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Subject: RE: using pax-logger/log4j in embedded felix (class loader issue)
Date: Mon, 7 Jul 2008 17:51:44 -0400
Message-ID: <9E8C2EE38A83804B8A7CD8F5C5D6BA91159688@exch01.praxislan01.com>
Thread-Topic: using pax-logger/log4j in embedded felix (class loader issue)
Thread-Index: Acjgb/cJPYrCvvvDTrydzU+Fa5jo0wACpoyQ
References: 
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From: "Craig Phillips" <lcphillips@praxiseng.com>
To: <dev@felix.apache.org>

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Hi,
=20
- I can't use Equinox because I discovered a Null Pointer Exception when =
attempting to restart it (in the same "context" loosely speaking -- =
embedded); Also, equinox as embedded is not really "instance" based out =
of the box; I logged a bug on the NPE and it's probably fixed by now, =
but it's not officially released as a non-beta and that presents issues =
for me that I plan to avoid;
- Since I wrote the small layer that creates the Felix instance, I have =
no problem tweaking that code to set the TCL to null (seems easier to do =
that than replicating a context finder);
=20
Dumb question - what is a TCL and how would I go about setting it to =
null (I could also ask the same of the context finder approach but =
setting something to null just sounds way easier) [I have written a =
class loader / jar loader or two or three in my time, so I'm not totally =
ignorant... just lazy if I can get away with it]...
=20
Thanks, Craig

________________________________

From: hedhman@gmail.com on behalf of Niclas Hedhman
Sent: Mon 7/7/2008 4:27 PM
To: dev@felix.apache.org
Subject: Re: using pax-logger/log4j in embedded felix (class loader =
issue)


On Tue, Jul 8, 2008 at 3:26 AM, Niclas Hedhman <niclas@hedhman.org> =
wrote:
> On Tue, Jul 8, 2008 at 1:31 AM, Karl Pauls <karlpauls@gmail.com> =
wrote:
>> Is it somehow possible that log4j is
>> doing some classloader magic itself and maybe is using the context
>> classloader? The context classloader would be the WebAppClassloader I
>> think ...
>
> Good Suggestion. Will investigate that initially.

Ok. Here is the funky bits. Me and Alin Dreghiciu have both been
looking at different problems today(!) turning out to be the same
thing.

Craig, if you took the time to try out Equinox, you would probably
find that it might work in Equinox (the Spring case does), but not in
Felix and not in Knopflerfish... Why?

Log4J (as Spring in Alin's case) try to use the thread context
classloader, if one is present, otherwise use the classloader of
"itself". Hence we are seeing the Webapp classloader being used in
Log4J's loading of resources, resulting in terrible mismatches.
So why does it work in Equinox. Equinox sets the thread context
classloader to something they call ContextFinder, which will look at
the call stack, and take the first classloader found other than system
CL, which would be the classloader of Pax Logging Service in your
case.

So, depending on how you are setting this up, you need to do
"something", like either replicate the ContextFinder approach, or
plainly set the TCL to null prior to loading up Felix.

I would be interested to hear if that helps. Also Many Thanks to Karl
who provided a 'short-cut', as the TCL was not the first thing on my
mind.


Cheers
Niclas


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