netty point

1. ChannelFuture的作用

channelFuture 是为了保存channel异步操作的结果的，在Netty中所有的IO操作都是异步的，因此，你不能立刻得知消息是否被正确处理，但是我们可以过一会等它执行完成或者直接注册一个监听，具体的实现就是通过Future和ChannelFuture,他们可以注册一个监听，当操作执行成功或失败时监听会自动触发。总之，所有的操作都会返回一个ChannelFuture。

2. ctx.close and ctx.channel.close的区别（引用自overflow）

Let’s say we have three handlers in the pipeline, and they all intercept the close() operation, and calls ctx.close() in it.

ChannelPipeline p = ...;
p.addLast("A", new SomeHandler());
p.addLast("B", new SomeHandler());
p.addLast("C", new SomeHandler());
...

public class SomeHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void close(ChannelHandlerContext ctx, ChannelPromise promise) {
        ctx.close(promise);
    }
}

Channel.close() will trigger C.close(), B.close(), A.close(), and then close the channel.

ChannelPipeline.context(“C”).close() will trigger B.close(), A.close(), and then close the channel.

ChannelPipeline.context(“B”).close() will trigger A.close(), and then close the channel.

ChannelPipeline.context(“A”).close() will close the channel. No handlers will be called.

So, when you should use Channel.close() and ChannelHandlerContext.close()? The rule of thumb is:

If you are writing a ChannelHandler and wanna close the channel in the handler, call ctx.close().
If you are closing the channel from outside the handler (e.g. you have a background thread which is not an I/O thread, and you want to close the connection from that thread.)

3. ChannelPromise的作用

Promise与future的不同在于它可以设置自己的状态，是在future之上的扩展。所以Promise是netty内部用到的，给返回给用户的对象Future是同一个。

4. 优雅关闭shutdownGracefully

这是一个有效的关闭客户端/服务端的方法，会向对方发送结束连接请求，同时关闭信道和Pipeline，同时退出循环(结束线程等待，向下执行)。