Aspect-Oriented Programming Considered Harmful

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."

Comments

Of course

[ravenspear]

The only thing worse would be GOTO + OOP, which of course = C++

Re:Of course

[Anonymous Coward]

Last I checked, assembler wasn't a high-level language.

And? High level languages have immediate targetted transfer of control instructions as well that one cannot easily get rid of. For example, C has break, continue, and the closing squiggles of for and while statements.

"High" level languages makes programmers not as aware as they should be when they're moving a different value into the instruction pointer, but their programs are still doing precisely that at various points.

One could argue that C and other compiled languages are low level, like machine code and assembler, and that interpreted things such as Java, C#, and BASIC are high level, but it wouldn't have much bearing on the difficulty of living without unconditional transfer.

Re:Of course

[drsquare]

Well, assembly is hardly a language that encourages large, well-organised programming projects, so you've pretty much supported his point.

Re:Of course

[Dachannien]

For example, C has break, continue, and the closing squiggles of for and while statements.

But all of those statements are used in the context of some logical block, and in some sense "operate" on that block. GOTO is/does not - in C/C++, at least, the only restriction is that you must GOTO a label that's in scope at the time.

"High" level languages makes programmers not as aware as they should be when they're moving a different value into the instruction pointer,

High level languages also relieve the programmer of the need of thinking about things like stack frames, heap management, and register usage. A programmer should in a general sense understand how these things work, but there doesn't need to be a constant awareness of those factors, because the compiler is designed to take care of them instead. Programmers working on hardcore optimization might have an interest in those low-level details, but a good optimizing compiler will take care of most of that before the programmer even starts to think about optimization.

The point in my original comment was that, in assembler, not using JMP reminds one of, "You can't get very far in life without saying 'is'". But the last time I've needed a GOTO was when I was a teenager naively programming in BASIC on my old C-64.

For those unfamiliar with AOP

[ravenspear]

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.

Re:For those unfamiliar with AOP

[starling]

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?

Re:For those unfamiliar with AOP

[Keeper]

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.

Re:For those unfamiliar with AOP

[snorklewacker]

If you want something to happen when a certain thing occurs, wouldn't you just call a function or a method when that whatever occurs? Why would you want a goto style statement there?

The whole idea of AOP is that it's not a GOTO. The "interceptor" (or whatever the jargon of the week is) gets called, does its thing, returns, and your original function runs. This is nothing, lisp does this with defadvice, even emacs lisp can do this (and indeed emacs lisp makes good use of it).

The neat thing about aspects is that you define them in ONE place. So you tell it "I want function setup_db_connection() and teardown_db_connection() called before every method on any Person, Room, and Resource" (imagine it's a scheduling app). This is just like subclassing, but you don't have to rewrite anything else in the app. In fact, VB programmers who did this sort of hook trickery actually did call it subclassing. If you use a factory pattern, you can do this sort of thing without AOP, but the idea of AOP is to not force you to rewrite everything to use factory patterns -- you just define the "aspect" once, in one place, and it affects everything you tell it to.

I find AOP is a great way to impose "external" conditions like debugging, extra asserts, and tests. I don't think it's necessarily a great way to structure an entire program, though someone with more experience in the methodology might make it work. As a new methodology, it's rife with shoddy implementations, buzzwords, overuse, poor use, and general immaturity. This is how all methodologies start out however -- even structured programming went overboard when it was first introduced to the mainstream.

Re:For those unfamiliar with AOP

[mysticgoat]

A better example might be a computerized patient record system (CPRS).

In a CPRS, it is clear that the core concerns involve information related to the patient's health. However if the institution is going to stay in business, there is a strong secondary concern with regard to diagnosis and treatment costs, billing, and so on. Obviously a healthy hospital has to deal with both of these.

AOP's promise in this situation is to allow these two mission-critical concerns to be temporarily divorced from each other, so that in a medical emergency the patient will get the treatment he needs in a timely way, without doctors and nurses having to deal with billing and cost issues (Clippy: "It looks like you want to defibrillate the patient. Would you like me to compare the costs of alternate treatment modalities?")

I've not run into this "AOP" thingee before. My impression is that the problems it is supposed to solve would be better approached by a change in the object hierarchy-- that its use would be sort of like adding epicycles to epicycles when a Copernican revolution is really needed. In the CPRS example, rather than having a PATIENT object with healthcare and billing concerns, it might work better to have TREATMENT and BILLING objects that were separate components of the PATIENT object.

I'm not convinced that the problems AOP is intended to solve couldn't be finessed by refactoring into a different hierarchy of objects.

$249

[Anonymous Coward]

who is going to shell out that cash for 5 pages?

Oh gee

[Anonymous Coward]

"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.

Re:Oh gee

[Bodrius]

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).

Considered... Harmful?

[Mancat]

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

[ZorbaTHut]

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.

Re:Good primer on aspect-oriented programming?

[Alien Being]

goatse.cx

Angry Flower

[maxjenius22]

http://angryflower.com/aposter.html

A bit sketchy to be called an article

[Goosey]

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

[idsfa]

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.

