[logback-user] Modifing MDCPropertyMap of LoggingEvents

[Cemo]

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;
   }
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