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Top 23 Java API Projects
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openapi-generator
OpenAPI Generator allows generation of API client libraries (SDK generation), server stubs, documentation and configuration automatically given an OpenAPI Spec (v2, v3)
To be fair I've never used Laravel - but I do have a memory of code generators/scaffolders inserting files that made clear they were responsible for work
/*
* NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
* Do not edit the class manually.
*/
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SaaSHub
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The code examples were written in kotlin using the JDA and jda-jtx libraries
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Apache Log4j 2
Apache Log4j is a versatile, feature-rich, efficient logging API and backend for Java.
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Flume
Apache Flume is a distributed, reliable, and available service for efficiently collecting, aggregating, and moving large amounts of log-like data
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swagger2markup
A Swagger to AsciiDoc or Markdown converter to simplify the generation of an up-to-date RESTful API documentation by combining documentation that’s been hand-written with auto-generated API documentation.
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microcks
The open source, cloud native tool for API Mocking and Testing. Microcks is a Cloud Native Computing Foundation incubating project 🚀
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Discord4J
Discord4J is a fast, powerful, unopinionated, reactive library to enable quick and easy development of Discord bots for Java, Kotlin, and other JVM languages using the official Discord Bot API.
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Standard Webhooks is still a good reference. Not every provider follows it, but it gives a good mental model for how webhooks should work.
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serverless-java-container
A Java wrapper to run Spring, Spring Boot, Jersey, and other apps inside AWS Lambda.
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cats
CATS is a REST API Fuzzer and negative testing tool for OpenAPI endpoints. CATS automatically generates, runs and reports tests with minimum configuration and no coding effort. Tests are self-healing and do not require maintenance. (by Endava)
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Elide
Elide is a Java library that lets you stand up a GraphQL/JSON-API web service with minimal effort.
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zerocode
zerocode-tdd is a community-developed, free, open-source, outcome-driven automated testing for microservices APIs, Kafka(Data Streams), Databases and Load scenarios. all defined in simple JSON or YAML — with zero coding.
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packetevents
PacketEvents is a protocol library tailored to Minecraft Java Edition, designed to facilitate the processing and transmission of packets.
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ByteChef
Open-source platform that unifies AI agent orchestration and workflow automation — autonomy and precision in one platform.
Project mention: Show HN: ByteChef – Open-source alternative to Zapier/n8n with Apache 2.0 core | news.ycombinator.com | 2026-04-23 -
jans
The Janssen Project is a home for open source IAM components, featuring Auth Server (OAuth/OpenID), Agama low-code identity orchestration, and the Cedarling policy decision point. The "Janssen Server" distributions bundle IAM components under one control plane.
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Membrane Service Proxy
Truly open source API gateway with native OpenAPI support. Written in Java, it is easily extensible, supports legacy XML and SOAP, and is optimized for container deployments.
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Para
Multitenant backend server for building web and mobile apps rapidly. The backend for busy developers. (self-hosted or hosted) (by Erudika)
NOTE:
The open source projects on this list are ordered by number of github stars.
The number of mentions indicates repo mentiontions in the last 12 Months or
since we started tracking (Dec 2020).
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A note from our sponsor - SaaSHub
www.saashub.com | 18 Jun 2026
Index
What are some of the best open-source API projects in Java? This list will help you:
| # | Project | Stars |
|---|---|---|
| 1 | openapi-generator | 26,395 |
| 2 | JDA | 4,660 |
| 3 | Apache Log4j 2 | 3,603 |
| 4 | docker-java | 3,186 |
| 5 | Flume | 2,560 |
| 6 | swagger2markup | 2,517 |
| 7 | microcks | 1,966 |
| 8 | Discord4J | 1,917 |
| 9 | standard-webhooks | 1,690 |
| 10 | serverless-java-container | 1,557 |
| 11 | crAPI | 1,523 |
| 12 | cats | 1,359 |
| 13 | github-api | 1,250 |
| 14 | openapi-diff | 1,086 |
| 15 | Elide | 1,023 |
| 16 | zerocode | 1,008 |
| 17 | packetevents | 872 |
| 18 | Javacord | 774 |
| 19 | ByteChef | 773 |
| 20 | jans | 632 |
| 21 | Membrane Service Proxy | 610 |
| 22 | PublicAPI | 581 |
| 23 | Para | 571 |
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