Javalin was inspired by https://sparkjava.com/ which was inspired by Sinatra (which I think also inspired Express?).

Anyway, libraries like this were only really feasible after Java 8 because of the reliance on lambdas. Having to instantiate anonymous nested classes for every "function" was a total pain before that.

Kinda now that Java has lambdas, but still async in that disn't work as easily as JS, which is important. This is only recently starting to change with Project Loom.