Different jruby behaviour that ruby 1.8.6/1.8.7/1.9 when running file that is named as one of required file.

I was doing some simple tests and named my file "thread.rb". In this file I required "thread" library and surprisingly it worked in ruby 1.8.6, 1.8.7, 1.9 but not in JRuby 1.1.5. I know that I shouldn't name my file as one of ruby std library name but I think that it should work the same way for all ruby interpreters.

require "thread"

puts Mutex

$ ruby1.8.6 -v
ruby 1.8.6 (2008-08-11 patchlevel 287) [x86_64-linux]
$ ruby1.8.6 thread.rb
Mutex

$ ruby1.8 -v
ruby 1.8.7 (2008-08-11 patchlevel 72) [x86_64-linux]
$ ruby1.8 thread.rb
Mutex

$ ruby1.9 -v
ruby 1.9.0 (2008-11-07 revision 20129) [x86_64-linux]
$ ruby1.9 thread.rb
Mutex

$ jruby -v
jruby 1.1.5 (ruby 1.8.6 patchlevel 114) (2008-11-06 rev 6586) [i386-java]
$ jruby thread.rb
null:1:in `const_missing': uninitialized constant Mutex (NameError)
from thread.rb:3
from thread.rb:1:in `require'
from thread.rb:1