Java Socket编程服务器客户端

Question

I'm trying to program a Server Client program where the CLIENT will be prompt if the SERVER closes or loses connection. What happens is once I connect the server and the client then disconnects the server it doesn't go to the ConnectException part example: I opened the Server and Client connects, in the Client it will show that "You are connected to the Server", then if the Server disconnects there should be a "Server is disconnected". and when the Server reopens it will prompt the Client that he's connected to the Server

How can I continuously check if the Server is open or disconnected

here's my code:

SERVER

   public class Server
{
    private static Socket socket;

    public static void main(String[] args)
    {
        try
        {
            int port = 25000;
            ServerSocket serverSocket = new ServerSocket(port);
            //Server is running always. This is done using this while(true) loop
            while(true)
            {
                //Reading the message from the client
                socket = serverSocket.accept();
                System.out.println("Client has connected!");
                InputStream is = socket.getInputStream();
                InputStreamReader isr = new InputStreamReader(is);
                BufferedReader br = new BufferedReader(isr);
                String number = br.readLine();
                System.out.println("Message received from client is "+number);

                //Multiplying the number by 2 and forming the return message
                String returnMessage;
                try
                {
                    int numberInIntFormat = Integer.parseInt(number);
                    int returnValue = numberInIntFormat*2;
                    returnMessage = String.valueOf(returnValue) + "\n";
                }
                catch(NumberFormatException e)
                {
                    //Input was not a number. Sending proper message back to client.
                    returnMessage = "Please send a proper number\n";
                }

                //Sending the response back to the client.
                OutputStream os = socket.getOutputStream();
                OutputStreamWriter osw = new OutputStreamWriter(os);
                BufferedWriter bw = new BufferedWriter(osw);
                bw.write(returnMessage);
                System.out.println("Message sent to the client is "+returnMessage);
                bw.flush();
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }
}

CLIENT

    public class Client
{
    private static Socket socket;

    public static void main(String args[])
    {
        Scanner input=new Scanner(System.in);

        try
        {
            String host = "localhost";
            int port = 25000;
            InetAddress address = InetAddress.getByName(host);
            socket = new Socket(address, port);
            System.out.println("Connected to the Server");
        }
        catch (ConnectException exception)
        {
            System.out.println("Server is still offline");
        }
        catch(IOException ex)
        {
            System.out.println("Server got disconnected");
        }
    }
}

Answer 1

Well, the best way to tell if your connection is interrupted is to try to read/write from the socket. If the operation fails, then you have lost your connection sometime.

So, all you need to do is to try reading at some interval, and if the read fails try reconnecting.

The important events for you will be when a read fails - you lost connection, and when a new socket is connected - you regained connection.

That way you can keep track of up time and down time.

you can do like this

try
{
    Socket s = new Socket("address",port);
    DataOutputStream os = new DataOutputStream(s.getOutputStream());
    DataInputStream is = new DataInputStream(s.getInputStream());
    while (true)
    {
        os.writeBytes("GET /index.html HTTP/1.0\n\n");
        is.available();
        Thread.sleep(1000);
   }
}
catch (IOException e)
{
    System.out.println("connection probably lost");
    e.printStackTrace();
}

or you can simply et connection time out like this socket.setSoTimeout(timeout); to check connectivity or you can use

socket.getInputStream().read()

makes the thread wait for input as long as the server is connected and therefore makes your program not do anything - except if you get some input and returns -1 if the client disconnected or what you can do is structure your code in this way

while(isConnected())
  {
      // do stuffs here
  }