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From BJ Hargrave <hargr...@us.ibm.com>
Subject Re: Confusion about concurrency in ServiceListener/ServiceTracker callbacks
Date Fri, 16 Feb 2007 18:37:37 GMT
This would be best asked in the osgi-dev list since it is not a felix 
specific issue.

But ServiceTracker is designed to be thread safe (I wrote most of the code 
and maintain it). Therefore STC implementations need to be thead safe as 
well.

> Section 6.1.21 of the OSGi R4 core specification reads:
> 
> ,----
> | When a ServiceEvent is fired, it is synchronously delivered to a
> | BundleListener.
> `----
> 

Wow. Change history for that file makes it my mistake... I have fixed it 
for 4.1. Thanks.

What the sentence is saying is that ServiceEvents are delivered 
synchronously from the action that caused the event (e.g. register 
service, etc.) and the action is blocked from completion until all 
interested service listeners have been called and returned. A service 
listener can be called by multiple threads delivering service events and 
can be reentered by the same thread if the listener takes some action 
which fires a service event.

Net: STC implementation must be thread safe.

BJ Hargrave
Senior Technical Staff Member, IBM
OSGi Fellow and CTO of the OSGi Alliance
hargrave@us.ibm.com

office: +1 386 848 1781
mobile: +1 386 848 3788




"Steven E. Harris" <seh@panix.com> 
02/16/2007 01:14 PM
Please respond to
felix-dev@incubator.apache.org


To
felix-dev@incubator.apache.org
cc

Subject
Confusion about concurrency in ServiceListener/ServiceTracker callbacks






I'm confused as to whether a ServiceListener can be called on from
multiple threads by the OSGi framework, say, if two service-related
events occurred "at the same" time by different threads registering
and unregistering services.

The question extends to ServiceTracker and its
ServiceTrackerCustomizer: Can the addingService(), modifiedService(),
and removedService() methods be called on concurrently, meaning that
their operation must be made thread-aware?

Section 6.1.21 of the OSGi R4 core specification reads:

,----
| When a ServiceEvent is fired, it is synchronously delivered to a
| BundleListener.
`----

Aside from it looking like "BundleListener" should probably be
"ServiceListener", the sentence sounds like it's saying that only one
ServiceEvent will be delivered to a ServiceListener at a time. Is that
true?

If so, can I also conclude that a ServiceTrackerCustomizer will only
be called on by one thread at a time? I ask because the
ServiceTrackerCustomizer implementation has some synchronized blocks
that are attempting to protect some of its data against concurrent
access, but the user-provided callback functions are called on outside
of those synchronized regions.

-- 
Steven E. Harris



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