Hello,
Seems I have been chewing on the service dynamics issue forever. Just as I
though I got a workable concept about tracking and releasing services I stumble
on a contradicting concept. The problem is this:
According to my understanding it is not acceptable importer behavior to ever
call service objects that are unregistered. Achieving this requires the service
to be tracked and every piece of code that calls the service be synchronized
with the tracking code. I'll name this the "dedicated lock approach":
private Object lock;
private Hello service;
void set(Hello service) {
synchronized (lock) {
this.service = service;
}
}
Hello get() {
synchronized (lock) {
if (service == null) {
throw new ServiceUnavailableException();
}
return service;
}
}
void usefulMethod() {
synchronized (lock) {
get().greet("World");
}
}
Some say we should never call out from the bundle while holding a lock but I
think we are safe if we use a dedicated private lock for every tracked service.
In any case I can't think of any other way to be safe at all times. Lately
however I have been encountering the "local-cache" approach, which seems to
state we don't need to be that paranoid. E.g.
void usefulMethod() {
Hello service = get();
/* At this spot right here the service can go down! */
service.greet("World");
}
Here we risk calling a service in an undetermined state. Do we expect every
exporter to invalidate his objects to keep us safe? E.g. the exporter must keep
a "closed" flag around, mark the service as invalid in a thread-safe manner, and
start tossing exceptions at callers from that point on.
iPojo follows the local cache approach - right Richard?
Peaberry follows the dedicated lock approach - right Stuart?
I'd be grateful if you help me compare these import modes. That is if I got it
right who uses what :)
Cheers,
Todor
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