Here I can solve socket programming for chat application between multiple client and server where client can send multiple message to the server. But now I want to solve a new problem where conversion of any string from any client [each client can send at most 2 messages] into a FULL UPPERCASE string with the help of the server. The server will be able to serve at most 5 clients.

This is my client coe.....

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.Socket;
import java.util.Scanner;

public class Client {
    public static void main(String[] args) throws IOException {
        System.out.println("Client started..");
        Socket socket = new Socket("127.0.0.1", 22222);
        System.out.println("Client Connected..");

        ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
        ObjectInputStream ois = new ObjectInputStream(socket.getInputStream());
        while (true) {
            Scanner sc = new Scanner(System.in);

            String message = sc.nextLine();

            if(message.equals("exit")){
                break;
            }

            //sent to server...
            oos.writeObject(message);

            try {
                //receive from server..
                Object fromServer = ois.readObject();
                System.out.println("From Server: "   (String) fromServer);

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

        socket.close();

    }
}

This is my server code........

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 Server Thread Start.....
            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) {
                //read from client...
                Object cMsg = ois.readObject();
                if (cMsg == null)
                    break;
                System.out.println("From Client: "   (String) cMsg);

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

                //send to client..
                oos.writeObject(serverMsg);
            }

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

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

CodePudding user response：

// Sever side

package sockettreading;
 
import java.io.*;
import java.net.*;
 
/**
 *
 * @author Amanur Rahman
 */
public class SocketTreading {
 
    public static void main(String[] args) throws IOException {
        ServerSocket ss = new ServerSocket(5050);
        System.out.println("Server is starting...");
        int cn=1;
        while (cn<=5) {
            Socket s = ss.accept();
            System.out.println("Client" cn " is connected \n"   s);
            DataInputStream dis = new DataInputStream(s.getInputStream());
            DataOutputStream dos = new DataOutputStream(s.getOutputStream());
            System.out.println("Assinging new thread for this client");
            Thread t = new ClientHandler(s, dis, dos);
            t.start();
            cn  ;
        }
        System.out.println("Client limit cross");
        
        ss.close();
 
    }
}
 
class ClientHandler extends Thread {
 
    final Socket soc;
    final DataInputStream input;
    final DataOutputStream output;
    int i = 1;
 
    public ClientHandler(Socket s, DataInputStream dis,
            DataOutputStream dos) {
        this.soc = s;
        this.input = dis;
        this.output = dos;
    }
 
    @Override
    public void run() {
        String received;
        String ends;
        String toreturn;
        while (i <= 2) {
            try {
                output.writeUTF("Please write your message : ");
                String str = input.readUTF();
                System.out.println("Client msg is: "  str.toUpperCase());
                output.writeUTF("Do you want to exit or continue ( if exit type \"ENDS\" ) ");
                ends = input.readUTF();
                if (ends.equals("ENDS")) {
                    System.out.println("Client"   this.soc   "send exit.");
                    this.soc.close();
                    System.out.println("Connection closed");
                    break;
                }
            } catch (IOException e) {
                System.out.println(e);
            }
            System.out.println(""   i);
            i  ;
        }
 
        try {
            this.input.close();
            this.output.close();
        } catch (IOException e) {
            System.out.println(e);
        }
    }
}

// client side

package sockettreading;
 
import java.io.*;
import java.net.Socket;
import java.util.Scanner;
 
/**
 *
 * @author Amanur Rahman
 */
public class ClientThreading {
 
    public static void main(String[] args) throws IOException {
        try(Socket s = new Socket("localhost", 5050);){
             System.out.println("Connected");
             Scanner scn = new Scanner(System.in);
        DataOutputStream dos = new DataOutputStream(s.getOutputStream());
        DataInputStream dis = new DataInputStream(s.getInputStream());
        int i = 1;
        while (i <= 2) {
            System.out.println(dis.readUTF());
            String num1 = scn.nextLine();
            dos.writeUTF(num1);
 
            System.out.println(dis.readUTF());
            String num2 = scn.nextLine();
            dos.writeUTF(num2);
            System.out.println(dis.readUTF());
            String ends = scn.nextLine();
            dos.writeUTF(ends);
            if (ends.equals("ENDS")) {
                System.out.println("Closing the connection"   s);
                s.close();
                System.out.println("Connection closed");
                break;
            }
            i  ;
        }
        System.out.println(dis.readUTF());
        s.close();
        dos.close();
        dis.close();
        }
        
        catch (IOException ex) {
            if(ex.getMessage()!=null){
                System.out.println("Already 5 clients are served so the server is closed: ");
            }else{
                System.out.println("Client 2 message already send. That's way the connection closed.");
            }
            
        }
 
       
    }
}