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Differences Between MRI And JRuby

Although 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.

Native C Extensions

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.

Continuations and Fibers

JRuby does not support continuations (Kernel.callcc).

Fibers (a form of delimited continuation) are supported on JRuby, but each fiber is backed by a native thread. This can lead to resource issues when attempting to use more fibers than the system allows threads in a given process.

Invoking external processes

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

Fork is not implemented

JRuby doesn't implement fork() on any platform, including those where fork() is available in MRI. This is due to the fact that most JVMs cannot be safely forked.

Native Endian is Big Endian

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.

Time precision

Since it is not possible to obtain usec precision under a JVM, cannot return values with microsecond precision.


=> 582000

Keep this in mind when counting on usec precision in your code.

Regular expressions

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(/^(.*)+$/)[1]
=> "a"

jruby-head > "a".match(/^(.*)+$/)[1]
=> ""

Thread priority

NOTE: from at least as early as JRuby 1.7.6, Ruby thread priorities are mapped to Java thread priorities, so this section isn't accurate -- you can use the same priority for MRI and JRuby.

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.

(See and


JRuby is not able to rescue from SystemStackError. If your code rely on this, you should rather try to catch a Java::JavaLang::StackOverflowError. See this ticket for further information.

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