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import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.Socket;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.spi.SelectorProvider;
import java.util.Date;
import java.util.Iterator;
import java.util.Set;

/**
 * sample code: https://docs.oracle.com/en/java/javase/19/core/non-blocking-time-server-nio-example.html
 */
public class NBTimeServer { // NonBlockingTimeServer
	private static final int DEFAULT_TIME_PORT = 8900;

	public NBTimeServer() throws Exception {
		acceptConnections(this.DEFAULT_TIME_PORT);
	}

	public NBTimeServer(int port) throws Exception {
		acceptConnections(port);
	}

	private static void acceptConnections(int port) throws Exception {
		// Create selector
		Selector acceptSelector = SelectorProvider.provider().openSelector();

		// Create serverSocketChannel and bind port
		ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
		serverSocketChannel.configureBlocking(false);

		InetAddress localhost = InetAddress.getLocalHost();
		InetSocketAddress isa = new InetSocketAddress(localhost, port);
		serverSocketChannel.socket().bind(isa);

		// Register accept event of serverSocketChannel to selector.
		// Receive selectionKey as a token for registration
		SelectionKey acceptKey = serverSocketChannel.register(acceptSelector, SelectionKey.OP_ACCEPT);

		int keysAdded = 0;

		// Blocking until an event occurs
		while ((keysAdded = acceptSelector.select()) > 0) {
			// Check occurred events
			Set<SelectionKey> readyKeys = acceptSelector.selectedKeys();
			Iterator<SelectionKey> i = readyKeys.iterator();

			// Process logic for each selectionKey with an event
			while (i.hasNext()) {
				SelectionKey sk = i.next();
				// Only one is registered, so if an event is received, it is the same as the selectionKey received at registration
				assert acceptKey == sk;

				// selectionKey: channel=sun.nio.ch.ServerSocketChannelImpl[/127.0.0.1:8900], selector=sun.nio.ch.KQueueSelectorImpl@57829d67, interestOps=16, readyOps=16
				System.out.println("selectionKey: " + sk);

				i.remove();// Remove selectKey to prevent reprocessing

				ServerSocketChannel nextReady = (ServerSocketChannel)sk.channel();
				Socket socket = nextReady.accept().socket();
				PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
				Date now = new Date();
				out.println(now);
				out.close();
			}
		}
	}

	public static void main(String[] args) {
		System.out.println("start");

		try {
			NBTimeServer nbt = new NBTimeServer();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}