SocketChannel in Java sends data, but it doesn't get to destination application

Question

Hi Everybody,

I'm suffering a lot to create a simple ChatServer in Java, using the NIO libraries. Wonder if someone could help me. I am doing that by using SocketChannel and Selector to handle multiple clients in a single thread. The problem is: I am able to accept new connections and get it's data, but when I try to send data back, the SocketChannel simply doesn't work. In the method write(), it returns a integer that is the same size of the data i'm passing to it, but the client never receives that data. Strangely, when I close the server application, the client receives the data. It's like the socketchannel maintains a buffer, and it only get flushed when I close the application.

Here are some more details, to give you more information to help. I'm handling the events in this piece of code:

private void run() throws IOException {

 ServerSocketChannel ssc = ServerSocketChannel.open();

 // Set it to non-blocking, so we can use select
 ssc.configureBlocking( false );

 // Get the Socket connected to this channel, and bind it
 // to the listening port
 this.serverSocket = ssc.socket();
 InetSocketAddress isa = new InetSocketAddress( this.port );
 serverSocket.bind( isa );

 // Create a new Selector for selecting
 this.masterSelector = Selector.open();

 // Register the ServerSocketChannel, so we can
 // listen for incoming connections
 ssc.register( masterSelector, SelectionKey.OP_ACCEPT );

 while (true) {
  // See if we've had any activity -- either
  // an incoming connection, or incoming data on an
  // existing connection
  int num = masterSelector.select();

  // If we don't have any activity, loop around and wait
  // again
  if (num == 0) {
   continue;
  }

  // Get the keys corresponding to the activity
  // that has been detected, and process them
  // one by one
  Set keys = masterSelector.selectedKeys();
  Iterator it = keys.iterator();
  while (it.hasNext()) {
   // Get a key representing one of bits of I/O
   // activity
   SelectionKey key = (SelectionKey)it.next();

   // What kind of activity is it?
   if ((key.readyOps() & SelectionKey.OP_ACCEPT) ==
    SelectionKey.OP_ACCEPT) {

    // Aceita a conexão
    Socket s = serverSocket.accept();

    System.out.println( "LOG: Conexao TCP aceita de " + s.getInetAddress() + ":" + s.getPort() );

    // Make sure to make it non-blocking, so we can
    // use a selector on it.
    SocketChannel sc = s.getChannel();
    sc.configureBlocking( false );

    // Registra a conexao no seletor, apenas para leitura
    sc.register( masterSelector, SelectionKey.OP_READ );

   } else if ( key.isReadable() ) {
    SocketChannel sc = null;

    // It's incoming data on a connection, so
    // process it
    sc = (SocketChannel)key.channel();

    // Verifica se a conexão corresponde a um cliente já existente

    if((clientsMap.getClient(key)) != null){
     boolean closedConnection = !processIncomingClientData(key);
     if(closedConnection){
      int id = clientsMap.getClient(key);
      closeClient(id);
     }
    } else {
     boolean clientAccepted = processIncomingDataFromNewClient(key);
     if(!clientAccepted){
      // Se o cliente não foi aceito, sua conexão é simplesmente fechada
      sc.socket().close();
      sc.close();
      key.cancel();
     }
    }

   }
  }

  // We remove the selected keys, because we've dealt
  // with them.
  keys.clear();
 }
}

This piece of code is simply handles new clients that wants to connect to the chat. So, a client makes a TCP connection to the server, and once it gets accepted, it sends data to the server following a simply text protocol, informing his id and asking to get registrated to the server. I handle this in the method processIncomingDataFromNewClient(key). I'm also keeping a map of clients and its connections in a data structure similar to a hashtable. Iḿ doing that because I need to recover a client Id from a connection and a connection from a client Id. This is can be shown in: clientsMap.getClient(key). But the problem itself resides in the method processIncomingDataFromNewClient(key). There, I simply read the data that the client sent to me, validate it, and if it's ok, I send a message back to the client to tell that it is connected to the chat server. Here is a similar piece of code:

private boolean processIncomingDataFromNewClient(SelectionKey key){
 SocketChannel sc = (SocketChannel) key.channel();
 String connectionOrigin = sc.socket().getInetAddress() + ":" + sc.socket().getPort();

 int id = 0; //id of the client

 buf.clear();
 int bytesRead = 0;
 try {
  bytesRead = sc.read(buf);
  if(bytesRead<=0){
   System.out.println("Conexão fechada pelo: " + connectionOrigin);
   return false;
  }
  System.out.println("LOG: " + bytesRead + " bytes lidos de " + connectionOrigin);

