private String getLogFileName() {
    String logFileName = getGemFireProperties().get(LOG_FILE);
    if (logFileName == null) {
      logFileName = DEFAULT_LOG_FILE_NAME;
    }
    return logFileName;
  }
Instance reference which is set in static initialization blocks of any subclasses.
extensions/geode-modules/src/main/java/org/apache/geode/modules/session/bootstrap/AbstractCache.java