Thanks for raising this, can you share your applications apache http dependencies? This discussion (java - ArrayStoreException making a post using apache httpasyncclient - Stack Overflow) suggests there might be a mismatch of the httpclient components that get pulled in. I'd like to check if those are the ones the ES rest client uses as transitive dependencies or if there is anything else in the classpath. If we (ES) pull in the dependencies we might have an incompatibility (but that would require further investigation then I think).