Static references in OSGi

[Atle Prange <at...@gmail.com>]

Hi,

i have a question about static references and how they are treated in OSGi.

Will singletons stay in memory after a bundle has been unloaded and
leak memory, or can i safely create huge amounts of static references?
And what about ThreadLocals? Can I use them safely as well? I see some
projecs use the "bundle activator singleton" pattern to access
resources throughout the code, but a tiny voice behind my ear tells me
that it might not be a good idea, is that voice right or wrong?

Thank you

-atle

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Re: Static references in OSGi

[David Savage <da...@paremus.com>]

On Fri, Jul 2, 2010 at 2:34 PM, Richard S. Hall <he...@ungoverned.org> wrote:
> On 7/2/10 9:06, Richard S. Hall wrote:
>>
>> On 7/2/10 2:20, Atle Prange wrote:
>>>
>>> Hi,
>>>
>>> i have a question about static references and how they are treated in
>>> OSGi.
>>>
>>> Will singletons stay in memory after a bundle has been unloaded and
>>> leak memory, or can i safely create huge amounts of static references?
>>> And what about ThreadLocals? Can I use them safely as well? I see some
>>> projecs use the "bundle activator singleton" pattern to access
>>> resources throughout the code, but a tiny voice behind my ear tells me
>>> that it might not be a good idea, is that voice right or wrong?
>>
>> As long as a bundle cleans up its threads and other bundles do not hold
>> onto stale references (i.e., objects from bundles that have been
>> uninstalled), then there shouldn't be any issues. The framework doesn't hold
>> onto bundle objects or class loaders after they are uninstalled (and
>> refreshed).
>
> Of course, I should have added, I don't recommend using statics, since they
> are just another form of global variables.

Completely agree with Richard, if you can avoid statics do. But think
it might be worth mentioning a minor addendum that bundle private
statics are "less awful" than bundle exported statics.

With private statics (i.e. objects that are referenced in private
packages or accessible only by private or package private accessors)
you are generally[1] in control and can work to ensure that when your
bundle is stopped/uninstalled then there are no more references to
those objects.

But with statics that are exported outside of the bundle the only way
to ensure these get cleared up is to have all code that is grabbing
hold of those static references to also listen for bundle lifecycle
events to know when to null references - which is a maintenance
nightmare waiting to happen...

2 cents anyway...

Regards,

Dave

[1] Generally - be wary of reflection that allows others to access
private vars...but definitely that's an edge case problem...

>
> -> richard
>
>>
>> -> richard
>>
>>> Thank you
>>>
>>> -atle
>>>
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>>>
>>
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Re: Static references in OSGi

["Richard S. Hall" <he...@ungoverned.org>]

On 7/2/10 9:06, Richard S. Hall wrote:
> On 7/2/10 2:20, Atle Prange wrote:
>> Hi,
>>
>> i have a question about static references and how they are treated in 
>> OSGi.
>>
>> Will singletons stay in memory after a bundle has been unloaded and
>> leak memory, or can i safely create huge amounts of static references?
>> And what about ThreadLocals? Can I use them safely as well? I see some
>> projecs use the "bundle activator singleton" pattern to access
>> resources throughout the code, but a tiny voice behind my ear tells me
>> that it might not be a good idea, is that voice right or wrong?
>
> As long as a bundle cleans up its threads and other bundles do not 
> hold onto stale references (i.e., objects from bundles that have been 
> uninstalled), then there shouldn't be any issues. The framework 
> doesn't hold onto bundle objects or class loaders after they are 
> uninstalled (and refreshed).

Of course, I should have added, I don't recommend using statics, since 
they are just another form of global variables.

-> richard

>
> -> richard
>
>> Thank you
>>
>> -atle
>>
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>
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Re: Static references in OSGi

["Richard S. Hall" <he...@ungoverned.org>]

On 7/2/10 2:20, Atle Prange wrote:
> Hi,
>
> i have a question about static references and how they are treated in OSGi.
>
> Will singletons stay in memory after a bundle has been unloaded and
> leak memory, or can i safely create huge amounts of static references?
> And what about ThreadLocals? Can I use them safely as well? I see some
> projecs use the "bundle activator singleton" pattern to access
> resources throughout the code, but a tiny voice behind my ear tells me
> that it might not be a good idea, is that voice right or wrong?
>    

As long as a bundle cleans up its threads and other bundles do not hold 
onto stale references (i.e., objects from bundles that have been 
uninstalled), then there shouldn't be any issues. The framework doesn't 
hold onto bundle objects or class loaders after they are uninstalled 
(and refreshed).

-> richard

> Thank you
>
> -atle
>
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> To unsubscribe, e-mail: users-unsubscribe@felix.apache.org
> For additional commands, e-mail: users-help@felix.apache.org
>
>    

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