Java ServerSocket OutputStream not flushing

Question

My ServerSocket (java.net.ServerSocket) only flushes the data if I close the OutputStream, but if I close the OutputStream, it closes the connection. Here is my code

Server:

ServerSocket server = new ServerSocket(7788);
Socket client = server.accept();

OutputStream os = client.getOutputStream();

os.write("Hello World".getBytes());
os.flush();
// os.close();

Client:

Socket client = new Socket("localhost", 7788);

byte[] bytes = client.getInputStream().readAllBytes();
String message = new String(bytes);

System.out.println(message);

If I close the OutputStream everything works fine, but I need to send data multiple times for my project. Is there a solution for this?

Answer 1

the problem is not the flush() on the OutputStream (server), but rather the readAllBytes() on the InputStream (client). from the docs (emphasis mine):

public byte[] readAllBytes() throws IOException

Reads all remaining bytes from the input stream. This method blocks until all remaining bytes have been read and end of stream is detected , or an exception is thrown.

end of stream is "sent" only on close() , that is why you are able to read (print) the message only after close() .

a very simple solution to exchange one-line textual data would be to send messages separated by '\\n' (new line) character and on the client part, read from the socket line-by-line, for example:

Server.java

import java.io.OutputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class Server
{
        public static void main(String[] args) throws Exception
        {
                ServerSocket server = new ServerSocket(7788);
                Socket client = server.accept();

                OutputStream os = client.getOutputStream();
                for (int i = 0; i < 5; i++) {
                        os.write(String.format("message %d\n", i).getBytes());
                        os.flush();

                        Thread.sleep(2000);
                }

                os.close();
        }
}

Client.java

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.Socket;

public class Client
{
        public static void main(String[] args) throws Exception
        {
                Socket client = new Socket("localhost", 7788);

                BufferedReader reader = new BufferedReader(new InputStreamReader(client.getInputStream()));
                String message = null;
                while ((message = reader.readLine()) != null)
                        System.out.println(message);

                reader.close();
        }
}

---

$ javac Server.java Client.java
$ java Server

---

$ java Client
message 0
message 1
message 2
message 3
message 4
$

if you run the above code, you will see that messages gets printed (received) one after the other with a 2 seconds delay and readLine() returns as soon as it read a \\n (new line) character.

if this is not feasible for your use case, I think you will need a more structured protocol.