Hi Matthew,
I’d be very glad if I could have a
custom JPSP from a factory. I think that the signatures should not be equal if
the source locations are not. The presence of line numbers is fairly common and
use in Java, although certainly not always there. I take the view that we
should provide as much information as possible and like the way that currently
with AspectJ we can.
From:
aspectj-users-bounces@xxxxxxxxxxx [mailto:aspectj-users-bounces@xxxxxxxxxxx] On Behalf Of Matthew Webster
Sent: Wednesday, October 25, 2006
12:26 AM
To: aspectj-users@xxxxxxxxxxx
Subject: RE: [aspectj-users]
Custom JoinPoints
Ron,
I
suggested that the _signatures_ of the JPSPs should be considered equal not the
JPSPs themselves.
JSPS
are transient singletons, a bit like the java.lang.reflect.* classes, and so
not appropriate for serialization. A custom factory would allow you to create
ones that are serializable. You are free to create your own implementations of
the relevant interfaces.
While
you can distinguish between the two join points using the runtime where you
can't with pointcuts I would still caution any reliance on this meta-data.
Changes to ajc will be of little value as the majority of Java code is compiled
by other compilers. I think you are essentially exploiting an artifact of the
runtime rather than something intentional in the AspectJ join point model.
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/[1]
"Ron Bodkin" <rbodkin@xxxxxxxxxxxxxx>
Sent
by: aspectj-users-bounces@xxxxxxxxxxx
20/10/2006 17:21
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RE: [aspectj-users] Custom JoinPoints
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Hi Matthew,
1. I agree with you that there are two distinct join point
static part instances.
2. However, I do not believe they should be equals either. I
think it’s valuable to be able to track the two join point static parts
separately say in a hash map to record the distinct timings for the distinct
join points. Moreover it would be nice if you could serialize and deserialize
the static parts and have thisJoinPointStaticPart in one execution be equals to
the deserialized value that was stored in a previous one. That’s how
I’d like equals to work and a use case that I see as valuable.
3. I agree you can’t advise one of them without
advising the other but nonetheless they are distinct join point static parts,
and I don’t see them as equal. I don’t think that two join points
that will always match the same advice should be viewed as equal. E.g.,
it’s still useful to know that one of them takes say 99% of the time and
the other 1% even if you don’t have the source location information to
distinguish them. Again consider a profiling scenario: it’s useful to
know that of the six calls in this method one took 75% so you can investigate
further. I agree that you can’t rely having on source location (even line
number) but we should provide as much information as possible (e.g., by adding
an attribute when compiling sources with ajc) and the equals model ought to be
consistent with how it would behave if you did. It would be very unfortunate if
the semantics of equals varied depending on what level of debug information was
included, and worse if the solution were to ignore source location altogether
in considering equals. If static parts had an equals method like this then it
will require use of identity maps to distinguish them when dealing with call
join points. In my view that would be unfortunate.
Let me ask it another way, is there a use case where having
these two static parts be considered equals is valuable?
One other thought, what is a reasonable way of handling a
persistent map from static parts to data, so that you can update information
about the static part across runs? If the exact same program runs, hopefully
you could use hash codes for the static parts. But if the program changes, what
kind of changes would (and should) give rise to new hash codes?
From:
aspectj-users-bounces@xxxxxxxxxxx [mailto:aspectj-users-bounces@xxxxxxxxxxx] On Behalf Of Matthew Webster
Sent: Friday, October 20, 2006 2:23 AM
To: aspectj-users@xxxxxxxxxxx
Subject: RE: [aspectj-users] Custom JoinPoints
Ron,
There are several points here:
1. You are right about hashcode (I got things the wrong way round) but if I
remove that remark from my statement the first sentence is correct:
In your example below the two invocations of go() are separate join points so
have their own JoinPoint.StaticPart instance. If they are written on separate
lines it becomes more obvious because their source locations will be different.
2. The third sentence is a problem. The signatures are _not_ equal eventhough I
think they should be: they look the same and they are matched by the same very
specific pointcut. I suspect we need to add some "equals()" methods
to the runtime classes.
The signatures for the 2 join points should be "equal()". If you
invoked "go()" in a loop then you would get the same StaticJoinPoint
instance.
3. The JoinPoint.StaticPart objects are not equal and shouldn't be. However
this distinction is below the level of granularity in the AspectJ join point
model _not_ a weakness in the weaving strategy, i.e. I cannot advise one
without advising both, and so cannot be relied upon: it is an artifact of the
runtime implementation. More importantly you cannot rely on source location: an
aspect should behave the same regardless of how it was woven and line numbers
can be omitted from the byte-code ("javac -o") so it's not just the
column that can be missing.
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/[1]
"Ron Bodkin" <rbodkin@xxxxxxxxxxxxxx>
Sent by: aspectj-users-bounces@xxxxxxxxxxx
19/10/2006 16:52
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Hi Matthew,
If the two static parts are considered equal in the sense that sp1.equals(sp2)
then they must have equal hash codes “If two objects are equal according
to the equals(Object) method, then calling the hashCode method on each of the
two objects must produce the same integer result.” See http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html#hashCode() (breaking this contract causes lots of problems, e.g., in
using the java.util collections).
However, I think they shouldn’t be equal either. Consider this case:
void workMethod() {
doWork(); for (int i=0; i<9999; i++) doWork();
}
I would like to distinguish the two join point static parts and they
shouldn’t equal in this respect (even though if they’re on the same
line we can’t distinguish which is which, which I think is more of a
limitation of the bytecode weaving strategy when we have source, that we
don’t implement SourceLocation.getColumn())
From: aspectj-users-bounces@xxxxxxxxxxx
[mailto:aspectj-users-bounces@xxxxxxxxxxx] On
Behalf Of Matthew Webster
Sent: Thursday, October 19, 2006 3:32 AM
To: wes@xxxxxxxxxxxxxx; aspectj-users@xxxxxxxxxxx
Subject: RE: [aspectj-users] Custom JoinPoints
Wes,
In your example below the two invocations of go() are separate join points so
have their own JoinPoint.StaticPart instance. If they are written on separate
lines it becomes more obvious because their source locations will be different.
The signatures for the 2 join points should be "equal()" although
their hashcodes will not be the same (but could be if we used String.intern()).
If you invoked "go()" in a loop then you would get the same
StaticJoinPoint instance.
package ajsandbox;
public class JPSP {
static void go() {}
public static void main(String[] args) {
go();
go();
}
static aspect A {
before() : call(void JPSP.go()) {
System.out.println(thisJoinPointStaticPart.hashCode());
}
}
}
>The solution is to permit the aspect programmer to say "I
>won't be using the join point instance any more" by setting
>the (currently local-final) variable to null; in theory, if
>that's the only use, then the context data associated with
>the join point is eligible for garbage collection.
Unfortunately this won't work because the JoinPoint instance is passed as a
argument to the advice method so is on the stack and so cannot be collected.
Setting a local variable to null will have no effect. You will have the same
problem if you use the this(), target() or args() pointcuts.
>Currently the same join point instance is shared by multiple advice
>in different aspects.
Correct, but this would have to change and any aspect associated with a custom
JoinPoint factory would get its own custom instances. How else would you
support multiple custom factories?
>So it should be the case, e.g., that a
>change made by around advice to replace an argument is visible to
>less precedent advice.
Don't confuse a JointPoint instance with what it refers to i.e.
this/target/args. You may have 2 separate JoinPoint instances, possibly created
by different factories both referring to the same context.
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/[1]_______________________________________________
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