Javalin is a lightweight web framework for Java and Kotlin, built on top of the Jetty HTTP server. It provides a fluent, consumer-based configuration API for defining HTTP routes, WebSocket endpoints, Server-Sent Events (SSE), static file serving, and more. The framework emphasizes simplicity and developer experience by avoiding annotations and reflection, instead using lambda handlers and a plugin system for extensibility. Core features include request/response handling with before/after lifecycle hooks, path parameter parsing, validation, JSON serialization (Jackson 2/3, Gson), compression (gzip, brotli, zstd), CORS, rate limiting, and SSL/TLS support. The architecture is modular, with separate packages for routing, configuration, plugins, and utilities, and optional satellite modules for metrics (Micrometer), template rendering (multiple engines), and testing tools. Javalin is designed as a library to be embedded in applications, not as a standalone server, and is distributed via Maven Central.