k.attachment();为什么就有值了 我们自己写的nioserversocketchannle 就是null 为啥 ???
safeExecute(task);
head->ServerBootstrapAcceptor->tail
①：java.nio.channels.spi.AbstractSelectableChannel#register
selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
②：io.netty.channel.nio.NioEventLoop#select
③：io.netty.channel.socket.nio.NioServerSocketChannel#doReadMessages