JRuby is the effort to recreate the Ruby interpreter in Java.

The Java version will be tightly integrated with Java to allow both to
script any Java class and to embed the interpreter into any Java
application.

JRuby based on Ruby 1.6.4, but it is written in 100% Java.

The project can be found at: http://jruby.sourceforge.net

You can download an alpha version from SourceForge by following this
link (Warning this is an _alpha_ version!):

http://sourceforge.net/project/showfiles.php?group_id=35413&release_i...

JRuby supports at the time:
        - if / else
        - while / until / redo / next / break
        - method calls
        - object instantiation
        - variables
                * local
                * global
                * instance
                * class
        - constants
         - blocks / iterators / for
        - method definition (Bugs)
        - class definition (Bugs)

        - builtin classes (supported methods [1])
                * Array (no methods)
                * Class (new)
                * FalseClass (to_s, type, &, |, ^)
                * Fixnum (to_i, to_s, +, -, *, /, **, %, ==, <=>, <, <=, >, >=)
                * Float (to_s, +, -, *, /, **, %, ==, <=>, <, <=, >, >=)
                * Integer (all methods)
                * Module (no methods)
                * NilClass (type, to_a, to_s, to_i, &, |, ^, nil?)
                * Numeric (all methods)
                * Object (type)
                * Range (each, initialize)
                * String (new, initialize, [], reverse, slice)
                * TrueClass (to_s, type, &, |, ^)
        - builtin modules (supported methods [1])
                * Comparator (all methods)
                * Kernel (print, puts)

[1] Methods which are "defined" and can be used in a Ruby script. There
are some more methods implemented.