Javalin — Simple Lightweight Web Framework for Java and Kotlin

Introduction

Javalin is a lightweight web framework that runs on the JVM, targeting both Java and Kotlin developers. It wraps Jetty into a clean API where routes are defined as lambdas, WebSocket handlers are first-class, and OpenAPI documentation can be generated from code. It fills the gap between bare-metal servlet programming and heavyweight enterprise frameworks.

What Javalin Does

• Creates HTTP and WebSocket endpoints with a simple lambda-based API
• Runs on an embedded Jetty server with no external container required
• Validates request data with built-in type-safe validators
• Generates OpenAPI 3.0 specifications from annotated handlers
• Supports both blocking and async request handling

Architecture Overview

Javalin is a thin layer over Jetty that translates servlet concepts into a simpler API. When a request arrives, it passes through a before-handler chain, matches against registered routes by method and path pattern, executes the handler, then runs after-handlers. The Context object wraps the servlet request and response, providing helpers for path parameters, query strings, JSON serialization (via Jackson or Gson), and file uploads.

Self-Hosting & Configuration

• Add the Maven or Gradle dependency; Jetty is included transitively
• Start a server with Javalin.create().start(port)
• Configure the embedded Jetty via Javalin.create(config -> config.jetty.server(...))
• Enable CORS with config.bundledPlugins.enableCors(cors -> cors.addRule(...))
• Add the OpenAPI plugin for automatic Swagger UI at /swagger-ui

Key Features

• First-class Kotlin support with extension functions and coroutine adapters
• WebSocket and Server-Sent Events built into the core API
• Plugin architecture for OpenAPI, SSL redirects, and rate limiting
• Type-safe path parameter and query parameter extraction
• Context-based API that avoids global state and thread-local patterns

Comparison with Similar Tools

• Spark Java — similar micro approach but Java-only, less active development
• Spring Boot — full enterprise framework with DI, far more setup for simple APIs
• Ktor — Kotlin-native async framework by JetBrains, more idiomatic for pure Kotlin projects
• Micronaut — compile-time DI and GraalVM support, steeper learning curve

FAQ

Q: Can Javalin be used with Kotlin coroutines? A: Yes. Javalin provides a coroutine plugin that lets you write suspend handlers and use structured concurrency.

Q: Does Javalin support GraalVM native images? A: Javalin can work with GraalVM, but Jetty requires reflection configuration. Community guides are available for setup.

Q: How does Javalin handle JSON serialization? A: It uses Jackson by default. You can swap in Gson or any other library by configuring a custom JSON mapper.

Q: Is Javalin suitable for microservices? A: Yes. Its small footprint, fast startup, and embedded server make it well suited for containerized microservices.

Sources

• https://github.com/javalin/javalin
• https://javalin.io