It seems quite mature already, runs Rails and passes 100% of the Ruby specs. I think it's going to become the default in JRuby, but probably not before Java9 c

It seems quite mature already, runs Rails and passes 100% of the Ruby specs. I think it's going to become the default in JRuby, but probably not before Java9 comes out, which includes a crucial component that enables Graal to sit on top of HotSpot (currently you need a modified VM).