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 <[email protected]>
> 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.
>
> _______________________________________________
> aspectj-users mailing list
> [email protected]
> https://dev.eclipse.org/mailman/listinfo/aspectj-users
>
>
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