  String msg = new String(buf.array(),0,bytesRead);

  // Do validations with the client sent me here
  // gets the client id

   }catch (Exception e) {
    e.printStackTrace();
    System.out.println("LOG: Oops. Cliente não conhece o protocolo. Fechando a conexão: " + connectionOrigin);
    System.out.println("LOG: Primeiros 10 caracteres enviados pelo cliente: " + msg); 
    return false;
   }
  }

 } catch (IOException e) {
  System.out.println("LOG: Erro ao ler dados da conexao: " + connectionOrigin);
  System.out.println("LOG: "+ e.getLocalizedMessage());
  System.out.println("LOG: Fechando a conexão...");

  return false;
 }

 // If it gets to here, the protocol is ok and we can add the client
 boolean inserted = clientsMap.addClient(key, id);
 if(!inserted){
  System.out.println("LOG: Não foi possível adicionar o cliente. Ou ele já está conectado ou já têm clientes demais. Id: " + id);
  System.out.println("LOG: Fechando a conexão: " + connectionOrigin);
  return false;
 }
 System.out.println("LOG: Novo cliente conectado! Enviando mesnsagem de confirmação. Id: " + id + " Conexao: " + connectionOrigin);

 /* Here is the error */
 sendMessage(id, "Servidor pet: connection accepted");

 System.out.println("LOG: Novo cliente conectado! Id: " + id + " Conexao: " + connectionOrigin);
 return true;
}

And finally, the method sendMessage(SelectionKey key) looks like this:

private void sendMessage(int destId, String msg) {
 Charset charset = Charset.forName("ISO-8859-1");
    CharBuffer charBuffer = CharBuffer.wrap(msg, 0, msg.length());
    ByteBuffer bf = charset.encode(charBuffer);

    //bf.flip();

    int bytesSent = 0;
    SelectionKey key = clientsMap.getClient(destId);

    SocketChannel sc = (SocketChannel) key.channel();

    try {
     /
     int total_bytes_sent = 0;
     while(total_bytes_sent < msg.length()){
      bytesSent = sc.write(bf);
      total_bytes_sent += bytesSent;

     }

     System.out.println("LOG: Bytes enviados para o cliente " + destId + ": "+ total_bytes_sent + " Tamanho da mensagem: " + msg.length());
    } catch (IOException e) {
     System.out.println("LOG: Erro ao mandar mensagem para: " + destId);
     System.out.println("LOG: " + e.getLocalizedMessage());
    }
}

So, what is happening is that the server, when send a message, prints something like this:

LOG: Bytes sent to the client: 28 Size of the message: 28

So, it tells that it sent the data, but the chat client keeps blocking, waiting in the recv() method. So, the data never gets to it. When I close the server application, though, all the data appears in the client. I wonder why.

It is important to say that the client is in C and the server JAVA, and I'm running both in the same machine, an Ubuntu Guest in virtualbox under windows. I also run both under windows host and under linuxes hosts, and keep getting the same strange problem.

I'm sorry for the great lenght of this question, but I already searched a lot of places for an answer, found a lot of tutorials and questions, including here at StackOverflow, but coundn't find a reasonable explanation. I am really not liking this Java NIO, and i saw a lot of people complaining about it too. I am thinking that if I had done that in C it would have been a lot easier :-D

So, if someone could help me and even discuss this behavor, it would be great! :-)

Thanks everybody in advance,

Péterson

Answer 1

try

System.out.println("LOG: " + bytesRead + " bytes lidos de " + connectionOrigin);
buf.flip();
String msg = new String(buf.array(),0,bytesRead);