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Aspect-Oriented Programming Considered Harmful 470

kupci writes "The 'x considered harmful' cry is a little overused, but there is a Forrester report that discusses some of the pro's and con's of Aspect-oriented Programming, and includes some interesting links. It is mainly based on papers from the University of Passau. It's worth it just for Clark's 'COME FROM' article." From the article: "Aspect-oriented programming (AOP) is intended to address common problems that object-oriented programming (OOP) doesn't address well, plus some problems that OOP itself created. However, AOP is a risky solution: It is a very generic mechanism for solving some very specific concerns and has been likened to a kind of "GOTO" statement for OOP. Like GOTO, it can cause more harm than good."
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Aspect-Oriented Programming Considered Harmful

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  • Of course (Score:5, Funny)

    by ravenspear ( 756059 ) on Sunday April 24, 2005 @02:33AM (#12327353)
    The only thing worse would be GOTO + OOP, which of course = C++
    • You insensitive clod!
    • by Anonymous Coward on Sunday April 24, 2005 @04:48AM (#12327736)

      You've attempted to assign C++ to a complex left-hand-side expression (GOTO+OOP). This is a classic blunder. What you probably meant was: "GOTO"+"OOP" == "C++".

      In addition, you're overloading the use of strings to have orthogonal meanings, which is frowned upon. After all, using the standard string operator overloading, your left-hand-side "GOTO"+"OOP" is simply "GOTOOOP", which is clearly not equal to "C++" -- a good comment compiler would notify you that this expression always evaluates to false. A further refinement would be to introduce your own class and write the expression as:
      LanguageFeature("GOTO")+LanguageFeature("OOP") == LanguageFeature("C++").

      In the future, I hope slashdot will provide a better comment compiler that can automatically detect such simple mistakes.

  • by ravenspear ( 756059 ) on Sunday April 24, 2005 @02:37AM (#12327370)
    Wikipedia linkage []

    In computing, the programming paradigm of aspect-oriented programming (AOP) centers on constructs called aspects, which treat concerns of objects, classes, or methods. The aim of AOP is to separate program code related to the main purposes of the application (its core concerns) from code related to secondary purposes (cross-cutting concerns).

    For example, a telecommunications application might have a core concern of routing calls, while code for timing and billing those calls would crosscut the whole object hierarchy. AOP aims to separate the billing concerns from the core concern. It moves code not related to solving the domain problem from the business logic into a separate module. The application code no longer contains pieces of crosscutting concerns scattered across modules; instead, programmers maintain crosscutting concerns in aspects; this makes it easier to maintain both core and crosscutting concerns.

    Any program has principled points (join points) where programmers can identify and modify the program semantics. In AOP, programmers specify join points using a language feature called a pointcut, and specify the behavior to join those points by using advice such as methods or functions. Some variants of AOP allow programmers to extend the types in the system. These features enable aspects to implement behavior for concerns that crosscut the core concern of the application.
    • by starling ( 26204 ) <> on Sunday April 24, 2005 @03:02AM (#12327458)
      Any program has principled points (join points) where programmers can identify and modify the program semantics. In AOP, programmers specify join points using a language feature called a pointcut, and specify the behavior to join those points by using advice such as methods or functions. Some variants of AOP allow programmers to extend the types in the system. These features enable aspects to implement behavior for concerns that crosscut the core concern of the application.

      Excuse me, but was that supposed to mean something?
      • by Keeper ( 56691 ) on Sunday April 24, 2005 @03:33AM (#12327555)
        Let's say you've got objects a, b, and c.

        Objects a and b do something useful (the core of the application)

        Object c (an aspect) wants to perform an operation when something happens in object a (a concern).

        Object c (an asepct) also wants to do different things when something happens in object a and b (crosscutting a concern).

        I look at this as a really complicated event notification system. Comments calling this 'goto for OOP' also seems rather appropriate.

    • CCITT Number 7.
      Signalling System 7.

      This beast takes care of the mission critical billing, timing, routing, SMS and a thousand other things, all in a single little packet switched channel. It's a small box with nice flashing LED's.

