While discussing some ongoing work, Pascal came up with a very interesting
idea: why won't we create a generic mechanism for "declarative"
registration of listeners?
The proposed event notification mechanism would be:
based on the "whiteboard" pattern [1], plus
a declarative way to register listeners, plus
ideas and implementation pieces from OSGi's EventAdmin [2].
To me, this sounds like a great idea. Listeners are ubiquitous and so are
the problems associated with their life cycle.
Details
In theory, OSGi's EventAdmin service should help work with listeners.
Implementations details aside (it is not in the SDK and needs to be tested
for scalability), it suffers from the same problem as OSGi services in
general: a listener's bundle has to be started and needs to register
listener with OSGi. This creates startup order problems and works against
lazy activation of bundles. For OSGi services add-on pieces that allow
declarative registration of services (Declarative Servieces, SPring, This
startup problem sounds similar to the OSGi services registration problem
for which we are starting to get "declarative" adds-ons such as DS and
Springs.
The solution to the listener mechanism proposed here is similar ? to have
a declarative way to describe listeners via XML or manifest file.
In this model listeners are likely would have to be singletons ("simple"
approach), or we could support user-provided factories to create listeners
("advanced" approach). We could add a flag to the declaration of the
listener to indicate if it should receive events in case its bundle has
not been activated yet. Most likely we'll need to support both synchronous
and asynchronous events, just like EventAdmin does.
To me, the declarative event processing mechanism truly sounds like a
component that would be used by a number of projects. What do you think
about this idea?
Sincerely,
Oleg Besedin
[1] The description of the "whiteboard" pattern can be found at
http://www.osgi.org/documents/osgi_technology/whiteboard.pdf .
[2] The OSGi specification can be downloaded from http://www.osgi.org/ ;
EventAdmin?s Javadoc can be found at
http://www2.osgi.org/javadoc/r4/org/osgi/service/event/EventAdmin.html .
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