@Override
        public void accept(final String fieldName, final byte[] fieldValue) {
            if (CollectionUtils.isEmpty(fieldValue)) {
                source.put(fieldName, Const.EMPTY_STRING);
            } else {
                source.put(fieldName, new String(Base64.getEncoder().encode(fieldValue)));
            }
        }
Stateful Hashmap based converter, from object to HashMap.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/storage/type/HashMapConverter.java