Interface ChannelEvent

All Known Subinterfaces:
ChannelStateEvent, ChildChannelStateEvent, ExceptionEvent, IdleStateEvent, MessageEvent, WriteCompletionEvent
All Known Implementing Classes:
DefaultChildChannelStateEvent, DefaultExceptionEvent, DefaultIdleStateEvent, DefaultWriteCompletionEvent, DownstreamChannelStateEvent, DownstreamMessageEvent, UpstreamChannelStateEvent, UpstreamMessageEvent

public interface ChannelEvent
An I/O event or I/O request associated with a Channel.

A ChannelEvent is handled by a series of ChannelHandlers in a ChannelPipeline.

Upstream events and downstream events, and their interpretation

Every event is either an upstream event or a downstream event. If an event flows forward from the first handler to the last handler in a ChannelPipeline, we call it an upstream event and say "an event goes upstream." If an event flows backward from the last handler to the first handler in a ChannelPipeline, we call it a downstream event and say "an event goes downstream." (Please refer to the diagram in ChannelPipeline for more explanation.)

When your server receives a message from a client, the event associated with the received message is an upstream event. When your server sends a message or reply to the client, the event associated with the write request is a downstream event. The same rule applies for the client side. If your client sent a request to the server, it means your client triggered a downstream event. If your client received a response from the server, it means your client will be notified with an upstream event. Upstream events are often the result of inbound operations such as InputStream.read(byte[]), and downstream events are the request for outbound operations such as OutputStream.write(byte[]), Socket.connect(SocketAddress), and Socket.close().

Upstream events

Event nameEvent type and conditionMeaning
"messageReceived" MessageEvent a message object (e.g. ChannelBuffer) was received from a remote peer
"exceptionCaught" ExceptionEvent an exception was raised by an I/O thread or a ChannelHandler
"channelOpen" ChannelStateEvent
(state = OPEN, value = true)
a Channel is open, but not bound nor connected Be aware that this event is fired from within the I/O thread. You should never execute any heavy operation in there as it will block the dispatching to other workers!
"channelClosed" ChannelStateEvent
(state = OPEN, value = false)
a Channel was closed and all its related resources were released
"channelBound" ChannelStateEvent
(state = BOUND, value = SocketAddress)
a Channel is open and bound to a local address, but not connected. Be aware that this event is fired from within the I/O thread. You should never execute any heavy operation in there as it will block the dispatching to other workers!
"channelUnbound" ChannelStateEvent
(state = BOUND, value = null)
a Channel was unbound from the current local address
"channelConnected" ChannelStateEvent
(state = CONNECTED, value = SocketAddress)
a Channel is open, bound to a local address, and connected to a remote address Be aware that this event is fired from within the I/O thread. You should never execute any heavy operation in there as it will block the dispatching to other workers!
"writeComplete" WriteCompletionEvent something has been written to a remote peer
"channelDisconnected" ChannelStateEvent
(state = CONNECTED, value = null)
a Channel was disconnected from its remote peer
"channelInterestChanged" ChannelStateEvent
(state = INTEREST_OPS, no value)
a Channel's interestOps was changed

These two additional event types are used only for a parent channel which can have a child channel (e.g. ServerSocketChannel).

Event nameEvent type and conditionMeaning
"childChannelOpen" ChildChannelStateEvent
(childChannel.isOpen() = true)
a child Channel was open (e.g. a server channel accepted a connection.)
"childChannelClosed" ChildChannelStateEvent
(childChannel.isOpen() = false)
a child Channel was closed (e.g. the accepted connection was closed.)

Downstream events

Event nameEvent type and conditionMeaning
"write" MessageEventSend a message to the Channel.
"bind" ChannelStateEvent
(state = BOUND, value = SocketAddress)
Bind the Channel to the specified local address.
"unbind" ChannelStateEvent
(state = BOUND, value = null)
Unbind the Channel from the current local address.
"connect" ChannelStateEvent
(state = CONNECTED, value = SocketAddress)
Connect the Channel to the specified remote address.
"disconnect" ChannelStateEvent
(state = CONNECTED, value = null)
Disconnect the Channel from the current remote address.
"close" ChannelStateEvent
(state = OPEN, value = false)
Close the Channel.

Other event types and conditions which were not addressed here will be ignored and discarded. Please note that there's no "open" in the table. It is because a Channel is always open when it is created by a ChannelFactory.

Additional resources worth reading

Please refer to the ChannelHandler and ChannelPipeline documentation to find out how an event flows in a pipeline and how to handle the event in your application.

  • Method Details

    • getChannel

      Channel getChannel()
      Returns the Channel which is associated with this event.
    • getFuture

      ChannelFuture getFuture()
      Returns the ChannelFuture which is associated with this event. If this event is an upstream event, this method will always return a SucceededChannelFuture because the event has occurred already. If this event is a downstream event (i.e. I/O request), the returned future will be notified when the I/O request succeeds or fails.