      It is perhaps as common in telecommunications now as McDonalds is a household name.
    • think xdoclet. or JDO byte code enhancements, or logging. the java logging api lets you log.entering("classname", "methodname", new Object[] { param} ); and similarly for exiting. but that's a terrible bore to have to go in and add by hand, and also a terrible waste of code-realestate. i just want the logging to be there implicityly. with AOP you could define the entry and exit logging as aspects of each method both in your code, and in any code libraries you are using.
  • $249 (Score:4, Interesting)

    by Anonymous Coward on Sunday April 24, 2005 @02:37AM (#12327372)
    who is going to shell out that cash for 5 pages?
    • Re:$249 (Score:3, Interesting)

      by Bodrius ( 191265 )
      Well, the papers linked in the psot describe the basic problem well enough and they're not charging 250 bucks for the pdfs... not sure what other research results are included in the forrester report, but I doubt there's much you wouldn't find in published academic material if your time is worth less than that... or if you just enjoy reading the research itself.

    • Re:$249 (Score:2, Funny)

      by bryan986 ( 833912 )
      Our Money-Back Gurantee If you are not completely satsified with your research document for any reason, you can return it for a full refund within three weeks of your online purchase.
  • Oh gee (Score:5, Interesting)

    by Anonymous Coward on Sunday April 24, 2005 @02:39AM (#12327380)
    "Powerful language constructs can be dangerous when misused"

    Thank you for the wonderful advice and welcome to 1971!!

    As for the GOTO comparison, it is disingenuous. The problem with GOTO is not that it is a flow control change, the problem is that it is an [i]unstructured[/i] flow control change. AOP is nothing if not structured, in fact its potential flaw lies in that the structures it represents can be so complicated the programmer can lose track. It's the absolute opposite of GOTO.
    • "in fact its potential flaw lies in that the structures it represents can be so complicated the programmer can lose track." That's GOTO
    • Re:Oh gee (Score:5, Insightful)

      by Bodrius ( 191265 ) on Sunday April 24, 2005 @04:55AM (#12327748) Homepage
      No, the problem with GOTO is that, as a consequence of being an unstructured flow control change, it makes it impossible to trust your reasoning about code because it destroys your ability to scope it.

      To quote: "... it becomes terribly hard to find a meaningful set of coordinates in which to describe the process progress ..."

      I think Dijkstra's point was that any construct that does this is harmful.

      Structured constructs are typically less liable to do this, but only because they are visible and the structure is known.

      AOP does have the ability to inject arbitrary code in your function that can change its semantics and you cannot see this when reading the affected code.

      No matter how structured that change is, its invisibility makes it impossible to fully understand your code from the source code without 'weaving in' all the aspects explicitly. Like GOTO, it destroys your ability to scope your reasoning of the code to the local view.

      As much as I love AOP for a class of problems, this is a very valid concern that needs to be addressed.

      It's certainly not the first language featureset that faces this problem; OOP can deal with its own issues with design by contract, either through documented best practices or by enforcing it in the language. I haven't seen much in AOP there, beyond the absence of problematic examples (which would typically come from real-world use).

      • Re:Oh gee (Score:3, Insightful)

        by maraist ( 68387 ) *
        AOP does have the ability to inject arbitrary code in your function that can change its semantics and you cannot see this when reading the affected code.

        Well, except that AOP can only modify code in a structured way.. You can replace a method (no more obscure than method over-loading), you can wrap a method, turning it's body into a nested scope (again similar to a method which calls super). Yes they have before and after methods, but these are mere conviniences to the wrapping of a method.

