netty-socketio

Netty-socketio is a Java implementation of the Socket.IO server protocol built on Netty's high-performance networking.

This lets your Java server speak Socket.IO to any Socket.IO client (JavaScript, Swift, Kotlin, etc.). You get namespaces, rooms, event-based messaging, and automatic transport fallback (WebSocket to long-polling). Netty underneath means it handles thousands of concurrent connections without breaking a sweat.

Fully free. Apache 2.0 license. No paid tier. Java teams building real-time features who don't want to run a separate Node.js service just for Socket.IO will find this useful. It's been around since 2012, so it's proven.

The catch: development has slowed significantly. It doesn't support the latest Socket.IO v4 protocol features. If you're starting fresh, consider whether you actually need Socket.IO compatibility or if plain WebSockets (with Spring WebSocket or Quarkus) would be simpler.