Server.java

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;

public class Server {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(22222);
        System.out.println("Server Started..");

        while (true) {
            Socket socket = serverSocket.accept();
            System.out.println("Client connected..");

            new ServerThread(socket);


        }
    }
}

class ServerThread implements Runnable {

    Socket clientSocket;
    Thread t;

    ServerThread(Socket clientSocket){
        this.clientSocket = clientSocket;
        t= new Thread(this);
        t.start();
    }


    @Override
    public void run() {

        try {
            ObjectInputStream ois = new ObjectInputStream(clientSocket.getInputStream());
            ObjectOutputStream oos = new ObjectOutputStream(clientSocket.getOutputStream());

            while (true) {

                Object cMsg = ois.readObject();
                if(cMsg==null)
                    break;
                System.out.println("From Client: " + (String) cMsg);

                String serverMsg = (String) cMsg;
                serverMsg = serverMsg.toUpperCase();

                oos.writeObject(serverMsg);
            }

        } catch (ClassNotFoundException | IOException e) {
            e.printStackTrace();
        }

        try {
            clientSocket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}