Java Concurrent Socket Programming

Question

Below is my code for a simple Concurrent Server. Whenever I run multiple clients, the server only prints out the input of the first client. I'm not sure what I've done wrong. Any help would be appreciated.

public static void main(String[] args) {
    try {
        ServerSocket serverSocket = new ServerSocket(8001);
        while (true){
            Socket clientSocket = serverSocket.accept();
            System.out.println(clientSocket);
            ConcurrentServer client = new ConcurrentServer(clientSocket);
            client.start();
        }
    } catch (IOException i){}
}

public void run(){
    try {
        inputStream = new BufferedReader(new InputStreamReader(concurrentSocket.getInputStream()));
        outputStream = new PrintWriter(new OutputStreamWriter(concurrentSocket.getOutputStream()));

        String testString = inputStream.readLine();
        System.out.println(testString);


    } catch (IOException i){}
}

Answer 1

This code might help you to understand how to run multiple clients concurrently. :)

What this code does? TCP Client sends a string to the server and TCP server sends back the string in UPPERCASE format & the server can do this concurrently with multiple connections.

I have included 3 files for the server and one more for testing the server with multiple clients(ClientTest.java)

Main.java

import java.io.IOException;

public class Main {

    public static void main(String[] args) {
        try {
            new Server(3000).start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Server.java

import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.logging.Logger;

public class Server {

    private ServerSocket sSocket;
    private boolean run;
    private int port;

    public Server(int port) throws IOException {
        this.port = port;
        this.sSocket = new ServerSocket(this.port);
    }

    public void start() {

        this.run = true;
        Logger.getLogger(getClass().getName()).info("Server is listening on port: " + port);

        try {
            while (run) {
                Socket cs = sSocket.accept();
                Logger.getLogger(getClass().getName())
                        .info("New Client Connected! " + cs.getPort());
                new Thread(new Client(cs)).start(); // Put to a new thread.
            }
        } catch (IOException e) {
            Logger.getLogger(getClass().getName()).severe(e.getMessage());
        }
    }

    public void stop() {
        this.run = false;
    }
}

Client.java (Client Process on server)

import java.io.*;
import java.net.Socket;
import java.util.logging.Logger;

public class Client implements Runnable {

    private Socket clientSocket;

    private DataOutputStream out; // write for the client
    private BufferedReader in; // read from the client

    public Client(Socket clientSocket) {
        this.clientSocket = clientSocket;
    }

    @Override
    public void run() {
        // Do client process
        outToClient(inFromClient().toUpperCase());
        closeConnection();
    }

    private String inFromClient() {

        String messageFromClient = "";

        /*
         *  Do not use try with resources because once -
         *  - it exits the block it will close your client socket too.
         */
        try {
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            messageFromClient = in.readLine();
        } catch (IOException e) {
            Logger.getLogger(getClass().getName()).severe("InFromClientErr - " + e.getMessage());
        }

        return messageFromClient.trim().equals("") ? "No Inputs given!" : messageFromClient;
    }

    private void outToClient(String message) {
        try {
            out = new DataOutputStream(clientSocket.getOutputStream());
            out.writeBytes(message);
        } catch (IOException e) {
            Logger.getLogger(getClass().getName()).severe("OutToClientErr - " + e.getMessage());
        }
    }

    private void closeConnection() {
        try {
            in.close();
            out.close();
            clientSocket.close();
        } catch (NullPointerException | IOException e) {
            Logger.getLogger(getClass().getName()).severe(e.getMessage());
        }
    }
}

ClientTest.java (For Testing clients)

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

public class ClientTest {

    public static void main(String[] args) {

        Socket clientSocket;

        try {
            clientSocket = new Socket("localhost", 3000);

            DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
            BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

            outToServer.writeBytes(new Scanner(System.in).nextLine() + '\n'); // Get user input and send.
            System.out.println(inFromServer.readLine()); // Print the server response.

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