import java.nio.*;
import java.nio.channels.*;
import java.net.*;
import java.util.*;
import java.io.IOException;

public class EchoServer {
    
  public static int DEFAULT_PORT = 7;
  
  public static void main(String[] args) {
  
    int port;
    try {
      port = Integer.parseInt(args[0]);
    }
    catch (Exception ex) {
      port = DEFAULT_PORT;   
    }
    System.out.println("Listening for connections on port " + port);

    
    ServerSocketChannel serverChannel;
    Selector selector;
    try {
	
	// create server channel
	serverChannel = ServerSocketChannel.open();
	// extract server socket of the server channel and bind the port
	ServerSocket ss = serverChannel.socket();
	InetSocketAddress address = new InetSocketAddress(port);
	ss.bind(address);

	// configure it to be non blocking
	serverChannel.configureBlocking(false);

	// register the server channel to a selector
	selector = Selector.open();
	serverChannel.register(selector, SelectionKey.OP_ACCEPT);
    }
    catch (IOException ex) {
		ex.printStackTrace();
		return;   
    }
    
    while (true) {
      
		try {
		// check to see if any events
			selector.select();
		}
		catch (IOException ex) {
			ex.printStackTrace();
			break;
		}
        
		// readKeys is a set of ready events
		Set readyKeys = selector.selectedKeys();

		// create an iterator for the set
		Iterator iterator = readyKeys.iterator();

		// iterate over all events
		while (iterator.hasNext()) {
        
			SelectionKey key = (SelectionKey) iterator.next();
			iterator.remove();

			try {
				if (key.isAcceptable()) {
					// a new connection is ready to be accepted

					ServerSocketChannel server = (ServerSocketChannel ) key.channel();
					// extract the ready connection
					SocketChannel client = server.accept();
					System.out.println("Accepted connection from " + client);

					// configure the connection to be non-blocking
					client.configureBlocking(false);

					// register the new connection with read and write events/operations
					SelectionKey clientKey = 
					 client.register(
					  selector, SelectionKey.OP_WRITE | SelectionKey.OP_READ);

					// attach a buffer to the new connection
					ByteBuffer buffer = ByteBuffer.allocate(100);
					clientKey.attach(buffer);
				} // end of isAcceptable

				if (key.isReadable()) {
					// a connection is ready to be read
					SocketChannel client = (SocketChannel) key.channel();
					ByteBuffer output = (ByteBuffer) key.attachment();
					client.read(output);
				} // end of isReadable

				if (key.isWritable()) {
					SocketChannel client = (SocketChannel) key.channel();
					ByteBuffer output = (ByteBuffer) key.attachment();
					output.flip();
					client.write(output);
					output.compact();
				} // end of if isWritable
			}
			catch (IOException ex) {
				key.cancel();
				try {
					key.channel().close();
				}
				catch (IOException cex) {}
			} // end of catch
	    
		} // end of while (iterator.hasNext()) {

    } // end of while (true)
  
  } // end of main

} // end of class