如何使用套接字获取Java，TCP / IP服务器/客户端模型中的远程IP地址？

Question

So i have a TCP/IP Server/Client model using Socket and ServerSocket in an desktop application (A game that's suposed to be played in network).

I need to get the servers Remote IP address so that the client application can connect to it at the opened Server application at the specific port opened.

public class ServerConnection {

private int PORT = 8100;
private ServerSocket serverSocket = null;

public void create() throws IOException {
    serverSocket = new ServerSocket();
    serverSocket.bind(new InetSocketAddress("localhost", PORT));
}

public void close() throws IOException {
    serverSocket.close();
}

public ClientConnection acceptRequest() throws IOException {
    Socket socket = serverSocket.accept();
    return new ClientConnection(socket);
}

public ServerConnection() throws IOException {
}
}
public class ClientConnection {

    private String adress = "127.0.0.1";
    private int PORT = 8100;
    private Socket socket = null;
    private PrintWriter out = null;
    private BufferedReader in = null;

    public ClientConnection() {

    }

    public ClientConnection(Socket socket) throws IOException {
        this.socket = socket;
        out = new PrintWriter(socket.getOutputStream(), true);
        in = new BufferedReader(new InputStreamReader(
                socket.getInputStream()));
    }

    public void connect() throws UnknownHostException, IOException {
        socket = new Socket(adress, PORT);
        out = new PrintWriter(socket.getOutputStream(), true);
        in = new BufferedReader(new InputStreamReader(
                socket.getInputStream()));
    }


    public void close() throws IOException {
        if (out != null) {
            out.close();
        }
        if (in != null) {
            in.close();
        }
        if (socket != null) {
            socket.close();
            socket = null;
        }
    }

    public void send(String request) {
        if (socket != null) {
            out.println(request);
        }            
    }

    public String receive() throws IOException {
        if (socket != null) {
            return in.readLine();
        }
        return null;
    }
}

It works well on localhost, but I want it to be capable to run anywhere (the server and the client). So i need a way for the server to find out it's current remote IP the user will send it via some comunication line (IM, E-mail etc...) and afterward the client will input the address and connect to the server. Therefore one single application can act as both server or client, so there is no need for a stable server application that will run continously and serve clients

Answer 1

If your client and the server live in the same network segment, Zeroconf is a very attractive zero configuration (well...) solution. you will find a Java implementation with JmDNS

Furthermore, it's not an easy task to implement a stable protocol from scratch. If it is not for education purposes i'd recommend relying on something like

• Apache Mina
• Grizzly
• Netty 3
• QuickServer
• xSocket

These libraries give you many features that are important here, like error handling and concurrency (non blocking calls).

If you want to reach your server from outside, you can not bind to "localhost". See ServerSocket.bind. You can bind to "null" and request the network adapter that has been used by default via ServerSocket.getInetAddress.

Answer 2

You need to make two adjustments:

• Server socket must be bound to something else than localhost. Typically you create server socket by using new ServerSocket(port) , without specifying a hostname. This way, server socket will listen on given port for all available addresses on the machine. If you bind your socket to 'localhost', then it will be accessible through localhost only. That's not what you want. After creating server socket, you can try to find out its SocketAddress by using serverSocket.getLocalSocketAddress() .

• Client will need to connect to the address that you've specified when creating/binding the server socket. If you have created server socket without specifying an address (ie binding to all available addresses), then client can connect to any of those (usually localhost + external ip address).

Servers typically don't need to know their address. They simply bind to <any address>:port , and client must know the address to connect to.