Differences Between MRI And JRubyAlthough ideally MRI and JRuby would behave 100% the same in all situations, there are some minor differences. Some differences are due to bugs, and those are not reported here. This page is for differences that are not bugs.
JRuby cannot run native C extensions. Popular libraries have all generally been ported to Java Native Extensions. Also, now that FFI has become a popular alternative to binding to C libraries, using it obviates the need to write a large chunk of native extensions.
On Microsoft Windows, JRuby is a little smarter when launching external processes. If the executable file is not a binary executable (.exe), MRI requires you give the file suffix as well, but JRuby manages without it.
For example, say you have file foo.bat on your PATH and want to run it.
system( 'foo' ) # works on JRuby, fails on MRI system( 'foo.bat' ) # works both in JRuby and MRI
JRuby doesn't implement fork() on any platform, including those where fork() is available in MRI.
Since the JVM presents a compatible CPU to JRuby, the native endianness of JRuby is Big Endian. This does matter for operations that depend on that behavior, like String#unpack and Array#pack for formats like I, i, S, and s.
Since it is not possible to obtain usec precision under a JVM, Time.now.usec cannot return values with microsecond precision.
> Time.now.usec => 582000
Keep this in mind when counting on usec precision in your code.
JRuby only has one regular expression engine, which matches Onigurama's behavior. It is not changed in --1.8 mode, so code depending on regular expressions behaving precisely as on MRI 1.8.n may fail on JRuby in --1.8 mode. For instance:
ruby-1.8.7-p302 > "a".match(/^(.*)+$/) => "a"
jruby-head > "a".match(/^(.*)+$/) => ""
In MRI, the Thread priority can be set to any value in Fixnum (if native threads are enabled) or -3..3 (if not). The default value is 0.
In JRuby, Threads are backed by Java threads, and the priority ranges from 1 to 10, with a default of 5. If you pass a value outside of this range to Thread#priority=, the priority will be set to 1 or 10.