How to create my own Appender in log4j?

I am new in log4j. Can anyone explain how to create my own Appender? i.e. how to implement the classes and interfaces and how to override it?

Update: the provided solution is valid for Log4J 1.x . If you're looking for 2.x versions, take a look at this article: How to create a custom appender in log4j2

You should extend AppenderSkeleton class, that (quoting javadoc) "provides the code for common functionality, such as support for threshold filtering and support for general filters."

If you read the code of AppenderSkeleton, you'll see that it handles almost all, leaving to you just:

1. protected void append(LoggingEvent event)
2. public void close()
3. public boolean requiresLayout()

The core method is append. Remember that you don't need to implement the filtering logic in it because it is already implemented in doAppend that in turn calls append. Here I made a (quite useless) class that stores the log entries in an ArrayList, just as a demo.

public /*static*/ class MyAppender extends AppenderSkeleton {
    ArrayList<LoggingEvent> eventsList = new ArrayList();

    @Override
    protected void append(LoggingEvent event) {
        eventsList.add(event);
    }

    public void close() {
    }

    public boolean requiresLayout() {
        return false;
    }

}

Ok, let's test it:

public static void main (String [] args) {

    Logger l = Logger.getLogger("test");

    MyAppender app = new MyAppender();

    l.addAppender(app);

    l.warn("first");
    l.warn("second");
    l.warn("third");

    l.trace("fourth shouldn't be printed");

    for (LoggingEvent le: app.eventsList) {
        System.out.println("***" + le.getMessage());
    }
} 

You should have "first", "second", "third" printed; the fourth message shouldn't be printed since the log level of root logger is debug while the event level is trace. This proves that AbstractSkeleton implements "level management" correctly for us. So that's definitely seems the way to go... now the question: why do you need a custom appender while there are many built in that log to almost any destination? (btw a good place to start with log4j: http://logging.apache.org/log4j/1.2/manual.html)

If you would like to do some manipulations or decisions you can do it like this:

@Override
protected void append(LoggingEvent event) {
		String message = null;
		if(event.locationInformationExists()){
			StringBuilder formatedMessage = new StringBuilder();
			formatedMessage.append(event.getLocationInformation().getClassName());
			formatedMessage.append(".");
			formatedMessage.append(event.getLocationInformation().getMethodName());
			formatedMessage.append(":");
			formatedMessage.append(event.getLocationInformation().getLineNumber());
			formatedMessage.append(" - ");
			formatedMessage.append(event.getMessage().toString());
			message = formatedMessage.toString();
		}else{
			message = event.getMessage().toString();
		}

		switch(event.getLevel().toInt()){
		case Level.INFO_INT:
			//your decision
			break;
		case Level.DEBUG_INT: 
			//your decision
			break;
		case Level.ERROR_INT:
			//your decision
			break;
		case Level.WARN_INT:
			//your decision
			break;
		case Level.TRACE_INT:
			//your decision
			break;
		default:
			//your decision
			break;
		}
}

I would like to expend @AgostinoX answer to support pro file configuration and the ability to start and stop the logging capture :

public class StringBufferAppender extends org.apache.log4j.AppenderSkeleton {

	StringBuffer logs = new StringBuffer();
	AtomicBoolean captureMode = new AtomicBoolean(false);
	
	public void close() {
		// TODO Auto-generated method stub
		
	}

	public boolean requiresLayout() {
		// TODO Auto-generated method stub
		return false;
	}


	@Override
	protected void append(LoggingEvent event) {
		if(captureMode.get())
			logs.append(event.getMessage());
	}
	
	public void start()
	{
		//System.out.println("[StringBufferAppender|start] - Start capturing logs");
		StringBuffer logs = new StringBuffer();
		captureMode.set(true);
	}
	
	public StringBuffer stop()
	{
		//System.out.println("[StringBufferAppender|start] - Stop capturing logs");
		captureMode.set(false);
		StringBuffer data = new StringBuffer(logs);
		logs = null;
		return data;
	}

	
}

Now all you have to do is to define in in the log4j.property file

log4j.rootLogger=...., myAppender  # here you adding your appendr name
log4j.appender.myAppender=com.roi.log.StringBufferAppender # pointing it to the implementation

than when ever you want to enable it during runtume:

Logger logger = Logger.getRootLogger();
		StringBufferAppender appender = (StringBufferAppender)logger.getAppender("myAppender");
		appender.start();

and while want to stop it:

StringBuffer sb = appender.stop();

To create a own Appender you just implement the Appender Interface and just override it. And also study this link start log