Evans,
On 28/08/2015 16:03, Evans Armitage wrote:
> 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_at_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.
I'm not familiar with that. Can you give a more detailed example of what you are suggesting?
The listener bean connects to the JMS provider and creates the JMS consumer during its @PostConstruct phase, using code
plugged in by the portable extension. What code would this use to discover what message selector to use and (say) save
it in a local variable?
> 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.
This is a problem even if the instance has not already been eagerly created. As I mentioned in my example below:
Inject Instance<MyDepScopeListenerBean> listenerProvider;
MyDepScopeJMSListener jmsListener1 = listenerProvider.get();
since the consumer is created during the bean's @PostConstruct stage then we need a way for the application to specify
the message selector etc before CDI actually creates the bean. Once the bean has been created it's too late. CDI allows
qualifiers to be specified before calling get(), but these annotations (@MessageSelector etc) are not qualifiers (i.e.
they're not used to determined which bean class to use).
Nigel
> Kind regards
> Evans Armitage
>
> On Fri, Aug 28, 2015 at 12:41 PM, Nigel Deakin <nigel.deakin_at_oracle.com <mailto: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 <mailto: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
>>
>
>