Automatically reload log4j configuration in tomcat

I was looking for a way to have Log4j reload its configuration file automatically when it changes, in Tomcat. Log4j can do this with the configureAndWatch method, but the default initialization procedure (simply putting a log4j.properties file in the classpath) doesn't use configureAndWatch. You have to write at least a little bit of code to get Log4j to do this. I found the easiest solution for integration with Tomcat to be to implement a Tomcat LifecycleListener.

import org.apache.catalina.Lifecycle;
import org.apache.catalina.LifecycleEvent;
import org.apache.catalina.LifecycleListener;
import org.apache.log4j.LogManager;
import org.apache.log4j.PropertyConfigurator;

public class Log4JInitializer implements LifecycleListener
{
    private String propertiesFile;

    public String getPropertiesFile()
    {
        return this.propertiesFile;
    }

    public void setPropertiesFile(String propertiesFile)
    {
        this.propertiesFile = propertiesFile;
    }

    @Override
    public void lifecycleEvent(LifecycleEvent event)
    {
        if (Lifecycle.BEFORE_START_EVENT.equals(event.getType()))
            initializeLog4j();
    }

    private void initializeLog4j()
    {
        // configure from file, and let log4j monitor the file for changes
        PropertyConfigurator.configureAndWatch(propertiesFile);

        // shutdown log4j (and its monitor thread) on shutdown
        Runtime.getRuntime().addShutdownHook(new Thread()
        {
            @Override
            public void run()
            {
                LogManager.shutdown();
            }
        });
    }
}

I simply listen for the "BEFORE_START_EVENT", and if that happens (which is once per Tomcat startup) I initialize Log4j using the configureAndWatch method. I also don't forget to install a shutdown hook to cleanup the thread Log4j creates to poll the configuration file for changes (I could also have chosen to listen to the "AFTER_STOP_EVENT" from Tomcat in stead).

Package this in a jar, put it on the Tomcat classpath, and now you can configure it in your Tomcat serverl.xml.

<Server>
  ...
  <Listener className="Log4JInitializer" propertiesFile="/path/to/log4j.properties"/>
</Server>

Can't be much easier, and it does what it has to do.