        My argument i
  • by Mancat ( 831487 ) on Sunday April 24, 2005 @02:42AM (#12327384) Homepage
    Really, I did read the article, but the headline is kind of discomforting. I mean, how is it harmful? If I were reading this headline while writing aspect-oriented code, should I stop immediately? I wouldn't want to die, you know. If I'm going to die, Slashdot, you need to let me know!
  • NEWS FLASH (Score:4, Funny)

    by ZorbaTHut ( 126196 ) on Sunday April 24, 2005 @02:42AM (#12327389) Homepage
    Useful technique, chemical, or product can be dangerous when misused! With this groundbreaking announcement, aspect-oriented programming joins up with jet planes, oxygen, razor blades, and almonds. Scientists urge all Americans to avoid anything that could, in any way, be harmful to anyone, and the government will begin production of mandatory Full Isolation Spheres (tm) within the week.
    • You get funny, but you'd be surprised how many people seem to think using OOP suddenly makes everything good, and anything that doesn't use it must be bad.
  • by maxjenius22 ( 560382 ) on Sunday April 24, 2005 @02:45AM (#12327398)
  • by Goosey ( 654680 ) on Sunday April 24, 2005 @02:46AM (#12327402) Homepage
    The one-paragraph 'article' is just a tagline for a 250 dollar document. Quite dissapointing to read that lead in and find that I won't actually be aquiring any new information on the topic. :\
  • Executive Whining (Score:5, Insightful)

    by idsfa ( 58684 ) on Sunday April 24, 2005 @02:46AM (#12327404)
    There's a new idea in field I am incapable of getting a job in that is gaining in popularity among people I wish cared about my opinions. Buzzword I am dissing is intended to address common problems that buzzword everyone likes doesn't address well, plus something into which I have no insight. However, buzzword I am dissing is a risky solution: Complaint which betrays my ignorance and has been likened to somthing everyone hates. attack by analogy. Buzzword I am dissing will find some uses among people I secretly envy, but for the needs of (me) typical application developers, language gurus would do better to write something I can understand. My paper is important, pay me $50/page to read it.
    • It's actually worse than goto.

      With goto you're branching to a specific label.

      With Aspects you're doing dynamic wildcard modifications of code based on procedure names!

      If that wasn't horrifying enough, it also circumvents the compiler (or interpreter)'s error checking.

      This is even scarier and less controllable than old COBOL self-modifying code feature
      (the much reviled ALTER X TO PROCEED TO Y ... [] statement),
      long since removed from the language.
  • There IS a GOTO statement in C++, an OOP language.

    So how can AOP be like GOTO for OOP when GOTO is already the GOTO for OOP?

  • I like GOTO! (Score:5, Insightful)

    by Wabbit Wabbit ( 828630 ) on Sunday April 24, 2005 @02:51AM (#12327420)
    At the risk of being modded troll, nuking my karma, being yelled at, laughed at, and otherwise folded, spindled, and mutilated, I'm gonna say it:

    I like GOTOs. GOTO has it's place. Even in C++. Sorry all you purists, but there are times when it just plain works.

    *runs and puts on Chinese wicker fighting suit and hides, trembling, behind a flame-retardant wall*
    • Yeah, I agree with you. Not sure whether your comment will be modded as offtopic or mine...but here we go

      GOTO statements are *BAD*, I had been taught. But there are places, I had to use it. Being hardcore fan of OO... I hated it. But no options.

      There are more shocks you see when u switch languages. Had big pains to implement frameworks in C++ while keeping options of callbacks. (If somebody knows, I will appreciate it) But I was shocked to see that CALLBACKS were put down by C++ purists, which is so com

      • Re:I like GOTO! (Score:3, Interesting)

        by top_down ( 137496 )
        But I was shocked to see that CALLBACKS were put down by C++ purists


        Callbacks are everywhere in C++ although these days they are often function objects instead of function pointers. Somebody who puts down callbacks is certainly no C++ purist.
    • Re:I like GOTO! (Score:5, Informative)

      by Keeper ( 56691 ) on Sunday April 24, 2005 @03:40AM (#12327571)
      The goto operation can be rather useful if used sparingly and correctly. If I see a goto jump up (earlier in the function) instead of down, I tend to get worried. But it is useful to break out of nested loops or complicated logic statements.

      It is also exceedingly useful for error handling, in that it allows you to skip the bulk of a function and still hit the cleanup section of the function. This gives you two things:
      1) you avoid a deep, deep mess of nested if statements
      2) you eliminate duplication of code in error handlers (and the bugs that occur later on when someone forgot to update one of those handlers to release some resource they just added)

    • The absence of "goto myfunc(param);" makes implementing coroutines or tail calls un-necessarily hard when using C (or compiling to C). There is no valid reason for a very-low-level language to hide features available from assembler.
      • by tepples ( 727027 )

        The absence of "goto myfunc(param);" makes implementing coroutines or tail calls un-necessarily hard when using C (or compiling to C).

