Nio uses threads to handle the writing problem of each read and write request

problem description

when writing the demo of java nio, I want to follow the React pattern so that every request that is ready is processed in a new thread. The specific code is shown below.

when a client disconnects after writing data, it will report an error:

java.nio.channels.ClosedChannelException

it is obvious that this way of writing is inappropriate or incorrect. See that the attach () method is used on the Internet

selectionKey.attach(new Acceptor(selector,selectionKey));

so:
question 1: what is the wrong writing method that caused the client to close the Times, and what is the cause of the error?
question 2: how do I use attach (), and why can this writing avoid turning off the client side to report an error?

related codes

use separate threads for select operations.

new Thread(() -> {
            try {
                while (selector.select() > 0) {
                    for (SelectionKey selectionKey : selector.selectedKeys()) {
                        try {
                            selector.selectedKeys().remove(selectionKey);
                            if (selectionKey.isAcceptable()) {
                                SocketChannel socketChannel = serverSocketChannel.accept();
                                socketChannel.configureBlocking(false);
                                socketChannel.register(selector, SelectionKey.OP_READ);
                            }
                            if (selectionKey.isReadable()) {
                                doRead(selectionKey);
                                
                            }
                        } catch (IOException e) {
                            selectionKey.cancel();
                        }
                    }
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }).start();

when a readable event is encountered, it is processed in a new thread.

public void doRead(SelectionKey selectionKey) {
        new Thread(() -> {
            try {
                SocketChannel socketChannel = (SocketChannel) selectionKey.channel();
                ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
                String content = "";
                int c = 0;
                while ((c = socketChannel.read(byteBuffer)) > 0) {
                    byteBuffer.flip();
                    content += charset.decode(byteBuffer);
                }
                System.out.println(content);
                selectionKey.interestOps(SelectionKey.OP_READ);
                if (c == -1) {
                    selectionKey.cancel();
                    socketChannel.close();
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        }).start();
    }

Java

Apr.02,2021

Iterator<SelectionKey> iterator = selector.selectedKeys().iterator(); while (iterator.hasNext()) { SelectionKey sk = iterator.next(); iterator.remove(); ..... }

selectionKey.interestOps(SelectionKey.OP_READ); if (c == -1) { selectionKey.cancel(); socketChannel.close(); }