Non-Blocking Time Server NIO Example

This example implements a non-blocking internet time server.

public class NBTimeServer {
    private static final int DEFAULT_TIME_PORT = 8900;

    // Constructor with no arguments creates a time server on default port.
    public NBTimeServer() throws Exception {

    // Constructor with port argument creates a time server on specified port.
    public NBTimeServer(int port) throws Exception {

    // Accept connections for current time. Lazy Exception thrown.
    private static void acceptConnections(int port) throws Exception {
        // Selector for incoming time requests
        Selector acceptSelector = SelectorProvider.provider().openSelector();

        // Create a new server socket and set to non blocking mode
        ServerSocketChannel ssc =;

        // Bind the server socket to the local host and port

        InetAddress lh = InetAddress.getLocalHost();
        InetSocketAddress isa = new InetSocketAddress(lh, port);

        // Register accepts on the server socket with the selector. This
        // step tells the selector that the socket wants to be put on the
        // ready list when accept operations occur, so allowing multiplexed
        // non-blocking I/O to take place.
        SelectionKey acceptKey = ssc.register(acceptSelector,

        int keysAdded = 0;

        // Here's where everything happens. The select method will
        // return when any operations registered above have occurred, the
        // thread has been interrupted, etc.
        while ((keysAdded = > 0) {
            // Someone is ready for I/O, get the ready keys
            Set<SelectionKey> readyKeys = acceptSelector.selectedKeys();
            Iterator<SelectionKey> i = readyKeys.iterator();

            // Walk through the ready keys collection and process date requests.
            while (i.hasNext()) {
                SelectionKey sk = (SelectionKey);
                // The key indexes into the selector so you
                // can retrieve the socket that's ready for I/O
                ServerSocketChannel nextReady = (ServerSocketChannel) sk
                // Accept the date request and send back the date string
                Socket s = nextReady.accept().socket();
                // Write the current time to the socket
                PrintWriter out = new PrintWriter(s.getOutputStream(), true);
                Date now = new Date();

    // Entry point.
    public static void main(String[] args) {
        // Parse command line arguments and
        // create a new time server (no arguments yet)
        try {
            NBTimeServer nbt = new NBTimeServer();
        } catch (Exception e) {