Spring MVC OutputStream buffer size / flush not blocking? - spring

Question

I have been trying to investigate the buffering behaviour of the HttpServletResponse OutputStream (Using tomcat in spring-boot). My expectation that flushing on the server side blocks until all data has been transmitted to the client.
I am trying to understand this, because I am noticing that clients seem to be processing much longer than the processing on the server.
My setup looks something like this:
Server:
#RequestMapping(path = "/some/url", method = RequestMethod.POST)
public void call(HttpServletRequest request, HttpServletResponse response)
{
OutputStream outputStream = response.getOutputStream();
... writing data here ...
outputStream.flush();
response.flushBuffer();
}
Some dummy client:
HttpURLConnection con = (HttpURLConnection) urlobj.openConnection();
con.setRequestMethod("POST");
con.setDoInput(true);
con.setDoOutput(true);
int nresp = 0;
final InputStream in = con.getInputStream();
while (true)
{
if (in.read() < 0)
break;
nresp++;
if ((nresp & 31) == 0)
Thread.sleep(1l); // slow down read
}
In this example, depending on the response size, the server finishes in 40ms, but the client is reading data for 35 seconds. Also in Wireshark I can see data being transmitted the entire time. Which means that the server is buffering the data.
I also tried with StreamingResponseBody as return value or with "response.setBufferSize(1024)", but the result is the same.
Does anybody understand where this buffering is occurring and if its behaving correctly?
Is there any way on the server to completely flush all buffers and blocking until data is really flushed to the client?
cheers, Christian