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[aspectj-users] Some questions/suggestions about AspectJ

Hello there,

I have a few questions and suggestions that I would like to share with the AspectJ community.

First of all, I would like to know why it isn't possible to inject exceptions in methods. I know it is not possible to make an advice throw an exception other than the ones in the contract of a method, which makes sense. However, why can't we make a method, annotated with a certain annotation, for example, throw a checked exception, through ITD? I am working on my master degree thesis project, about access control, and I am using annotations to specify the points in the code that require some kind of permission. This way, I am doing something like this on methods:

@AccessControlled(requires = "foo")
public void foo() {
 ...
}

In order for client code to be able to deal with the lack of permissions by the principals, I would like to make it possible to throw a checked exception (at the moment, I am just throwing an unchecked exception, which is not what we want here!). Shouldn't this be possible? Something like:

declare throws mypackage.MyClass.foo() : AccessControlException;


Second: I would also like to know why isn't possible to extend concrete aspects. I have one situation in which I would like to make the aspect application mandatory (a concrete aspect) but, at the same time, make it possible to "refine" the pointcuts definitions in it (an abstract aspect). More concretely, I have created an aspect which enforces some policies, which must always be applied. However, I would like to make it possible to switch from warning to error messages, as wanted. Example:


public aspect MyAspect {

 public final pointcut all() : !none();

 public final pointcut none();

 public pointcut declareWarningScope() :    all();

 public final pointcut declareErrorScope() :    !declareWarningScope();

 declare warning :    declareWarningScope() && ... : "Warning: Blabla";
  declare error :    declareErrorScope() && ... : "Error: Blabla";

}

This aspect should always be applied but I would like to make it possible to redefine the declareWarningScope() pointcut. However, I cannot extend that aspect so I have a problem! To fix this, I have created a workaround, together with my thesis advisor (Manuel Menezes de Sequeira), which I would like to share and, maybe, get some feedback about it.

The idea is that we put the pointcuts definitions (the declareWarningScope and declareErrorScope pointcuts) in a class, which the aspect extends (not an interface so that the declareErrorScope() can be final):

public abstract class MyClass {
  public pointcut declareWarningScope() :    mypackage.AnAspect.all();

 public final pointcut declareErrorScope() :    !declareWarningScope();

}

public aspect MyAspect extends MyClass {

 declare warning :    declareWarningScope() && ... : "Warning: Blabla";
  declare error :    declareErrorScope() && ... : "Error: Blabla";

}

Then, when we make the project available for others, we create two JAR files: one with everything, except the class MyClass, jar-base.jar, and another one with just the class MyClass, jar-ext.jar.

When we want to refine the pointcuts definition, we don't use the second JAR, which will cause a compile time error, due to the unexistence of the class MyClass. That forces the user to create that class in the specified package, where he may define the pointcut as wanted. If he does not want a redefinition of those pointcuts, he simply uses both JAR's. What do you think about this solution? Is there one better for this problem?


Finally, I am using annotations in packages to declare the permissions for all members of that same package. This way, I try to verify if the permission requirements for a certain method have been declared or not in a certain class or package. However, when those are declared in packages, I cannot guarantee that the package have been actually loaded: only if a class in that package have been loaded, right? This way, I force a class to be loaded in a certain package. The convention was that one aspect in my project, when instantiated, is passed a list of classes that should be instantiated, so that the packages and their annotations are loaded and stored in that aspect. However, if we have a class A in the package pt.iscte but no class in the package pt, the package pt will not be recognized. So, my question is: would it be possible to use AspectJ to declare classes on packages? If that would be possible, I would not have to create a class myself but I could use AspectJ to create it, and then pass it to that Aspect that loads the classes.


I hope I have made myself clear and sorry for the mail size...

Thanks for your attention and interest.

Cheers,

Paulo Zenida

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