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

[Simon Nash <oasis@cjnash.com>]

Jim Marino wrote:
> Hi,
> 
> Quick question inline....
> 
> Jim
> 
> On Feb 13, 2009, at 6:39 AM, Simon Nash wrote:
> 
>> 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").
>>
> 
> What happens for methods that exist both on the forward bidirectional 
> interface and another, "unidirectional" interface?
> 
If invoked via the forward bidirectional interface there would be
a valid @Callback injected and it would be valid for the method to make
callbacks.  If invoked via the unidirectional interface there would
not be a callback available and it would not be valid for the method
to attempt to make callbacks.

Presumably there would be some application business logic that would
enable the method implementation to determine which of these modes it
is running in.  If the business logic is unable to tell the difference,
then the method could call RequestContext.getCallback() and test the
result for null.

   Simon

>>  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
>>> ---------------------------------------------------------------------
>>> To unsubscribe from this mail list, you must leave the OASIS TC that
>>> generates this mail.  Follow this link to all your TCs in OASIS at:
>>> https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
>>
>>
>>
>> ---------------------------------------------------------------------
>> To unsubscribe from this mail list, you must leave the OASIS TC that
>> generates this mail.  Follow this link to all your TCs in OASIS at:
>> https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
> 
> 
> ---------------------------------------------------------------------
> To unsubscribe from this mail list, you must leave the OASIS TC that
> generates this mail.  Follow this link to all your TCs in OASIS at:
> https://www.oasis-open.org/apps/org/workgroup/portal/my_workgroups.php
> 
>