Here is the example of socket application in java that server communicates with the client by localhost in port 2004.

Socket Server:

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

/**
 *
 * @author Administrator
 */
public class Provider {
    
    ServerSocket providerSocket;
    Socket connection = null;
    ObjectOutputStream out;
    ObjectInputStream in;
    String message;
    Provider(){}
    void run() {
        try{
            //1. creating a server socket
            providerSocket = new ServerSocket(2004, 10);
            
            //2. Wait for connection
            System.out.println("Waiting for connection");
            connection = providerSocket.accept();
            System.out.println("Connection received from " + connection.getInetAddress().getHostName());
            //3. get Input and Output streams
            out = new ObjectOutputStream(connection.getOutputStream());
            out.flush();
            in = new ObjectInputStream(connection.getInputStream());
            sendMessage("Connection successful");
            //4. The two parts communicate via the input and output streams
            do{
                try{
                    message = (String)in.readObject();
                    System.out.println("client> reply " + message);
                    sendMessage("Server> reply " + message.replaceAll("server", "client"));
                    if (message.equals("bye"))
                        sendMessage("bye");
                } catch(ClassNotFoundException classnot){
                    System.err.println("Data received in unknown format");
                }
            }while(!message.equals("bye"));
        } catch(IOException ioException){
            ioException.printStackTrace();
        } finally{
            //4: Closing connection
            try{
                in.close();
                out.close();
                providerSocket.close();
            } catch(IOException ioException){
                ioException.printStackTrace();
            }
        }
    }
    void sendMessage(String msg) {
        try{
            out.writeObject(msg);
            out.flush();
            System.out.println("server>" + msg);
        } catch(IOException ioException){
            ioException.printStackTrace();
        }
    }
    public static void main(String args[]) {
        Provider server = new Provider();
        while(true){
            server.run();
        }
    }
}

Socket Client


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

/**
 *
 * @author Administrator
 */
public class Requester {
    
    Socket requestSocket;
    ObjectOutputStream out;
    ObjectInputStream in;
    String message;
    Requester(){}
    void run() {
        try{
            //1. creating a socket to connect to the server
            requestSocket = new Socket("localhost", 2004);
            System.out.println("Connected to localhost in port 2004");
            //2. get Input and Output streams
            out = new ObjectOutputStream(requestSocket.getOutputStream());
            out.flush();
            in = new ObjectInputStream(requestSocket.getInputStream());
            //3: Communicating with the server
            do{
                try{
                    message = (String)in.readObject();
                    System.out.println("server>" + message);
                    sendMessage("Hi my server 1");
                    message = (String)in.readObject();
                    System.out.println(message);
                    sendMessage("Hi my server 2");
                    message = (String)in.readObject();
                    System.out.println(message);
                    sendMessage("Hi my server 3");
                    message = (String)in.readObject();
                    System.out.println(message);
                    sendMessage("Hi my server 4");
                    message = (String)in.readObject();
                    System.out.println(message);
                    sendMessage("Hi my server 5");
                    message = (String)in.readObject();
                    System.out.println(message);
                    
                    message = "bye";
                    sendMessage(message);
                } catch(ClassNotFoundException classNot){
                    System.err.println("data received in unknown format");
                }
            }while(!message.equals("bye"));
        } catch(UnknownHostException unknownHost){
            System.err.println("You are trying to connect to an unknown host!");
        } catch(IOException ioException){
            ioException.printStackTrace();
        } finally{
            //4: Closing connection
            try{
                in.close();
                out.close();
                requestSocket.close();
            } catch(IOException ioException){
                ioException.printStackTrace();
            }
        }
    }
    void sendMessage(String msg) {
        try{
            out.writeObject(msg);
            out.flush();
            System.out.println("client got>" + msg);
        } catch(IOException ioException){
            ioException.printStackTrace();
        }
    }
    public static void main(String args[]) {
        Requester client = new Requester();
        client.run();
    }    
    
}
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