[batista]

>Let's say I want to plugin some Java lib for image manipulation. How? And who will guarantee that these libs will be concurrent and/or non-blocking as well?

Well, this is the JVM, not a single threaded javascript engine. As the other guy says below:

"You don't need to necessarily have non-blocking libraries to take advantage of the async I/O. Netty uses worker threads to prevent blocking the server event loop and I assume Vert.X does as well. You can do blocking JDBC queries, Redis, etc."

And threadsafe libs --of which are tons--, usually advertise it on "the box.