@Override
        public void onComplete() {
            final String content = dataList.stream().map(HttpData::array)
                                           .reduce(Bytes::concat)
                                           .map(bytes -> new String(bytes, StandardCharsets.UTF_8))
                                           .orElse(null);
            future.complete(content);
        }
A subscriber of data chunk that accumulates bytes to a single String.
core/src/test/java/com/linecorp/armeria/common/multipart/MultipartEncoderTest.java