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Question

What happens if I close the channel right after writing the response? Is the response still delivered?

This http upload example seems to say that but I'm not sure if it's just a bug.

writeResponse(ctx.channel());
ctx.channel().close();

Answer 1

If you close a channel using .close() just after writing, then you create a race condition in with either all the data is written, half of the data, or even none, depending on the length of the message.

This happens because all written messages end up into a queue, and depending if the current thread is a Netty thread or not, it either processes the data directory, or just returns.

Since in most conditions you only want to close the channel after all writes as completed, you should use the following code when writing your response:

ctx.writeAndFlush(protocolSpecificPacket)
    .addListener(ChannelFutureListener.CLOSE);

While this always works, you don't always have access to the last write, in these cases you can also send an empty ByteBuf , and then add the listener to that:

ctx.writeAndFlush(Unpooled.EMPTY_BUFFER)
    .addListener(ChannelFutureListener.CLOSE);

Answer 2

No it not a bug,but not the best way to close channel,I assume you writeResponse perform like this:

ctx.channel().write(msg)

It's sent asynchronisly,actuallay the message will offer to a writeBufferQueue,and the write IO thread will be wake up to do the actual write.

Check the ChannelFuture returned by ctx.channel().write(msg),you can wait on that object.

the best way to close a netty channel,I think,would be :

ctx.channel().write(a emptybuffer).addFutureListener(Channels.CloseFuture);

since you are using netty 4.0,you may neeed to search something similar with above.