OpenAPITools/openapi-generator ? reverse-engineered prompt

Build me a Java based tool that takes an OpenAPI spec, either v2 or v3, and generates useful code and docs from it. I want it to be able to create client SDKs, server stubs, and simple API documentation from the same input, so someone can point it at a spec file and get a working starting point for their API project.

Please set it up so it can run from the command line, be easy to configure, and support different output styles for common languages and frameworks where possible. Keep the generated code clean and practical, and make sure the project includes a few sample specs and example outputs so it is easy to test. If you need to check current usage details or best practices, look them up online first.

I also want the repo to be organized so contributors can understand it, run tests, and build it locally without too much hassle.