一种关联网络套接字或能够进行I/O 操作（例如读取，写入，连接和绑定）的组件的组件
channel提供给用户以下用途：
channel的当前状态
channel的配置参数
channel支持的I/O操作（例如：读、写、连接、绑定）
channel关联的能够处理所有I/O事件和请求的ChannelPipeline
所有的I/O操作都是异步的
channel是分层的
* <p>
* A {@link Channel} can have a {@linkplain #parent() parent} depending on
* how it was created. For instance, a {@link SocketChannel}, that was accepted
* by {@link ServerSocketChannel}, will return the {@link ServerSocketChannel}
* as its parent on {@link #parent()}.
一个Channel
* <p>
* The semantics of the hierarchical structure depends on the transport
* implementation where the {@link Channel} belongs to. For example, you could
* write a new {@link Channel} implementation that creates the sub-channels that
* share one socket connection, as <a href="http://beepcore.org/">BEEP</a> and
* <a href="http://en.wikipedia.org/wiki/Secure_Shell">SSH</a> do.
*
* <h3>Downcast to access transport-specific operations</h3>
* <p>
* Some transports exposes additional operations that is specific to the
* transport. Down-cast the {@link Channel} to sub-type to invoke such
* operations. For example, with the old I/O datagram transport, multicast
* join / leave operations are provided by {@link DatagramChannel}.
*
* <h3>Release resources</h3>
* <p>
* It is important to call {@link #close()} or {@link #close(ChannelPromise)} to release all
* resources once you are done with the {@link Channel}. This ensures all resources are
* released in a proper way, i.e. filehandles.
*/