Hi Nigel,
For the problem of how to specify the message selector when there are
multiple callback methods, an approach could be using the unified
expression language @MessageSelector(method="#{someBean.method}") which
allows selectors to be reused in other listeners. The evaluation of the EL
expression would be delegated to the CDI container.
Making the injecting application decide the selector might not be feasible
because depending on the scope e.g @ApplicationScoped and @SessionScoped
the listener instance might already have been created and receiving
messages before it is injected at any injection point. This is assuming
that @Eager like behavior gets implemented as default.
Kind regards
Evans Armitage
On Fri, Aug 28, 2015 at 12:41 PM, Nigel Deakin <nigel.deakin_at_oracle.com>
wrote:
> Evans,
>
> Agreed. The current proposals is that the listener bean class specifies
> the destination, connection factory, destination type, acknowledge mode,
> subscription durability, clientId, subscription name and message selector
> using annotations, which means these are set at compile time.
>
> It would be desirable to allow these to be specified at runtime, for each
> listener bean instance separately.
>
> I like your suggestion of allowing the listener bean itself to have
> callbacks which return these values.
>
> @SessionScoped
> public class MyCDIBean21 {
>
> @JMSListener(lookup="java:global/java:global/Trades",type=JMSListener.Type.TOPIC )
> @JMSConnectionFactory("java:global/MyCF")
> @MessageSelector("ticker='ORCL'")
> public void processNewsItem(String newsItem) {
> ...
> }
>
> @GetMessageSelector
> public void returnMessageSelector(){
> // some logic to work out message selector
> return ...
> }
>
> This would allow the bean initialisation code to find out what message
> selector to use. However what only works if there is a single callback
> method. The current proposals suggest that a bean can have multiple
> callbacks, each listening to different destinations or using different
> message selectors. To handle that we'd need something like:
>
> @GetMessageSelector
> public void returnMessageSelector(Method m){
> // some logic to work out message selector for the specified method
> return ...
>
> It would be simpler overall if we allowed these beans to define just a
> single callback method. (The same issue applies to the new-style MDBs).
>
> There's a second, and more important, issue with these callback methods.
> Although it allows the listen bean itself to decide (say) the message
> selector, it doesn't provide a way for the application which is injecting
> it to decide the message selector. I think that's probably a bigger
> requirement, but I'm not sure the best way to achieve that.
>
> CDI provides a way to programmatically obtain an instance of the listener
> bean. I describe this here:
>
> https://java.net/projects/jms-spec/pages/CDIBeansAsJMSListeners#JMS_listener_bean_with_dependent_scope_and_explicit_lifecycle_management
>
> Inject Instance<MyDepScopeListenerBean> listenerProvider;
> MyDepScopeJMSListener jmsListener1 = listenerProvider.get();
>
> However since the consumer is created during the bean's @postCreate stage
> then we need a way for the application to specify the message selector etc
> before we actually create the bean. CDI allows qualifiers to be specified
> before calling get(), but these annotations are not qualifiers. Ideas
> welcome.
>
> Nigel
>
>
>
> On 26/08/2015 12:36, Evans Armitage wrote:
>
> Hi Nigel,
> I was asking more around the definition of the destinations themselves.
> Currently those properties are static and thus do not allow the developer
> to decide at runtime which messages get delivered. In the @SessionScoped
> scenario it is likely that if a developer needs a listener to be
> @SessionScoped then they intend for the messages to be delivered to the
> current session user only. They would thus likely appreciate some
> customization on their activation config (especially for message selector)
> to allow them to specify the selector dynamically through a call back
> method to select messages for, say, the current user principal.
> It's just a thought triggered by your suggested addition. If the targets
> for delivery are now going to be dynamic would it also be feasible to make
> the message selector dynamic?
>
> Evans
> On 26 Aug 2015 12:34 PM, "Nigel Deakin" <nigel.deakin_at_oracle.com> wrote:
>
>> Evans,
>>
>> On 26/08/2015 05:44, Evans Armitage wrote:
>>
>>> I take it container discovery and validation still happens at startup
>>> even for these CDI listeners?
>>>
>>
>> My proposal was that "JMS listener beans" would be completely ordinary
>> CDI managed beans, created in the same way. The new portable extension
>> would simply extend their postCreate behaviour to create a JMS consumer,
>> and extend their preDestroy behaviour to close that consumer. I'm not
>> familiar with those specific events, but I'm not proposing anything to
>> interfere with them working as normal.
>>
>> Is there anything that can be added to allow an @SessionScoped listener
>>> that only delivers messages for the current
>>> session? I think that will be a more common reason to use a
>>> @SessionScoped jms listener (i.e only deliver when user
>>> session exists AND only messages intended for the current user).
>>>
>>
>> A JMS listener bean could have any scope, including @SessionScoped. So
>> once it was injected and created by the application it would listen for
>> messages until the scope ends and the bean was destroyed.
>>
>> Over on the cdi-dev list people have suggested we provide some way of
>> automatically creating a listener bean whenever a new scope starts, so that
>> the application wouldn't have to inject it. If that were the case then an
>> instance of the listener would be automatically created whenever a new
>> SessionScope starts. (I'm still thinking about this).
>>
>> Would adding a message selector callback approach be too much work for
>>> container developers for too little gain?
>>>
>>>
>> I don't understand your question. Can you explain it a bit more?
>>
>> Nigel
>>
>>
>