All the node-like environments (nodyn, vert.x) are ports or emulations; your mileage may vary.
You can also start the node process as an external process and proxy to it over HTTP.
You can use a worker and job queue system to communicate with node processes, especially if you start a service-oriented architecture.
You can have Node be a proxy to your other application, running as an app server along side of your Java app server.
You can integrate purely through a load balancer or front-end proxy, if your apps don't need to talk to each other, just be merged into a cohesive whole.
It really depends on WHAT you're doing. Node doesn't even imply 'web', so knowing what you're doing would be easier to give specific answers for.
Aria