Environment

• JRuby: jruby 9.1.15.0 (2.3.3) 2017-12-07 929fde8 Java HotSpot(TM) 64-Bit Server VM 25.151-b12 on 1.8.0_151-b12 +jit [linux-x86_64]
• Kernel: Linux u186024434db159d25c92 4.10.0-38-generic #42~16.04.1-Ubuntu SMP Tue Oct 10 16:32:20 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
• Distro: Ubuntu 16.04.3 LTS

Expected Behavior

Example code:

puts RUBY_DESCRIPTION

puts Thread.new { sleep }.backtrace.inspect

Output on MRI 2.0 to 2.5 (this seems to have changed from 1.9, where it returned nil):

ruby 2.4.2p198 (2017-09-14 revision 59899) [x86_64-linux]
[]

(This is of course a race and may sometimes yield non-empty results)

Actual Behavior

Output on JRuby:

jruby 9.1.15.0 (2.3.3) 2017-12-07 929fde8 Java HotSpot(TM) 64-Bit Server VM 25.151-b12 on 1.8.0_151-b12 +jit [linux-x86_64]
nil