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Re: [ajdt-dev] Load-Time weaving questions
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> why aspects need to remain
> constant?
If the aspect doesn't apply to an entire namespace, then
many pointcuts and advice won't work as expected. For example:
before() : call(void setX(Integer i)) { ... }
would not work on *all* calls, only on the classes woven.
Effectively, the decision to apply aspects is a bounds on
the AspectJ language, which can make it very difficult to
reason about and understand. There are specific situations
where that's intended and could be well-understood; for
those situations, a pointcut can and should be used.
{warning: long-winded elaboration...}
Historically, this was an engineering requirement as well
as a semantic one; even though it's circumventable,
defining aspects first and applying them to the
namespace has the correct semantics and is more usable.
In the AspectJ 1.0 days, we used to do "callee-side call
join points", where the call join point could be implemented
in the target class. This required some coordination between
the caller and the callee, as well as whole-program analysis
by the compiler. In that situation, weaving some classes but
not others would break the pointcut. The weaver has since
been redesigned to support incremental compilation, so each
class can be separately woven. (However, there may still
be situations where the engineering won't work. For example,
our implementation of declaring members on interfaces works
on all top-level classes implementing the interface; it would
fail when some implementing classes were not woven.)
You can, of course, get the partial-weaving effect by
segregating classes into jars, one woven and one not,
during a build. And the WeavingURLClassLoader I think
permits you to add aspects; that means you can weave with
one set of aspects and then with a superset, effectively
omitting the classes loaded before the superset from
being woven with the added aspects. So if you can try
it and it sometimes work, why not?
Semantically, the idea is to consolidate all crosscutting
specifications and make them susceptible to both human
and machine processing - to put them all in {an} aspect{s}.
With build configuration files and LTW support in aop.xml,
the consolidation isn't perfect, but still is intended to
apply to a namespace in its entirety.
The interesting questions come with binary aspects,
where the use-model is for one person to write the aspect
and another person applies it using a concrete subaspect.
Assuming the pointcut should specify the target classes
protected pointcut p() : within(com.foo.bar..*);
That could be saying the same thing as "weave only these
classes with this aspect." But since there's a way to
say that with AspectJ, and since weaving some but not other
classes in a namespace can make things hard to understand,
we adopted the guidance about weaving namespaces as a
whole with the same set of aspects. (You lose mainly where
the aspect writer didn't anticipate that the aspect shouldn't
apply to certain classes, and you're stuck with the aspect.)
Wes
P.S. -
You say
> > that should be ok using it
> > programatically just to load certain classes at
> > runtime, right?
With hierarchical classloaders you can carve off an namespace
which has a constant set of aspects, but only applies to
specified classes. (All other classes would be loaded by the
parent loader, and the parent loader should not be loading
the specified classes.) The difference here is that anything
that sees those specified classes is also in that namespace.
However, that's a hard way to try associating specific aspects
with specific classes.
> ------------Original Message------------
> From: Marius Danciu <mdanciusv@xxxxxxxxx>
> To: "AspectJ Development Tools developer discussions" <ajdt-dev@xxxxxxxxxxx>
> Date: Tue, Aug-30-2005 6:55 AM
> Subject: Re: [ajdt-dev] Load-Time weaving questions
>
> Thank you Matthew.
>
> You're saying
> "This mechanisms ensures that all classes loaded by a
> classloader are woven with the same constant set of
> aspects (very important)."
>
> This means that aspects provided to classloader weaves
> class A and then when loading class B you can not
> change the set of spects to weave class B? ... even if
> aspects are already loaded by ExtensionClassLoader
> (for example) ? ... why aspects need to remain
> constant?
>
> Regards,
> Marius
>
> --- Matthew Webster <matthew_webster@xxxxxxxxxx>
> wrote:
>
> >
> >
> >
> >
> > Marius,
> >
> > The interface you need is
> > org.aspectj.weaver.loadtime.Aj in the "loadtime"
> > project (no examples yet). You cannot replace
> > aop.xml yet but I can imagine
> > supplying Aj with a "provider" interface allowing
> > you to programmatically
> > determine the set of aspects to be woven. This
> > mechanisms ensures that all
> > classes loaded by a classloader are woven with the
> > same constant set of
> > aspects (very important).
> >
> > Matthew Webster
> > AOSD Project
> > Java Technology Centre, MP146
> > IBM Hursley Park, Winchester, SO21 2JN, England
> > Telephone: +44 196 2816139 (external) 246139
> > (internal)
> > Email: Matthew Webster/UK/IBM @ IBMGB,
> > matthew_webster@xxxxxxxxxx
> > http://w3.hursley.ibm.com/~websterm/
> >
> > Marius Danciu <mdanciusv@xxxxxxxxx>@eclipse.org on
> > 19/08/2005 14:05:02
> >
> > Please respond to AspectJ Development Tools
> > developer discussions
> > <ajdt-dev@xxxxxxxxxxx>
> >
> > Sent by: ajdt-dev-bounces@xxxxxxxxxxx
> >
> >
> > To: ajdt-dev@xxxxxxxxxxx
> > cc:
> > Subject: [ajdt-dev] Load-Time weaving questions
> >
> >
> > I've been using load-time weaving with AspectJ5 M2
> > and
> > it worked great.
> >
> > Is there any way of doing this programatically? ...
> > What I mean is if you use
> > org.aspectj.weaver.WeavingURLClassLoader as system
> > class-loader, that should be ok using it
> > programatically just to load certain classes at
> > runtime, right? ... furthermore is there any API
> > exposed that you can use to replace aop.xml role?
> > something like using WeavingURLClassLoader to load
> > class A and use a certain aspect or aspects to weave
> > the bytecode of class A...
> >
> > Thank you so much,
> > Marius
> >
> >
> >
> > ____________________________________________________
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> >
> > _______________________________________________
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> >
> >
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> >
>
>
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