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Re: [aspectj-users] Weaving code that comes from an Annotation's value
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Wes,
I'm sorry but I've never used AspectJ before. Would it, using your approach,
be possible to execute the code from the annotation without using a
preprocessor other than ajc or an expression evaluator as Dean described?
Which drawbacks would your solution have?
If it's impossible using only AspectJ, I'll probably use Sun's apt to
generate the required aspects.
Thanks once again
Michael Herrmann
> Three comments, to stir up this old pot...
>
> (1) if() pointcuts are evaluated before each join point, so you could do
>
> before() : foo() && if(contractsEnforced &&
> contract(thisJoinPointStaticPart)) {}
>
> This would be a terrible thing for many reasons, but it kind of works.
> Instead of
> writing
>
> @Test("test code here") void foo() {...}
>
> you'd write
>
> void foo() {..}
>
> @Test("execution(void Bar.foo())")
> void fooTest() { ... }
>
> (2) perhaps (less terribly and sticking closer to AspectJ)
>
> @Before("Contract4J.enforcing() && execution(void Bar.foo())")
>
> (look familiar?)
>
> And if you're nice about IDE integration, you can have
> - links back from fooTest() to foo() [free with @Before, soon?)
> - context menu item at foo() to create fooTest() and annotation.
>
> My design criterion for DBC is that the contracts need to be expressed in
> the
> programming language and get all the usual (IDE) support, so I much prefer
> associating code to developing a new domain language (which you can do
> using
> library API's).
>
> (3) The AspectJ AST is becoming much more usable, mainly driven by the
> desire to
> support refactoring, but someone could probably read comments or
> annotations
> during compilation, evaluate them using the standard compiler, and update
> the
> emitted code accordingly. That give you all the power of Java in the
> expressions,
> and could work both in AJDT and on the command line.
>
> Cheers,
> Wes
>
> > ------------Original Message------------
> > From: "Michael Herrmann" <Michael_Herrmann@xxxxxx>
> > To: aspectj-users@xxxxxxxxxxx
> > Date: Sat, Feb-4-2006 9:04 AM
> > Subject: Re: [aspectj-users] Weaving code that comes from an
> Annotation's value
> >
> > 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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> >
>
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