Ask HN: OS Chat server implementations?

Looking for Open Source chat server that can be extended to integrate or embed with products. Preferably mature and active project. I am kind of surprised that there's not much available out there.

requirements:

- Mature, not one of hundreds "node.js chat server in 15 minutes"

- based on mainstream language platform, Node/JS or Java

What is available so far:

There's Matrix Synapse https://github.com/matrix-org/synapse but it's based on Python stack and really cumbersome to deploy.

Another one is Signal server but it's tightly coupled with Signal specific features, like requirement to register via SMS, etc.