Netty
An event-driven asynchronous network application framework
...It wraps Java NIO with a channel/handler pipeline model, enabling clean composition of codecs, TLS, and application logic with minimal overhead. Its ByteBuf abstractions, buffer pooling, and zero-copy techniques help reduce GC pressure and improve throughput under heavy I/O. Support for common protocols—HTTP/1.1, HTTP/2, WebSocket, and raw TCP/UDP—makes it a foundation for RPC frameworks, proxies, and messaging systems. Pluggable transports and native integrations (like epoll or kqueue where available) allow fine-tuning for different operating systems. With robust resource management, leak detection, and extensive testing, Netty is widely chosen when low latency, scalability, and control over the I/O stack are required.