I’m trying to upload large files to S3 like this:
https://www.baeldung.com/aws-s3-multipart-upload

// Prepare the parts to be uploaded
List<CompletedPart> completedParts = new ArrayList<>();
int partNumber = 1;
ByteBuffer buffer = ByteBuffer.allocate(5 * 1024 * 1024); // Set your preferred part size (5 MB in this example)

// Read the file and upload each part
try (RandomAccessFile file = new RandomAccessFile(filePath, "r")) {
    long fileSize = file.length();
    long position = 0;

    while (position < fileSize) {
        file.seek(position);
        int bytesRead = file.getChannel().read(buffer);

        buffer.flip();
        UploadPartRequest uploadPartRequest = UploadPartRequest.builder()
            .bucket(existingBucketName)
            .key(keyName)
            .uploadId(uploadId)
            .partNumber(partNumber)
            .contentLength((long) bytesRead)
            .build();

        UploadPartResponse response = s3.uploadPart(uploadPartRequest, RequestBody.fromByteBuffer(buffer));

Is it possible to also have a Spring REST API receive those chunks from a client? E.g. the client would send chunk 1 5MB then chunk 2 5MB but I would start uploading chunk 1 to S3 before chunk 2 is received.

How would that client be configured? It says here that RestTemplate can be used for Multipart file requests for this purpose:

Multipart file requests break a large file into smaller chunks and use
boundary markers to indicate the start and end of the block.

I assume I can use a multipart Spring API like this if the client is configured correctly?
https://spring.io/guides/gs/uploading-files