TL;DR: HubSpot is open-sourcing its fast, Jinja-compatible templating language runtime for Java.
HubSpot's Content Optimization System (COS) was originally developed in python, and the templating engine chosen to power it was Jinja. Jinja provides a robust and powerful foundation in which to craft page templates, in a syntax derived from django templates. While python was a fine choice at the start, and allowed us to rapidly iterate on the COS, we started to run into growing pains and scalability issues with python and django. Over the past year, we've been reimplementing parts of the system in Java.