Introducing Threads in Socket Programming in Java

// Java implementation of  Server side
// It contains two classes : Server and ClientHandler
// Save file as Server.java

import java.io.*;
import java.text.*;
import java.util.*;
import java.net.*;

// Server class
public class Server 
{
    public static void main(String[] args) throws IOException 
    {
        // server is listening on port 5056
        ServerSocket ss = new ServerSocket(5056);
        
        // running infinite loop for getting
        // client request
        while (true) 
        {
            Socket s = null;
            
            try 
            {
                // socket object to receive incoming client requests
                s = ss.accept();
                
                System.out.println("A new client is connected : " + s);
                
                // obtaining input and out streams
                DataInputStream dis = new DataInputStream(s.getInputStream());
                DataOutputStream dos = new DataOutputStream(s.getOutputStream());
                
                System.out.println("Assigning new thread for this client");

                // create a new thread object
                Thread t = new ClientHandler(s, dis, dos);

                // Invoking the start() method
                t.start();
                
            }
            catch (Exception e){
                s.close();
                e.printStackTrace();
            }
        }
    }
}

// ClientHandler class
class ClientHandler extends Thread 
{
    DateFormat fordate = new SimpleDateFormat("yyyy/MM/dd");
    DateFormat fortime = new SimpleDateFormat("hh:mm:ss");
    final DataInputStream dis;
    final DataOutputStream dos;
    final Socket s;
    

    // Constructor
    public ClientHandler(Socket s, DataInputStream dis, DataOutputStream dos) 
    {
        this.s = s;
        this.dis = dis;
        this.dos = dos;
    }

    @Override
    public void run() 
    {
        String received;
        String toreturn;
        while (true) 
        {
            try {

                // Ask user what he wants
                dos.writeUTF("What do you want?[Date | Time]..\n"+
                            "Type Exit to terminate connection.");
                
                // receive the answer from client
                received = dis.readUTF();
                
                if(received.equals("Exit"))
                { 
                    System.out.println("Client " + this.s + " sends exit...");
                    System.out.println("Closing this connection.");
                    this.s.close();
                    System.out.println("Connection closed");
                    break;
                }
                
                // creating Date object
                Date date = new Date();
                
                // write on output stream based on the
                // answer from the client
                switch (received) {
                
                    case "Date" :
                        toreturn = fordate.format(date);
                        dos.writeUTF(toreturn);
                        break;
                        
                    case "Time" :
                        toreturn = fortime.format(date);
                        dos.writeUTF(toreturn);
                        break;
                        
                    default:
                        dos.writeUTF("Invalid input");
                        break;
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        
        try
        {
            // closing resources
            this.dis.close();
            this.dos.close();
            
        }catch(IOException e){
            e.printStackTrace();
        }
    }
}

// Java implementation for a client
// Save file as Client.java

import java.io.*;
import java.net.*;
import java.util.Scanner;

// Client class
public class Client 
{
    public static void main(String[] args) throws IOException 
    {
        try
        {
            Scanner scn = new Scanner(System.in);
            
            // getting localhost ip
            InetAddress ip = InetAddress.getByName("localhost");
    
            // establish the connection with server port 5056
            Socket s = new Socket(ip, 5056);
    
            // obtaining input and out streams
            DataInputStream dis = new DataInputStream(s.getInputStream());
            DataOutputStream dos = new DataOutputStream(s.getOutputStream());
    
            // the following loop performs the exchange of
            // information between client and client handler
            while (true) 
            {
                System.out.println(dis.readUTF());
                String tosend = scn.nextLine();
                dos.writeUTF(tosend);
                
                // If client sends exit,close this connection 
                // and then break from the while loop
                if(tosend.equals("Exit"))
                {
                    System.out.println("Closing this connection : " + s);
                    s.close();
                    System.out.println("Connection closed");
                    break;
                }
                
                // printing date or time as requested by client
                String received = dis.readUTF();
                System.out.println(received);
            }
            
            // closing resources
            scn.close();
            dis.close();
            dos.close();
        }catch(Exception e){
            e.printStackTrace();
        }
    }
}