I used / hacked on nailgun some 9y ago it was a pleasant experience, not sure if something has changed though these days I'm not much bothered by startup time. One can learn to tackle the problems that make restarting a JVM perceivedly necessary in the first place. Then your repl session can last weeks/months, as Sean C. shares over here from time to time :) Some random examples: run tests from the repl, learn to avoid defprotocol pitfalls, learn to avoid dangling resources, add abundant, commonly-used libs to the classpath (which doesn't affect startup time!) in advance

I don't leave things running that don't need to be, that just wastes power