Client socket connection refused

Question

I am trying to understand how sockets work in Java, thus I wanted to take a look at a client-server implementation. I found this: Creating a Simple Java TCP/IP Server and Client Socket.

The server seems to work properly if I pass localhost or 127.0.0.1 however the client will refuse to connect on either one of those, throwing a connection refused exception, although I started the server before the client.

Server output:

Running Server: Host=127.0.0.1 Port=5069

Client output:

Exception in thread "main" java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.connect0(Native Method)
    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:79)
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
    at java.net.Socket.connect(Socket.java:589)
    at java.net.Socket.connect(Socket.java:538)
    at java.net.Socket.<init>(Socket.java:434)
    at java.net.Socket.<init>(Socket.java:244)
    at Client.<init>(Client.java:12)
    at Client.main(Client.java:29)

My Java code:

public class Server {
    private ServerSocket server;

    public Server(String ipAddress) throws Exception {
        if (ipAddress != null && !ipAddress.isEmpty()) {
//            System.out.println(InetAddress.getByName(ipAddress));
            this.server = new ServerSocket(0, 1, InetAddress.getByName(ipAddress));
        } else {
            this.server = new ServerSocket(0, 1, InetAddress.getLocalHost());
        }
    }

    private void listen() throws Exception {
        String data = null;
        Socket client = this.server.accept();
        String clientAddress = client.getInetAddress().getHostAddress();
        System.out.println("\r\nNew connection from " + clientAddress);

        BufferedReader in = new BufferedReader(
                new InputStreamReader(client.getInputStream()));
        while ((data = in.readLine()) != null) {
            System.out.println("\r\nMessage from " + clientAddress + ": " + data);
        }
    }

    public InetAddress getSocketAddress() {
        return this.server.getInetAddress();
    }

    public int getPort() {
        return this.server.getLocalPort();
    }

    public static void main(String[] args) throws Exception {
        Server app = new Server("localhost");
        System.out.println("\r\nRunning Server: " +
                                   "Host=" + app.getSocketAddress().getHostAddress() +
                                   " Port=" + 50696);

        app.listen();
    }

}

public class Client {
    private Socket socket;
    private Scanner scanner;

    private Client(InetAddress serverAddress, int serverPort) throws Exception {
        this.socket = new Socket(serverAddress, serverPort);
        this.scanner = new Scanner(System.in);
    }

    private void start() throws IOException {
        String input;
        while (true) {
            input = scanner.nextLine();
            PrintWriter out = new PrintWriter(this.socket.getOutputStream(), true);
            out.println(input);
            out.flush();
        }
    }

    public static void main(String[] args) throws Exception {
//        System.out.println( InetAddress.getByName("localhost"));
        Client client = new Client(
                InetAddress.getByName("127.0.0.1"), 50696);

        System.out.println("\r\nConnected to Server: " + client.socket.getInetAddress());
        client.start();
    }
}

Answer 1

If you are trying to connect to the wrong port, then the connection will be refused (see this answer). Your server is not listening on the port 5069. You obviously set it to listen on port 0 right here:

if (ipAddress != null && !ipAddress.isEmpty()) {
    this.server = new ServerSocket(0, 1, InetAddress.getByName(ipAddress));
} else {
    this.server = new ServerSocket(0, 1, InetAddress.getLocalHost());
}

Look at the Javadoc for that constructor you used:

public ServerSocket(int port,
                    int backlog,
                    InetAddress bindAddr)
             throws IOException

Parameters:

port - the port number, or 0 to use a port number that is automatically allocated.

backlog - requested maximum length of the queue of incoming connections.

bindAddr - the local InetAddress the server will bind to

That first parameter is the port number, and you pass 0 for both constructors.