I understand your concern, though it is a question of what is more
confusing...
Experts,
Before we decide on *how* to specify a single MDB instance, do you think
it *is* useful to be able to have one?
thanks,
-marina
Antonio Goncalves wrote:
> Like Carlo I was thinking of reusing @Singleton. The EE platform
> already has 3 (javax.ejb.Singleton & javax.inject.Singleton &
> @ApplicationScoped) which already confuses people. I understand the
> technical issues that you express Marina, I'm just a bit disapointed
> to think that we have another way to have singletons.
>
> Antonio
>
> On Wed, Sep 19, 2012 at 1:05 AM, Marina Vatkina
> <marina.vatkina_at_oracle.com <mailto:marina.vatkina_at_oracle.com>> wrote:
>
>
> Carlo de Wolf wrote:
>
> Thinking out loud I would find it counter-intuitive, rather I
> would think @Singleton @MessageDriven. Taking it a bit farther
> @MessageDriven can actually be construed to be a view instead
> of a component type. The component type would, by default, be
> like a stateless session bean.
>
>
> Let's not go too far from the current MDB setup ;) As we discussed
> it before, any major changes to MDBs would require substantial
> changes in the JCA spec, which is not possible in the MR release
> (i.e. without a JCA EG).
>
>
> Even without going over that hurdle, the question also raises
> thinking about locking. Sooner or later somebody is going to
> make that leap.
>
>
> Right. This is why I was suggesting an attribute on a
> @MessageDriven. It can have a simpler requirements and no
> confusion with a singleton *session* bean (that a @Singleton defines).
>
> Theoretically speaking, even now (see the example in David's
> proposal), a connector impl gets a hold of the proxy and use it
> however it sees it fit (in David's example it uses it as a
> singleton), and cause a deadlock or any non-thread-safe behavior.
> So if the single-instance MDB is defined as only a responsibility
> of the MDB container not to create a new instance and serialize
> access to it, it will solve the ordering problem without bringing
> in additional features.
>
> Does it make sense?
>
> thanks,
> -marina
>
>
> Carlo
>
> On 09/15/2012 02:18 AM, Marina Vatkina wrote:
>
> Experts,
>
> While you are thinking about David's proposal (as I hope
> you all do ;) ), here is another question: do we need to
> provide a standard way to guarantee a single MDB instance
> running in a server instance? I do not propose support for
> all other features available to the singleton session
> beans, just a guarantee of a single MDB instance.
>
> In GlassFish (RI) we had this request to support message
> ordering processed by the MDB.
>
> If you think that it is useful, we can add an attribute
> (e.g. 'isSingleton') to the @MessageDriven annotation that
> would default to false for backward compatibility.
>
> thanks,
> -marina
>
>
>
>
>
>
> --
> Antonio Goncalves
> Software architect and Java Champion
>
> Web site <http://www.antoniogoncalves.org> | Twitter
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