5 alternatives to NodeJS for Java
The next thing you will need, is some sort of module system. CommonJS defines a module system, which is what NodeJS uses. It's wise to focus on that since it's basically the standard. There are several CommonJS implementations for Rhino that works pretty well. If you are using Nashorn, you will need to build this yourself since all the other implementations out there are either faulty or simply too slow.
Next, you will probably need an http-bridge and some sort of pluggability. Then, unless you are fully NodeJS compliant, you will have to add reusable modules to make it work for your own and other's projects.
Everything in Vert.x is reactive. That means it is non-blocking by nature, like NodeJS. This programming technique can be difficult for Java programmers, at least to switch to thinking in a reactive way. Some features are not implemented in a straight forward way in the code, so the code tends to be hard to read unless you are not used to it.
Vert.x is very pluggable and has a lot of existing modules. It's nicely separated into a core, language extensions and other extensions. This means that even the language support is pluggable, which is very cool indeed.
One of the problems with Vert.x, as I see it, is that you pretty much need to implement your entire application on it. I know you can mix and match, but I found it hard to make it play nicely with my existing application. For a micro-service architecture it probably does not matter since your "glue" is the network.
I would really like to see it using Nashorn instead of the old Rhino. Also it seems a bit foreign for us Java developers. It feels like investing in a NodeJS replacement, instead of just an alternative.
PurpleJS is another alternative for NodeJS running on Java. It's one of the projects I have been working on, so I will try to be very objective when writing about it. As stated earlier there are a lot of inner workings that you have never considered when trying to create such a project. Luckily for me I had hands on experience building another framework inside a well known Web Platform, so I took all the knowledge that I had from that project, put it into PurpleJS - and improved it for the better.
For now, PurpleJS uses a multi-threaded model and the http-layer is not async or reactive in itself. This can be a limitation for some, but I find it easier to build and debug applications using a more straight-forward model.