classes folder of your webapp.Sometimes you may need externalize Log4j configuration out of your application archive. As you know, it supports two kind of configuration file: *.properties and *.xml. Below is showed how to tell to Log4j which configuration file load.
Properties configuration
PropertyConfigurator.configure("log4j.properties");
XML configuration
DOMConfigurator.configure("log4j.xml");
References
http://www.javabeat.net/2008/07/baisc-steps-to-configure-log4j-using-xml-and-properties-file/