        If your C compiler does not optimize obvious tail calls (e.g. return myfunc(param);), then your C compiler has a bug.

        There is no valid reason for a very-low-level language to hide features available from assembler.

        C is not a very low level language. You're thinking of C-- [].

    • Re:I like GOTO! (Score:3, Interesting)

      by zorander ( 85178 )
      Goto only has a place because C++ is a weak language. If c++ had a proper runtime exception mechanism or support for continuations (which can be used to implement the former but generally only serve to obfuscate code), then gotos would be wholely unneccesary.

      Yes it works, but it's not clean. The only reason I use gotos in c/c++ code is to break out of multiple levels of scope or for a basic 'goto end' pattern in a long/complex routine that needs to ensure cleanup before returning. I suppose I'd also use it
  • by sfcat ( 872532 ) on Sunday April 24, 2005 @02:53AM (#12327425)
    I like OO but is isn't great for every task. For instance, logging (which is the default example for AOP) is really bad in OO. This is because you have to add something like

    Log.printLog("some message " + someObject.toString());

    everywhere you want to log something. But with AOP you can used kleen operators (*, +, ?, etc) to add Log.printLog() to certain methods of certain objects. Aspects allow you to inject code into method call boundaries. But like all programming constructs it can be abused. This is because you might get an exception stack trace that indicates a specific method call has thrown an exception when the exception came from code inserted by an aspect. But other languages can be abused too (Perl anyone). So I think this type of critism of aspects says more about the critic than about the programming construct. I don't use C++ to implement logic evaluators (I would use Prolog or another logic or unification based language). Nor do I try to write OO code in LISP (despite the horrid OO extensions for LISP) because each language has it strengths and weaknesses. For each application choose the language that best suites the task.

    On another note, I think these types of critisms are really from people who are afraid of learning new languages or skills. They have worked long and hard to get good at C (or Java or some other language) and don't want to have to learn another language. Well suck it up, there aren't many Fortran programmers anymore and if you want to keep working, you must keep learning new languages and skills. Not that you should jump on every bandwagon, but if I'm writing something that needs to be really, really fast I use C. If I'm writing something that needs to be maintainable forever and speed isn't that important I use Java. The best tool for the task.

    • I like OO but is isn't great for every task. For instance, logging (which is the default example for AOP) is really bad in OO. This is because you have to add something like

      Log.printLog("some message " + someObject.toString());


      Log "some message " someObject endl;


    • But what does AOP solve in that logging scenario? As far as I can tell, it lets you change this:

      Log.log("Something went wrong!");

      Into this:

      log("Something went wrong!");

      Four less characters, whoop-di-shit.

      And that's not even going into the fact that static import brings exactly the same benefit.

    • The problem is too many people do jump on every bandwagon. I think that's what the author is warning of. AOP is not a panacea and shouldn't be seen as one.
    • Funny, this is not the first time I heard of this complaint and, for example, I wouldn't think Anders Hejlsberg [] was afraid of learning a new language feature considering his career... (the question on AOP is asked within the first two mins if you don't want to watch the whole thing).

      The concern predates the report for quite a bit, and from some reliable sources. This is not a surprise, as I would think the role of companies like forrester is to communicate trends like these, not to create them out of the b
    • if you don't try to write OO code in lisp, how do you know it cannot be done (and done well)?

      hint: AOP is just half-assed CLOS.

      (no worries, i don't use any of this stuff, either. ;-)

    • You lost your credibility when you wrote: "Nor do I try to write OO code in LISP (despite the horrid OO extensions for LISP) because each language has it strengths and weaknesses." Given that Common Lisp is actually CLOS, an object-oriented language (one of the first ones, and one that has been called one of the few real OO language by Kay), I find it difficult to consider the standard as an "horrid extension".

