Simple Echo Websocket using Java NIO

Server.java

package org.viewpoint;

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.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Base64;
import java.util.Set;

// 1st References to  https://developer.mozilla.org/ko/docs/WebSockets/Writing_WebSocket_servers#Step_4_Closing_the_connection
// 2nd References to https://www.websocket.org/aboutwebsocket.html
// 3rd References to https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API/Writing_WebSocket_server
public class Server {
    private static final String MAGIC_NUMBER = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11";

    private static String encodeSha1AndBase64(String secWebSocketKey) {
        String string = secWebSocketKey + MAGIC_NUMBER;
        MessageDigest digest = null;
        try {
            digest = MessageDigest.getInstance("SHA-1");
            byte[] sha1Result = digest.digest(string.getBytes());
            return Base64.getEncoder().encodeToString(sha1Result);
        } catch (NoSuchAlgorithmException e) {
        }
        return null;
    }

    public static void main(String[] args) throws IOException {
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.bind(new InetSocketAddress(8000));

        Selector selector = Selector.open();
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
        while (true) {
            int select = selector.select(1);

            Set<SelectionKey> selectionKeys = selector.selectedKeys();
            for (SelectionKey key : selectionKeys) {
                if (key.isAcceptable()) {
                    SocketChannel channel = serverSocketChannel.accept();
                    if (channel != null) {
                        channel.configureBlocking(false);
                        channel.register(selector, SelectionKey.OP_READ);
                    }
                }
                if (key.isReadable()) {
                    SocketChannel socketChannel = (SocketChannel) key.channel();
                    ByteBuffer buffer = ByteBuffer.allocate(2048);
                    int count = socketChannel.read(buffer);
                    buffer.flip();

                    //String byteString = toByteString(buffer);
                    String string = new String(buffer.array());
                    if (string.startsWith("GET / HTTP/1.1")) {
                        int swkIndex = string.indexOf("Sec-WebSocket-Key:");
                        int endIndex = string.indexOf("\r\n", swkIndex);
                        String swk = string.substring(swkIndex + 19, endIndex);

                        String s = encodeSha1AndBase64(swk);

                        String response = "HTTP/1.1 101 Switching Protocols\r\n" +
                                "Upgrade: websocket\r\n" +
                                "Connection: Upgrade\r\n" +
                                "Sec-WebSocket-Accept: " + s + "\r\n\r\n";
                        socketChannel.write(ByteBuffer.wrap(response.getBytes()));
                    } else {
                        if (buffer.limit() <= 0)
                            continue;

                        byte firstByte = buffer.get();
                        byte secondByte = buffer.get();

                        byte fin = (byte) ((firstByte & 128) >> 7);
                        byte rsv1 = (byte) ((firstByte & 64) >> 6);
                        byte rsv2 = (byte) ((firstByte & 32) >> 5);
                        byte rsv3 = (byte) ((firstByte & 16) >> 4);

                        byte opCode = (byte) ((firstByte & 8) | (firstByte & 4) | (firstByte & 2) | (firstByte & 1));

                        byte mask = (byte) ((secondByte & 128) >> 7);
                        long length = (long) ((secondByte & 64) | (secondByte & 32) | (secondByte & 16) | (secondByte & 8) | (secondByte & 4) | (secondByte & 2) | (secondByte & 1));

                        if (length == 126) {
                            length = buffer.getShort();
                        } else if (length == 127) {
                            length = buffer.getLong();
                        }

                        byte[] dataBytes = new byte[(int) length];

                        byte[] maskValue = null;
                        if (mask == 1) {
                            maskValue = new byte[4];
                            buffer.get(maskValue);

                            for (int i = 0; i < length; i++) {
                                byte data = buffer.get();
                                dataBytes[i] = (byte) (data ^ maskValue[i % 4]);
                            }
                        }
                        System.out.println(new String(dataBytes));
                    }
                }
            }
        }
    }

    private static String toByteString(ByteBuffer buffer) {
        StringBuilder builder = new StringBuilder();
        int count = buffer.remaining();
        for (int i = 0; i < count; i++) {
            if (i > 0)
                builder.append(',');
            builder.append(buffer.get());
        }
        return builder.toString();
    }
}