spring boot integration ip (tcp) force flush in order to respond

Question

I am now making TCP socket server using Spring boot integration ip.

public AbstractServerConnectionFactory serverConnectionFactory() {
    TcpNioServerConnectionFactory serverConnectionFactory = new TcpNioServerConnectionFactory(port);
    serverConnectionFactory.setUsingDirectBuffers(true);
    serverConnectionFactory.setSoTcpNoDelay(true);
    serverConnectionFactory.setSoKeepAlive(true);
    serverConnectionFactory.setSoTimeout(5);
    serverConnectionFactory.setSingleUse(false);


    // I use ByteArrayStxEtxSerializer provided by Spring boot.
    serverConnectionFactory.setSerializer(SERIALIZER);
    serverConnectionFactory.setDeserializer(SERIALIZER);

    return serverConnectionFactory;
}

My test client code is like this:

Socket sock = new Socket("127.0.0.1", 8081);

OutputStream os = new BufferedOutputStream(sock.getOutputStream());
BufferedReader br = new BufferedReader(new InputStreamReader(sock.getInputStream()));

ByteArrayOutputStream baos = new ByteArrayOutputStream();

baos.write(0x02); // STX
baos.write(0x41); // A
baos.write(0x03); // ETX

System.out.println(Arrays.toString(baos.toByteArray()));
os.write(baos.toByteArray());
os.flush();

String line = null;

while ((line = br.readLine()) != null) {
    System.out.println("from server: " + line);
}

os.close();
br.close();
sock.close();

In order to flush output stream into client, I need to set setSingleUse(true) or setSoTimeout(n). Otherwise, it blocks for a while.

I read the following from the official document:

An outbound TCP gateway is provided. It allows for simple request-response processing. If the associated connection factory is configured for single-use connections, a new connection is immediately created for each new request. Otherwise, if the connection is in use, the calling thread blocks on the connection until either a response is received or a timeout or I/O error occurs.

I think my test client waits until timeout for about 2 minutes if I don't set single-use or timeout.

How can I force the server to flush output stream into socket instead of single-use or timeout?

Thank you.