https://github.com/apache/incubator-druid/pull/6417 proposes upgrading to netty 4.1.x
A lot of the prior issues are likely resolved. Things like java-util are part of the druid repository now, the dependent libraries which were still using 4.0.x are upgraded (in the PR) to ones using 4.1.x, and Spark's laster major version (2.3.x) has netty 4.1 I propose giving netty 4.1.x another shot. Sound good? Charles Allen
