ChannelOutboundHandler本应该只关注outbound事件，但是它却声明了一个read方法：

/**
* Intercepts {@link ChannelHandlerContext#read()}.
*/
void read(ChannelHandlerContext ctx) throws Exception;

有人在stackoverflow问了这个问题，trustin给出了回答：

Inbound handlers are supposed to handle inbound events. Events are triggered by external stimuli such as data received from a socket. Outbound handlers are supposed to intercept the operations issued by your application.

所以ChannelOutboundHandler上的read方法，如其注释所述，是为了拦截ChannelHandlerContext.read()操作。也就是说，ChannelOutboundHandler可以通过read()方法在必要的时候阻止向inbound读取更多数据的操作。这个设计在处理协议的握手时非常有用。