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Subject: Re: [sca-j] NEW ISSUE: @Callback injection could be NULL
If a component is providing both bidirectional and non-bidirectional
services isn't it an error in implementation if a method from
non-bidirectional service is using a callback?
Also, a method from bidirectional service would expect access to a callback
service and if the callback service is not available, it may not be able to
serve the request. This situation of NULL callback may not arise when
bidirectional service is invoked through SCA mechanism, as the interface of
service and reference will have to match. (One of the situations I see is
with EJBs providing bidirectional services. What should happen when the EJB
is invoked as an EJB, not as an SCA service is out of the scope of the
current discussion I guess.)
++Vamsi
Apache Tuscany Committer http://tuscany.apache.org
Apache Geronimo Committer and Member of PMC http://geronimo.apache.org
David Booz
<booz@us.ibm.com>
To
11/02/2009 20:12 sca-j@lists.oasis-open.org
cc
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
(Embedded image moved to file: pic01519.jpg)Simon Nash ---02/10/2009
11:18:56 AM---David Booz wrote: > TARGET: Java CAA, section 6.2.4
"Accessing Callbacks" [1]
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Subject:
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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