Implements observer pattern on socket client server

Question

Each client is registered as an observer once it connected to the server. When any client did changes, another clients would be notified.

My problem is how could I keep the socket connected?

Can I store all connections like Socket[] and check their InputStream every second?

Answer 1

I don't know if i got your problem right... But I'll give it a shot.

The Problem sems to be, that you have 1 ServerSocket and multiple sockets (one for each client) and now you want to get notified/informed about acitvity on these sockets. so you plan on iterating through the list of sockets?

The keyword is nonblocking I/O. Search for the Keyword "selector" or "multiplexing". I'll try to put up a simple example.

I build a really minimal example. But it is a start. This is the whole

package server;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.channels.spi.SelectorProvider;
import java.util.Iterator;
import java.util.Set;

public class Server {

public Server() throws IOException{
    Selector selector = SelectorProvider.provider().openSelector();
    ServerSocketChannel ssc = ServerSocketChannel.open().bind(new InetSocketAddress(9000));
    ssc.configureBlocking(false);       
    ssc.register(selector, 
              SelectionKey.OP_ACCEPT);  


    while(true) {

      int readyChannels = selector.select();

      if(readyChannels == 0) continue;


      Set<SelectionKey> selectedKeys = selector.selectedKeys();

      Iterator<SelectionKey> keyIterator = selectedKeys.iterator();

      while(keyIterator.hasNext()) {

        SelectionKey key = keyIterator.next();

        if(key.isAcceptable()) {
           System.out.println("acceptable");
           SocketChannel socketChan =  ((ServerSocketChannel)key.channel()).accept();
           socketChan.configureBlocking(false);
            socketChan.register(selector, SelectionKey.OP_READ);

        } else if (key.isConnectable()) {
            // a connection was established with a remote server.

        } else if (key.isReadable()) {
            System.out.println("Processing reading...");

            ByteBuffer buf = ByteBuffer.allocate(1024);
            int readedBytes = ((SocketChannel)key.channel()).read(buf);
            System.out.println("Readed: " + readedBytes);
            buf.flip();

            for(byte b : buf.array()) {
                System.out.print((char) b);
            }

        } else if (key.isWritable()) {
            // a channel is ready for writing
        }

        keyIterator.remove();
      }
    }
}

public static void main(String[] args) throws IOException {
    Server server = new Server();

}

}

I can run this, and connect via netcat on port 9000 to it, and send messages from there. Just ONE Thread, with as many client connections as you like....

I used this ressources/examples

http://tutorials.jenkov.com/java-multithreaded-servers/thread-pooled-server.html and http://docs.oracle.com/javase/7/docs/technotes/guides/io/example/index.html