In 2025 there is a new answer, Flavour
Flavour is an open-source, batteries-included framework for making fast-loading, single-page apps with Java for business logic (and HTML for templates).
Pros:
True open source (Apache licensed)
Fast downloads and fast builds
Well documented: read The Flavour Book
Share models and validation between frontend and backend
Example projects built with Flavour:
Wordii: A 5-letter word game
Castini: Make your podcast from your browser
Tea Sampler: Like SwingSet for Flavour