@Override
  public Object evaluate(DeferredObject[] arguments) throws HiveException {
    Object valObject = arguments[0].get();
    if (valObject == null) {
      return null;
    }

    try {
      String text = inputConverter.convert(valObject).toString();
      if (text.trim().length() == 0) {
        return null;
      }
      return jsonReader.parseStruct(text);
    } catch (Exception e) {
      throw new HiveException("Error parsing json: " + e.getMessage(), e);
    }
  }
Parses a json string representation into a Hive struct.
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFJsonRead.java