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From "Juozas Baliuka" <bali...@centras.lt>
Subject Re: commons-logging & classloading
Date Fri, 10 Oct 2003 19:25:31 GMT


There are no good workarounds for classloading.
 "LogFactory.class.getClassLoader()" has meaning too, it will use the same
class loader for logging and "pluggin".
 "class.getClassLoader()"  must be the same as "Class.forName(name)" if you
call it  in static initializer, but I do not think it will work for all of
use cases too.
I think the best way is to use classloader as parameter or configuration
option, without workarounds.



> 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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