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Aspects and Modularity:The Hope and the Challenge

Jonathan Aldrich

Institute for Software Research InternationalSchool of Computer ScienceCarnegie Mellon University

April 20, 2023 2

Why Modularity?

• Protect client code from changes to design decisions encapsulated within a module• [Parnas, others]

• Protect invariants of module code from ill-behaved clients• [Milner, others]

April 20, 2023 3

Aspects: A New Hope

Key benefit: capture concerns more locally and consistently

• Better protects clients from change by localizing crosscutting concerns

• Can be used to consistently enforce invariants of a module

April 20, 2023 4

Aspects: A Double-Edged Light-Saber?

• This works when you use AOP to modularize a concern

• What happens when the client of a module uses AOP?

April 20, 2023 5

Aspects: The Phantom Menace

• Aspect-oriented clients can become dependent on details of a module’s implementation

• Aspect-oriented clients can break a module’s invariants

April 20, 2023 6

Non-Solution: Read-Only Aspects

• AOP clients may still depend on information that might change

• Even non-mutator AOP clients can break invariants• Untrusted code can view sensitive data• Aspects can create new objects with

broken invariants

• Many concerns require mutator aspects

April 20, 2023 7

Non-Solution: Semantic Pointcuts

• Control flow, data flow pointcuts are more robust• Still can be broken if control/data flow

does not exactly match problem

• Only one kind of semantic pointcut is a solution:

Do what I mean!We say this already with interfaces!

April 20, 2023 8

Non-Solution: Semantic Pointcuts

• Control flow, data flow pointcuts are more robust• Still can be broken if control/data flow

does not exactly match problem

• Only one kind of semantic pointcut is a solution:

Do what I mean!We say this already with interfaces!

April 20, 2023 9

Non-Solution: Semantic Pointcuts

• Control flow, data flow pointcuts are more robust• Still can be broken if control/data flow

does not exactly match problem

• Only one kind of semantic pointcut is a solution:

Do what I mean!We say this already with interfaces!

April 20, 2023 10

Solution: Interfaces• Use Interfaces to Mediate Interaction

• Aspects may advise entry/exit join points of a module• Aspects may advise pointcuts explicitly exported by a

module• “do what I mean” by this pointcut

• Other advice is prohibited

• Module changes which preserve interface pointcuts also preserve client behavior• Can be formalized and proved for simple pointcuts

• Aldrich, FOAL ‘04, ECOOP ’05• Work needed to extend to practical system

• Interfaces also enforce internal invariants

This story is old…but controversial in AOP!

April 20, 2023 11

Solution: Interfaces• Use Interfaces to Mediate Interaction

• Aspects may advise entry/exit join points of a module• Aspects may advise pointcuts explicitly exported by a

module• “do what I mean” by this pointcut

• Other advice is prohibited

• Module changes which preserve interface pointcuts also preserve client behavior• Can be formalized and proved for simple pointcuts

• Aldrich, FOAL ‘04, ECOOP ’05• Work needed to extend to practical system

• Interfaces also enforce internal invariants

This story is old…but controversial in AOP!

April 20, 2023 12

Obliviousness is Bunk!

• Well-known AOP “requirement”• Code does not have to be specifically prepared to

receive AOP enhancements

• Bunk because aspects depend on details of code

• As soon as you advise it, code is no longer oblivious!• Can’t change code or rely on invariants obliviously

(without knowing about aspects)

April 20, 2023 13

Obliviousness is Bunk!

• A proxy for what you really want: agility• Ease of change of crosscutting concerns

• Modify a pointcut declaratively and locally within a module

• After-the-fact enhancement

Tools could give it to you transparentlyView or modify a pointcut in an interface

Even if it’s a reflection of a more global pointcutAdd or modify a pointcut that cuts across interfaces

This may require negotiation with another stakeholder (other developers, the user, the application architect)

April 20, 2023 14

Obliviousness is Bunk!

• A proxy for what you really want: agility• Ease of change of crosscutting concerns

• Modify a pointcut declaratively and locally within a module

• After-the-fact enhancement

• Maybe tools could give this to you transparently• View or modify a pointcut in an interface

• Even if it’s a reflection of a more global pointcut• Add or modify a pointcut that cuts across interfaces

• This may require negotiation with another stakeholder (other developers, the user, the application architect)

• Insufficient to calculate one from the other• Both must be editable if you want both modularity and

agility• Changes to pointcuts in interfaces of module require

negotiation