I have added running Puma server tests under JRuby 9k (see puma/puma#701 including Travis https://travis-ci.org/puma/puma/jobs/63116729). There is one test there causing that hanging: https://github.com/puma/puma/blob/master/test/test_puma_server.rb#L207

I haven't debug it deeply but it seems it is caused by reading from a socket. I got the same issue as on Travis on my local machine with these settings:

$ ruby -v
jruby 9.0.0.0.pre2 (2.2.2) 2015-04-28 2755ae0 Java HotSpot(TM) 64-Bit Server VM 25.45-b02 on 1.8.0_45-b14 +jit [darwin-x86_64]