Java.nio select() method return 0 in my client application

Question

Hello,
I'm developing a simple chat application who echo messages
But my client application loop because the select() method return 0
This is my code
// SERVER
package test;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;
public class Server {
     private int port = 5001;
     public void work() {               
          try {
               ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
               serverSocketChannel.configureBlocking(false);
               InetSocketAddress isa = new InetSocketAddress(port);               
               serverSocketChannel.socket().bind(isa);
               Selector selector = Selector.open();
               serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
               System.out.println("Listing on "+port);
               while(selector.select()>0) {
                    Set keys = selector.selectedKeys();
                    for(Iterator i = keys.iterator(); i.hasNext();) {
                         SelectionKey key = (SelectionKey) i.next();
                         i.remove();
                         if (key.isAcceptable()) {
                              ServerSocketChannel keyChannel = (ServerSocketChannel)key.channel();                              
                              SocketChannel channel = keyChannel.accept();
                              channel.configureBlocking(false);                              
                              channel.register(selector, SelectionKey.OP_READ );
                         } else if (key.isReadable()) {
                              SocketChannel keyChannel = (SocketChannel) key.channel();
                              String m = Help.read(keyChannel );
                              Help.write(m.toUpperCase(), keyChannel );
          } catch (IOException e) {                                             
               e.printStackTrace();                         
     public static void main(String[] args) {
          Server s = new Server();
          s.work();
// CLIENT
package test;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
public class Client extends JFrame  {
     private String host = "localhost";
     private int port = 5001;
     private SocketChannel socketChannel;
     private Selector selector;
     public void work() {               
          try {
               socketChannel = SocketChannel.open();
               socketChannel.configureBlocking(false);
               InetSocketAddress isa = new InetSocketAddress(host, port);               
               socketChannel.connect(isa);
               selector = Selector.open();
               socketChannel.register(selector, SelectionKey.OP_CONNECT | SelectionKey.OP_READ );
               while(true) {
                    selector.select();
                    Set keys = selector.selectedKeys();
                    for(Iterator i = keys.iterator(); i.hasNext();) {
                         SelectionKey key = (SelectionKey) i.next();
                         i.remove();
                         if (key.isConnectable()) {
                              SocketChannel keyChannel = (SocketChannel) key.channel();
                              if (keyChannel.isConnectionPending()) {
                                   System.out.println("Connected "+keyChannel.finishConnect());                                                                           
                         } else if (key.isReadable()) {                                                                                                                                                           
                              SocketChannel keyChannel = (SocketChannel) key.channel();                                             
                              String m = Help.read(keyChannel);
                              display(m);                                                                                                                                                                                                                   
          } catch (IOException e) {                                             
               e.printStackTrace();                         
     private void display(final String m) {
          SwingUtilities.invokeLater(new Runnable() {
               public void run() {
                    area.append(m+"\n");
                    textFieed.setText("");
     private void sendMessage(final String m) {
          Thread t = new Thread(new Runnable() {               
               public void run() {                                                                                
                    try {                         
                         Help.write(m, socketChannel);
                    } catch (IOException e) {               
                         e.printStackTrace();
          t.start();                    
     public Client() {
          addWindowListener(new WindowAdapter() {
               public void windowClosing(WindowEvent e) {
                    System.exit(1);
          textFieed.addKeyListener(new KeyAdapter() {
               public void keyPressed(KeyEvent e) {
                    if (e.getKeyCode()== KeyEvent.VK_ENTER) {
                         String m = textFieed.getText();
                         sendMessage(m);     
          area.setEditable(false);
          getContentPane().add(textFieed, "North");
          getContentPane().add(new JScrollPane(area));
          setBounds(200, 200, 400, 300);
          show();
     private String messageToSend;
     private JTextArea area = new JTextArea();
     JTextField textFieed = new JTextField();
     public static void main(String[] args) {
          Client s = new Client();
          s.work();
// HELPER CLASS
package test;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;
public class Help {
     private static Charset charset = Charset.forName("us-ascii");
     private static CharsetEncoder enc = charset.newEncoder();
     private static CharsetDecoder dec = charset.newDecoder();
     private static void log(String m) {
          System.out.println(m);
     public static String read(SocketChannel channel) throws IOException {
          log("*** start READ");                              
          int n;
          ByteBuffer buffer = ByteBuffer.allocate(1024);
          while((n = channel.read(buffer)) > 0) {
               System.out.println("     adding "+n+" bytes");
          log("  BUFFER REMPLI : "+buffer);
          buffer.flip();               
          CharBuffer cb = dec.decode(buffer);          
          log("  CHARBUFFER : "+cb);
          String m = cb.toString();
          log("  MESSAGE : "+m);          
          log("*** end READ");
          //buffer.clear();
          return m;                    
     public static void write(String m, SocketChannel channel) throws IOException {          
          log("xxx start WRITE");          
          CharBuffer cb = CharBuffer.wrap(m);
          log("  CHARBUFFER : "+cb);          
          ByteBuffer  buffer = enc.encode(cb);
          log("  BUFFER ALLOUE REMPLI : "+buffer);
          int n;
          while(buffer.hasRemaining()) {
               n = channel.write(buffer);                         
          System.out.println("  REMAINING : "+buffer.hasRemaining());
          log("xxx end WRITE");

