For a side project I've deployed some simple endpoints to Heroku and used Spark (http://sparkjava.com/). Spark is a Java Sinatra clone rather than full Rails.
That said, not sure I'd pick it for simple side-projects. Part of the benefit here is how it works for a large dev team, with a large codebase, etc.
That said, not sure I'd pick it for simple side-projects. Part of the benefit here is how it works for a large dev team, with a large codebase, etc.