Re: [aspectj-users] Super calls to inter-type methods

[Dean Wampler <dean@xxxxxxxxxxxxxxxxxxxxx>]

There's a limited way to do it if you use the Template Method Pattern. However, it's got "issues", too.

Comments inline:

On Mar 12, 2008, at 11:23 AM, Dave Whittaker wrote:

Hi there.  I'm having trouble figuring out the best method to do something and I was hoping someone out there with more AspectJ experience might be able to help.

I am trying to use inter-type declarations on interfaces to simulate multiple inheritance.  The trouble comes in when I want to override the inherited method from an interface.  For instance if I have:

public interface SelectAction<T>{

public void select(T selection);

}

public aspect SelectAspect{

private T SelectAction<T>.selection;

public void SelectAction<T>.select(T selection){
this.selection = selection;
}

}

Try this instead:

public aspect SelectAspect {

private T SelectAction<T>.selection;

// Template Method pattern:

public void SelectAction<T>.select(T selection) {

before(selection);

this.selection = selection;

after(selection);

}

// Must declare these default implementations public:

public void before(T selection) {}

public void after(T selection) {}

}

then:

public class StringSelectAction implements SelectAction<T>{...}

Causes StringSelectAction to inherit the select method correctly, but what if I wanted to override the inherited method and say check permissions before selecting an object?  I can't have:

public void select(String selection){
checkPermissions(selection);
super.select(selection);
}

Instead of overriding "select", override "before":

public void before(String selection) {

checkPermissions(selection);

}

because the select method is not a member of the super class.  Is there a syntax in AspectJ to indicate I want to call the inherited method?  If not, what is the best practice for this type of thing?

This avoids having to call super. In fact, I almost always use Template Method in cases like this, with pure objects, because calling super is a bit of an "anti-pattern", because it would be easy to do the wrong thing; (i) forget to call super, (ii) call it at the wrong time, (iii) change the "contract" of the method unexpectedly, etc. Template Method helps nail those problems down.

The two big drawbacks of this "trick" are (i) the Eclipse java editor shows an error indicator on the class name, claiming that it needs to implement the methods defined by the interface, even though everything builds fine if you use ajc, (ii) it can be confusing to the person who gets to maintain the code after you. ;)

dean

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Dean Wampler, Ph.D.

dean at objectmentor.com

http://www.objectmentor.com

See also:

http://www.aspectprogramming.com  AOP advocacy site

http://aquarium.rubyforge.org     AOP for Ruby

http://www.contract4j.org         Design by Contract for Java5