Answer 1

Here's the fix for that old problem. Change the work method to do the following
- don't register interest in things that can't happen
- when you connect register based on whether the connection is complete or pending.
- add the OP_READ interest once the connection is complete.
This doesn't fix all the other problems this code will have,
eg.
- what happens if a write is incomplete?
- why does my code loop if I add OP_WRITE interest?
- why does my interestOps or register method block?
For code that answers all those questions see my obese post Taming the NIO Circus
Here's the fixed up Client code
// CLIENT
package test
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
import java.util.Set;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
public class Client extends JFrame  {
     private String host = "localhost";
     private int port = 5001;
     private SocketChannel socketChannel;
     private Selector selector;
     public void work() {
          try {
               socketChannel = SocketChannel.open();
               socketChannel.configureBlocking(false);
               InetSocketAddress isa = new InetSocketAddress(host, port);
               socketChannel.connect(isa);
               selector = Selector.open();
               int interest = 0;
               if(socketChannel.isConnected())interest = SelectionKey.OP_READ;
               else if(socketChannel.isConnectionPending())interest = SelectionKey.OP_CONNECT;
               socketChannel.register(selector, interest);
               while(true)
                    int nn = selector.select();
                    System.out.println("nn="+nn);
                    Set keys = selector.selectedKeys();
                    for(Iterator i = keys.iterator(); i.hasNext();)
                         SelectionKey key = (SelectionKey) i.next();
                         i.remove();
                         if (key.isConnectable())
                              SocketChannel keyChannel = (SocketChannel) key.channel();
                              System.out.println("Connected "+keyChannel.finishConnect());
                              key.interestOps(SelectionKey.OP_READ);
                         if (key.isReadable())
                              SocketChannel keyChannel = (SocketChannel) key.channel();
                              String m = Help.read(keyChannel);
                              display(m);
          } catch (IOException e) {
               e.printStackTrace();
     private void display(final String m) {
          SwingUtilities.invokeLater(new Runnable() {
               public void run() {
                    area.append(m+"\n");
                    textFieed.setText("");
     private void sendMessage(final String m) {
          Thread t = new Thread(new Runnable() {
               public void run() {
                    try {
                         Help.write(m, socketChannel);
                    } catch (IOException e) {
                         e.printStackTrace();
          t.start();
     public Client() {
          addWindowListener(new WindowAdapter() {
               public void windowClosing(WindowEvent e) {
                    System.exit(1);
          textFieed.addKeyListener(new KeyAdapter() {
               public void keyPressed(KeyEvent e) {
                    if (e.getKeyCode()== KeyEvent.VK_ENTER) {
                         String m = textFieed.getText();
                         sendMessage(m);
          area.setEditable(false);
          getContentPane().add(textFieed, "North");
          getContentPane().add(new JScrollPane(area));
          setBounds(200, 200, 400, 300);
          show();
     private String messageToSend;
     private JTextArea area = new JTextArea();
     JTextField textFieed = new JTextField();
     public static void main(String[] args) {
          Client s = new Client();
          s.work();