Re: [sca-j] NEW ISSUE: @Callback injection could be NULL

[Simon Nash <oasis@cjnash.com>]

David Booz wrote:
> I'm going to try to state this again without using the word injection 
> because it seems to have too many connotations.....and the email thread 
> is wandering all over the place.
> 
> The point I'm worried about is a setter method that is annotated with 
> @callback (or even a 0..1 @reference as pointed out earlier).
> 
> I'm concerned that we provide a consistent model to component 
> developers, and therefore the runtime MUST NOT skip invocation of the 
> setter method. It may be very important for the logic in the setter 
> method to understand that there is no reference target or callback. It's 
> the 'skipping of the setter' that is the problem.
> 
I don't think we need to drive all these setters with null arguments
just in case they have some important initialization to do.  My reason
for saying this is that we also have the @Init method, which is called
after all the setters have (or have not) run.

So the consistent model is that all setters with something to set
would run, and then the @Init method would run.  This combination
allows the implementation code to handle all possible cases.
For example, the setters could each set a flag to say "OK, done my
thing", and the @Init method could check these flags to find out
if further processing is needed because some setter did not run.

I was looking to see what Spring does.  AFAICT it does not drive
dependency injection setters when the dependency is missing (though
it does pass NULL for missing dependencies when calling constructors).
See this article:
  http://blog.springsource.com/2007/07/11/setter-injection-versus-constructor-injection-and-the-use-of-required/
The article also mentions the @Init-style approach that I described
above as one way to check for missing dependencies.

