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?
>