I like GOTO!

[Wabbit Wabbit]

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*

Re:I like GOTO!

[evvk]

Plain C:

void foo()
{
do some thing
if(error)
goto err1;

do something more;
if(error)
goto err2; ....

return;

err2:
free stuff
err1:
free more stuff
}

For anything else I shun goto.

Re:I like GOTO!

[Keeper]

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)

Re:I like GOTO!

[Keeper]

Gotos in C++ are limited to the same method, so no stack problems there.

Exceptions incur an obscene performance hit, and should be used for 'exceptional' conditions and not expected/handled errors. You also have to worry about the case where someone else adds another exception handler that inadvertantly 'catches' your thrown exception (which makes for maintainance problems later down the road).

Re:I like GOTO!

[evvk]

The example was in __plain C__. And besides, exceptions are worthless in a non-garbage collected language. (Ugly kludges in C++ don't count as proper GC.) You have to catch them at every point to free stuff, and the nested structures become even uglier than gotos.

Re:I like GOTO!

[top_down]

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!

[top_down]

You have to catch [execeptions] at every point to free stuff, and the nested structures become even uglier than gotos.

The freeing of resources, all resources not just memory, is automatic in C++ once you learn to use RAII.

Look it up. It is one of major improvements of standard C++ over C/C++.

Re:I like GOTO!

[top_down]

Show me code how to assign callbacks.

Sure. This is an example using the boost::function and boost::bind libraries which have both been available for a long time and are in the process of becoming part of the standard C++ library.

#include <iostream>
#include <boost/function.hpp>
#include <boost/bind.hpp>

using namespace std;
using boost::function;
using boost::bind;

void hello() { cout << "hello from function" << endl; }

struct Object {
void hello() { cout << "hello from member function" << endl; }
};

int
main()
{
// callback with a normal function
function<void()> f(hello);
f();

// callback with a member function
Object o;
f = bind(&Object::hello, &o);
f();
}

If you don't want to use libraries just assign function objects or static class functions.

If you want to do several callbacks at once check out one of the many available signal/slot libraries.

Re:I like GOTO!

[maxwell demon]

Callbacks are everywhere in C++ although these days they are often function objects instead of function pointers

Great! I am looking for an example.

Ok, here you go:

#include <vector>
#include <iterator>
#include <algorithm>
#include <iostream>
#include <cmath>

bool is_negative(double x) { return x < 0; }

int main()
{
std::vector<double> v;

// read numbers from standard input
std::copy(std::istream_iterator<double>(std::cin),
std::istream_iterator<double>(),
std::back_inserter(v));
// those iterators are actually callbacks packed into an iterator interface:
// istream_iterator encapsulates the input operator on streams,
// back_insert_iterator (the iterator which the function back_inserter
// returns) encapsulates a callback to a container's push_back member function

// replace all negative numbers with zero
std::replace_if(v.begin(), v.end(), is_negative, 0.0);
// is_negative is of course a classic callback (alternatively,
// a function object obtained by binding the second argument
// of std::less<double> to 0.0 could have been used)

// calculate square_roots
std::transform(v.begin(), v.end(), std::sqrt);
// and here we have a classic callback to std::sqrt

// sort from largest to smallest
std::sort(v.begin(), v.end(), std::greater<double>);
// std::greater<T> is a function object which just encapsulates
// the greater than operator

// output the results
std::copy(v.begin(), v.end(),
std::ostream_iterator<double>(std::cout);
// the ostream_iterator again is just a callback to stream output,
// encapsulated in an iterator interface
}

All standard C++, using the C++ standard library. And also it's all genuine C++ code, not just C code moved to C++.

Show me code how to assign callbacks.

I don't know what it gives you, but here you are:

callback = f;

I'm of course assuming that callback and f are of compatible types.

These example should be pure OO.

Why? If you haven't yet found out, C++ is not a pure object-oriented language, but a multi-paradigm language. The code I've shown actually uses generic programming (ok, it just scratches at the surface of GP, since it only uses the STL algorithms, but doesn't define one on it's own).

But then, if you want a typical OO version of callback, just look up the observer pattern. (And no, I'm not going to show that here as well, it's already hard enough to get the above code through Slashdot's lame[ness] filter!)

NO C global functions. (Every function is part of some object.)

Well, in the example above, I've had only 3 C global functions: is_negative (but this would probably be better done with standard function objects and binding anyway), std::sqrt (the standard library just has this as function, but it could be encapsulated as object as well), and main (whiuch you just cannot avoid in C++ anyway). The rest are either function templates or objects (which is typical for generic programming).

Re:I like GOTO!

[antientropic]

While anything allocated on the stack will be destroyed when an exception is thrown, any resources allocated during execution will leak (ex: handles or memory allocated on the heap).

