Atmosphere Framework is described as 'atmosphere - Realtime Client Server Framework for the JVM, supporting WebSockets and Cross-Browser Fallbacks Support'. There are four alternatives to Atmosphere Framework for a variety of platforms, including Linux, Mac, Windows, Android and Android Tablet. The best alternative is Socket.io, which is both free and Open Source. Other great apps like Atmosphere Framework are Pusher (Paid), Lightstreamer (Paid) and Hydra by DataSecs (Free, Open Source).
Lightstreamer is a server for delivering real-time messages to browser-based and mobile applications, using WebSockets and HTTP. Features include dynamic throttling, bandwidth and frequency control, delta delivery, and total control over security and the quality of service.
Hydra is built upon Netty. It is supposed to simplify the process of socket setup in Java. Hydra is a lightweight library that is supposed to help the developer. Hydra is designed to be an easy and fast library. And it's developed with care and love.