Re: [aspectj-users] Choosing which mixin among a collection of possible

[Simone Gianni <simoneg@xxxxxxxxxx>]

Ciao Alessio,

what you created are in fact two ITD methods, and since both of them implement the same method, the compiler will complain.

Mixins in AspectJ are not dynamic, they are static type changes, cause Java is statically typed and AspectJ does not change this.

For example, suppose you have an interface named Timestamped, that declares :

public interface Timestamped {
  public void setTimestamp();
  public long getTimestamp();
}

As a contract, when the setTimestamp is called, the instance should record the current milliseconds, and return it when getTimestamp is called. 

As obvious, it is a tedious and error prone work to implement the same two methods in every class that needs to implement such interface, and to keep them up to date when specifications change, not to mention that probably there are many other similar interfaces, like Loggable with its setLogger and getLogger etc..

In Java one of these interfaces can be implemented in an abstract class, but since each class can only extend one other class, if you need both Timestamped and Loggable you'll end up having to implement one of the two.

In AspectJ you can declare an aspect for this :

public aspect TimestampedImpl {
  private long Timestamped.timestamp = 0;
  public void Timestamped.setTimestamp() {
    timestamp = System.currentTimeMillis();
  }
  public long Timestamped.getTimestamp() {
    return timestamp;
  }
}

And do the same for Loggable in a LoggableImpl and so on for any other interface.

Now, you can simply write :

public class MyClass implements Timestamped, Loggable { }

And see that MyClass "inherits" setTimestamp, getTimestamp etc... from the Timestamped interface, and setLogger and getLogger from Loggable, that is multiple inheritance.

Also, you can override those methods by simply declaring them in the target class .. if for example you have a corner case where MyClass wants to return always 0 for getTimestamp :

public class MyClass implements Timestamped, Loggable { 
  public long getTimestamp() { return 0; }
}

The way it works is applying (copying verbatim) the bytecode of the method declared in TimestampedImpl inside classes implementing Timestamped, unless the method is already there.

If you have more than one TimestampedImpl aspects in your classpath, there is no way to select one or the other at run time, cause the modification is at bytecode level so the type is statically determined.

In that case, you can include only one or the other TimestampedImpl implementation using the aop.xml file, or placing them in two different .jar files and adding only one of them in the classpath at build/runtime (depending on the type of weaving you are using), or do the same with Maven creating two different artifacts and including them as dependencies in two different profiles .. anyway, once the application is built, only one of the two implementations will have been woven in.

Hope this helps,
Simone

2010/3/4 Alessio Pace <alessio.pace@xxxxxxxxx>

Hi,

I was trying to make some simple mixins using AspectJ.

My scenario is the following. I have an (abstract) class, let's say Foo, that implements the interface Bar, and that has a template method in which it calls the methods provided by the Bar interface.

interface Bar {

  String barMethod();

}

abstract class Foo implements Bar {

  public void run() {

    System.out.println( barMethod() );

  }

}

so, I would like to have various mixins which can provide the Bar implementation, and only choose one of them to be actually merged in.

I was trying to adopt the approach of writing them in the following way:

public privileged aspect BarAspect_First {

public String Foo.barMethod() {

           return "First";

        }

}

public privileged aspect BarAspect_Second{

public String Foo.barMethod() {

           return "Second";

        }

}

but clearly Eclipse/AJDT complains about conflict inter-type declarations.

So going back to my question, how do I declare the mixins and them be able to specify which one to actually be used? 

Thanks in advance for any suggestion.

Regards,

Alessio Pace.

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