如何理解 NIO 中 attach 以及如何正确注册 write 事件？

我开始以为 attach 是分读的附件和写的附件的，但写了测试代码发现并不是。 服务端代码：

package NonBlocking;

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.Iterator;

public class NonBlockingServer {
    public static void main(String[] args) throws IOException {
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
        serverSocketChannel.configureBlocking(false);
        serverSocketChannel.bind(new InetSocketAddress(8888));

        Selector selector = Selector.open();
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);  //首先注册 ACCEPT 事件

        int result = 0; int i = 1;
        while(true) {  //遍历获得就绪事件
            result = selector.select();
            System.out.println(String.format("selector %dth loop, ready event number is %d", i++, result));
            if (result == 0) {
                continue;
            }
            Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();
            while(iterator.hasNext()){  //就绪事件可能不止一个
                SelectionKey sk=iterator.next();

                if(sk.isAcceptable()){  //如果是 ACCEPT，那么与之关联的 channel 肯定是个 ServerSocketChannel
                    System.out.println("服务端有 ACCEPT 事件就绪");
                    ServerSocketChannel ss = (ServerSocketChannel)sk.channel();
                    SocketChannel socketChannel = ss.accept();
                    socketChannel.configureBlocking(false);  //也切换非阻塞
                    socketChannel.register(selector, SelectionKey.OP_READ);  //注册 read 事件
                }
                else if(sk.isReadable()){    //如果是 READ，那么与之关联的 channel 肯定是个 SocketChannel
                    System.out.println("服务端有 READ 事件就绪");
                    SocketChannel socketChannel = (SocketChannel)sk.channel();
                    ByteBuffer buf=ByteBuffer.allocate(1024);
                    int len=0;
                    StringBuilder sb = new StringBuilder();
                    while((len=socketChannel.read(buf))>0){
                        buf.flip();
                        String s  = new String(buf.array(),0,len);
                        sb.append(s);
                        buf.clear();
                    }
                    //服务端开始响应消息
                    ByteBuffer readAtta = (ByteBuffer)sk.attachment();
                    if (readAtta != null) {
                        System.out.println("lasttime readAtta string is: "+new String(readAtta.array()));
                    } else {
                        System.out.println("lasttime readAtta is null ");
                    }
                    sk.attach(ByteBuffer.wrap(sb.toString().getBytes()));
                    sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE);
                    String sendStr = "您的消息'"+sb.toString()+"'我已经收到了";
                    System.out.println("接下来 attach 的是："+sendStr);
                    sk.attach(ByteBuffer.wrap(sendStr.getBytes()));
                }
                else if(sk.isWritable()){
                    System.out.println("服务端有 WRITE 事件就绪");
                    SocketChannel socketChannel = (SocketChannel)sk.channel();

                    ByteBuffer writeAtta = (ByteBuffer) sk.attachment();
                    if (writeAtta != null) {
                        System.out.println("lasttime writeAtta string is: "+new String(writeAtta.array()));
                    } else {
                        System.out.println("lasttime writeAtta is null ");
                    }

                    sk.interestOps(sk.interestOps() & ~SelectionKey.OP_WRITE);
                }
                iterator.remove();
                System.out.println("after remove key");
            }
        }
    }
}

客户端代码（这个不重要，就放链接里了）： https://paste.ubuntu.com/p/58P39BQQTm/ 使用方法：客户端在控制台每输入一句话，再去服务端看执行效果:

selector 1th loop, ready event number is 1
服务端有 ACCEPT 事件就绪
after remove key
selector 2th loop, ready event number is 1
服务端有 READ 事件就绪
lasttime readAtta is null 
接下来 attach 的是：您的消息'你好'我已经收到了
after remove key
selector 3th loop, ready event number is 1
服务端有 WRITE 事件就绪
lasttime writeAtta string is: 您的消息'你好'我已经收到了
after remove key

selector 4th loop, ready event number is 1
服务端有 READ 事件就绪
lasttime readAtta string is: 您的消息'你好'我已经收到了
接下来 attach 的是：您的消息'他好吗'我已经收到了
after remove key
selector 5th loop, ready event number is 1
服务端有 WRITE 事件就绪
lasttime writeAtta string is: 您的消息'他好吗'我已经收到了
after remove key

• 可以发现，attach 其实针对的是一个对象。换句话说，它是针对的同一个 channel 的附件。这样理解对吗？
• 是否可以认为，sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE);和sk.interestOps(sk.interestOps() & ~SelectionKey.OP_WRITE);是正确的注册和反注册 write 事件的姿势？

然后我进行服务端代码修改：

• 把sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE);换成socketChannel.register(selector, SelectionKey.OP_WRITE)
• 把sk.interestOps(sk.interestOps() & ~SelectionKey.OP_WRITE);换成socketChannel.register(selector, SelectionKey.OP_READ)

再看服务端效果则是这样了：

selector 1th loop, ready event number is 1
服务端有 ACCEPT 事件就绪
after remove key
selector 2th loop, ready event number is 1
服务端有 READ 事件就绪
lasttime readAtta is null 
接下来 attach 的是：您的消息'你好'我已经收到了
after remove key
selector 3th loop, ready event number is 1
服务端有 WRITE 事件就绪
lasttime writeAtta string is: 您的消息'你好'我已经收到了
after remove key


selector 4th loop, ready event number is 1
服务端有 READ 事件就绪
lasttime readAtta is null 
接下来 attach 的是：您的消息'他好吗'我已经收到了
after remove key
selector 5th loop, ready event number is 1
服务端有 WRITE 事件就绪
lasttime writeAtta string is: 您的消息'他好吗'我已经收到了
after remove key

可以发现：

• 经过socketChannel.register(selector, SelectionKey.OP_READ);后，下一次再执行 attachment，附件就丢了，变成 null 了。
• 但经过socketChannel.register(selector, SelectionKey.OP_WRITE);后，下一次再执行 attachment，附件却不会丢。 这是为什么呢？