Java NIO中的Selector是什么？它在多路复用中的作用是什么？

Selector selector = Selector.open();
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.bind(new InetSocketAddress(8080));
serverSocketChannel.configureBlocking(false);

serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

while (true) {
    selector.select();  // 阻塞直到至少一个通道就绪
    Set<SelectionKey> selectedKeys = selector.selectedKeys();
    Iterator<SelectionKey> iterator = selectedKeys.iterator();

    while (iterator.hasNext()) {
        SelectionKey key = iterator.next();
        iterator.remove();

        if (key.isAcceptable()) {
            // 处理连接请求
            ServerSocketChannel server = (ServerSocketChannel) key.channel();
            SocketChannel client = server.accept();
            client.configureBlocking(false);
            client.register(selector, SelectionKey.OP_READ);
        }

        if (key.isReadable()) {
            // 处理读取请求
            SocketChannel client = (SocketChannel) key.channel();
            ByteBuffer buffer = ByteBuffer.allocate(256);
            client.read(buffer);
            // 处理读取到的数据
        }
    }
}

// 一个简单的基于Selector的网络聊天服务器示例
Selector selector = Selector.open();
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
serverSocketChannel.bind(new InetSocketAddress(8080));
serverSocketChannel.configureBlocking(false);
serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

while (true) {
    selector.select();  // 阻塞直到有通道准备就绪

    Set<SelectionKey> selectedKeys = selector.selectedKeys();
    Iterator<SelectionKey> iterator = selectedKeys.iterator();

    while (iterator.hasNext()) {
        SelectionKey key = iterator.next();
        iterator.remove();

        if (key.isAcceptable()) {
            // 处理新连接
            ServerSocketChannel server = (ServerSocketChannel) key.channel();
            SocketChannel client = server.accept();
            client.configureBlocking(false);
            client.register(selector, SelectionKey.OP_READ);
        }

        if (key.isReadable()) {
            // 读取客户端消息
            SocketChannel client = (SocketChannel) key.channel();
            ByteBuffer buffer = ByteBuffer.allocate(256);
            client.read(buffer);
            // 将消息广播给其他客户端
        }
    }
}