@Override
        public byte[] getBytes(final String fieldName) {
            final String value = (String) source.get(fieldName);
            if (StringUtil.isEmpty(value)) {
                return new byte[] {};
            }
            return Base64.getDecoder().decode(value);
        }
Stateful Hashmap based converter, build object from a HashMap type source.
oap-server/server-core/src/main/java/org/apache/skywalking/oap/server/core/storage/type/HashMapConverter.java