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Re: [aspectj-dev] interfaces, pointcuts, and libraries
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Hi Arno -
You point out that we often want to recompose a pointcut.
One way is to redefine a concrete pointcut:
------ SuperPointcut.java
abstract aspect AA {
pointcut publicMethodCalls() : call(public * *(..))
&& !within(AA+);
before() : publicMethodCalls() {
System.out.println("public method call: " +
thisJoinPointStaticPart);
}
}
//aspect A extends AA {}
aspect B extends AA {
pointcut publicMethodCalls() : AA.publicMethodCalls()
&& !call(void java..*.*(..));
}
public class SuperPointcut {
public static void main(String[] a) {
new C().run();
}
}
class C {
public void run(){
System.out.println("run()"); // advise this?
}
}
------
The aspect A would advise the call to System.out.println(String),
while the aspect B does not.
Do you think that would help? Your library aspects can have
concrete pointcuts which are referred to and qualified by
the subaspects for a particular configuration.
Wes
Arno Schmidmeier wrote:
Hi all,
Hello Wes,
the current abstract pointcut behaviour is fine with me, asl long as I use it
as a real abstract pointcut.
However I often have to abuse abstract pointcuts to emulate something what I
have named extensible pointcuts. (One of my requests for 1.1)
I have the impression, that this feature would be give Adrian, all what he
needs to implement and would fit more clearly into the language. Please tell
me if I am wrong.
Unfortunatly I can imagine, that this causes some headache with incremental
compile, weave. ...
Some motivations examples:
1. Exception logging.
I found very often a general rule, that all caught runtimeexception, which
might have caused some harm, should be logged to stderr, ...
so following advice with pointcut would do it:
before(Exception ex):handler(Exception)&&args(ex){
ex.printStackTrace(); // some other stuff...
}
It is pretty naturally to move this logic to a kind of library aspect.
Unfortunatly, there are always some exceptions form the general rule in the
code base. Places, where the advice should not be applied.
So I would like to say:
abstract aspect LogExceptions{
abstract pointcut mustNotLog();
before(Exception ex):handler(Exception)&&args(ex)&&!mustNotLog(){
ex.printStackTrace(); // some other stuff...
}
So I can have one aspect per project, where I limit the scope for the library
aspect. (with a concrete aspect).
In practical code, this causes a very ugly concrete aspect.
What I would prefere to have is something like:
abstract aspect LogExceptions{
extensible pointcut mustNotLog();
before(Exception ex):handler(Exception)&&args(ex)&&!mustNotLog(){
ex.printStackTrace(); // some other stuff...
}
}
somewhere else:
declare pointcut LogException() extends: &&within(SomeClasss);
and somewhere else:
declare pointcut LogException() extends: &&within(SomeClasss);
This specific feature can be also used the other way round, motivated by my
original post:
To add some joinpoint to an aspect. For this case I have found some idioms to
work around, however I find them everything else than handy.
However for the case, where I want to exclude some joinpoints with extensible
pointcuts I do not jet know a solution.
kind regards
Arno
}
On Saturday 26 July 2003 23:09, Wes Isberg wrote:
Another question is whether abstract aspects could play the same role.
While extending aspects is less flexible, it has the property that
the implementation is determinate for incremental weaving.
Here's some pseudo-code for a programmer using a library of J2EE
pointcuts that is implemented by different vendors. The last step
can be the selection of the vendor, when deploying:
------- pseudo-code...
---- AspectJ J2EE library
abstract aspect EjbPointcuts {
abstract pointcut ejbInit();
abstract pointcut ejbActivation();
abstract pointcut ejbPassivation();
abstract pointcut ejbPersisting();
abstract pointcut ejbMessage();
...
}
abstract aspect JmsPointcuts { ... }
abstract aspect ServletPointcuts { ... }
abstract aspect JSPPointcuts { ... }
---- {vendor} J2EE library
abstract aspect VendorEjbPointcuts extends EjbPointcuts {
{define pointcuts}...
