I have a layout which is generating JSON from logging events and MDC Context. MDC is great and I want to add parameters to MDC Context conditionally. For example, I would like add request information as JSON into MDC conditionally on Error Level log entries. I am not familiar with Logback internals but gave a try with Logback Filter implementation. I have used:

   public FilterReply decide(ILoggingEvent event) {

      // simplified for demo
      // conditionally I am putting some attributes here
      Map<String, String> mdcPropertyMap = event.getMDCPropertyMap();
      if (event.getLevel().isGreaterOrEqual(Level.ERROR)) {
         mdcPropertyMap.put("demo", "demo");
      }
      return FilterReply.ACCEPT;
   }

I am expecting mdcPropertyMap to be cleared for each event but it does not. LoggingEvent has a internal CACHED_NULL_MAP map and its modified per touch.

 private static final Map<String, String> CACHED_NULL_MAP = new HashMap<String, String>();

I could not be sure but it seems a little bid buggy to me. Can someone verify this behaviour?
For those who are interested why I am not using MDC.put in a Servlet Filter implementation because I am conditionally adding some attributes. Serialising HttpRequest into JSON is not cheap task but having Http Request information is giving great insights about error. Using Filter is the only way I found to add conditionally attributes so far. What I would like to use is:

   public FilterReply decide(ILoggingEvent event) {
      if (event.getLevel().isGreaterOrEqual(Level.WARN)) {
         MDC.put("demo", "demo");
      }
      return FilterReply.ACCEPT;
   }