[aosd-discuss] "AOP considered harmful"

[Bill Burke]

Ron, I agree with all your statements, but can we expect the average 
developer to follow all these good practices and rules?  Is there any 
good research out there on what should be removed/restricted from AOP 
frameworks/languages?

This is where I come back to JBoss's long ago message of stressing the 
use annotations+AOP.  I think this is an easier sell to these analyst types.



Bill



Ron Bodkin wrote:
> First off, I'd say that the best tool for understanding a system as a whole
> is a good design. AOP improves the modularity and supports traceable
> implementations, making it easier to understand without tools support. An
> overly complicated OO system can be effectively impossible to reason about,
> too.
> 
> I question this in practice in large OO systems. Without tools support,
> finding the possible subclasses or implementers of something is quite
> difficult, so knowing what actual implementation code will execute is hard.
> The idea of "looking at your code" breaks down on the kind of real (large)
> systems where modularity most matters. Of course for systems that might be
> extended by 3rd parties, it's not possible if you don't have their
> extensions.
> 
> And of course, identifying the possible implementations that might get
> assigned to a given field can require reading all the code (possibly
> including XML configuration files). In the case of worker objects like
> Runnable or Action, in practice you often can't reasonably figure out what
> will execute by static analysis either.
> 
> Re: the question of dynamic parts to pointcuts, that to me is analogous to
> the fact that in general one knows which branches of an if test might
> potentially execute. 
> 
> Indeed, having good tools to see dynamic behavior is often a requirement to
> understand real code. A good debugger is helpful for this, and, in fact, AOP
> makes it easier to trace code (even incomprehensible pure OO code) to
> understand what's happening.
> 
> I'd also note that, in practice, large project use all kinds of special-case
> obscure artifacts to address crosscutting, that are also hard to follow
> without tools support, but (to my knowledge), don't offer good tools support
> either, e.g.,
> 
> - dynamic proxies (either created by the JDK or using bytecode libraries
> like ASM)
> - code generators like Xdoclet (sure you can read the _generated_ code, but
> how do you trace it back to the sources?)
> - execution "containers" (e.g., Servlet and EJB)
> 
> You might argue that these aren't OO, but any real critique of AOP needs to
> address the shortcomings that motivated such alternatives.
> 
> 
> -----Original Message-----
> From: discuss-bounces at aosd.net [mailto:discuss-bounces at aosd.net] On Behalf
> Of Eric Tanter
> Sent: Monday, April 25, 2005 10:11 AM
> To: Mira Mezini
> Cc: discuss at aosd.net
> Subject: Re: [aosd-discuss] "AOP considered harmful"
> 
> 
>>>as one of the authors of the referenced paper, just a short note: This
>>>paper was written for a panel discussion, and indeed intended to be
>>>provocative.
>>>
>>>However, I think that AOP as we see it now can result in reduced 
>>>program
>>>understandability, if used in the wrong way. Additionally AOP imho
>>>strongly depends on tool support (thanks for ajdt, great tool in that
>>>context!).
>>
>>I disagree with the above. AOP makes great tool support like ajdt 
>>*possible*
>>rather than "strongly depending on them"! Pointcuts make the 
>>crosscutting
>>structure, which implicitly exists anyway in an OO equivalent, 
>>explicit and
>>hence displayable by a "stupid" tool. By this, it actually improves 
>>overall
>>program understanding rather than reducing it, IMHO.
> 
> 
> But what Max said -which I believe is true- is that without tool 
> support, you cannot determine what is going to happen at execution just 
> by looking at your code unless you're good enough to 'expand' 
> predicates in your mind at any time. Type polymorphism in OOP is 
> arguably much easier to grasp in one's head that pointcut shadows.
> So yes, good tools support is even more necessary for AOP I guess. And 
> still, another point that is made in the paper is that even with tool 
> support, if your pointcuts have dynamic parts, you can only know what 
> aspects 'potentially' apply.
> 
> -- Eric
> 
> 
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-- 
Bill Burke
Chief Architect
JBoss Inc.