1. Spring Boot & Spring Cloud The de facto standard for building Java microservices — Spring Boot for rapid application setup and Spring Cloud for distributed system patterns like service discovery, config, and circuit breaking.
2. Micronaut A modern JVM framework for building modular, easily testable microservice and serverless applications — with ahead-of-time compilation for fast startup and low memory footprint.
3. Dropwizard A Java framework for developing ops-friendly, high-performance RESTful web services — bundling Jetty, Jersey, Jackson, and Metrics into a production-ready package.
4. Eclipse MicroProfile An open specification optimising Enterprise Java for microservices architectures — providing portable APIs for health checks, metrics, fault tolerance, JWT, and OpenAPI.
5. Eclipse Vert.x A polyglot, event-driven toolkit for building reactive applications on the JVM — offering high concurrency with non-blocking I/O, suitable for microservices and real-time systems.
6. Grails A powerful Groovy-based web application framework for the JVM built on top of Spring Boot — enabling convention-over-configuration development with rapid productivity.
7. Restlet Framework An open source Java solution for building and consuming RESTful APIs — with a lightweight, component-based architecture suited for microservices and API-first development.
8. Quarkus A Kubernetes-native Java stack tailored for OpenJDK HotSpot and GraalVM — delivering supersonic startup times and low memory usage by crafting from best-of-breed Java libraries.
9. Helidon A collection of Java libraries for writing microservices that run on a fast, reactive web core powered by Netty — available in both SE (functional) and MP (MicroProfile) flavours.
10. Spark A micro-framework for creating web applications in Kotlin and Java 8 with minimal effort — designed for simplicity, rapid prototyping, and lightweight REST service development.
11. Axon Framework & Axon Server A framework and infrastructure platform for building event-driven microservices using CQRS and Event Sourcing patterns — with Axon Server for scalable event routing and storage.
12. Lagom An open source framework for building systems of Reactive microservices in Java or Scala — with first-class support for event sourcing, CQRS, and service communication.
13. WSO2 Microservices Framework for Java A lightweight, high-performance Java framework from WSO2 for developing and deploying microservices — built on Netty with support for REST, WebSockets, and circuit breaking.
14. Ktor A Kotlin-native framework for creating asynchronous client and server applications — from microservices to multiplatform HTTP client apps, with a flexible, idiomatic API.
15. Javalin A simple, lightweight microframework for Kotlin and Java developers who want extreme simplicity and no "magic" — ideal for REST APIs and small, focused microservices.