Socket between 2 devices

Question

I would like to send data between different machines, between two computers, between a computer and a Android for example.

I did the example internally, on the same computer, but when I separate (the server stays on one computer and the client goes to another. It doesn’t work).

Server:

public static void main(String[] args) {
        try {
            // Instancia o ServerSocket ouvindo a porta 12345
            ServerSocket servidor = new ServerSocket(12345);
            System.out.println("Servidor ouvindo a porta 12345");
           // servidor.bind(new InetSocketAddress("192.168.5.1", 0));

            InetAddress inet = servidor.getInetAddress();
            System.out.println("HostAddress="+inet.getHostAddress());
            System.out.println("HostName="+inet.getHostName());
            while(true) {
              // o método accept() bloqueia a execução até que
              // o servidor receba um pedido de conexão
              Socket cliente = servidor.accept();
              System.out.println("Cliente conectado: " + cliente.getInetAddress().getHostAddress());
              ObjectOutputStream saida = new ObjectOutputStream(cliente.getOutputStream());
              saida.flush();
              saida.writeObject(new Date());
              saida.close();
              cliente.close();
            }  
        }   
        catch(Exception e) {
            System.out.println("Erro: " + e.getMessage());
        }
    }

Client:

public static void main(String[] args) {
        try {
             Socket cliente = new Socket("0.0.0.0",12345);
            InetAddress inet = cliente.getInetAddress();
            System.out.println("HostAddress="+inet.getHostAddress());
            System.out.println("HostName="+inet.getHostName());
            ObjectInputStream entrada = new ObjectInputStream(cliente.getInputStream());
            Date data_atual = (Date)entrada.readObject();
            JOptionPane.showMessageDialog(null,"Data recebida do servidor:" + data_atual.toString());
            entrada.close();
            System.out.println("Conexão encerrada");
        }
        catch(Exception e) {
          System.out.println("Erro: " + e.getMessage());
        }
    }

The strange thing is that the server output is:

Servidor ouvindo a porta 12345
HostAddress=0.0.0.0
HostName=0.0.0.0

Then when I try to put the client in another computer it does not work, even if inserting (in the client) the IP where the server is.

Obs.: The machines are in different networks.

Obs².: I even tested with the firewall of Windows deactivated and did not work.

Answer 1

About the server output:

The output is correct, as you did not specify which IP was to bind, it gave bind to all of the network interface.

About the code:

Socket cliente = new Socket("0.0.0.0",12345);

I just changed the "0.0.0.0" to the IP of my remote server and the code worked correctly. You must have some firewall lock.

Answer 2

In the client you need to pass the IP address or hostname of the server.

Socket cliente = new Socket("0.0.0.0",12345);  // <<< errado

I don’t know Java, but the line above should be

Socket cliente = new Socket("127.0.0.1",12345);  // <<< mesma máquina

or

Socket cliente = new Socket("10.172.36.130",12345);  // <<< outra máquina

where "10.172.36.130" is just one example, you need to replace it with the IP (or name) of the machine your server is running on.