From matt at new-bamboo.co.uk  Tue Mar 25 22:26:08 2008
From: matt at new-bamboo.co.uk (Matthew Ford)
Date: Wed, 26 Mar 2008 02:26:08 +0000
Subject: [Aquarium-users] Advise that catches one type of exception and
	raises another
Message-ID: <4b542e970803251926p7a062e10qd09f5eeac4331977@mail.gmail.com>

Hi,

Im trying to write an aspect which catches one error and then throws
another, but im having some difficulty

Here is my aspect (pointcut on methods in my controller):
Aspect.new :after_raising => DataMapper::ObjectNotFoundError,
:pointcut => @pointcut do |join_point, object, *args|
      object rescue DataMapper::ObjectNotFoundError
        raise Exceptions::NotFound
    end

Im trying to avoid this exception, Exception raised while executing
"before" advice for "Users#show".

Any help would be greatly appreciated thanks,
Matt

-- 
Matthew Ford

New Bamboo Web Development Ltd.
Creating fresh, flexible and fast-growing web applications is our passion.

Tel. +44 (0)20 7099 7486
Mob.+44 (0)78 8212 4376
E-mail. matt at new-bamboo.co.uk
http://www.new-bamboo.co.uk

From dean at aspectprogramming.com  Wed Mar 26 11:00:11 2008
From: dean at aspectprogramming.com (Dean Wampler)
Date: Wed, 26 Mar 2008 10:00:11 -0500
Subject: [Aquarium-users] Advise that catches one type of exception and
	raises another
In-Reply-To: <4b542e970803251926p7a062e10qd09f5eeac4331977@mail.gmail.com>
References: <4b542e970803251926p7a062e10qd09f5eeac4331977@mail.gmail.com>
Message-ID: <F5095C51-B17F-419D-ADDA-CD51F25313FF@aspectprogramming.com>

Matthew,

By "some difficulty", I assume that you don't get the exception  
rescuing you expect, but the original exception gets raised. True?

You have to use :around advice for exception "wrapping" like this. The  
reason is that :after and :after_raising will still raise the original  
exception. Only around advice is designed to change the control flow.

Here's the idiom you want (and I'll add an example of this in the next  
minor release - next week?)

     Aquarium::Aspects::Aspect.new :around, :pointcut => @pointcut do | 
join_point, obj, *args|
       begin
         jp.proceed   # call the wrapped join point
       rescue DataMapper::ObjectNotFoundError => e
         raise Exceptions::NotFound
       end
     end

It may be confusing that you can't handle the exception  
in :after_raising advice. I followed the AspectJ model. It would also  
be confusing if this type of after advice let you circumvent the  
control flow while other types of advice (except for around) didn't  
allow this.

However, what you tried to do is so natural that I think it would be  
nice to have a special type of advice for this purpose. I posted a  
feature request, #19119 for this purpose:
Provide an "after_raising" type of advice that lets you handle the  
exception.

Feel free to comment. A possibility might be ":rescue", so your  
example would become:

     Aquarium::Aspects::Aspect.new :rescue =>  
DataMapper::ObjectNotFoundError, :pointcut => @pointcut do | 
join_point, obj, *args|
         raise Exceptions::NotFound
     end

Yea, that's a lot simpler, isn't it??

Finally, at first, I was a little suspicious of storing the pointcut  
in an attribute "@pointcut". The aspect is created as it's parsed  
(like a method definition). Is it possible this attribute is not  
initialized at that time?
Also, I didn't understand the first line in the advice:
>      object rescue DataMapper::ObjectNotFoundError

Was that supposed to be a comment?

HTH,
dean

On Mar 25, 2008, at 9:26 PM, Matthew Ford wrote:

> Hi,
>
> Im trying to write an aspect which catches one error and then throws
> another, but im having some difficulty
>
> Here is my aspect (pointcut on methods in my controller):
> Aspect.new :after_raising => DataMapper::ObjectNotFoundError,
> :pointcut => @pointcut do |join_point, object, *args|
>      object rescue DataMapper::ObjectNotFoundError
>        raise Exceptions::NotFound
>    end
>
> Im trying to avoid this exception, Exception raised while executing
> "before" advice for "Users#show".
>
> Any help would be greatly appreciated thanks,
> Matt
>
> -- 
> Matthew Ford
>
> New Bamboo Web Development Ltd.
> Creating fresh, flexible and fast-growing web applications is our  
> passion.
>
> Tel. +44 (0)20 7099 7486
> Mob.+44 (0)78 8212 4376
> E-mail. matt at new-bamboo.co.uk
> http://www.new-bamboo.co.uk
> _______________________________________________
> Aquarium-users mailing list
> Aquarium-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/aquarium-users

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



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