ChannelEvent是和这个Channel相关的IO事件和请求，会由各个ChannelHandler来处理。
事件分为上行（Upstream）和下行（Downstream）两种。
当服务器从客户端收到一个消息，那么与之相关的就是一个上行事件（Upstream Event），流水线（Pipeline）中的UpstreamChannelHandler会处理它；如果服务器要响应这个客户端，那么与响应消息对应的就是下行事件（Downstream Event），流水线（Pipeline）中的DownstreamChannelHandler会处理它。

org.jboss.netty.channel.ChannelEvent源码：

public interface ChannelEvent {

    /**
     * Returns the {@link Channel} which is associated with this event.
     */
    Channel getChannel();

    /**
     * Returns the {@link ChannelFuture} which is associated with this event.
     * If this event is an upstream event, this method will always return a
     * {@link 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.
     */
    ChannelFuture getFuture();
}

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

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

Downstream events