[cdi-dev] JMS 2.1: Proposal to allow any CDI managed bean in a Java EE application to listen for JMS messages

[John D. Ament]

On Wed, Aug 26, 2015 at 6:53 AM Nigel Deakin <nigel.deakin at oracle.com>
wrote:

> On 26/08/2015 10:51, arjan tijms wrote:
> > On Wed, Aug 26, 2015 at 11:09 AM, Romain Manni-Bucau
> > <rmannibucau at gmail.com> wrote:
> >> Agree for provided scope but JMS + short time scopes will not match
> well in
> >> practise so i would worry more about not "default" scopes which can miss
> >> these events.
> >
> > Short lived scopes like @RequestScoped may not be the best match indeed.
> >
> > Additionally, @RequestScoped is kinda assumed to be an "@ThreadScoped"
> > thing, e.g. there's the expectation that only the current thread will
> > access it. If the JMS provider will asynchronously call a method on
> > the bean instance from another thread, then this breaks this
> > assumption.
>
> That's an interesting point. Is there anything in the CDI spec which would
> forbid the use of a @RequestScoped JMS
> listener bean from another thread?
>

The CDI spec still mandates that a request scope is started for delivery to
MDBs.  See http://docs.jboss.org/cdi/spec/1.2/cdi-spec.html#request_context

The JMS provider would be responsible for retrieving a contextual reference
to the bean and then invoking the method.  The container should still be
responsible for starting the request context (If I read this correctly).

Likewise, we could say that a @TransactionScoped bean also applies here,
since a JTA transaction should have been started by the RA.


>
> Perhaps all that is needed is to make it clear to users that this is what
> would happen. The same issue would arise if
> the user injected a dependent-scoped JMS listener bean into a
> @RequestScoped bean.
>
> It's worth considering threading more generally. Although the JMS provider
> (resource adapter, actually) can make sure
> that the callback method won't be called from multiple JMS provider
> threads at the same time, it can't guarantee that an
> application thread won't be calling a business method on the bean at the
> same time. (Would it want to?)
>
> Nigel
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