Any web service needs to export their public API if consumers want to make the best use of that service. A developer-friendly approach to do so if you work in the Java ecosystem is to package DTOs and endpoint interfaces in an API jar file and use the Retrofit framework to create type-safe clients for integration testing. This article discusses a complete example.
If you’ve worked in enterprise Java projects you will remember good old Web Services Description Language, the XML based format for describing network services by IBM and Microsoft. Maybe you still work with it? WSDL and its twin XML Schema are among those W3C standards that seasoned developers love to hate. Its specification files are not very human readable, let alone human writable. Fortunately you don't have to. They can be generated by your server endpoint and fed straight into a code generator to create transfer objects (DTOs) and service stubs.