I'm not sure whether 9.0.x is still maintained, but let me raise an incompatibility issue that seems to be introduced in 9.0.5.0 and already fixed in 9.1.x.

• JRuby version (jruby -v): jruby 9.0.5.0 (2.2.3) 2016-01-26 7bee00d Java HotSpot(TM) 64-Bit Server VM 25.112-b16 on 1.8.0_112-b16 +jit [darwin-x86_64]

• Operating system and platform (e.g. uname -a): Darwin myhostname.local 15.6.0 Darwin Kernel Version 15.6.0: Thu Sep 1 15:01:16 PDT 2016; root:xnu-3248.60.11~2/RELEASE_X86_64 x86_64

When loading a Struct via autoload and assigning it to a constant in JRuby 9.0.5.0, the loaded Struct is not associated with the Constant.

For example, having the following t.rb, y.rb, and z.rb:

# t.rb
X = Struct.new(:x)
require_relative 'y'
autoload :Z, "#{__dir__}/z.rb"

p x: X, y: Y, z: Z

# y.rb
Y = Struct.new(:y)

# z.rb
Z = Struct.new(:z)

MRI 2.2+, JRuby 9.0.4.0, and JRuby 9.1.7.0 prints {:x=>X, :y=>Y, :z=>Z} but only JRuby 9.0.5.0 prints {:x=>X, :y=>Y, :z=>#<Class:0x2b2948e2>}.