http://www.javatastic.com/2007/08/22/initialize-log4j-in-a-web-application/

gistfile1.java

package some.package;

import java.io.File;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import org.apache.log4j.PropertyConfigurator;
import org.apache.log4j.LogManager;
import org.apache.log4j.xml.DOMConfigurator;
/**
 * ServletContextListener to initialize Logging for the application
 */

public class Log4jConfigListener implements ServletContextListener
{
    private static long DEFAULT_INTERVAL = 300000;
    public void contextDestroyed(ServletContextEvent event)
    {
        //Turn off logging when the Servlet Context is destroyed
    	LogManager.shutdown();
    }

public void contextInitialized(ServletContextEvent event)
    {
        //Read configuration properties from web.xml
        String file = event.getServletContext().getInitParameter("LOG4J_CONFIG_LOCATION");
        String interval = event.getServletContext().getInitParameter("LOG4J_REFRESH_INTERVAL");
        long delay = DEFAULT_INTERVAL; //Default delay is 5 minutes

//Check for interval parameter
        if (interval != null && !"".equals(interval))
        {
            try
            {
                delay = Long.parseLong(interval);
            }
            catch(NumberFormatException e)
            {
                //Can't really log the error since we haven't initialized Log4J
                //Will use the default value
                delay = DEFAULT_INTERVAL;
            }
        }

//Check for file parameter
        if(file != null && !"".equals(file))
        {
            if(!(new File(file).exists()))
            {
                throw new IllegalArgumentException("Invalid 'LOG4J_CONFIG_LOCATION' parameter value '"
                                                                 + file + "'.");
            }
if(file.toLowerCase().endsWith(".xml"))
            {
                DOMConfigurator.configureAndWatch(file, delay);
            }
            else
            {
                PropertyConfigurator.configureAndWatch(file, delay);
            }
        }
    }
}

gistfile2.xml

<!-- Log4j Initialization Parameters -->
<context-param>
  <!-- absolute path to log4j config file (*.properties or *.xml) -->
<param-name>LOG4J_CONFIG_LOCATION</param-name>
<param-value>
  ABSOLUTE_PATH_TO_YOUR_LOG4J_CONFIGURATION_FILE
  </param-value>
</context-param>
<context-param>
  <!-- number of milliseconds to wait before checking LOG4J_CONFIG_LOCATION for updates -->
  <param-name>LOG4J_REFRESH_INTERVAL</param-name>
  <param-value>(e.g. 300000)</param-value>
</context-param>

<listener>
  <listener-class>
    some.package.Log4jConfigListener
  </listener-class>
</listener>