Shyness in Programming

1. DEMETER DHMHTRA Shyness in Programming Karl Lieberherr Demeter Research Group Northeastern University Boston

2. 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. DEMETER DHMHTRA MIT Technology Review2001 • Aspect-oriented programming is called “adaptive programming” at Northeastern University.

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 aspect1 aspect2 aspect3 class diagram 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. DEMETER DHMHTRA • AP-Concern-shy • 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.

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. 2200 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 (inAspectJ) • 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. 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.

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. 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 B A C Middle graph: Abstraction barrier B A C Bottom tree: select subtrees B c1:C c2:C c3:C A

18. DEMETER DHMHTRA 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.

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

34. The End

35. Crosscutting in Demeter generated Java program Demeter program structure-shy functionality structure replicated! 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… signaturesw/ 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 class members 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 definedThings 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. definedThings = from EquationSystem bypassing Expression to Variable M1: Equation System EquationSystem equations Equation_List Ident * lhs Equation Variable Numerical rhs Simple args Expression_List Expression S T * op Add Compound D B

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

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

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

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

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