Use Java Selector to handle multiple clients with single thread

Client.java

package com.franzwong.selector;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SocketChannel;

public class Client {
    private static final Logger LOGGER = LoggerFactory.getLogger(Client.class);

    public static void main(String[] args) throws Exception {
        try (var socket = SocketChannel.open(new InetSocketAddress("localhost", 8080))) {
            var buffer = ByteBuffer.allocate(128);
            byte[] remaining = new byte[Long.BYTES];

            try {
                boolean running = true;
                while (running) {
                    socket.read(buffer);

                    buffer.flip();
                    int remainingBytes = buffer.remaining();
                    while (remainingBytes >= Long.BYTES) {
                        long counter = buffer.getLong();
                        LOGGER.info("Counter: {}", counter);
                        remainingBytes = buffer.remaining();

                        if (counter >= 1_000_000) {
                            running = false;
                            break;
                        }
                    }

                    if (remainingBytes > 0) {
                        buffer.get(remaining, 0, remainingBytes);
                        buffer.clear();
                        buffer.put(remaining, 0, remainingBytes);
                    } else {
                        buffer.clear();
                    }
                }
            } catch (Exception e) {
                LOGGER.error("Error occurs", e);
            }
        }
    }
}

Server.java

package com.franzwong.selector;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.concurrent.atomic.LongAdder;

public class Server {

    private static final Logger LOGGER = LoggerFactory.getLogger(Server.class);

    private static class LongHolder {
        long value = 0;
    }

    public static void main(String[] args) throws IOException  {
        int port = 8080;
        try (var selector = Selector.open();
             var serverSocket = ServerSocketChannel.open()) {
            serverSocket.configureBlocking(false);
            serverSocket.register(selector, SelectionKey.OP_ACCEPT);

            serverSocket.socket().bind(new InetSocketAddress(port));
            var buffer = ByteBuffer.allocate(128);

            while (true) {
                selector.select();

                var selectedKeys = selector.selectedKeys();
                var iter = selectedKeys.iterator();

                while (iter.hasNext()) {
                    var key = iter.next();
                    iter.remove();  // next selectedKeys call will return the same key if not removed

                    if (key.isAcceptable()) {
                        LOGGER.info("Client is connected");

                        var counter = new LongHolder();
                        var client = serverSocket.accept();
                        client.configureBlocking(false);
                        client.register(selector, SelectionKey.OP_WRITE, counter);
                    }

                    if (key.isWritable()) {
                        var client = (SocketChannel) key.channel();
                        try {
                            var counter = (LongHolder) key.attachment();
                            counter.value++;
                            long counterValue = counter.value;

                            buffer.clear();
                            buffer.putLong(counterValue);
                            buffer.flip();
                            client.write(buffer);

                            if (counterValue >= 1_000_000L) {
                                LOGGER.info("Counter reaches 1,000,000");
                                client.close();
                            }
                        } catch (Exception e) {
                            LOGGER.error("Error occurs", e);
                            client.close();
                        }
                    }
                }
            }
        }
    }
}