I'm stronlgy in favour of Contract-First Development, when in comes to APIs. All invested parties, including your future self, agree on a neutral format, that both API providers and consumers will stick to. For REST APIs that is the OpenAPI spec

A popular critique of that approach is that it reeks of Big Design Upfront, happily skipping over the fact that nothing stops the teams to iterate over the specification too, one path, one schema at the time

The source of truth

The specification becomes the single authorative source for endpoints, security requirements, data formats and responses. While it is possible to generate the spec from source code, like. e.g. Spring or Quarkus, I see clear advantages to provide the specification standalone. Create it with a tool like Apicur.io, Stoplight or APIGit. Or use a plugin (or another) in your IDE or the other one

Once you have your first draft, you want to implement it server side. Eclipse vert.x offers the Vert.x OpenAPI Router for exactly that. You can get more details from my OpenAPI talk, or by peeking into the sample project (which used vert.x inside Quarkus).