@amiwrong123

* Registers this channel with the given selector, returning a selection
* key.
*
* <p> An invocation of this convenience method of the form
*
* <blockquote><tt>sc.register(sel, ops)</tt></blockquote>
*
* behaves in exactly the same way as the invocation
*
* <blockquote>
<tt>sc.{@link
* #register(java.nio.channels.Selector,int,java.lang.Object)
* register}(sel, ops, null)</tt></blockquote>

还是我原来说的，你仔细看你的代码：

//sk.interestOps(sk.interestOps() | SelectionKey.OP_WRITE);
socketChannel.register(selector, SelectionKey.OP_WRITE); 👈

sk.attach(ByteBuffer.wrap(sendStr.getBytes())); 👈

注册写，attachment 置 null，然后调用了 attach()，attachment 为 sendStr 。

//sk.interestOps(sk.interestOps() & ~SelectionKey.OP_WRITE);
socketChannel.register(selector, SelectionKey.OP_READ); 👈

注册读，attachment 置空，没有调用 attach()，所以 attachment 依然为 null 。