Rhino JS is well integrated with the JVM, so it's easy to call back and forth between Java and JS. You basically have access to everything Java has. JS doesn't

Rhino JS is well integrated with the JVM, so it's easy to call back and forth between Java and JS. You basically have access to everything Java has. JS doesn't have an equivalent to "synchronized", but it could easily be added as a function call (since JS has real functions). However, I wouldn't do that way since sticking synchronized blocks all over the place is the wrong solution anyway. Just write your code so that separate threads aren't stepping on eachother's data (and do cross-thread stuff in a carefully controlled manner). I'm doing my development in Java+JS and it's a pretty nice combination.