Channel全名是 io.netty.channel.Channel
是netty通信的载体，是netty网络操作的抽象接口，包含了JDK提供的Channel的功能，还额外聚合了一组功能。
Chnanel包含的东西相当庞杂，这里只做一个简介，当一回源码的搬运工。
Channel 源码上的说明：（英语战五渣，全靠翻译工具）
io.netty.channel.Channel
A nexus to a network socket or a component which is capable of I/O operations such as read, write, connect, and bind.
【连接到网络socket或组件的连接，它能够进行I/O操作，如读、写、连接和绑定。】
A channel provides a user: 【一个channel 能提供：】
- the current state of the channel (e.g. is it open? is it connected?), 【channel当前的状态，例如 是否开启，是否连接】
- the configuration parameters of the channel (e.g. receive buffer size), 【channel的配置数据，例如 接收的buffer长度】
- the I/O operations that the channel supports (e.g. read, write, connect, and bind), 【channel支持的I/O操作，例如，读、写、连接、绑定】
- the ChannelPipeline which handles all I/O events and requests associated with the channel.【获取关联的ChannelPipeline，使用上面注册的handler处理所有的I/O事件】
1、All I/O operations are asynchronous. 【所有的 I/O 操作都是异步的】
All I/O operations in Netty are asynchronous. 【所有的 I/O 操作在netty中都是异步的】
It means any I/O calls will return immediately with no guarantee that the requested I/O operation has been completed at the end of the call. 【这意味着任何I/O调用都会立即返回，不能保证请求的I/O操作在调用结束时完成】
Instead, you will be returned with a ChannelFuture instance which will notify you when the requested I/O operation has succeeded, failed, or canceled.【相反，会返回一个ChannelFuture 对象，它会在I/O操作成功、失败、取消时通知你】
2、Channels are hierarchical.【Channel是有 等级/层次 的】
A Channel can have a parent depending on how it was created.【谁创建了它，谁就是它的父channel】
For instance, a SocketChannel, that was accepted by ServerSocketChannel, will return the ServerSocketChannel as its parent on parent().【例如，ServerSocketChannel通过accept()方法接受一个SocketChannel后，调用SocketChannel的 parent()会返回 ServerSocketChannel对象】
The semantics of the hierarchical structure depends on the transport implementation where the Channel belongs to. 【层次结构的语义取决于通道所属的Channel实现】
For example, you could write a new Channel implementation that creates the sub-channels that share one socket connection, as BEEP and SSH do.【例如，你可以编写一个新的Channel实现，它创建一个子Channel，它与父Channel共享一个socket的内存，例如BEEP和 SSH do】
3、Downcast to access transport-specific operations.【向下转型获得子类的特殊操作】
Some transports exposes additional operations that is specific to the transport. 【某些子类传输会提供一些特定的操作】
Down-cast the Channel to sub-type to invoke such operations.【向下转型成子类传输以获得这些操作】
For example, with the old I/O datagram transport, multicast join / leave operations are provided by DatagramChannel.【例如，UDP传输有特定的 jion() 和 leave() 操作，可以向下转型成 DatagramChannel获得这些操作】
4、Release resources.【释放资源】
It is important to call close() or close(ChannelPromise) to release all resources once you are done with the Channel. 【一旦你使用完Channel，必须调用 close() 或 close(ChannelPromise)方法释放一些重要的资源】
This ensures all resources are released in a proper way, i.e. filehandles.【确保这些资源以一个适当的方式释放，比如文件句柄】
1 public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel>
继承了三个接口
- AttributeMap ：简单理解为一个存放属性的map
- Comparable ：表明Channel是可以比较的
- ChannelOutboundInvoker ：网络通信
ChannelOutboundInvoker
public interface ChannelOutboundInvoker {
//绑定本地地址
ChannelFuture bind(SocketAddress localAddress);
ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise);
//连接远程地址
ChannelFuture connect(SocketAddress remoteAddress);
ChannelFuture connect(SocketAddress remoteAddress, ChannelPromise promise);
ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress);
ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);
//解除连接
ChannelFuture disconnect();
ChannelFuture disconnect(ChannelPromise promise);
//关闭Channel
ChannelFuture close();
ChannelFuture close(ChannelPromise promise);
//与EventLoop解除注册
ChannelFuture deregister();
ChannelFuture deregister(ChannelPromise promise);
/**
* Request to Read data from the {@link Channel} into the first inbound buffer,
* triggers an {@link ChannelInboundHandler#channelRead(ChannelHandlerContext, Object)} event if data was read,
* and triggers a {@link ChannelInboundHandler#channelReadComplete(ChannelHandlerContext) channelReadComplete} event so the
* handler can decide to continue reading. If there‘s a pending read operation already, this method does nothing.
