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https://issues.apache.org/jira/browse/CONFIGURATION-596?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Vladimir Sitnikov updated CONFIGURATION-596:
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Attachment: CoWArrayList1.jpg
dataflow_commons_configuration_cowarraylist.png
{quote}Do you have concrete numbers that these objects acutally cause a
measurable overhead?{quote}
That's a good question. Those CowLists alone are 1-2% of all the allocations --
see JFR report attached.
Note, that each CowList always creates ReentrantLock and that ReentrantLock
creates Sync inside.
So the overall impact is higher (it is not included .
{quote}We could create them with a low initial capacity (as most applications
are not expected to register many listeners), but this will only save a few
bytes.{quote}
I do not think you can. There is no such thing as "initial capacity" for copy
on write lists :)
{quote} null checks would have to be added, and you would need a means of
synchronization to ensure thread-safe access{quote}
I believe null checks would not have overhead since the runtime anyway performs
the check (to throw NPE if the value is null).
{quote} you would need a means of synchronization to ensure thread-safe
access{quote}
Sure. However COWList is properly synchronized, so we need just enable
properly-synchronized instantiation.
Either double-checked locking (volatile COWList + synchronize on EventSource)
or volatile COWList + AtomicReferenceFieldUpdater would do.
{quote}In addition, some concrete configuration implementations register a
special error listener which just logs occurring exceptions; in such cases,
nothing is gained{quote}
Do you mean there are cases when at least a single listener is added?
I'll check my heap dumps for the cases of initialized vs empty lists.
> Allow event listener Collections to be lazily created in event.EventSource
> --------------------------------------------------------------------------
>
> Key: CONFIGURATION-596
> URL: https://issues.apache.org/jira/browse/CONFIGURATION-596
> Project: Commons Configuration
> Issue Type: Improvement
> Components: Events & Notifications
> Affects Versions: 1.9
> Reporter: Vladimir Sitnikov
> Attachments: CoWArrayList1.jpg,
> dataflow_commons_configuration_cowarraylist.png
>
>
> It turns out every instance of {{AbstractConfiguration}} creates a couple of
> {{CopyOnWriteAraryLists}} for {{event.EventSource}}.
> In our usage patterns in most of the cases the lists are not used.
> Can the lists be lazy-initialized? (that is created on the first addListener
> request)
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