      I think that, a priori, we should read criticism as an opportunity to improve concepts, not merel
  • by LadyLucky ( 546115 ) on Sunday April 24, 2005 @02:55AM (#12327432) Homepage
    What the submitter failed to mention is that to read this article you have to stump up with $250. The comment in the submission is merely from the executive summary.

    'It's worth it'. Come on, how many people here read the article, let alone will pay $250 for it?

    • It's likely that the OP'er works for or attends an institution which has a site license for Forrester's content, and so they were able to read the article for free. It probably didn't even occur to them that other people wouldn't be able to access the article. The same thing happens when people link to scientific journals in their /. submissions.

    • No. The whole concept has a merit, and it works. You're just missing what it is.

      It's not a viable programming technique, but it's a viable method for siphoning money into their account. And as thus, a valid business method.
  • by the-build-chicken ( 644253 ) on Sunday April 24, 2005 @02:56AM (#12327437)
    Two fifty to hear some twat tell you that a new technology is good, but only in the right circumstances used by trained people.

    What bollocks.

    Do any CIOs still buy this crap? Do the sensationalist headlines do the job and actually sell this bs? "AOP considered harmful" very afraid!

    I personally don't have a week go by when we don't find another great use for AOP...and we write financial that blows his 'frameworks only' theory out of the water.

    But I suppose "AOP, useful sometimes in the right circumstances if you make sure you train your people on how to use it, not so useful in others" doesn't exactly sell those $250 a pop articles, does it Carl
  • Looks to me like the most harmful part of it is that it's patented, or at least parts of it are.
  • Side effects++ (Score:5, Insightful)

    by evvk ( 247017 ) on Sunday April 24, 2005 @03:33AM (#12327557)
    The bad thing about AOP is that it adds lots of side effects to function calls and can thus make the program very hard to reason about. This is completely orthogonal to (purely) functional programming that intends to remove side effects thus making it easier to reason about programs.

    Functional programming: f(1) == f(1) always, with no other effects to "global state" etc.

    Imperative programming without AOP: f(1) != f(1) necessarily at every point of the code, as the function can access globals. It can also change the global state, so its effect is more than returning its value; it can have side effets.

    Imperative programming with AOP: Same as above, plus the side effects may happen somewhere unrelated to the definition of f itself.
    • Of course, a PURELY functional language is worthless for real world use. Even languages that are usually considered pure funtional like haskell have some sort of 'cheat' to allow changing the global state.
      • Re:Side effects++ (Score:2, Interesting)

        by evvk ( 247017 )
        It may seem like a cheat at first sight, but the haskell approach to IO/global state is really elegant infact, and stays within the boundaries of a purely functional language. Most other functional programming languages do use a cheat, and are thus impure (and strict instead of lazy).

        What haskell infact lets you do with its IO monad is construct instructions for a separate IO interpreter (that is not pure) within the Haskell runtime. It is a kind of DSL (domain-specific language), a concept that is used a
        • DSL? (Score:2, Funny)

          by tepples ( 727027 )

          What haskell infact lets you do with its IO monad is construct instructions for a separate IO interpreter (that is not pure) within the Haskell runtime. It is a kind of DSL (domain-specific language), a concept that is used a lot in Haskell programs.

          Then what are the impure languages? Dial-up?

  • 10 GOTO 10 (Score:3, Interesting)

    by John Bokma ( 834313 ) on Sunday April 24, 2005 @03:42AM (#12327579) Homepage
    GOTO doesn't cause harm (or its syntax sugared grandchildren), it's the programmer who is not able to understand when to use it or not. Such programmers are a harm by themselves :-)
  • by tyates ( 869064 ) on Sunday April 24, 2005 @04:14AM (#12327677) Homepage
    The big IT consultancies like Forrestor, Meta, gartner, etc are intellectually bankrupt. If you want an example, read the free article off of Forrestor's web page on IT metrics. It's absolutely worthless. Believe it not, it says that you should evaluate your IT department based on a balanced scorecard that is calculated by, among other things, the number of steering commitee meetings you have. Yes, you read that right, the more meetings the better your department is.
    These consultancies ran out of ideas a long time ago, and are trying to turn IT ito some Six Sigma pseudo-science. It doesn't work, but idiots still buy into it because it sounds impressive.
  • by po8 ( 187055 ) on Sunday April 24, 2005 @04:43AM (#12327724)

    How do I get suckers^H^H^H^H^H^H^Hfolks to pay $250 for my 5 page speculations on computer science topics? Seriously. I'm an academic, and I write 20+ pages per month of this stuff for free. If I could get even 100 people to pay $50 per page each for it, that would be $100K per month: that's way more than I make in a year at my current job. Leaving aside the question of the price of my immortal soul, it doesn't sound like too bad a gig.

