LOG4J: Modify logged message using custom appender

Question

For security reasons, I need to look at every logged message in my app and possibly modify it before it goes to the log file. I figured I could write a custom appender (exte

Answer 1

Thats how I did since I am working with an older version of log4j

Instead of customizing appender, I customized the layout and referred that layout in whichever Appender i needed this feature

public class LogValidatorLayout extends PatternLayout {

    public LogValidatorLayout() {
        super();
    }

    public LogValidatorLayout(String pattern) {
        super(pattern);
    }

    @Override
    public String format(LoggingEvent event) {

        // only process String type messages
        if (event.getMessage() != null && event.getMessage() instanceof String) {

            String message = event.getMessage().toString();
            message = StringUtils.trim("Some custom text --->>"+message);

            // earlier versions of log4j don't provide any way to update messages,
            // so use reflections to do this
            try {
                Field field = LoggingEvent.class.getDeclaredField("message");
                field.setAccessible(true);
                field.set(event, message);
            } catch (Exception e) {
                // Dont log it as it will lead to infinite loop. Simply print the trace
                e.printStackTrace();
            }

        }

        return super.format(event);
    }

}

And in your log4j.properties or xml, register this Layout

log4j.appender.STDOUT.layout=a.b.c.package.LogValidatorLayout