logback-android/src/main/java/ch/qos/logback/classic/sift/MDCBasedDiscriminator.java
public String getDiscriminatingValue(ILoggingEvent event) {
    // http://jira.qos.ch/browse/LBCLASSIC-213
    Map<String, String> mdcMap = event.getMDCPropertyMap();
    if (mdcMap == null) {
      return defaultValue;
    }
    String mdcValue = mdcMap.get(key);
    if (mdcValue == null) {
      return defaultValue;
    } else {
      return mdcValue;
    }
  }
public String getDiscriminatingValue(ILoggingEvent event) {
    // http://jira.qos.ch/browse/LBCLASSIC-213
    Map<String, String> mdcMap = event.getMDCPropertyMap();
    if (mdcMap == null) {
      return defaultValue;
    }
    String mdcValue = mdcMap.get(key);
    if (mdcValue == null) {
      return defaultValue;
    } else {
      return mdcValue;
    }
  }
Return the value associated with an MDC entry designated by the Key
property. If that value is null, then return the value assigned to the
DefaultValue property.