Charles Nutter, of the core JRuby team, writes:

One of the largest problems plaguing Ruby implementations [..] is the ever-painful story of “extensions”. In general, these take the form of a dynamic library, usually written in C, that plugs into and calls Ruby’s native API as exposed through ruby.h and libruby.

The many compiled bridges between external libraries and Ruby pose a problem for alternate implementations like JRuby, because of the complexity involved in exposing internals of the implementation or expensive serialization in both directions. Instead, an interface is necessary so that instead of developing libraries that act only as unique bridges to others, we can just have one library that provides the interface to any arbitrary library of our choice. Read More