Java Language NIO - Networking


SelectionKey defines the different selectable operations and information between its Selector and Channel. In particular, the attachment can be used to store connection-related information.

Handling OP_READ is pretty straight-forward. However, care should be taken when dealing with OP_WRITE: most of the time, data can be written to sockets so the event will keep firing. Make sure to register OP_WRITE only before you want to write data (see that answer).

Also, OP_CONNECT should be cancelled once the Channel has connected (because, well, it is connected. See this and that answers on SO). Hence the OP_CONNECT removal after finishConnect() succeeded.

Using Selector to wait for events (example with OP_CONNECT)

NIO appeared in Java 1.4 and introduced the concept of "Channels", which are supposed to be faster than regular I/O. Network-wise, the SelectableChannel is the most interesting as it allows to monitor different states of the Channel. It works in a similar manner as the C select() system call: we get woken-up when certain types of events occur:

  • connection received (OP_ACCEPT)
  • connection realized (OP_CONNECT)
  • data available in read FIFO (OP_READ)
  • data can be pushed to write FIFO (OP_WRITE)

It allows for separation between detecting socket I/O (something can be read/written/...) and performing the I/O (read/write/...). Especially, all I/O detection can be done in a single thread for multiple sockets (clients), while performing I/O can be handled in a thread pool or anywhere else. That allows for an application to scale easily to the number of connected clients.

The following example shows the basics:

  1. Create a Selector
  2. Create a SocketChannel
  3. Register the SocketChannelto the Selector
  4. Loop with the Selector to detect events
Selector sel =; // Create the Selector
SocketChannel sc =; // Create a SocketChannel
sc.configureBlocking(false); // ... non blocking
sc.setOption(StandardSocketOptions.SO_KEEPALIVE, true); // ... set some options

// Register the Channel to the Selector for wake-up on CONNECT event and use some description as an attachement
sc.register(sel, SelectionKey.OP_CONNECT, "Connection to"); // Returns a SelectionKey: the association between the SocketChannel and the Selector
System.out.println("Initiating connection");
if (sc.connect(new InetSocketAddress("", 80)))
    System.out.println("Connected"); // Connected right-away: nothing else to do
else {
    boolean exit = false;
    while (!exit) {
        if ( == 0) // Did something happen on some registered Channels during the last 100ms?
            continue; // No, wait some more
        // Something happened...
        Set<SelectionKey> keys = sel.selectedKeys(); // List of SelectionKeys on which some registered operation was triggered
        for (SelectionKey k : keys) {
            System.out.println("Checking "+k.attachment());
            if (k.isConnectable()) { // CONNECT event
                System.out.print("Connected through select() on "" -> ");
                if (sc.finishConnect()) { // Finish connection process
                    k.interestOps(k.interestOps() & ~SelectionKey.OP_CONNECT); // We are already connected: remove interest in CONNECT event
                    exit = true;
                } else
            // TODO: else if (k.isReadable()) { ...
        keys.clear(); // Have to clear the selected keys set once processed!
System.out.print("Disconnecting ... ");
sc.shutdownOutput(); // Initiate graceful disconnection
// TODO: emtpy receive buffer

Would give the following output:

Initiating connection
Checking Connection to
Connected through 'select()' on java.nio.channels.SocketChannel[connection-pending] -> done!
Disconnecting ... done