mrniko/netty-socketio

This project is a Java library that lets you run a Socket.IO server in your Java application. Socket.IO is a protocol that makes it easy to push messages from a server to connected clients in real time, which is the kind of thing you need for live chat, multiplayer games, dashboards that update without refreshing, and similar features. Most Socket.IO implementations are written in JavaScript for Node.js servers, this one brings that same capability to the Java world. Under the hood, the library is built on top of Netty, a Java networking framework known for handling large numbers of simultaneous connections efficiently. The library supports both WebSocket connections and older polling-based connections, so it works even in environments where WebSocket traffic is restricted. It also supports namespaces and rooms, which let you group clients and broadcast messages only to relevant subsets of connected users. For teams running multiple servers, the library can coordinate broadcasts across nodes using Redis (via Redisson) or Hazelcast, so a message sent on one server reaches clients connected to any other server in the cluster. It also supports SSL for encrypted connections, Spring integration for projects already using that framework, and annotations for defining event handlers without a lot of wiring code. Performance figures shared by users over the years are notable: one company reported handling 30,000 simultaneous WebSocket clients with peaks of around 140,000 messages per second. The library has been maintained continuously since roughly 2012 and supports Socket.IO client versions 1.x through 4.x, so it works with both older and current client-side Socket.IO code. To add it to a Maven project, you include one dependency block in your pom.xml and the library is available as a standard Java package. It requires Java 8 or later to run. The source code is released under the Apache 2.0 license.