   Simon

> The spec text you point out does not clearly say what to do. The text is 
> clearly written assuming everything is wired up (I'm guilty of thinking 
> that way myself).
> 
> Dave Booz
> STSM, BPM and SCA Architecture
> Co-Chair OASIS SCA-Policy TC and SCA-J TC
> "Distributed objects first, then world hunger"
> Poughkeepsie, NY (845)-435-6093 or 8-295-6093
> e-mail:booz@us.ibm.com
> 
> Inactive hide details for Simon Nash ---02/13/2009 09:41:55 AM---David 
> Booz wrote: > I'm glad we're doing this in email, and noSimon Nash 
> ---02/13/2009 09:41:55 AM---David Booz wrote: > I'm glad we're doing 
> this in email, and not on the call. We need to do
> 
> 
> From:	
> Simon Nash <oasis@cjnash.com>
> 
> To:	
> sca-j@lists.oasis-open.org
> 
> Date:	
> 02/13/2009 09:41 AM
> 
> Subject:	
> Re: [sca-j] NEW ISSUE: @Callback injection could be NULL
> 
> ------------------------------------------------------------------------
> 
> 
> 
> David Booz wrote:
>  > I'm glad we're doing this in email, and not on the call. We need to do
>  > more of this.
>  >
>  > We need to avoid uninitialized fields and so am asserting that NULL
>  > would in fact be injected, ensuring that a test for NULL in the app was
>  > always going to give the correct answer. This makes the two cases
>  > parallel (which they should be). It doesn't do the app any good to leave
>  > uninitialized fields laying around.
>  >
>  > Good catch on the "should".
>  >
>  > I knew about the 76 overlap in the example and chose to ignore it given
>  > that it hasn't been accepted/applied yet. In any case, I am fine if we
>  > omit the example updates from the proposal.
>  >
>  > This then changes the proposal to:
>  >
>  > In section 6.2.4, at the end of the first paragraph (line 395 [1] ) add
>  > the following:
>  >
>  > A field or setter method annotated with @callback is injected or set to
>  > NULL if the implementation is invoked through a non-bidirectional
>  > service or when invoked as a callback. It is RECOMMENDED that the
>  > component implementation checks for null before using a callback
>  > reference in this situation (See also the getCallbackReference() method
>  > in section 8.2 "RequestContext").
>  >
> Apart from the question of whether or not NULL gets injected, I don't
> think we should make the component implementation logic into an RFC2119
> conformance target.  I'd like to proposed alternative wording that
> addresses these two points:
> 
> A field or setter method annotated with @Callback will not be injected
> if the implementation is invoked through a non-bidirectional service or
> when invoked as a callback. To allow for this, the component implementation
> can check for null before using a callback reference in this situation
> (See also the getCallbackReference() method in section 8.2 
> "RequestContext").
> 
>   Simon
> 
>  >
>  > Dave Booz
>  > STSM, BPM and SCA Architecture
>  > Co-Chair OASIS SCA-Policy TC and SCA-J TC
>  > "Distributed objects first, then world hunger"
>  > Poughkeepsie, NY (845)-435-6093 or 8-295-6093
>  > e-mail:booz@us.ibm.com
>  >
>  > Inactive hide details for Simon Nash ---02/10/2009 11:18:56 AM---David
>  > Booz wrote: > TARGET: Java CAA, section 6.2.4 "AccessingSimon Nash
>  > ---02/10/2009 11:18:56 AM---David Booz wrote: > TARGET: Java CAA,
>  > section 6.2.4 "Accessing Callbacks" [1]
>  >
>  >
>  > From:
>  > Simon Nash <oasis@cjnash.com>
>  >
>  > To:
>  > sca-j@lists.oasis-open.org
>  >
>  > Date:
>  > 02/10/2009 11:18 AM
>  >
>  > Subject:
>  > Re: [sca-j] NEW ISSUE: @Callback injection could be NULL
>  >
>  > ------------------------------------------------------------------------
>  >
>  >
>  >
>  > David Booz wrote:
>  >  > TARGET: Java CAA, section 6.2.4 "Accessing Callbacks" [1]
>  >  >
>  >  > DESCRIPTION: There are situations where the field or setter with an
>  >  > @callback annotation (when used to obtain a reference to a callaback
>  >  > instance) can be injected with NULL. As noted in section 8.2
>  >  > RequestContext, the getCallback() method might return NULL in certain
>  >  > situations, and those situations might also occur in combination with
>  >  > the use of @callback. For example, suppose a component implements two
>  >  > services, one has a bidirectional interface and the other does 
> not. The
>  >  > component impl could use @callback to get a callback instance. When
>  >  > invoked over the bidirectional service, the @callback will 
> non-null, but
>  >  > will be null when the non-bidirectional service is invoked. AFAIK, the
>  >  > specs don't prohibit the example (nor do I think they should).
>  >  >
>  > This case isn't quite the same as RequestContext.getCallback().
>  > In that case, the API can return NULL.  In this case, the callback
>  > would not be injected, and so the field would be uninitialized.
>  >
>  >  > PROPOSAL:
>  >  > There is no pdf for [1] so bear with me:
>  >  > In section 6.2.4, at the end of the first paragraph (line 395) add the
>  >  > following:
>  >  >
>  >  > A field or setter method annotated with @callback might be injected or
>  >  > set to NULL if the implementation is invoked through a 
> non-bidirectional
>  >  > service or when invoked as a callback.
>  >  >
>  > As I said above, it's not correct to say "...might be injected or
>  > set to NULL...".  It's really "...might not be injected...".
>  >
>  >  >                                        The component implementation
>  >  > should always check for null before using a callback reference 
> (See also
>  >  >
>  > You've got a non-RFC2119 "should" here.  Even if we found another
>  > word, I think it's going too far to say that this should *always*
>  > be done.  IMO it should *only* be done if the component logic
>  > allows this case to arise, which I believe will be a minority of
>  > all the cases where callbacks are used.
>  >
>  >  > the getCallbackReference() method in section 8.2 "RequestContext").
>  >  >
>  >  > In section 6.2.4, the first example (line 400) replace the example 
> with
>  >  > the following:
>  >  >
>  >  > @Callback
>  >  > protected ServiceReference<MyCallback> callback;
>  >  >
>  >  >       *public void *someMethod() {
>  >  >
>  >  >       *if* (callback) {
>  >  >       MyCallback myCallback = callback.getCallback(); …
>  >  >
>  >  >       myCallback.receiveResult(theResult);
>  >  >       }
>  >  >       }
>  >  >
>  > These changes overlap with the proposed changes for JAVA-76,
>  > and they carry forward one of the problems that is the subject
>  > of JAVA-76 (incorrectly using getCallback() where getService()
>  > should be used).  Also, bearing in mind my comment above, I'm
>  > not convinced that this example needs to show the "if".  I think
>  > it's enough to describe the possibility in added text.
>  >
>  >   Simon
>  >  >
>  >  >
>  >  > [1]
>  >  >
>  > 
> http://www.oasis-open.org/committees/download.php/31121/sca-javacaa-1.1-spec-cd02-rev2.doc
>  >  >
>  >  > Dave Booz
>  >  > STSM, BPM and SCA Architecture
>  >  > Co-Chair OASIS SCA-Policy TC and SCA-J TC
>  >  > "Distributed objects first, then world hunger"
>  >  > Poughkeepsie, NY (845)-435-6093 or 8-295-6093
>  >  > e-mail:booz@us.ibm.com
>  >
>  >
>  >
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