DataInputStream.readFully() doesn't have data to read when reading an HTTP POST request with no Expect: 100-continue

Question

I have a simple HTTP server that must manually implement handling for POST requests that send in a file. The problem is, if the header does not include an Expect: 100-continue and thus sends the HTTP body along with the headers, when I call DataInputStream.readFully() to read, there's no data to be read and it blocks indefinitely.

I make the POST request via cURL: curl "localhost:40000/test1.txt" --data-binary @./test.txt --trace dump, sending a small text file containing only hello world in it.

private static void postRequest(InputStream input, OutputStream output, BufferedReader reader, String fileName) throws IOException {
    var dataInput = new DataInputStream(input);
    String line;
    byte[] byteArray = new byte[0];
    int length = 0;
    var writer = new BufferedWriter(new OutputStreamWriter(output));

    while ((line = reader.readLine()) != null && !line.isEmpty()) {
        String[] split = line.split(" ");
        if (split[0].equals("Content-Length:")) {
            length = Integer.parseInt(split[1]);
            byteArray = new byte[length];
        } else if (line.equals("Expect: 100-continue")) {
            writer.write(CONTINUE_HEADER); // HTTP/1.1 100 Continue
            writer.flush();
        }
        System.out.println(line);
    }
    System.out.println(line);
    writeToFile(fileName, dataInput, byteArray, length, writer);
}

private static void writeToFile(String fileName, DataInputStream dataInput, byte[] byteArray, int length, BufferedWriter writer) throws IOException {
    dataInput.readFully(byteArray, 0, length);

    try (var stream = new FileOutputStream(fileName)) {
        stream.write(byteArray);
        System.out.println("File successfully saved to server.");
    } catch (IOException e) {
        System.out.println("Could not write to file.");
        writer.write(INTERNAL_SERVICE_ERROR_HEADER);
    }
    writer.write(CREATED_HEADER);
}

I've tried calling DataInputStream.available() right before calling readFully(), and it always returns 0. Calling BufferedReader.ready() returns false if there's an Expect: 100-continue, but true otherwise. However, it still doesn't have any data to read.