There's a really sweet web application framework called Helma that lets you write entire web apps in javascript, and gives you the JDK through Rhino. The documentation is a little slim, but it's my framework of choice. I don't know why it isn't more popular.
Sure, we might go look at framework code sometimes. But it just looks like, well, code. There's usually nothing particularly famous-looking or even glamorous about it. Go look at the source code for Emacs or Rails or Python or Firefox, and it's just a big ball of code. In fact, often as not it's a big hairy ball, and the original author is focused on refactoring or even rewriting big sections of it.
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.
http://dev.helma.org/