public static String getString(JsonNode jsonNode, String fieldName, String defaultValue) {
    Preconditions.checkNotNull(jsonNode, "JsonNode should not be null");
    JsonNode textNode = jsonNode.get(fieldName);

    if (textNode == null) {
      return defaultValue;
    }
    return textNode.asText();
  }
Convert an java object(usually java bean) to JsonNode.
amoro-common/src/main/java/org/apache/amoro/utils/JacksonUtil.java