Muitiplexed socket communication in Java

Question

I am writing a server program that can accept communication from multiple (but fixed) number of clients. I want to keep the program single-threaded. To do so, I am using non-blocking socket to iterate over each client, but each client's channel uses blocking mode. Here's my server code:

class server {
    public static void main(String args[])
         throws Exception {

        ServerSocketChannel channel = ServerSocketChannel.open();
        channel.configureBlocking(false);
        channel.socket().bind(new java.net.InetSocketAddress("localhost", 8005));
        System.out.println("Server attivo porta 8005");
        Selector selector = Selector.open();
        channel.register(selector, SelectionKey.OP_ACCEPT);

        for(;;) {
          selector.select();
          Set keys = selector.selectedKeys();
          Iterator i = keys.iterator();

          while(i.hasNext()) {
            SelectionKey key = (SelectionKey) i.next();

            i.remove();

            if (key.isAcceptable()) {
              SocketChannel client = channel.accept();
              client.configureBlocking(true);

              ObjectInputStream ois = new ObjectInputStream(
                    client.socket().getInputStream());
              String s = (String)ois.readObject();
              System.out.println(s);
            }
          }
        }
    }
}

The client uses simple blocking I/O, as shown here:

class client {
    public static void main(String args[]) throws Exception {
        SocketChannel channel = SocketChannel.open();

        channel.configureBlocking(true);

        channel.connect(new java.net.InetSocketAddress("localhost", 8005));

        ObjectOutputStream oos = new ObjectOutputStream
              (channel.socket().getOutputStream());

        for (int i = 0; i < 100; i++) {
             oos.writeObject(new String("Hello " + i));
             System.out.println(i);
         }
    }
}

The problem is that although the client wants to write 100 times, the server is reading the message just once. Neither the server nor the client is giving any exception, but I am only getting the output "Hello 0" from the server. Is there any problem in what I am doing here? If so, what alternatives do I have?

Thanks.

Update: Closing the ObjectInputStream within the server's loop gives a BrokenPipeException by the client (the server behaves in the same way).

Answer 1

The issue is that you're just checking for new connections with key.isAcceptable() . You also need to check for reads with key.isReadble() . You should only be doing connection setup from key.isAcceptable() .

See Java ServerSocketChannel SocketChannel (Callback)

Answer 2

The problem is that the server is not waiting for the client to send all it's data. In client server programs, what you need to do is to establish a clear protocol between both so that they are in sync when data is transmitted/received. This is usually done by signaling an end of transmission by either side by sending a designated symbol or closing the connection when they are done