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From "Justin Edelson (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SLING-6852) A question about reliability of keeping non DS tracked references
Date Fri, 12 May 2017 10:53:04 GMT

    [ https://issues.apache.org/jira/browse/SLING-6852?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16007947#comment-16007947
] 

Justin Edelson commented on SLING-6852:
---------------------------------------

it would be best to ask generic OSGi questions on the osgi-dev mailing list.

In this specific example, I'm not sure what you mean by "listener services". ModelAdapterFactory
creates a BundleListener *object* whose lifecycle it controls. In turn, the BundleListener
registers a number of services, but again it controls their lifecycle. So you could say that
ModelAdapterFactory is responsible for the lifecycle of those services, but there's no "listener
services" per se.

I should note that while this pattern is relatively common in Sling, it isn't something recommended
for "normal" users of OSGi. Sling is a provider of infrastructure and so ends up using more
low-level features than normal applications.

> A question about reliability of keeping non DS tracked references
> -----------------------------------------------------------------
>
>                 Key: SLING-6852
>                 URL: https://issues.apache.org/jira/browse/SLING-6852
>             Project: Sling
>          Issue Type: Task
>          Components: Best practices
>    Affects Versions: Sling Models Impl 1.2.0
>            Reporter: Dominik Smogór
>            Priority: Minor
>
> It's a generic best practice question but illustrated on an example within sling models.
> We have 2 kind osgi services being in parent - child 
> 1. a factory service that programmatically registers number listener services 
> 2. the listener services.
> The listener services are called asynchronously (usually in a white board manner) by
some 3rd party components. 
> Now, the listener services implementations are keeping a simple object references to
the parent service. Osgi is not aware of this link and is not tracking it in any way.
> An example of such a relation is here http://grepcode.com/file/repo1.maven.org/maven2/org.apache.sling/org.apache.sling.models.impl/1.2.0/org/apache/sling/models/impl/ModelAdapterFactory.java#900:
> with the ModelAdapterFactory creating instances of  ModelPackageBundleListener which
self register themselves with BundleTracker.
> No the question is: what guarantee do we have that the child services will never use
defunct instances of parent objects after they have been deactivated by OSGI (for whatever
reason)?
> Isn't it a potential for race conditions (that partner is used frequently throughout
sling code base) during after related to bundle / component restarts?
> (disclaimer: I've used the highest sling models version that's there on grepcode)



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