I have seen that on the web there are few guide on how to extend Log4j.
Now i show to you how to create an extension with a timer function for Log4J.

If you have extended yet a class but have error on “repository nulla” like:
NullPointerException when extending Log4j Logger class

You must to see this extension and resolve that problem. Because you must to use the LogManager istance.

import org.apache.log4j.Logger;

public class LogHelper extends Logger {
    final int _whitespace = 20;
    protected Logger target;

    public LogHelper(Logger target) {
        super(target.getName());
        this.target = target;
    }

    public void timer(String msg, Long timerMillisecond) {
        _timer(msg, timerMillisecond);
    }

    public void timer(Long timerMillisecond) {
        _timer("", timerMillisecond);
    }

    private void _timer(String msg, Long timerMillisecond)
    {
        StringBuilder builder = new StringBuilder(_whitespace);
        String strLog = "";
        strLog = "TIMER: "+timerMillisecond+"ms";
        builder.append(strLog);

        int whiteSpace = _whitespace-builder.length();
        for(int i = 0; i <whiteSpace ; i++) {
            builder.append(' ');
        }
        strLog = builder.toString();

        if(msg != null &amp;&amp; msg != "")  strLog = strLog.concat(" || "+msg);

        if(timerMillisecond < 2000) {
            target.info(strLog);
        } else {
            target.warn(strLog);
        }
    }

    public void error(String message){
        if (target != null)
            target.error(message);
    }

    public void error(Object message, Throwable t) {
        if (target != null)
            target.error(message, t);
    }
}

Refer Logger

The important was to set the refer to a Logger

public LogHelper(Logger target) {
        super(target.getName());
        this.target = target;
    }

Wrap the log error to avoid the NullPointerException on Repository

public void error(String message){
    if (target != null)
    target.error(message);
}

public void error(Object message, Throwable t) {
    if (target != null)
    target.error(message, t);
}