* This will result in having the
* {@link ChannelOutboundHandler#read(ChannelHandlerContext)}
* method called of the next {@link ChannelOutboundHandler} contained in the {@link ChannelPipeline} of the
* {@link Channel}.
*/
/**
* 从channel中读取数据到第一个 InboundBuffer，
* 1、触发 ChannelInboundHandler#channelRead(ChannelHandlerContext, Object)事件，在有数据的情况下
* 2、触发ChannelInboundHandler#channelReadComplete(ChannelHandlerContext)事件，handler继续读取
* 如果有read操作已经挂起，则不执行任何操作
* 实际调用：Channel——>ChannelPipeline——>ChannelOutboundHandler——>ChannelOutboundHandler#read(ChannelHandlerContext)——>read()
* ChannelHandlerContext继承了ChannelOutboundInvoker，它的子类实现read()方法，最后ctx.read()。
* */
ChannelOutboundInvoker read();
//写
ChannelFuture write(Object msg);
ChannelFuture write(Object msg, ChannelPromise promise);
//将数据冲刷到Channel
ChannelOutboundInvoker flush();
//write + flush
ChannelFuture writeAndFlush(Object msg);
ChannelFuture writeAndFlush(Object msg, ChannelPromise promise);
ChannelPromise newPromise();
ChannelProgressivePromise newProgressivePromise();
ChannelFuture newSucceededFuture();
ChannelFuture newFailedFuture(Throwable cause);
ChannelPromise voidPromise();
}
本人英语渣渣，源码中每个方法都有注释，但是每个方法中的this will result... 这一段让我困惑，捣鼓了好久才明白它的含义并写在代码上，其他方法的翻译也可以按照这个格式。
ChannelOutboundInvoker主要是定义一些 I/O的操作，扩展在Channel接口中。
Channel
public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel> {
//get属性
ChannelId id(); //获得一个唯一的channelId
EventLoop eventLoop();//获得关联的EventLoop
Channel parent();//父Channel
ChannelConfig config();//获得配置参数
ChannelMetadata metadata();//获得元数据
SocketAddress localAddress();//获得本地地址
SocketAddress remoteAddress();//获得远端地址
ChannelFuture closeFuture();//获得Channel关闭时的异步结果
ChannelPipeline pipeline();//获得事件管道，用于处理IO事件
ByteBufAllocator alloc();//获得字节缓存分配器
Unsafe unsafe();//获得Unsafe对象
//状态查询
boolean isOpen();//是否开放
boolean isRegistered();// 是否注册到一个EventLoop
boolean isActive();// 是否激活
boolean isWritable();// 是否可写
long bytesBeforeUnwritable();
long bytesBeforeWritable();
@Override
Channel read();
@Override
Channel flush();
}
Unsafe
public interface Channel extends AttributeMap, ChannelOutboundInvoker, Comparable<Channel> {
....
interface Unsafe {
RecvByteBufAllocator.Handle recvBufAllocHandle();//当接受数据时返回它，用于分配ByteBuf
SocketAddress localAddress();
SocketAddress remoteAddress();
void register(EventLoop eventLoop, ChannelPromise promise);
void deregister(ChannelPromise promise);
void bind(SocketAddress localAddress, ChannelPromise promise);
void connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);
void disconnect(ChannelPromise promise);
void close(ChannelPromise promise);
void closeForcibly();//当注册失败时强制关闭连接
void beginRead();
void write(Object msg, ChannelPromise promise);
void flush();
ChannelPromise voidPromise();//返回一个特殊的可重用的ChannelPromise，它仅作为一个容器不用于操作成功或失败的通知器
ChannelOutboundBuffer outboundBuffer();//返回消息发送缓冲区
}
}
unsafe ? 不安全的？ 一开始我是懵逼的，看了《netty权威指南》才知道这个不安全是相对于程序员而言的，不应该直接被程序员调用。它的方法与ChannelOutboundInvoker中的方法大部分重叠，实际上Channel的I/O读写操作都是由它来完成。