Multi threading has zero overhead in normal Java web frameworks. It's thread-per-request so you don't need to worry about thread safety any more than you do in JS. That you didn't know that implies you haven't worked with many Java web frameworks.

JS has become a huge language, just as big as Java. So is the runtime, V8 is gigantic, comparable to JVM. I use both every day.

Vert.X is nearly identical to Express JS including all the callback stuff. And that's one of the harder Java frameworks to use