Java Two-Way Socket Connection

Question

I'm working on producing a basic two-way socket connection between a client and server in Java. Following some guides I found elsewhere online, I have it working one way, with the server receiving messages from the client, but I was curious on how I would go about making it so the client and server can send messages to each other. I'm also working on implementing a payload, but I'm not worrying about that until after I manage to get the two-way connection working.

This is my code for the server:

import java.net.*;
import java.io.*;
public class ServerDemo {
    private Socket mySocket = null;
    private ServerSocket server = null;
    private DataInputStream input = null;
    
    public ServerDemo(int port)
    {
         try
         {
               server = new ServerSocket(port);
               System.out.println("Server started");
               System.out.println("Waiting for a client ...");
               mySocket = server.accept();
               System.out.println("Client accepted");
               input = new DataInputStream(
                        new BufferedInputStream(mySocket.getInputStream()));
               String newLine = "";
               while (!newLine.equals("Over"))
               {
                    try
                    {
                        newLine = input.readUTF();
                        System.out.println(newLine);
                    }
                    catch(IOException i)
                    {
                        System.out.println(i);
                    }
               }
               System.out.println("Closing connection");
               mySocket.close();
               input.close();
         }
         catch(IOException i)
         {
              System.out.println(i);
         }
    }
 
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ServerDemo server = new ServerDemo(5000);
    }

}

This is the code for the client:

import java.net.*;
import java.util.Scanner;
import java.io.*;
public class ClientDemo {
    private Socket mySocket = null;
    private BufferedReader input= null;
    private DataOutputStream output = null;

    public ClientDemo(String address, int port)
    {
        try
        {
            mySocket = new Socket(address, port);
            System.out.println("Connected");
            input = new BufferedReader(new InputStreamReader(System.in));
            output = new DataOutputStream(mySocket.getOutputStream());
        }
        catch(UnknownHostException u)
        {
            System.out.println(u);
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
        String newLine = "";
        while (!newLine.equals("Over"))
        {
            try
            {
                newLine = input.readLine();
                output.writeUTF(newLine);
            }
            catch(IOException i)
            {
                System.out.println(i);
            }
        }
        try
        {
            input.close();
            output.close();
            mySocket.close();
        }
        catch(IOException i)
        {
            System.out.println(i);
        }
    }

    public static void main(String[] args) {
        String response="";
        Scanner myScanner = new Scanner(System.in);
        System.out.println("Please enter a server to connect to.");
        response=myScanner.next();
        ClientDemo client = new ClientDemo(response, 5000);
    }
}

I would appreciate any advice on how to convert this into two-way communication.

Answer 1

In order for the server to send a message back to the client, you have to respond from the server (ServerDemo) to the client using mySocket.getOutputStream()

There are multiple ways of doing it,

Using OutputStreamWriter

BufferedWriter buffer = new BufferedWriter(new OutputStreamWriter(mySocket.getOutputStream(), "UTF8"));
buffer.append(stringToSend).append("\n");
buffer.append("Over").append("\n");
buffer.flush();

Using PrintWriter

PrintWriter out = new PrintWriter(mySocket.getOutputStream());
String response = "Data received successfully";
out.print(response);

To print the response from the server following code snippet can be used in the ClientDemo try/catch block where the socket is closed.

BufferedReader bufferIn=new BufferedReader(new InputStreamReader(mySocket.getInputStream()));
String message;
while(!(message = bufferIn.readLine()).equals("Over")){
    System.out.println(message);
}