Hi Peter,
It’s true that there aren’t method
arguments at a field set join point, but I’d argue that the value being set has
the semantics of being an argument in a field set operation, whereas the current
value does not. Likewise the use of args in a handler join point in capturing the
handled throwable is semantically an argument.
Another way of stating my point about
matching for args is that currently in AspectJ args(S) and args(S, T) will
NEVER match the same join point. The first one will match only join points with
a single argument of type S and the second only join points with two arguments
of types S and T. I think it’s highly desirable to preserve a definition of
args that is an ordered list without optional members.
You’re right that exposing the java.lang.reflect.Field
with thisJoinPoint.getField() could break with obfuscated code. If a field() primitive
pointcut designator returned the Field in question it also would have that
problem. Indeed, you could pretty easily calculate the Field associated with a
join point static part, so providing this would be a minor optimization.
It would be nice from a developer’s view
point if AspectJ had a primitive pointcut designator fieldval(X) that would be
defined as the current value of the related field and which would only be
defined at field get and field set join points. Perhaps this is what Matthew
was suggesting with field(). However, this would have the odd behavior of being
mutable at a join point, i.e., the value for fieldval could change during the
join point. I also suspect it would be hard to expose this value through
thisJoinPoint (to do so without using reflection would require updating the
value of fieldval multiple times).
You do make a good argument for providing
the feature. In many use cases, use of reflection is quite performant so in
past when people have asked for optimized access to the current field value,
the response has been to demonstrate a compelling example where the performance
is unacceptable for the use. With respect to allowing reweaving, the AspectJ
approach to reweaving statically woven code at load-time is to reweave unwoven code
with all relevant aspects included, i.e., it doesn’t try to weave woven field
access code in a second stage. With respect to obfuscation, is it really true
that the obfuscator just breaks when reflection is used? That seems like a big
limitation given how much reflection is used in libraries.
From:
aspectj-users-bounces@xxxxxxxxxxx [mailto:aspectj-users-bounces@xxxxxxxxxxx] On Behalf Of Peter Murray
Sent: Saturday, September 16, 2006
4:28 AM
To: aspectj-users@xxxxxxxxxxx
Subject: Re: Add argument to field
set joinpoint ? Was: Re:[aspectj-users]Question re: declare error
Ron,
Please excuse me if I'm not making sense - AspectJ is still a bit fresh to me.
Isn't it the case that "arguments" at a set join point are already
arbitrary as there are really no official arguments as there are in a method
joinpoint. Therefore it's up to the framework to define what the
arguments are at that point. I do agree that calling proceed with three
parameters in this case would 'feel' a bit odd. I'm not sure I understand
your point on the positional matching of args.
It would be best to avoid using reflection for the following reasons:
- for performance reasons, at some level
- to allow woven classes to be obfuscated
- to allow woven classes to be further woven at
run-time if desired
Once we resort to
reflection, the woven code is 'disconnected' from the structure of the class
and it's field accesses and invocations cannot be intercepted by examining the
bytecode. Sometimes this may be desireable, but it should be the decision
of the developer which way to go.
It seems to me that it would be possible to modify AspectJ to allow for
non-reflective access to existing field values during a set operation.
The question is - what would be the most consistent way to do this?
Cheers,
pete
peter m. murray
pete@xxxxxxxxxxxx
On 9/15/06, Ron
Bodkin <
rbodkin@xxxxxxxxxxxxxx> wrote:
I think it would be inconsistent because args is really about
arguments at a join point whereas this value of old state isn't an argument.
And allowing args(x) or args(x,y) to match is inconsistent because args matches
positionally (e.g., args(x, *)), so the optional extra parameter would be
surprising. I certainly agree that retaining backward compatibility would be
important.
What about having a getField() method on thisJoinPoint? You
could imagine having a subclass of JoinPoint for FieldAccessJoinPoint (much as
Signature has subtypes). Failing that, it could be null if there isn't a java.lang.reflect.Field
associated with the given join point. At least in this way you preserve
pointcut orthogonality and the kinded-specific join point information is
accessed through the API.
Subject: Add argument to
field set joinpoint ? Was: Re: [aspectj-users]Question re: declare error
Matthew,
What about the idea of adding a second "argument" to the set
joinpoint that would be optionally bound (to be backwards compatible) and, if
present, would pass in the current value of the field? The only slightly
weird thing about that woudl be that the proceed() would also contain that
argument which would essentially be ignored.
What do you think about that idea?
Cheers,
pete
peter m. murray
pete@xxxxxxxxxxxx
Pete,
When used with get/set the pointcut this & target match the object
accessing the field and the one that defines it respectively.There is no
pointcut to match the field itself although you could image one:
"field()" for example. But there are problems. Like this & target
it must be typed but you can't just use Object because of primitive fields. But
it gets worse because in AspectJ any pointcut combination is possible - they
are orthogonal - although certain combinations are not very useful because they
match nothing e.g. call & execution. The "field()" pointcut would
be _undefined_ at certain join points which is messy.
