NIO first problems

Question

Hi guys,
I experience very strange problem. Once a new connection is established and the key for the channel is registered for READ operations, the selector starts to continuosly provide me with the key having OP_READ set up:
if (selector.select() == 0)
    continue;
Iterator<SelectionKey> it = selector.selectedKeys().iterator();
while (it.hasNext())
    SelectionKey key = (SelectionKey) it.next();
    // Accept new connection
    if (key.isAcceptable())
        ServerSocketChannel server = (ServerSocketChannel) key.channel();
        SocketChannel channel = server.accept();
        if (channel == null)
            continue;
        // Set the new channel nonblocking
        channel.configureBlocking(false);
        // Register it with the selector
        SelectionKey newKey = channel.register(selector, SelectionKey.OP_READ);
        // initialization of attachment ...
    if (key.isReadable())
        System.out.print("session index = "  +((ConnectionDescriptor)key.attachment()).session.sessionIndex);+
+System.out.print(", counter "+  ++counter);+
+System.out.println(", ops="+  key.readyOps());                   
        // read the data from channel
        if( counter == 10 ) {
            setEnabled(false);
            break;
    it.remove();
}I use a python script in order to test the server:
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect("localhost", 28000)
s.send(b'\x00\x01')
s.close()When I run the script I see the following output from the server:
session index = 1, counter 1, ops=1
session index = 1, counter 2, ops=1
session index = 1, counter 3, ops=1
header limit = 2
body limit = 0
process message 1
session index = 1, counter 4, ops=1
session index = 1, counter 5, ops=1
session index = 1, counter 6, ops=1
session index = 1, counter 7, ops=1
session index = 1, counter 8, ops=1
session index = 1, counter 9, ops=1
session index = 1, counter 10, ops=1some parts of the code are omitted in order to keep it clear. If I do not stop it at 10th iteration it runs into endless loop.
Why the key always says that the channel is "ready" to be read while I actually have already read the data?
Thanks a lot in advance.
Edited by: coderman on Jan 28, 2010 5:44 AM

Answer 1

This is an example class you can reproduce the issue with:
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.ClosedChannelException;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Collection;
import java.util.Iterator;
public class MyServer
    private Selector selector;
    private int sessionSequence;
    public MyServer() throws IOException
        final ServerSocketChannel mainServerChannel = ServerSocketChannel.open();
        ServerSocket mainSocket = mainServerChannel.socket();
        mainSocket.bind(new InetSocketAddress(28000));
        mainServerChannel.configureBlocking(false);
        selector = Selector.open();
        mainServerChannel.register(selector, SelectionKey.OP_ACCEPT);
        int counter = 0;
        while (true)
            if (selector.select(1000) == 0)
                continue;
            Iterator<SelectionKey> it = selector.selectedKeys().iterator();
            while (it.hasNext())
                SelectionKey key = (SelectionKey) it.next();
                if (key.isAcceptable())
                    ServerSocketChannel server = (ServerSocketChannel) key.channel();
                    SocketChannel channel = server.accept();
                    if (channel == null)
                        continue;
                    channel.configureBlocking(false);
                    SelectionKey newKey = channel.register(selector, SelectionKey.OP_READ );
                if (key.isReadable())
                    System.out.print("counter " + counter);
                    System.out.print(", ops=" + key.readyOps());
                    ByteBuffer header = ByteBuffer.allocateDirect(10);
                    try
                        ((SocketChannel) key.channel()).read(header);
                    } catch (ClosedChannelException cce)
                        // the channel has been closed by the server
                    } catch (IOException ex)
                        // networking problem. Might be also connection lost.
                    header.flip();
                    System.out.println(", header limit = " + header.limit());
                    if (++counter == 15)
                        System.exit(0);
                it.remove();
    public static void main(String[] argv)
        try
            new MyServer();
        } catch (IOException ex)
            ex.printStackTrace();
}Here is the python script:
import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect(("localhost", 28000))
s.send(b'\x00\x01\x02')
s.shutdown(1)This is what I get on my computer:
counter 0, ops=1, header limit = 3
counter 1, ops=1, header limit = 0
counter 2, ops=1, header limit = 0
counter 3, ops=1, header limit = 0
counter 4, ops=1, header limit = 0
counter 5, ops=1, header limit = 0
counter 6, ops=1, header limit = 0
counter 7, ops=1, header limit = 0
counter 8, ops=1, header limit = 0
counter 9, ops=1, header limit = 0
counter 10, ops=1, header limit = 0
counter 11, ops=1, header limit = 0
counter 12, ops=1, header limit = 0
counter 13, ops=1, header limit = 0
counter 14, ops=1, header limit = 0