NIO sockets

Question

Hello,
Im having difficulty understanding the WRITE issue of nio sockets. Ive read most of the nio-related posts on this forum but I still cant figure it out. Ive also looked at some examples by PkWooster but they seems really complicated.
I have a server which accepts connections and echos messages to all clients. Everything works fine except my client cannot write to the server. I know I need to switch back and forth between Op interests since they can block the cpu if set incorrectly but I dont really understand what to set when. Since my server seems to be working fine Ill post my Clients code:
import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.net.InetSocketAddress;
import java.util.*;
import java.nio.charset.*;
public class GameClient implements Runnable {
  Game game;
  SocketChannel sc;
  ByteBuffer buffer;
Selector selector;
ByteBuffer sendBuffer=null;
    String host = "192.168.1.102";
    int port = 8888;
private CharsetDecoder asciiDecoder;
private CharsetEncoder encoder;
private LinkedList sendQ = new LinkedList();
SelectionKey sk;
LinkedList invocations;
public GameClient (Game g) {
game = g;
    buffer = ByteBuffer.allocateDirect(1024);
          Charset charset = Charset.forName("ISO-8859-1");
          asciiDecoder = charset.newDecoder();
          encoder = charset.newEncoder();
game.CharMSGArrival("Starting...");  // transfers data to the game thread
public void run(){
try{
    // Create a nonblocking socket channel and connect to server.
    sc = SocketChannel.open();
    sc.configureBlocking(false);
game.CharMSGArrival("Connecting to server..."+"\n");
    InetSocketAddress addr = new InetSocketAddress(host, port);
    sc.connect(addr);    // Nonblocking
    while (!sc.finishConnect()) {
game.CharMSGArrival("I am waiting ..."+"\n" );
game.CharMSGArrival("Connection acqired"+ "\n" );
    // Send initial message to server.
    buffer.put((new Date().toString()+ "\n").getBytes());
    buffer.flip();
    sc.write(buffer);
    // Create a new selector for use.
    selector = Selector.open();
    // Register the socket channel with the selector.
    sk = sc.register(selector, SelectionKey.OP_READ);
    while (true) {
        //System.out.println("Listening for server on port " +
         //                   remotePort);
        // Monitor the registered channel.
        int n = selector.select();
        if (n == 0) {
            continue;   // Continue to loop if no I/O activity.
        // Get an iterator over the set of selected keys.
        Iterator it = selector.selectedKeys().iterator();
        // Look at each key in the selected set.
        while (it.hasNext()) {
            // Get key from the selected set.
            SelectionKey key = (SelectionKey) it.next();
            // Remove key from selected set.
            it.remove();
            // Get the socket channel from the key.
            SocketChannel keyChannel = (SocketChannel) key.channel();
            // Is there data to read on this channel?
            if (key.isReadable()) {
                replyServer(keyChannel);
            if (key.isWritable()) {
                doSend();
       } catch (IOException ioe) {
  private void replyServer(SocketChannel socketChannel)
                      throws IOException {
    // Read from server.
buffer.flip( );
    int count = socketChannel.read(buffer);
    if (count <= 0) {
        // Close channel on EOF or if there is no data,
        // which also invalidates the key.
        socketChannel.close();
        return;
buffer.flip( );
String str = asciiDecoder.decode(buffer).toString( );
     game.CharMSGArrival(str);
public void doSend(){
sk.interestOps(SelectionKey.OP_WRITE);     
if(sendBuffer != null)write(sendBuffer);
writeQueued();     
public void trySend(String text){ // this method is invoked when a send buttong is pressed in the 'game' thread
          sendQ.add(text);     // first put it on the queue
          writeQueued();          // write all we can from the queue
     private void writeQueued()
          while(sendQ.size() > 0)     // now process the queue
               String msg = (String)sendQ.remove(0);
               write(msg);     // write the string
private void write(String text)
          try
          ByteBuffer buf = encoder.encode(CharBuffer.wrap(text));
               write(buf);
          catch(Exception e){e.printStackTrace();}
         * write out a byte buffer
     private void write(ByteBuffer data)
          SocketChannel scc = (SocketChannel)sk.channel();
          if(scc.isOpen())
               int len=0;
               if(data.hasRemaining())
                    try{len = sc.write(data);}
                    catch(IOException e){e.printStackTrace(); //closeComplete();
               if(data.hasRemaining())     // write would have blocked
game.CharMSGArrival("write blocked"+"/n");
                    //writeReady = false;
     sk.interestOps(SelectionKey.OP_READ | SelectionKey.OP_WRITE);     // select OP_WRITE
                    sendBuffer = data;          // save the partial buffer
               else {sendBuffer = null;
sk.interestOps(SelectionKey.OP_READ);     }
          else game.CharMSGArrival("write closed"+"/n");
}

Answer 1

public void doSend(){
sk.interestOps(SelectionKey.OP_WRITE);     
if(sendBuffer != null)write(sendBuffer);
writeQueued();     
}This method makes no sense. You only get into it if the key is writable, which means it is already registered for OP_WRITE, so why would you register it again?
Also you're frequently ignoring the result of many write calls, of which you have too many.
See the thread 'Taming the NIO circus': http://forum.java.sun.com/thread.jspa?threadID=459338,
ignoring all the contributions except those by pkwooster and me.
I'm not that crazy about Mina myself. It has its own buffer classes and it quietly uses threads, i.e. missing the entire point of NIO.