[PRL] AspectJ "feature"

[Karl Lieberherr]

Hi John and Matthias:

This is bad. Gregor is aware of this problem. 

We work on a partial solution to this by studying the unsatisfiability
problem for pointcuts. Whenever you change your program and one of your
pointcuts becomes unsatisfiable, the compiler will throw a red flag. Not
perfect, but a step in the right direction.

-- Karl

-----Original Message-----
From: prl-bounces at lists.ccs.neu.edu
[mailto:prl-bounces at lists.ccs.neu.edu] On Behalf Of Matthias Felleisen
Sent: Tuesday, April 05, 2005 12:14 PM
To: John Clements
Cc: PRL Mailing List
Subject: Re: [PRL] AspectJ "feature"


We don't need a semantics for our languages, because we don't need to 
reason about our programs.

On Apr 5, 2005, at 11:22 AM, John Clements wrote:

> It's rude of me to crow, but I noticed (and Jeff Palm confirmed) a 
> bizarre "feature" of AspectJ that explains something about how they 
> think.
>
> Here's a simple "Counter" class.  Increment mutates x, and legalDx (if

> it were called) checks that dx is > 0.
>
> public class Counter {
> 	int x;
> 	Counter(int x) {this.x = x;}
> 	
> 	void increment(int dx){this.x += dx;}
> 	boolean legalDx(int dx){return dx > 0;}}
>
> Here's a tiny example of creating a counter and calling increment:
>
> public class CounterTest extends TestCase {
> 	public void test(){
> 		Counter c = new Counter(14);
> 		c.increment(-4);}}
>
> Here's an aspect that adds a check on the argument passed to
increment:
>
> public aspect CheckDx {		
> 	before(Counter c, int dx): target(c) && args(dx) && call(void 
> increment(int)) {
> 		if (! c.legalDx(dx)) {
> 			System.out.println("Illegal value for x");
> 			return;}}}
>
> So, if increment is called with negative dx (as it is above), you'll 
> get output on stdout.
>
> All fine, and works as expected.
>
> QUIZ QUESTION: what happens if you make a mistake in the name of the 
> method called inside the advice?  E.G:
>
> ...
> 		if (! c.legalwhoopsbadnameDx(dx)) {
> ...
>
>
> What happens is...
>
>
> nothing.  No compile error, runs fine.  The advice is _not_ applied, 
> no output to stdout.  I can only guess that type information from the 
> body (in this case, the existence of a 'legalwhoopsbadnameDx' method) 
> is implicitly added to the pointcut,
>
> Now imagine you're working on a 100KLoc program, and you change the 
> name of a method.  You change all the uses of the method (using 
> compiler error messages, say), until it compiles.  And runs!  
> Unfortunately, your aspects no longer apply to the same join points 
> that they used to.  Yikes!
>
> Is this something that Gregor et. al. consider "the right thing?"
>
>
> john
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