Based on all that, I set about writing a small test app to see what it’s like to program a Swing app with JS. After a day or so of work (spread over a few months), I offer my Rhino Swing Test App:
The documentation for interaction between Java and JS is limited, but sufficient. For simplicity, I used Rhino as an interpreter, I did not compile to java bytecode. Nonetheless, the animation runs about as smoothly in JS as it does in Java, because the heavy lifting is done by the JDK classes.
Still, I recommend a look at this approach if you are fond of dynamic languages but need to build on the Java platform, and I intend to investigate server-side JS development also.