private void cleanUpAfterException()  {
        for (Future<Long> file : partFiles) {
            file.cancel(false);
        }
        download.setState(Transfer.TransferState.Failed);
    }
Helper class to merge all the individual part files into a destinationFile.
aws-java-sdk-s3/src/main/java/com/amazonaws/services/s3/transfer/internal/CompleteMultipartDownload.java