Discussion of Steimann’s Essay:“The Paradoxical Success of

Aspect-Oriented Programming”Gary T. Leavens

Department Of Computer Science, Iowa State University

OOPSLA, October 26, 2006

Supported in part by the US NSF

October 25, 2006 Discussion2

Outline

SummaryImportant pointsPerspective: what questions?Questionable pointsConclusion

October 25, 2006 Discussion3

Summary: Definition and Paradox

Need definition of AOP languagesDefinition offered ≈ implicit invocationParadox:

Ok in moderation, but then no contributionHurts modularity if more developed

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Summary:Modularity of AOP

Information hiding (Parnas)“Main concern” (5.1.2):

Strong coupling of aspects to baseImplicit interface, often not publicImpairs independent development

Explicit interfaces for aspects in base: Less obliviousnessMore scattering

October 25, 2006 Discussion5

Summary:Locality of AOP

Implicit invocation= less direct control flowContext exposure= more global access to variables

October 25, 2006 Discussion6

Summary:Utility of AOP

Generated code weavingForming new components (glue code)

Dismantle componentsReassemble to form new components

Observed uses:LoggingSecurityRuntime assertion checking

October 25, 2006 Discussion7

Summary:Prospects for AOP

“Tremendous success”CachetOOP is old, need something new

Paradox (section 8):Aims to “modularize crosscutting concerns”“Its very nature … breaks modularity”

October 25, 2006 Discussion8

Important Points:What is AOP?

Focus on mechanisms (vs. goals)Need definition of AOP languages

MechanismsIndependent of goals

Implicit invocation definition sensible

October 25, 2006 Discussion9

Important Points:Modularity of AOP

Interfaces are crucialExplicit interfaces for aspects in base:

Less obliviousnessMore scattering

October 25, 2006 Discussion10

Important Points:Utility of AOP

AOP especially useful as notedGenerated code weaving

Aids modularity of generatorAspectJ = Assembly language of 2000’s

Glue code is important

October 25, 2006 Discussion11

Important Points:Prospects for AOP

AOP has been a success

October 25, 2006 Discussion12

Perspective

Flon’s axiom (SIGPLAN, Oct. 1975):“There does not now, nor will there ever, exist a programming languagein which it is the least bit hard to write bad programs.“

October 25, 2006 Discussion13

Perspective

Flon’s axiom (SIGPLAN, Oct. 1975):Can write bad programs in any language

October 25, 2006 Discussion14

Perspective:What are the right questions?

Flon’s axiom (SIGPLAN, Oct. 1975):Can write bad programs in AOP languages

Can we write good ones?Without scattering and tangling ofcrosscutting concernsWithout excess coupling of aspects to base

October 25, 2006 Discussion15

Perspective:All or Nothing?

ModularitySystem is modular or not

ObliviousnessMechanisms are oblivious or not

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Perspective:Questions of Degree

Coupling

ScatteringTangling

October 25, 2006 Discussion17

Perspective:Perfect AOP Measures

Coupling

ScatteringTangling

October 25, 2006 Discussion18

Perspective:Without AOP

Coupling

ScatteringTangling

October 25, 2006 Discussion19

Perspective:Compromise

Coupling

ScatteringTangling

October 25, 2006 Discussion20

Perspective:N-Dimensions of Modularity

Coupling

ScatteringTangling

Reasoning Difficulty

Heap interference...

October 25, 2006 Discussion21

Questionable Points:Coupling of Aspects and Base

Section 5.1.2:Code “moved out of its context …”“it must take (a reference to) the context that it depends on with it,”“thereby establishing coupling”

How often?How much coupling?

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Questionable Points:Reducing Coupling

XPIs (Griswold et al.)Adding interfaces for crosscuttingExtra indirection reduces coupling

Open Modules (Aldrich)Explicit interface to adviceReduces obliviousness

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Questionable Points:Paradox

Due to all-or-nothing usage of:ModularityQuantification and obliviousnessAOP’s contribution

Due to multiple senses of “modularity”Information hiding (lack of coupling)Encapsulation (lack of scattering)

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Questionable Points:Lack of Imagination

“I find it difficult to imagine …” (4.2)“I cannot see how ... ” (4.3)“it may be my lack of imagination” (5.1.7)

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Questionable Points:Lack of Imagination

Most technology nay-sayers wrongSome promising directions

Tool support (Eclipse AJDT)XPIsOpen ModulesStatic analysisModel-driven architectureAnnotations

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Conclusions

Thought-provokingRead it!Agree: AOP ≈ implicit invocationPerspective on “paradox”:

N-dimensions of modularityEach a scale, not all-or-nothingCompromise!Research: how to do better

October 25, 2006 Discussion27

Time for Discussion

Coupling

ScatteringTangling

Reasoning Difficulty

Heap interference...