users@javaee-spec.java.net

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

From: Iida, Minehiko <iida.minehiko_at_jp.fujitsu.com>
Date: Tue, 9 Oct 2012 07:02:58 +0000

Hi all,

Adding new annotation @Metatype is difficult to accept.

Annotation duplication with different names may confuse Java EE developers.
If @Stereotype is lack of features Java EE specs require,
we should add the features to @Stereotype.

Java EE application developers generally use @Stereotype by default.
And if necessary, other existing annotations can be used.

Minehiko

> -----Original Message-----
> From: Bill Shannon [mailto:bill.shannon_at_oracle.com]
> Sent: Friday, September 21, 2012 3:56 PM
> To: jsr342-experts_at_javaee-spec.java.net
> Subject: [jsr342-experts] Re: allowing stereotypes to be used more widely
>
> 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?
> >