Wow, this is stunningly wrong. That's what destructors are for - they will free any resources that your local objects allocated/acquired. That's the resource acquisition is initialisation paradigm, and it ensures that resources are always freed when you leave a scope. This is contrary to using a goto for error handling, where you still have to be careful that every execution path goes through the cleanup code. And of course, how are you going to ensure that some future maintainer doesn't add a "return" somewhere, bypassing your cleanup code altogether?

This is one of the really nice features of C++, by the way. It's much cleaner than finally blocks.

Re:I like GOTO!

[Pete]

ur comment Callbacks are everywhere in C++ although these days gives an impression as if, callbacks are part of standard C++ way.

Well, callbacks are a fairly straightforward concept. They're most commonly used in things like GUI programming, where you do a lot of (asynchronous) event handling.

But more generally, callbacks are just a very specific example of functional programming, which is just another style of programming that emphasises functions as first-class elements. And because C++ supports several programming paradigms, you can do object-oriented programming, functional programming and/or generic programming - often all at the same time.

Just like in Perl, there's more than one way to do it. And there isn't any one "correct" way - whatever gets the job done is fine.

If u r aware, OOP was first implemented on pascal (rather than C).

No.

Object-oriented programming is widely recognised as first being implemented on a language called Simula in the 1960s. 1962 to be precise - for Simula I - though Bjarne Stroustrup based C++ most closely on the OO concepts in Simula 67 (1967).

Pascal was first implemented in 1970, though the original Pascal didn't support object-oriented programming.

And Pascal had inherent capability, while C++ did not.

I'm not quite sure what you mean by that. Most implementations of Pascal don't support object-oriented programming at all, much less have "inherent capability". Those implementations that do support OO-style programming usually aren't called Pascal - eg. Delphi, Oberon, Modula-2.

I could go with the list of many things. For example: C++ did not provide any mechanism to identify whether this object is instance of *THAT CLASS*.

You're using your terminology confusingly here, so it's difficult to work out exactly what you're saying. But I can say that I've only seen a few occasions where it'd be useful to work out if Base* p is actually pointing at a Derived object, and usually those cases arise when you've started out with a bad design or bad logic. Usually the whole point of having a (pointer-to) Base is that you're supposed to rely on using Base functionality. You're not supposed to care if it might actually be a Derived, the only thing that should matter is that it is substitutable-for a Base. If you need to access Derived functionality, you should have a Derived.

But anyway, in those cases where you're dealing with someone else's badly designed code, a dynamic_cast should be enough to get around it. Take your Base* and try to dynamic_cast it to a Derived* - if that fails, you haven't got a valid pointer-to-Derived. Go find the person that built the code you're using and kick them. Hard. :-)

It's not considered in the context of OOP

[ravenspear]

Because OOP purists would mysteriously erupt in convulsive fits mixed with violent outbursts of anger resembling a sex crazed bigfoot/unicorn hybrid if you actually used a GOTO statement while writing OOP code.

best tool for the task

[sfcat]

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.

Buy this document? 249.00???

[the-build-chicken]

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"...be afraid...be 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 apps...so 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

Side effects++

[evvk]

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.

10 GOTO 10

[John Bokma]

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 :-)

Re:AOP?!

[sv0f]

OP is not specific to Java -- I've seen it done with Python

And it was invented by Kizcales working in Common Lisp. For example, see this brief history.

Those [like the grandparent] who do not know history...

Re:Reminds me of my teen years

[Fjornir]

C:\>copy con autoexec.bat
@echo off
echo ^G^GSYSTEM ERROR
autoexec.bat
^Z
1 file(s) copied.
C:\>copy con t.bat
echo Fuck you.
t.bat
^Z
1 file(s) copied.
C:\>t.bat

Forrester is just another clueless IT consultancy

[tyates]

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.

classic newbie blunder

[Anonymous Coward]

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.

Very true. That's what i told them at IBM HRL

[dharmax]

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?

Software Patents Considered Harmful

[Minna Kirai]

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?

AOP sound in principle

[triksox]

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!

Re:Where are all the aspect gurus?

[Hast]

When I tried AOP a few years back it was before they added terms like joint and concern. The principle is the same though.

We were doing a compiler in that project and that was a very good time to use AOP. From the CC programs you get a bunch of object code, but you don't want to edit these files as they are generated by other programs. Instead we used aspects which are then "weaved" into the generated code (this is what is called joints and concerns now). In reality it's pretty much a bunch of pre-compiler steps.

Eg we did an Aspect of the compiler code called Interpret. This aspect added a method "interpret()" in all classes, and you can easily define it's default behaviour as well as adding specific behaviours for some classes. So in our case we defined expressions to return their values and print statements to do a println and so on.

Run the pre-compiler steps, then compile and you've got a compiler that can parse and interpret code. Now add a new stamtement type to the grammar, and do it again and you have a new compiler working. No need to hand edit a bunch of files.

Naturally since this was in the early stages one problem was that looking for compiler errors in generated code was a bit of a bother. All in all it was a great help.

Objective-C has these, and no one bats an eyelash.

[israfil_kamana]

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.

AOP has its uses (without going to production!)

[bsletten]

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.