We've been very lucky to have a lot of ingenious JRuby users help port some of those extensions or produce API-compatible wrappers around equivalent Java libs.

We've been very lucky to have a lot of ingenious JRuby users help port some of those extensions or produce API-compatible wrappers around equivalent Java libs. There's still C exts we don't have equivalent JRuby exts for, but things are much better than they used to be.