
0..*
NonTerm S T
Concrete
Abstract D B

44. CS1:Grammar usedThings = from Grammar through Body to NonTerm S T D B
0..*
Entry
EParse
entries
Grammar
BParse
Production
Body
rhs
parts
Part
lhs
0..*
NonTerm S T
Concrete
Abstract D B

45. Software Structure with ACs Software Structure with ACs P1 P1 P1 P2 P2 P2 P3 P3 P3 P1 P1 P1 P4 P4 P4 P5 P5 P5 P3 P3 P3 P2 P2 P2 P2 P2 P2 P6 P6 P6 P1 P1 P1 45 45 45

46. AP-structure-shy: notion of crosscutting
B2 = program and its execution trees or an abstraction thereof: UML class diagram and its object diagrams.
A program b1*b2 is aspect-oriented if it is crosscutting.
Examples: Adaptive, aspect-oriented programs: Policies (Concurrency, Distribution, Authentication, Logging), Adaptive Method, Law of Demeter Checker in AspectJ

47. Scattering
B2 is a graph. Count number of nodes and edges that are enhanced. Scat(b1*b2) = number of nodes and edges in b2 enhanced by b1. The higher the number, the more the crosscutting.
A program b1*B2 is crosscutting if there is an infinite sequence R1, R2, … of B2 so that Scat(b1*R1), Scat(b1*R2), … is strictly increasing.