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
Yes, but that solution requires specific knowledge of the field you are
pointcutting on. In general, I need to write one pointcut that services
many objects with many different fields. Otherwise I might as well
manually inline my undo / transaction rollback facility.
-pete
On 9/14/06, Vincent Jorrand <vjorrand@xxxxxxxxx >
wrote:
Matthew Webster's post actually mentions 2 solutions, one of which uses a
priviledged aspect and no reflection.
The following is copied from his post:
public class SomeClass {
private int intField;
public void setInt (int i) {
intField = i;
}
}
public privileged aspect PrivilegedAspect {
pointcut intFieldSet(SomeClass obj, int newValue) :
set(* intField) && target(obj)
&& args(newValue)
&& withincode(* SomeClass+.set*(..))
;
before (SomeClass obj, int newValue) : intFieldSet
(obj,newValue) {
int oldValue = obj.intField;
System.out.println("?
beforeIntFieldSet() oldVaue=" + oldValue
+ ", newValue=" + newValue);
}
}
Vincent
----- Original Message ----
From: Peter Murray <pete@xxxxxxxxxxxx
>
To: aspectj-users@xxxxxxxxxxx
Sent: Thursday, September 14, 2006 5:12:00 PM
Subject: Re: [aspectj-users] Question re: declare error
I wonder if there would be support in the AspectJ community for adding an
additional "argument" to the set property that could optionally be
bound using syntax like
... set(* *) && args(newValue, oldValue) ...
which would send in the old value?
Any thoughts? Am I off in the weeds - or is this technically possible if
we expand the definition of the set joinpoint?
-pete
peter m. murray
pete@xxxxxxxxxxxx
On 9/14/06, Ron Bodkin < rbodkin@xxxxxxxxxxxxxx>
wrote:
Hi Peter: the topic of getting old values of a
field during set has come up before on the mailing list and as you've
determined the only way to do it where you advise different fields is via
reflection (e.g., see Matthew Webster's post at http://dev.eclipse.org/mhonarc/lists/aspectj-users/msg04126.html
). I see how the obfuscator might
break with reflective code (though I'd hope that an obfuscator wouldn't: lots
of libraries now rely on reflection). One idea might be to track the old value
of the field also in advice so you can compare the previous value to the
current value (e.g., using your map that associates static parts with state).
Ron,
Thanks a bunch! That does the trick. I was suspicious that there
might be an operator for "subtypes".
Now, the real question is - how can I get ahold of the original value of a
field during around set advice so I can use AspectJ to create the ValueChange
objects and post them based on the @Transactional annotation. It seems
the only way is through reflection which will break once we obfuscate...
Any thoughts on that?
Cheers,
pete
peter m. murray
pete@xxxxxxxxxxxx
I would use within(Change+) instead of this(Change). That
isn't the exact same semantics as using this (*) but in most cases it would
capture your intent: it makes it an error to set the field outside of code
that's in a subtype of Change.
(*) it's based on the join points being lexically located
inside Change. So it can differ because it won't allow setting the fields in a
base class that might be extended by a type that extends Change. It also
doesn't allow for setting inside of inter-type declared methods (you could
extend the rule to allow those if that matters to you).
Subject: Re: [aspectj-users] Question re:
declare error
Thanks
for your reply, Elizabeth.
That makes sense - do you see a way to accomplish what I am trying to do?
Cheers,
pete
peter m. murray
pete@xxxxxxxxxxxx
As I understand it, declare is a compile time error or
warning; this(), target() and args() are all run time checks. Except in the
simplest of cases, it is not possible to tell at compile time what the type of
this, target or args will be.
I'd like to declare an error if my @Transactional attributed instance variables
are being set outside of the context of a Change object. In other words,
I'd like to have an error in this case:
public class Foo
{
@Transactional
String name
public void setName(String newName)
{
name = newName;
}
}
But not in this case:
public class Foo
{
@Transactional
String name
public void setName(String newName)
{
new ValueChange<String>(name,
newName)
{
public void set(String
value)
{
name = value;
}
}.post();
}
}
It seems like this kind of aspect should do that:
public aspect FieldChangeAspect
{
declare error :
set(@Transactional * *) &&
!this(Change) : "Set of @Transactional
variable not in Change object";
}
But this gives me an error saying "this() pointcut designator cannot be used
in declare statements." BTW, target(), and args() also give
the same error for declare statements i guess, so you can't filter on types of
this, target, or args in declares.
Am I missing something? Is there a reason for this or could these be enhancements?
Cheers,
--
-pete
peter m. murray
pete@xxxxxxxxxxxx
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