Subscribing to push service for web application

I am creating a web application in which the front end is made of HTML and javaScript and the back-end(including the servlets) is created in Java. I am using the REST API of Google's Weave servers. I was reading the documentation of How to subscribe for the push service to get notified when any changes occur (instead of constantly querying the server). The document reads - "Weave handles notifying any client applications subscribed to the device state." My question is how can My Java back-end subscribe to it to get push notifications.