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

[Mike Edwards <mike_edwards@uk.ibm.com>]

Folks,

I compare the case of Callback injection
with that of Reference injection.  They are directly comparable.

In the case of a Reference, the Reference
can either be required - 1..1 or 1..n multiplicity - or it can be
optional - 0..1 or 0..n multiplicity.
 For the required cases, then the SCA runtime is obliged to inject
something.
If nothing is available to inject (no
wiring) then it is an error and the component must not be run.  In
the optional
case, nothing need be injected and the
component should be coded to deal with this appropriately.

I think that an implementation which
implements only bidirectional services, it can be written to expect that
a callback will be injected. If the
implementation has both bidirectional and non-bidirectional services then
this is like an optional callback and
the implementation must be written to deal with the case of not getting
a
callback injected for invocations of
methods belonging to the non-bidirectional services.


Yours,  Mike.

Strategist - Emerging Technologies, SCA & SDO.
Co Chair OASIS SCA Assembly TC.
IBM Hursley Park, Mail Point 146, Winchester, SO21 2JN, Great Britain.
Phone & FAX: +44-1962-818014    Mobile: +44-7802-467431  
Email:  mike_edwards@uk.ibm.com

From:	David Booz <booz@us.ibm.com>
To:	sca-j@lists.oasis-open.org
Date:	11/02/2009 15:02
Subject:	Re: [sca-j] NEW ISSUE: @Callback injection could be NULL

---

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").


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

Simon
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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