Re: [aspectj-users] re: Possible?

[David Vollbracht <alt@xxxxxxxxxxx>]

I think this does effective what you want, but to make this work
you have to make C extend B (declared in the aspect) so it can be
used as that type. Since they have the same public interface that
shouldn't cause a problem, but you do have to add a no-arg constructor
to B (again, declared in the aspect) since C's constructor knows nothing
about B. Here a modified version of your sample code that does something
close to what you want, I think.

public class A
{
 	private B m_b;
 	
 	public void method()
 	{
 		m_b = new B( "Test" );
 		m_b.method();
 	}

   public static void main( String[] args )
   {
      A a = new A();
      a.method();
   }
}

class B extends D
{
   public B(String string)
   {
   }
   
   public void method()
   {
      System.out.println( "In B" );
   }
}

class C extends D
{
   public C(String string)
   {
   }
   
   public void method()
   {
      System.out.println( "In C" );
   }
}

class D
{
}

aspect BToC
{
   declare parents: C extends B;

   /**
    * No-arg constructor for class B so Class C doesn't
    * need to be changed
    */
   public B.new()
   {
      this( "Dummy String" );
   }

   pointcut createB( String string ) :
      call( B.new( String ) ) && args( string );

   B around( String string ) : createB( string )
   {
      return new C( string );
   }
}

When you run class A it does indeed print "In C".  Is this sufficient to
answer your question?

David Vollbracht

On Tue, 17 Jun 2003 13:12:07 -0500
"Anagnost, Ted (MED, GEMS-IT)" <Theodore.Anagnost@xxxxxxxxxx> wrote:

> Sorry if this is a repeat of my previous post:
> 

-- original code example removed --

> > 
> > Is it possible to change A so that when it tries to do this:
> > 
> > 		m_b = new B( "Test" );
> > 		m_b.method();		
> > 		
> > it actually effectively does this:
> > 
> > 		m_b = new C( "Test" );
> > 		m_b.method(); // calls C's method()		
> > 	
> > In other words can I used AspectJ to substitute another class C
> > for class B (as long as it has the same inheritance model and public 
> > interface) without changing the original source code?
> > 
> > 
> > If the above is impossible, a related question is:
> > 
> > Can I create an aspect such that if there is a call to a method, that
> > method call 
> > is redirected to another object (that has a method of the same
> > signature) instead?
> > 
> > And then when that other object's method call is finished, the
> > original object's 
> > method's code is bypassed altogether?
> > 
> > Thanks,
> > 
> > Ted
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