[logback-dev] App Servers and Thread.contextClassLoader

[Ceki Gulcu]

Robert Elliot wrote:
> Hi Ceki,
> 
> 
> On 30 Jul 2009, at 14:45, Ceki Gulcu wrote:
> 
>>
>> my recommendation would be to ignore the
>> logging separation issue by delegating it to SLF4J.
>>
> 
> I intend to - the problem I'm trying to solve is that the webapp which 
> replaces System.out will replace it with an instance of a class loaded 
> by that webapp's classloader, which will load SLF4J and whatever binding 
> it is loading.  So which ever web app is the last to replace System.out 
> will get all System.out calls in all webapps (and indeed in the app 
> server) redirected to the logging system specific to that webapp.  Which 
> isn't what I want.  I've got a theory about it, hopefully I can get 
> something working soon and show it, as real code is a better basis for 
> discussion.

Real code is indeed a good basis for discussion. I see several possible 
techniques which may or may not help you in your design.

T1) Once System.out is redirected from a web-app, you will be redirecting 
System.out for all web-apps, which may be deemed somewhat pushy. However, as 
long as you revert to the original System.out when your web-app is stopped, it 
should be OK/acceptable.

Note that application servers, just hijack System.out for all hosted applications.

T2) If logback is the underlying logging system, without any change in code (in 
your System.out redirection implementation and assuming you use SLF4J as the 
logging API) logback could redirect System.out on a per web-application basis. 
See the Chapter 9 of the logback manual on "Context Selectors": 
http://logback.qos.ch/manual/loggingSeparation.html  .

T3) After redirection, the original System.out may continue to be fed
with data. (Nothing says that the data flow to the original System.out
must be dried out.)

>> As for your goal, most application servers have their own way of
>> capturing and redirecting the console. So you may be facing a big
>> hurdle in order to get an app server to adopt your library.
> 
> Oh, I'm not aiming that high!  Just to allow the writer of a webapp to 
> configure where System.out goes without it screwing up every other 
> webapp deployed on the app server.

I understand. One way to achieve this is after hijacxking System.out,
to redirect the console to SLF4J within your web-app and to redirect
to the original System.out outside your web-app. It's a variant of
technique number 3 (T3) mentioned above.  You would need to put things back
to their original state when your application is stopped (T1).

>>
>> Anyway, is it correct to assume that you wish to tackle bug 110 [1]
>> outside SLF4J?
> 
> No, I'd still like to donate it  if you want it - just life has got in 
> the way, I've embarked on a fairly major refactor and encountered issues 
> like the one described above and a classloader memory leak that would 
> mean anyone using the library and redeploying the webapp dynamically 
> would have got PermGen issues.  I've got another idea about the 
> exception handling issue, too.  Hopefully have something out soon.

I am very happy to see you working on this topic and would like to see
the results donated to the SLF4J project, subject of course to your
agreement.

> Rob
-- 
Ceki Gülcü
Logback: The reliable, generic, fast and flexible logging framework for Java.
http://logback.qos.ch