Netty exceptionCaught 异常机制

Netty handler的exceptionCaught 只会catch inbound handler的exception, outbound exceptions 需要在writeAndFlush方法里加上listener来监听消息是否发送成功，
最好在每个outbound handler的处理类里加上try catch，只是处理由于程序异常导致的发包失败，由于网络原因没有发送成功，最终会被nettychannel异常检测机制检测到，反馈到channel inactivate事件上，这样能够处理大部分case，但是如果需要保证消息的最终送达，不允许丢失，则需要业务层自己保证。

https://github.com/netty/netty/issues/4721

exceptionCaught(...) is only called for inbound exceptions. All outbound exceptions must by handled in a listener by design. If you always want to have exceptions handled in exceptionCaught(...) just add a ChannelOutboundHandler that will an listener for every outbound operation

Netty outbandHandler write方法抛出的Exception需要加listener监听操作结果，来处理exception，The exceptionCaught() method is only invoked for exceptions from inbound events like channelRead(), channelActive() etc.

参考：

https://github.com/netty/netty/issues/2357

推荐阅读：

https://stackoverflow.com/questions/30994095/how-to-catch-all-exception-in-netty

放在pipline的最后
public class ExceptionHandler extends ChannelDuplexHandler {

    private static final Logger logger = LoggerFactory.getLogger(ExceptionHandler.class);

    @Override
    public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
        ctx.write(msg, promise.addListener((ChannelFutureListener) future -> {
            if (!future.isSuccess()) {
                logger.error("send data to client exception occur: ", future.cause());
            }
        }));
    }
}