Javalin
Lightweight web framework for Java and Kotlin built on Jetty and Kotlin. Javalin follows the Sinatra/Express philosophy — minimal DSL for routing, no annotations, no magic. Define routes with app.get('/path') { ctx -> ctx.json(result) }, add WebSocket handlers, serve OpenAPI docs with swagger-ui. Zero magic, explicit configuration, and works identically in Java and Kotlin.
Score Breakdown
⚙ Agent Friendliness
🔒 Security
Minimal framework with small attack surface. TLS via Jetty. No built-in auth — implement carefully. Community-maintained with regular updates.
⚡ Reliability
Best When
You want a simple, readable Kotlin/Java HTTP server without Spring's complexity — great for microservices, tool servers, and prototypes.
Avoid When
You need Spring ecosystem libraries, JPA/Hibernate integration, or enterprise patterns — use Spring Boot instead.
Use Cases
- • Build simple agent API servers with minimal boilerplate — Javalin's flat route definitions are readable and require no framework-specific annotations
- • Prototype agent tool servers quickly — a working HTTP endpoint with JSON response is 5 lines of code in Javalin
- • Add WebSocket support to agent communication channels with Javalin's built-in WebSocket handler
- • Generate OpenAPI documentation automatically for agent APIs using Javalin's swagger-ui and openapi plugin
- • Embed Javalin in test suites as a lightweight mock server for agent integration testing without Docker or external dependencies
Not For
- • Large enterprise applications requiring Spring Boot's ecosystem (Spring Data, Spring Security, Spring Batch, Spring Cloud)
- • Teams that prefer annotation-based MVC — Javalin is explicit/functional; Spring MVC @RestController is more familiar for enterprise Java devs
- • Applications requiring complex AOP, transaction management, or enterprise patterns — use Spring Boot for these requirements
Interface
Authentication
No built-in auth — auth is implemented via Javalin's before() filter. Bearer token and API key patterns implemented manually. OAuth via external libraries.
Pricing
Community-maintained open source. Apache 2.0 license.
Agent Metadata
Known Gotchas
- ⚠ Javalin's ctx.json() uses Jackson by default — if you want Gson or another serializer, configure JavalinJackson or JavalinGson in the config block
- ⚠ Route handlers execute on the same thread as the request — blocking operations (DB calls, HTTP calls) in handlers block Jetty threads; use ctx.future() for async responses
- ⚠ Javalin 5 → 6 changed the configuration API significantly — Javalin.create { config -> } DSL replaced the older method-chain API; check version when copying examples
- ⚠ WebSocket message handlers and HTTP handlers share the same Javalin instance but are separate — WebSocket close events must be handled separately from HTTP lifecycle events
- ⚠ ctx.bodyAsClass<T>() deserializes the request body and throws InternalServerErrorResponse on malformed JSON — wrap in try-catch or use ctx.bodyValidator() for proper error responses
- ⚠ Javalin's before() filters run before ALL matching routes including static files — auth filters that throw UnauthorizedResponse block static file serving if misconfigured
Alternatives
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Scores are editorial opinions as of 2026-03-07.