private void fillEmptyFields() {
      if (fileNameGenerator == null) {
        fileNameGenerator = DefaultsFactory.createFileNameGenerator();
      }
      if (backupStrategy == null) {
        backupStrategy = DefaultsFactory.createBackupStrategy();
      }
      if (cleanStrategy == null) {
        cleanStrategy = DefaultsFactory.createCleanStrategy();
      }
      if (flattener == null) {
        flattener = DefaultsFactory.createFlattener2();
      }
      if (writer == null) {
        writer = DefaultsFactory.createWriter();
      }
    }
Build configured {@link FilePrinter} object.

@return the built configured {@link FilePrinter} object
xlog/src/main/java/com/elvishew/xlog/printer/file/FilePrinter.java