    I started to list my qualifications, but got tired of typing. Suffice it to say that they're way more than that Forrester guy has. Let me know folks: if 100 of you pay me $50 per page each, I'll research pretty much whatever you want.

  • by dharmax ( 878508 ) on Sunday April 24, 2005 @04:48AM (#12327738) Journal
    When i worked at IBM research labs, i criticized AspectJ using almost exactly the same words: they try to solve specific issues (logging, persistency, etc), that deserve specific solutions, using a completely generic approach. They failed to provide more examples for those specific problems that AspectJ will solve. Of course, i was silenced - because that's the way it works in IBM Haifa - they just can't deal with criticism :)

    The concepts behind AspectJ, TMO, are not bright. They break encapsulation, they are too ambitious and far from real-world understanding of programming concerns. All in all, they introduce too many problems.

    TMHO,Aspect Oriented Programming should be different, and should be based on something much simpler, much more down-to-earth ideas and more consistent with real-world needs.

    Today it is easily possible to do using Java Annotations, that will specify class member's affinity with an aspect, and thus provide:

    A) a mechanism by which the compiler could limit access (errors & warnings) to members according to their affinity with a common aspect or aspects (common to it and its caller);

    B) a programmer, using a proper IDE, can view a breakdown of his code according to aspects.

    C) in runtime, the current aspect should be visible to the program thus extending the ability to: I) log, and trace errors; II) affect work-flow according to the aspect in action (that's an intense feature so im not too sure about that)

    Doing more than the above, looks to be like an abuse of proper programming concepts.

    The Annotation mechanism introduced in Java 5 is quite powerful and maybe already provide all that is needed for the job, including hierarchical arrangement of aspects (slash annotations), attributes, etc. Very little is needed in order to implement what i propose.

    Maybe i'll propose is to the JCP. What'd ya think?

  • Quite confused (Score:3, Interesting)

    by t_allardyce ( 48447 ) on Sunday April 24, 2005 @05:25AM (#12327826) Journal
    So let me get this straight..

    AOP basically means that when an object is created, or a method is called you wan't to run some code?

    So if we tried to put this into a non-OOP example you would basically be calling something else whenever certain functions were called? Eg calling a 'save', 'load' or 'start' function would result in running the 'logging' function too?

    But as I gather (and its pretty hard to find a simple explanation around here) the idea of AOP is that it allows you to do this without having to put any function calls in your code, ie it 'captures events'. So if for example you had to implement some security checks but you were worried that someone in your project would forget to put the 'if(secure)' lines into their modules you could rest assured that AOP would be on the case.

    Then theres COME FROM which is basically an event handler that says 'when you get to this line/label in your code, come over here and run me.

    I see no problem with this in a relatively high-level environment where the goal is to write less code and where so much crap is going on in the background anyway (dynamic this and managed that) that another overlay is just going to make things look simpler.

    Isn't this however almost exactly the same as an interrupt?
  • All these arguments about different methodologies and techniques being harmful or not boil down to one thing:

    "There are no good constructs, there are no bad constructs; a compiler in the hands of a bad programmer is a bad thing, a compiler in the hands of a good programmer is no threat to anyone - except the patent system."
  • by Minna Kirai ( 624281 ) on Sunday April 24, 2005 @06:00AM (#12327883)
    And since Aspect-Oriented programming is a patented technique [], basically nobody can legally use it unless you're a personal friend of the inventor.

    So, who really cares if its theoretically any good, when legally it is worthless?
  • by triksox ( 614286 ) on Sunday April 24, 2005 @06:01AM (#12327884)

    I've been following AOP (cautiously) for some years now. Here's a few salient points for those who don't have $250 to splash out.

