<conversionRule conversionWord="message" converterClass="org.sample.DebugViewMessageConverter" />

    <appender name="ConsoleAppender1" class="ch.qos.logback.core.ConsoleAppender">
        <encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
            <pattern>[2]%-5level %message\n</pattern>
        </encoder>
    </appender>


public interface IDebugView {

    public String getDebugView();

}

public class DebugViewMessageConverter extends ClassicConverter {

    @Override
    public String convert(ILoggingEvent event) {
        Object[] argumentArray = event.getArgumentArray();
        if (argumentArray != null) {
            boolean hasDebugView = false;
            for (int i = 0; i < argumentArray.length; i++) {
                if (argumentArray[i] instanceof IDebugView) {
                    argumentArray[i] = ((IDebugView) argumentArray[i]).getDebugView();
                    hasDebugView = true;
                }
            }
            if (hasDebugView) {
                // Need to store it in Event so multiple loggers will use this message, otherwise  toString() of the objects will be used in message.
                LoggingEventAccess.setFormattedMessage((LoggingEvent) event, MessageFormatter.arrayFormat(event.getMessage(), argumentArray).getMessage());
            }
        }
        return event.getFormattedMessage();
    }
}

 
public class DebugObject implements IDebugView {

    private final String text;

    public DebugObject(String text) {
        this.text = text;
    }

    public String getDebugView() {
        return text;
    }

    @Override
    public String toString() {
        return "!!not this!!";
    }

}
log.info("test message: {}", new DebugObject("print this"));

result:
INFO  test message: print this