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[aspectj-dev] interfaces, pointcuts, and libraries


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