@Override
  public CharSequence flatten(long timeMillis, int logLevel, String tag, String message) {
    String flattenedLog = pattern;
    for (ParameterFiller parameterFiller : parameterFillers) {
      flattenedLog = parameterFiller.fill(flattenedLog, timeMillis, logLevel, tag, message);
    }
    return flattenedLog;
  }
Try to create a message filler if the given parameter is a message parameter.

@return created message filler, or null if the given parameter is not a message parameter
xlog/src/main/java/com/elvishew/xlog/flattener/PatternFlattener.java