import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;
public class NioFileServer {
    private static final int PORT = 8080;
    public static void main(String[] args) throws IOException {
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.bind(new InetSocketAddress(PORT));
        serverSocketChannel.configureBlocking(false);
        Selector selector = Selector.open();
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        System.out.println("Server is listening on port " + PORT);
        while (true) {
            if (selector.select() > 0) {
                for (SelectionKey key : selector.selectedKeys()) {
                    if (key.isAcceptable()) {
                        SocketChannel socketChannel = serverSocketChannel.accept();
                        socketChannel.configureBlocking(false);
                        socketChannel.register(selector, SelectionKey.OP_READ);
                    } else if (key.isReadable()) {
                        SocketChannel socketChannel = (SocketChannel) key.channel();
                        ByteBuffer buffer = ByteBuffer.allocate(1024);
                        int bytesRead = socketChannel.read(buffer);
                        if (bytesRead == -1) {
                            socketChannel.close();
                        } else {
                            buffer.flip();
                            Files.write(Paths.get("received_file.txt"), buffer.array(), StandardOpenOption.CREATE, StandardOpenOption.APPEND);
                        }
                    }
                }
                selector.selectedKeys().clear();
            }
        }
    }
}

import java.io.*;
import java.nio.*;
import java.nio.channels.*;
import java.nio.file.*;
public class NioFileClient {
    private static final String SERVER_HOST = "localhost";
    private static final int SERVER_PORT = 8080;
    public static void main(String[] args) throws IOException {
        Path filePath = Paths.get("file_to_send.txt");
        FileChannel fileChannel = FileChannel.open(filePath, StandardOpenOption.READ);
        SocketChannel socketChannel = SocketChannel.open();
        socketChannel.connect(new InetSocketAddress(SERVER_HOST, SERVER_PORT));
        socketChannel.configureBlocking(false);
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        while (fileChannel.read(buffer) != -1) {
            buffer.flip();
            socketChannel.write(buffer);
            buffer.clear();
        }
        fileChannel.close();
        socketChannel.close();
    }
}