Java NIO select() returns without selected keys - why?

java, nio, select

Solution

Short answer: remove `OP_CONNECT` from the list of operations you are interested in for the accepted connection -- an accepted connection is already connected.

I managed to reproduce the issue, which might be exactly what's happening to you:

import java.net.*;
import java.nio.channels.*;


public class MyNioServer {
  public static void main(String[] params) throws Exception {
    final ServerSocketChannel serverChannel = ServerSocketChannel.open();
    serverChannel.configureBlocking(true);
    serverChannel.socket().bind(new InetSocketAddress("localhost", 12345));
    System.out.println("Listening for incoming connections");
    final SocketChannel clientChannel = serverChannel.accept();
    System.out.println("Accepted connection: " + clientChannel);


    final Selector selector = Selector.open();
    clientChannel.configureBlocking(false);
    final SelectionKey clientKey = clientChannel.register(selector, SelectionKey.OP_READ | SelectionKey.OP_CONNECT);
    System.out.println("Selecting...");
    System.out.println(selector.select());
    System.out.println(selector.selectedKeys().size());
    System.out.println(clientKey.readyOps());
  }
}

After the above server receives a connection, the very first `select()` on the connection exits without blocking and there are no keys with ready operations. I don't know why Java behaves in this way, but it appears many people get bitten by this behavior.

The outcome is the same on Sun's JVM 1.5.0_06 on Windows XP as well as Sun's JVM 1.5.0_05 and 1.4.2_04 on Linux 2.6.

Problem

In writing some test code I have found that Selector.select() can return without Selector.selectedKeys() containing any keys to process. This is happening in a tight loop when I register an accept()ed channel with ``` SelectionKey.OP_READ | SelectionKey.OP_CONNECT ``` as the operations of interest. According to the docs, select() should return when: 1) There are channels that can be acted upon. 2) You explicitly call Selector.wakeup() - no keys are selected. 3) You explicitly Thread.interrupt() the thread doing the select() - no keys are selected. If I get no keys after the select() I must be in cases (2) and (3). However, my code is not calling wakeup() or interrupt() to initiate these returns. Any ideas as to what is causing this behaviour?

Original source