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[javaee-spec users] [jsr342-experts] Re: Re: allowing stereotypes to be used more widely

From: Yoon Kyung Koo <kyungkoo_yoon_at_tmax.co.kr>
Date: Thu, 11 Oct 2012 12:23:08 +0900

Hi, all.

I think extending CDI stereotypes is good and the right way to go.
Sorry for late feedback.

-- 
--------------------
 Software Innovation Evangelist
 Researcher & Executive Director / WAS Lab / TmaxSoft R&D Center
 PGP  http://www.javadom.com/personal/yoonforhatgmaildotcom.asc
2012. 9. 21.,  3:56, Bill Shannon <bill.shannon_at_oracle.com> ۼ:
> 
> As with my previous message, I'd like to hear from the rest of the
> expert group on this topic.
> 
> Bill Shannon wrote on 08/30/2012 01:58 PM:
>> Some time ago David Blevins started a discussion in the EJB expert
>> group about "meta-annotations".  The thread starts here:
>> http://java.net/projects/ejb-spec/lists/jsr345-experts/archive/2011-12/message/21
>> 
>> Several of us have been discussing this idea privately on and off
>> for some time, and it's time to bring that discussion to this expert
>> group.  The general idea is to allow developers to create their own
>> annotations that are combinations of existing annotations.  You can
>> think of this as a limited "macro" facility for annotations.
>> 
>> For example:
>> 
>> @Metatype
>> @Stateless
>> @TransactionAttribute(TransactionAttributeType.NOT_SUPPORTED)
>> public @interface NonTransactional { }
>> 
>> @NonTransactional
>> public class MyStatelessEJB { ... }
>> 
>> 
>> This is essentially CDI stereotypes, which leads to a counter-proposal
>> to simply allow CDI stereotypes to be used more widely.
>> 
>> The issues with using stereotypes more widely are:
>> 
>> - Stereotypes are implemented by CDI, but (typically) the Java EE
>>  deployment processing has no knowledge of CDI when it's looking
>>  for Java EE annotations.  Integrating with CDI so that stereotypes
>>  could be considered during this deployment-time annotation processing
>>  would require a new CDI SPI.
>> 
>> - CDI extensions can change the definition of a stereotype.  Because
>>  most Java EE annotations are processed at deployment time, before
>>  any application code (including CDI extensions) can run, any dynamic
>>  changes to stereotypes can't effect the deployment process.
>> 
>> - The additional processing required at deployment time to handle
>>  stereotypes could have a non-trivial impact on deployment performance.
>> 
>> To address these issues, we could introduce a new @Metatype annotation
>> with most of the same functionality as stereotypes, but without the
>> ability to change them dynamically.  Of course, introducing a new
>> annotation has issues of its own:
>> 
>> - Developers will be confused as to when to use @Stereotype and when
>>  to use @Metatype.  In most, but not all, cases they will behave
>>  similarly.
>> 
>> - Introducing a new annotation, without a common place to handle
>>  processing of that annotation, will likely lead to inconsistencies
>>  in its implementation.
>> 
>> 
>> Our preliminary analysis suggests that the performance impact of
>> handling stereotypes when processing deployment time annotations
>> will not be significant.  While there's a very small incremental
>> cost to be *able* to handle stereotypes, most of the actual cost
>> is only incurred if applications *use* stereotypes.  And in any
>> event, the cost would be essentially the same as the @Metatype
>> approach.
>> 
>> Based on our experience so far, very few developers make use of the
>> dynamic capabilities of stereotypes.  That fact, along with the
>> potential confusion of having two annotations that are almost but
>> not exactly the same, makes it attractive to consider enhancing
>> the definition of stereotypes to indicate that when they're used
>> with Java EE annotations, the definition of the stereotype is static
>> at deployment time.  Of course, this also requires a tighter
>> integration of CDI with the rest of the Java EE platform, which
>> seems to be the direction we're moving on several fronts.  (More
>> on that later.)
>> 
>> Using stereotypes for this purpose would only work when CDI is
>> enabled.  Separately, the CDI expert group is considering whether we
>> should change the default and enable CDI by default.  Doing so would
>> make this approach more attractive, although it may also introduce
>> additional performance issues that would need to be addressed.
>> 
>> Allowing the use of stereotypes for this purpose requires changing
>> many existing annotations to include ANNOTATION_TYPE as a @Target.
>> 
>> Many existing implementations would need to be changed to understand
>> how to expand stereotypes.  Requiring every technology to do this
>> itself will almost certainly lead to inconsistencies.  Since stereotypes
>> are a CDI feature, CDI will provide a simple replacement for the
>> java.lang.reflect methods such as getAnnotations that takes into
>> account stereotypes.
>> 
>> Some technologies will not want to have a hard dependency on CDI so
>> we'll need to provide a simple way for them to conditionally invoke
>> these new methods only if CDI is present, falling back to java.lang.reflect
>> if not.  This seems straightforward.  In this case, the functionality of
>> @Stereotype would not be available to applications that chose to run
>> without CDI.
>> 
>> What do you think of the above approach?  Is expanding the use of
>> @Stereotype the best approach?  Or are the issues with that approach
>> significant enough that we should consider introducing a new annotation
>> for this purpose?
>> 
>