JRuby 10.1.0.0 Released — JRuby.org

JRuby 10.1.0.0 has been released, targeting Ruby 4.0 compatibility. Key improvements include reduced object sizes (from 32 to 24 bytes, or 16 bytes with JVM Compact Object Headers via Project Lilliput), specialized numeric representations splitting RubyFixnum into LongFixnum and ShortFixnum, support for the cross-platform Prism Ruby parser (with native builds for Windows/Linux/macOS and WASM fallback), and groundwork for direct argument passing on the call stack to eliminate boxing overhead. Internal implementations of RubyArray and RubyHash have been made abstract classes, and RubyString constructors deprecated, to support upcoming optimized variants. JRuby 9.4 moves to soft EOL, JRuby 10.0 becomes the LTS release until April 2028, and 10.1 is the experimental tip release. Production users are advised to stay on 10.0 while 10.1 stabilizes. A new LTS cadence of every two years starting spring 2027 is also announced.