    The underlying principle of AOP is about "separation of concerns", a term introduced by Dijkstra back in 1974. Separation of concerns is a Good Thing[tm], but there's more than one way to do it. It's a conceptual thing more than it is any one particular implementation technique.

    Both structured and OO programming offer techniques that allow the hacker to more clearly separate concerns: by organising their code into subroutines, modules, objects, methods and so on. The problem with OOP is that real world problems don't always break down into a set of clearly defined, independant object classes. In some cases you can end up with a problem fragmented into so many small pieces that you can no longer see the wood for the trees.

    AOP tries to address this by allowing you to identify those concerns that don't fit neatly into an object model. These "cross-cutting" concerns are typically things like logging, debugging and security that affect many of the objects in your system. If you decide to change the way logging is handled, for example, you don't want to have to go and edit every single object that generates logging information. But that's often what happens in OO based systems - you design your class hierarchy with Products, Customers, Orders and other real-world entities in mind and implement them as "black-boxes" with internal functionality neatly hidden away. That's fine when the functionality really is local to the object, but not when it relates to a system-wide aspect like logging, etc. These are the kind of undesirable artifacts that can arise from the decomposition of a problem into objects.

    However, that's not to say that there aren't ways of achieving the good parts of AOP in a non-AOP language. Many Design Patterns are examples of separating concerns. The Model/View/Controller and Model/Visitor patterns come immediately to mind. Going back to the early logging example, we could implement this in AOP fashion in an OO language, by creating a "Logger" object which implements all the logging functionality. Just make sure all your other objects delegate to the logger for logging rather than trying to do it themselves. Now you have all your logging code in one place, and you just have to worry about how you're going to pass the logger object around so that all your other objects can call on it... (and this is often the start of the rest of the problems...)

    So AOP-a-like can be done in OO languages, but most OO languages aren't really cut out for it - you have to code the magic manually if you want it. Hence the rise of AOP languages (usually just bolt-on syntax additions to existing languages) that make this process that little bit easier.

    AOP in Java does smell a little like GOTO, IMHO. In brief, it uses "join points" to connect different aspects together (e.g. call this logging method just before calling that other method). One can certainly argue that it's a more structured form of GOTO, but I believe the same fundamental problems remain: control flow jumping all over the place, with actions-at-a-distance waiting to catch out the unsuspecting programmer.

    So my advice on AOP would be to treat it like OOP, XML, Java, and all the other "silver bullets" that over the years have claimed to be the next big thing that will save our collective software sanity. Recognise the problem that it's trying to solve, realise the benefits of the particular solution(s) presented, and ignore all the hype!

  • A very well respected (if a bit obscure) OO language is Objective-C. It has a few features that are analogous to aspects.

    First it has categories, which allow you to add new methods, or replace the implementation of a method in a given class. You add a category to that class that includes, say, a replacement description method, and the new implementation replaces the old. Or you can add a path utility methods to the String class, as another example.

    It also has "poseAs", which allows you to insert an object into a hierarchy. Say you have Class B which extends Class A. But you want custom behaviour to be accessible to all As children (including, for example, B). You create Class C, which extends A, and make it poseAs Class A. Now the runtime hooks all clesses that inherit A off of C, which replaces A. A still exists, but is in the background, providing normal inherited functionality to C.

    These abilities were one of the things that made Objective C a very powerful OO language. In particular, it allows a particular kind of reuse, by virtue of allowing one to patch someone else's library without having source. So if you want to add the aforementioned path utilities to NSString, you just added a Category in your own code, and your code would run with the extended functionality in NSString (no one elses would, unless they used your category too). This means that often you can make the most out of someone else's code without a complete rewrite.

    It isn't all powerful. There are access and visibility restrictions. It is also used to partition code for ease of organization and better modularization. Here's the point:

    Because it is a well known feature of the language, you think in terms of categories, and you know to look for categories. This can be aided by tools, but you most definitely need to pay attention to them. But because of this, they are only "dangerous" if you aren't paying attention. And guess what! That's true of programming, period.

