Hi Robert, Robert Elliot wrote:
Slightly odd question, but it is logback related. I'm writing a library to redirect System.out and System.err to SLF4J. I'm envisaging this being used in application servers. I'd like it to work "correctly" with SLF4J implementations like Logback, so if the app server has child first class loading and SLF4J and logback are in webapp A's lib dir, a call to System.out.println within webapp A will be handled as per the logback.xml in webapp A's classes folder, regardless of other webapps with different config.
A per class-loader approach is similar to that of commons-logging. As you know, SLF4J has a different philosophy whereby it will ignore the value returned by Thread.currentThread().getContextClassLoader() and only use the class loader which loaded SLF4J itself (i.e. the class loader which loaded the SLF4J classes). The general problem is called the logging separation problem which is deemed to be exceedingly difficult to solve correctly for all environments. As such, my recommendation would be to ignore the logging separation issue by delegating it to SLF4J.
The only way that is possible (as far as I can see) is to work out from the thread calling the System.out methods which web app it was that made the call - and I've found the method getContextClassLoader on Thread which looks hopeful! However, I'm not completely confident that app servers that implement child first class loading will set that to the a web app specific class loader.
Yes, as far as I know, the app server will set the web app specific class loader for the current thread. If you generalize the context, for example to include build systems (Ant, Maven, Ivy, etc), then things get a little muddier. For example, I believe that earlier versions of Ant did not set the thread context class loader. 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. Anyway, is it correct to assume that you wish to tackle bug 110 [1] outside SLF4J? [1] http://bugzilla.slf4j.org/show_bug.cgi?id=110 -- Ceki Gülcü Logback: The reliable, generic, fast and flexible logging framework for Java. http://logback.qos.ch