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From "Shapira, Yoav" <Yoav.Shap...@mpi.com>
Subject RE: commons-logging & classloading
Date Fri, 10 Oct 2003 17:01:42 GMT

Howdy,
I'm sure other developers like myself have been following your thread.
Personally, I'm tried of commons-logging classloading issues and don't
use it unless required.  I use log4j directly and am happy with my
experience.

Search the archives of this, the log4j-user, and the tomcat-user mailing
lists, as many people have run into problem using commons-logging in
webapps, and several creative solutions have been offered, including by
the authors of commons-logging.

Yoav Shapira
Millennium ChemInformatics


>-----Original Message-----
>From: Will Jaynes [mailto:jaynes@umich.edu]
>Sent: Friday, October 10, 2003 1:00 PM
>To: Jakarta Commons Developers List
>Subject: Re: commons-logging & classloading
>
>I was hoping some developers would weigh in on this issue. I expect
that
>this has been discussed before, but I can't find any references.
>
>I believe the classloading in commons-logging is broken. The web app
use
>case I describe below is a demonstration of why c-l is broken. The way
>classloading is implemented makes it impossible to to share components
>at the J2EE server level. It's clear from looking at the
>LogFactory.getContextClassLoader() method that the developers
explicitly
>want to use the thread context classloader. But it just causes
problems.
>
>I went in and changed LogFactory.getContextClassLoader() to simply
>return LogFactory.class.getClassLoader(), and all my logging problems
>went away. I can put slide, HttpClient, commons-logging at the server
>level; and struts, commons-logging, log4j in the web apps. And things
>work fine and just as I expect them to.
>
>So, can someone please explain why commons-logging is implemented as it
>is? Will anyone consider changing it?
>
>Thanks, Will
>
>Will Jaynes wrote:
>
>>  >> Why does LogFactoryImpl in commons-logging
>>  >> try to load the Log implementation class first
>>  >> from thread classloader and then loader that loaded this class?
>>  >>
>>  >> Is there some kind of design pattern behind this?
>>  >>
>>  >
>>  >One very common :-) use case for commons-logging is inside web
>>  >applications, where the servlet container provides a class loader
per
>>  >webapp (pointing at the classes in /WEB-INF/classes and
/WEB-INF/lib),
>>  >plus normally a parent class loader for shared classes and
resources.
>>  >The container is required to set the Thead context class loader for
the
>>  >current webapp prior to handing the request off to the servlet.
>>  >
>>  >The lookup design pattern in LogFactoryImpl allows webapps to use
their
>>  >own version of the log implementation classes.
>>
>> With regard to this web app use case, the problem I'm seeing is that
I
>> can't share components at the server level if they use
commons-logging.
>> What must the configuration of jars and property files look like if I
>> have components that use commons-loggin both at the server and web
app
>> levels?
>>
>> So far, nothing works properly unless all components are at the web
app
>> level (in WEB-INF/lib).
>>
>>
>> Here's an example of what can go wrong:
>> I have slide and HttpClient in at the server level in resin/lib.
>> HttpClient uses commons-logging, so I have to add commons-logging to
>> resin/lib. My web app uses Struts, so I've got commons-logging in
>> WEB-INF/lib, and I use log4j, so log4j.jar is also in WEB-INF/lib.
(by
>> the way, I'm using Java 1.4)
>>
>> As soon as my web app trys to use HttpClient I get a exception :
"Class
>> org.apache.commons.logging.impl.Log4JLogger does not implement Log".
I
>> believe that what is happening is this: HttpClient loads with the
server
>> classloader. HttpClient wants to log, so it causes Log and LogFactory
to
>> be loaded with the server classloader. LogFactory specifically uses
the
>> thread context classloader to look a log factory. The thread context
>> classloader is the web app's classloader, so it finds LogFactoryImpl
and
>> log4j and then loads Log4JLogger, but it is still using the thread
>> context classloader, so it finds the Log4JLogger in the WEB-INF/lib.
It
>> then does a check with Log.class.isAssignableFrom() on Log4JLogger,
but
>> since Log and Log4JLogger were loaded with different classloaders the
>> test fails and the exception is thrown.
>>
>> After a lot of experimentation, the only configuration of jars that
>> works properly is to put everything in the WEB-INF/lib of each web
app.
>>    Commons-loggin has made it impossible to deploy slide and
HttpClient
>> at the server level.
>>
>> Am I missing something in how to configure this use case?
>>
>> Will
>>
>>
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>>
>
>
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