Re: [aspectj-users] Weaving code that comes from an Annotation's value

["Michael Herrmann" <Michael_Herrmann@xxxxxx>]

Hi Dean, 

thank you very much for your detailed explanation. We've already talked on
the SF.net forum of Contract4J where I suggested Contract4J V3, do you
remember? I'm asking this very question because I'm thinking about writing
my own implementation of Design by Contract. 
For this, I wouldn't want to use an expression evaluator, because I'd like
to avoid the 'hassles' you mentioned. I'd rather write a preprocessor that
generates .aj files in order to get around it. 


Does anybody else maybe know of a possibility of how to achieve this without
using a preprocessor?

Thanks all
Michael Herrmann

> This is pretty much what I do in Contract4J
> (http://www.contract4j.org), where the @Pre annotations triggers
> before advice. In this case, the value of the annotation is a java
> expression and it is evaluated with Jakarta Jexl's expression
> evaluator.
> 
> I'm actually talking about the newest version that I am finishing, not
> the two previous releases on the website. (ETA for the new release is
> ~1 week).
> 
> Writing the aspect is not difficult. Evaluating the expression is
> harder, depending on how "sophisticated" it needs to be.
> 
> Concerning the expression in the annotation value, does it vary with
> each usage of the aspect or is it fixed? Your example suggests you
> want to do tracing, but maybe that's just the example. If you have a
> fixed java expression or one that varies in "simple" ways, it would be
> far easier to omit it from the annotation and put it in the aspect
> itself.  Consider the tracing aspect and ignoring the
> thisJoinPointStaticPart.toString() method ;), you could write tracing
> advice that does something like this:
>    before(Object o, <other args>): mypointcut(o,<...>) {
>      System.out.format ("class name = %s, stuff = %s",
> o.getClass().getName(), ...);
>    }
> Even if the expression changes a lot, maybe it would be simpler to
> write a base aspect and override it for each class/method, where the
> advice for class "Foo.foo" contains the expression. I'm just trying to
> save you the hassles of integrating an expression evaluator, having
> gone through that pain myself, if you don't really need it.
> 
> Actually, it wouldn't be that bad if your expressions don't require
> access to local context data, like Foo fields or methods. The pain
> comes in setting up Jexl's context correctly.
> 
> So, with that digression, the aspect would be something like
> 
> aspect BeforeExecutor {
>   pointcut execBefore (Object o, ExecuteBefore eb):
>     execution(@ExecuteBefore * *.*(..)) && this(o) && @annotation(eb);
> 
>   before (Object o, ExecuteBefore eb): execBefore (o, eb) {
>     evalExpression(eb.value());
>   }
> 
>   void evalExpression(String expr) { ... }
> }
> 
> Of course, "evalExpression" is the hard bit.
> 
> I can send you a preview of the newest C4J if you'd like to see what I
> did with Jexl. In my case, I need to do some fancy footwork to setup
> Jexl's "context" for evaluating the expression. It can be done!
> 
> Good luck,
> dean
> 
> --
> Dean Wampler
> http://www.aspectprogramming.com
> http://www.newaspects.com
> http://www.contract4j.org
> 
> 
> On 2/4/06, Michael Herrmann <Michael_Herrmann@xxxxxx> wrote:
> > Hi,
> >
> > I'd like to be able to annotate my methods with code that should then
> always
> > be called before the method itself is called. Is it, through AspectJ,
> > possible to treat the value of a (String) element of an annotation as
> code?
> >
> > For example:
> > public interface Foo {
> >         @ExecuteBefore("System.out.println('hi');");
> >         public void foo();
> > }
> >
> > I now want System.out.println('hi') to always be called before the
> > implementation of foo() is called. You might recommend using abstract
> > classes in this case, but this is just an extremely oversimplified
> example
> > to show you what I mean and using abstract classes or anything similar
> won't
> > help me, unfortunately.
> >
> > Thank you very much for your time
> > Michael Herrmann

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