As introduced in Netty New and noteworthy in 4.0:
channelOpen, channelBound, and channelConnected have been merged to channelActive. channelDisconnected, channelUnbound, and channelClosed have been merged to channelInactive. Likewise, Channel.isBound() and isConnected() have been merged to isActive().
Note that channelRegistered and channelUnregistered are not equivalent to channelOpen and channelClosed. They are new states introduced to support dynamic registration, deregistration, and re-registration of a Channel, as illustrated below:
IMHO, in scenario of TCP/IP, when a channel is unregistered, the corresponding socket is closed, how could it be re-register again?