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Re: Basic AspectJ Question

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Arved Sandstrom <asandstrom3minus1@eastlink.ca> wrote in
news:gfg2r.15444$kv1.3503@newsfe03.iad: 

> On 12-02-25 04:07 PM, Novice wrote:
>> I'm finding AspectJ very hard to get into, mostly because the manual 
>> (AspectJ Language Guide) seems very weak to me. 
>> 
>> For example, a construct they use all the time in examples is
>> "target()". Clearly, this is an important thing. But I'm darned if I
>> can find anything in the manual that actually says what "target()" is
>> or does! They just seem to assume that it's obvious. But I've got a
>> pretty good imagination and I can imagine lots of things it might be,
>> each of which has different implications and significance....
>> 
>> Can anyone enlighten me on what "target()" does? Even just an
>> indication of where it is explained in the manual would be fine. A
>> link to a better manual or tutorial on AspectJ, if there is such a
>> thing, would also be greatly appreciated. 
> 
> I assume you meant the Programming Guide. 

I've got two manuals in my Eclipse installation and they're called 
"AspectJ Development User Guide" and "AspectJ Language Guide" in the high 
level help index. (I'm running Eclipse 3.7.1 and installed the latest 
version of AJDT a couple of days ago.) So that's why I'm calling it the 
Language Guide. But I see that there are some lesser books _within_ the 
Language Guide, including a Programming Guide, so we are really talking 
about the same thing, more or less.

> If you're looking at that in
> examples, it should be Right-In-Your-Face (TM) evident

:-)

> that target() is a pointcut. After that you refer to Appendix B      
> Language Semantics,
> and find target(). The section on State-based Pointcuts explains
> this(0 and target() pointcuts quite well.
>
Yes, I see target() discussed in the Pointcuts section of Language 
Semantics. 

I didn't phrase my original question/remark as well as I should have. 
Yes, the first page where I encountered target() ("Introduction to 
AspectJ") explicitly says it's a pointcut but, to make a long story 
short, I didn't quite see what it meant or how to use it. I kept reading 
forward, hoping for clarification, and just kept encountering more and 
more things that raised my eyebrows. I see know that I should have just 
headed for Language Semantics.
 
> You should get from that explanation that if you wanted your pointcut
> to apply when execution is about to transfer to a Java object of type
> org.novice.MyClass, say, that target(org.novice.MyClass) would be
> useful. This particular primitive pointcut is particularly useful when
> combined with others.
>
As I expected, target() has a perfectly reasonable meaning/purpose. I was 
just getting frustrated trying to find out where that meaning/purpose was 
explained. The examples, as frequent as they are, weren't explained in 
much detail so they had a sense of "Hey, kids, here's something cool you 
can do!" without giving me much insight into how the code actually 
worked.
 
>> And if anyone is aware of a manual or tutorial that explains how to
>> do X in Aspect J where there is a long list of X's, that would be
>> REALLY helpful. For instance, I'd like to write an "entering" log
>> entry for each method (and constructor) as I execute it and I want
>> that log entry to include the name of the class and method so I need
>> a pointcut and advice that is able to determine the class name and
>> method name that is being executed so that I can put it in the
>> logging statement. I'm picturing something like:
>> 
>> pointcut entering() : execution ( * * (..));
>> 
>> before() : entering() {
>>          this.logger.entering(className, methodName); 
>>       }
>> 
>> I know that some context information is available in each method but
>> I'm not sure how to get the class and method names so that I can put
>> them in the advice.
> 
> Pointcut parameters. Each join point has 3 items of state: current
> object, target object, and arguments. What you do with pointcut
> parameters is to "publish" context from the selected join points. In
> your example you have
> 
> pointcut entering() : ...
> 
> so because of the empty parentheses you are publishing nothing.
> 
I didn't mean to present that as if it seemed like a complete solution. I 
just meant it as a skeleton that I knew had to be fleshed out. ;-)

> This is where the pointcut primitives this(), target() and args() also
> come in real handy, if you use them with Java identifiers rather than
> types. 
> 
> In your example you'd like the advice to be
> 
> before(String className, String methodname) :
> entering(className, methodName) {
>     log(className, methodName);
> }
>
I hadn't come across anything yet that said what you could/should put in 
the brackets after the "before". That was frustrating me too!
 
> and you can do that by writing the pointcut as
> 
> pointcut entering(String className, String methodName) : execution( *
> * (..)) && args(className, methodName);
> 
> This will work OK with the '..' wildcard for the parameters, but
> you'll want to be sure that the method join points that get picked
> actually do have those 2 arguments.
> 
Well, I'm just playing around with the idea of doing logging from 
aspects. Many of my current methods have this as the first two lines: 

String methodName = "foo()";
this.logger.entering(this.CLASS_NAME, methodName); 

[where className is a class variable defined as final String CLASS_NAME = 
getClass().getName();]

I was trying to figure out how I'd write that in an aspect so that I 
could remove the "entering" statement from each method.

> As a complete aside, using the this(), target() and args() pointcuts
> like this sort of reminds me of Prolog unification. :-)
> 
Sorry, that one went right over my head. I know Prolog is a programming 
language but I've never used it and have no idea what Prolog unification 
is ;-)

>> That's why I'd love to find a "How to do X" list with lots of
>> different X's in it. With a bit of luck, what I'm trying to do would
>> be on the list and I get to what I want to do a little more quickly
>> than churning through the Language Guide...
>> 
> Getting Started With in the Programming Guide should help, as should
> the FAQ. I think you're getting a bit ahead of yourself, maybe trying
> to get too much coded up with aspects before doing some solid,
> necessary reading and experimenting. Seriously though, the existing
> AspectJ docs are simply loaded with good examples.
> 
I do tend to be impatient when learning something new. I'm in something 
of a hurry to improve my current logging and I'm hoping to get that done 
very quickly and then come back and explore AspectJ in more depth as time 
permits. I was looking at the manual hoping to encounter a way of doing 
logging and developing a little prototype that would get the essence of 
the code I need. 

Thanks for your assistance with this Arved. I'll look to Language 
Semantics to help me over unclear bits of the manual and, of course, post 
questions about things that aren't clear.

-- 
Novice

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Basic AspectJ Question Novice <novice@example..com> - 2012-02-25 20:07 +0000
  Re: Basic AspectJ Question Arne Vajhøj <arne@vajhoej.dk> - 2012-02-25 18:04 -0500
    Re: Basic AspectJ Question Novice <novice@example..com> - 2012-02-26 01:43 +0000
  Re: Basic AspectJ Question Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2012-02-25 21:52 -0400
    Re: Basic AspectJ Question Novice <novice@example..com> - 2012-02-26 14:34 +0000

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