"that's stupid" is a pretty broad statement. There are many reasons why one may choose to write new software in JS while taking advantage of existing investment in Java. If it doesn't make sense for you, don't do it. But for many it may.
As for coding Java on Nodyn, well, yes you can. That's kind of the point in some ways. You can write Java, package it as a jar and access this from Nodyn. Nodyn itself does this for the Buffer implementation.
Yes, vert.x already supports JS (I maintain the mod-lang-js, mod-lang-rhino and mod-lang-dynjs language modules for vert.x). The idea behind Nodyn is to provide drop-in Node.js compatibility in the Vert.x environment.
As the creator of Nodyn, but being late on the draw, I'll just have to echo what everyone else has said here. Java provides a large and rich ecosystem that is either unavailable or prohibitively difficult to bundle into a native runtime.
Nodyn is not yet complete, though I'd like to have a release in the next month that is capable of running express.js apps. It is slower than Node.js, but of course I'd like that to change; and with the JVM underneath, beating Node.js on performance is possible.
Nodyn is built on top of Vert.x (http://vertx.io) an asynchronous polyglot platform which excels on the Techempower benchmarks. The javascript runtime is DynJS - a new JS runtime for the JVM which is independent of Oracle. Though it's admittedly slower than Nashorn at the moment, we hope for that to change.
As for coding Java on Nodyn, well, yes you can. That's kind of the point in some ways. You can write Java, package it as a jar and access this from Nodyn. Nodyn itself does this for the Buffer implementation.
Yes, vert.x already supports JS (I maintain the mod-lang-js, mod-lang-rhino and mod-lang-dynjs language modules for vert.x). The idea behind Nodyn is to provide drop-in Node.js compatibility in the Vert.x environment.