  • by bsletten ( 20271 ) on Sunday April 24, 2005 @09:50AM (#12328574)
    AOP is not the answer to everything, but one of the main points of a talk I give at the NFJS [] show is that you can find tons of uses w/ development aspects without ever having to migrate them into production.

    You can enforce (and detect violations of) various architectural decisions (i.e. no direct JDBC connectivity without going through a facade), define and enforce contracts (and leave them out of production), detect threading issues (How would you find all Swing thread abuse cases in a large, multi-threaded application?), get a sense of code coverage, etc. around any arbitrary cut of your system.

    If you take the time to build around interfaces and support Object decoration via some variant of factories, you can do some of these things using Decorator and Dynamic Proxies, but you don't always have that luxury.

    Tool support has been fundamentally lacking until things like recent versions of the AJDT. It still isn't perfect, but should serve to answer many of the early critics complaining about debuggability and losing track of what is going on.

    I am an advocate of AOP in production systems, but you don't have to drink the whole glass to find aspects tremendously useful in development alone. With support for Ant-based builds and whatnot, it isn't even really an onerous task to give it a try.

    Don't use AOP for things it isn't a good fit for, but there are quite a few things that can't be done as easily and elegantly without it (at least in Java). As others have pointed out, languages like Lisp and Objective-C have support for AOPish things built in.
  • by MemoryDragon ( 544441 ) on Sunday April 24, 2005 @01:37PM (#12330035)
    Just like goto, which I would not totally consider harmful. Goto has its place in programs which need a precise command which maps directly to a processor jump instruction, therefore goto or similar constructs are widely used in system near programming.

    I have to admit, I stayed away from AOP for a long time, but that stuff can be really useful if applied in the proper manner. The usual application has lots of sideconstraints which basically stain your algorithms up to a point, where you end up having 2/3rd side constraint code, 1/3rd algorithmic code. The side constraint is constantly the same over and over again, only altered slightly according to the state of the algorithm.

    Now if you move that code in a well documented and clean manner into aspects, you can gain a lot. Aspects however are a deadly construct, if you go the AOP is everywhere manner and dont limit yourself to the basic rule, use it seldom and use it wisely but use it whenever you can achieve a cleaner code with it. If you go the aspect here aspect there route, you end up in a bigger mess than with a huge spaghetti code and 100 gotos in there.

    Perfect examples for AOP are, to move the error handling code of checked exceptions out of the algorithms into aspects, another one would be to get a clean semi automatic transactional scope in methods which are called from the outside. Both things can be achieved with aspects in a much cleaner way, than with traditional programming, but if you use this stuff in that manner, document it and comment it, otherwise the persons who will follow you cannot see that the method is in a transactional scope.

    So aspects are definitely worthwile using (I learned that once I jumped over the wall of considering it a deadly construct), but you really have to be careful to limit it to a few problem domains.
  • My Take on Aspects (Score:3, Interesting)

    by nimblebrain ( 683478 ) on Sunday April 24, 2005 @02:58PM (#12330692) Homepage Journal

    Aspects 'inject' behaviour into your classes. This is fine if the behaviour is completely orthogonal to the workings of your class, e.g. you're injecting enter/exit logging. If you start to inject functionality, however, the workings of the class can be substantially obscured. A number of the examples I've seen can be as properly done with more broken-down normal methods and a naming convention (e.g. before_x, after_x).

    That said, chances are that the true benefits of aspect-oriented programming are not going to come from it being a mere curiosity to add to object-oriented programming. Someone is going to have to discover how to program in AOP where the focus is on AOP.

    Possibly, it will have a journey like object-oriented programming. Started out with being used as object-based programming (just a more convenient way to associate data with functions), grew to some standard uses, then exploded into class libraries and eventually patterns.

    Even 'interface-oriented programming' requires a slightly different tack than plain object-oriented programming does.

    That's not to say that AOP will be the next actual big thing, or even really find its niche. There are plenty of good ideas that never really 'made it'. Time, and a whole passel of people with a lot of on their hands, will tell :)

If I had only known, I would have been a locksmith. -- Albert Einstein