}
...
---- programmer writes to AspectJ J2EE library
abstract aspect BankTransactions extends EjbPointcuts {
Object around() : ejbMessage()
&& call(void Account.deposit(int)) {
...
}
...
}
---- application assembler/deployer hooks everything up
aspect BankTransactionsAssembly extends BankTransactions {
declare parents: BankTransactions extends VendorEjbPointcuts;
}
------- end of pseudo-code...
This approach poses single-inheritance problems that might drive
towards a single abstract aspect and deter reuse of other
abstract aspects, so it's obviously not ideal. It might lead to
a practice of using the library pointcuts from other aspects:
----
-- programmer code
/** concretize the EjbPointcuts for use */
abstract aspect BankTransactionPoints extends EjbPointcuts {
...
public pointcut depositTransaction() : ejbMessage()
&& call(void Account.deposit(int);
}
aspect BankTransactions extends Transactions {
/** concretize the abstract pointcuts in Transactions */
pointcut transactionalMessage() :
BankTransactionPoints.depositTransaction();
...
}
-- application assembler/deployer
aspect BankTransactionsAssembly extends BankTransactions {
declare parents: BankTransactionPoints extends VendorEjbPointcuts;
}
----
This approach localizes the pointcut definitions in the vendor aspect,
which is loaded before any aspect or class that is affected by
the aspect, so it preserves incremental load-time weaving.
(Indeed, a vendor controlling the weaver could implement custom
join points not based on the normal Java programming model without
affecting the AspectJ language.)
So before I'd want to support interface pointcuts that prevented
implementations of AspectJ from doing incremental weaving, I'd like
to see a few libraries using abstract aspects and know that they
can't do what people would like to do with libraries.
Wes
Adrian Colyer wrote:
AspectJ lets us declare pointcuts within aspects, classes and interfaces.
To create 'library' pointcuts I need to be able to declare pointcuts in
an interface (and let users of the library program to the interface), and
then have implementers of the interface provide concrete implementations
of those pointcuts. Exploring the behaviour of AspectJ 1.1 I see that we
are part of the way there, but not fully. What I am about to describe is
partially bug and partially feature request...
Today I can write:
public interface Foo {
public pointcut bar();
}
This compiles happily, and I can refer to Foo.bar() in advice (it doesn't
match any joinpoints).
If I write
class C implements Foo {}
this does not cause a compilation error (I believe it should, since C
does not define pointcut bar which it's interface contract says it
should).
If I write
class C implements Foo {
public pointcut bar() : execution(... ...);
}
this compiles happily. Writing advice against Foo.bar does not match
anything, writing advice against C.bar() matches the execution
joinpoints. The desired behaviour is that writing advice against Foo.bar
should match against the C definition.
If I write
aspect A implements Foo {}
this does not cause a compilation error (I believe it should, since C
does not define pointcut bar()).
If I change the interface definition to
public interface Foo {
public abstract pointcut bar();
}
then compilation of A fails with "inherited abstract pointcut Foo.bar()
is not made concrete in A" (good, but tells me that the pointcut is not
being implicitly made abstract when defined in an interface). Compilation
of the empty C declaration still does not produce the compilation error.
How I think I would like this to behave is that pointcuts declared in
interfaces are implicitly abstract (just like method definitions in
interfaces). If a class or aspect declares that it implements the
interface without providing a concrete definition of the pointcut then
this is a compilation error. Clients should be able to write advice
against the interface, and the advice will apply to joinpoints matching
any of the concrete implementations of interface in the system (same
rules as for abstract / concrete aspect pairs).
Why this is important:
* I can create a standard interface that clients program to
* Multiple parties can implement the interface to provide concrete
implementations that make sense within their system. These can even be
binary so that implementation details are never exposed to clients.
What do others think?
-- Adrian
Adrian_Colyer@xxxxxxxxxx
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