Support one-time configuration of expression parameters

Problem

Let’s say I have a request template that looks like this:

@RequestLine("GET /{env}/{resource}")
void getResource(@Param("env") String env, @Param("resource") String resource);

In the application that defines this template, env is configured at startup and will only ever have 1 value (e.g. "production" or "qa") during the application’s lifespan. It would make sense to not have to pass this same value into each call of getResource().

Default methods are of no help here because env still needs to be dynamically set.

Proposal

Support on-time configuration of expression parameters via configuration methods on the client interface:

@Configuration
void setEnv(@Param("env") String env);

@RequestLine("GET /{env}/{resource}")
void getResource(@Param("resource") String resource);

Implementing this is pretty easy (~150 lines).

N.B. this proposal is somewhat similar to https://github.com/OpenFeign/feign/issues/1359.

[kdavisk6]

The more I think about this, the more I like the idea of expanding @Param to be used at the method and class level. Here’s an example:

@Param(value = "tenant", expander = TenantExpander.class)
interface MultiTenantResourceApi {

   @Param(value = "env", expander = EnvironmentExpander.class)
   @RequestLine("GET /{tenant}/{env}/{resource}")
   void getResource(@Param("resource") String resource);
}

This approach meets the initial request, can be included by Contract extensions already out in the ecosystem without modification, and provides full control over how the parameter is resolved.

Changes to make this possible would be limited refactoring the current @Param annotation processor into a dual purpose process that can be registered on the class, method, and parameter:

class ParamAnnotationProcessor implements ParameterAnnotationProcessor, AnnotationProcessor {

   public void process(E annotation, MethodMetadata metadata, int paramIndex) {
   
   }

   public void process(E annotation, MethodMetadata metadata) {

   }
}

@hellproxy does this meet your needs? @velo any feedback?

[kdavisk6]

I don’t see any point on having the @Param if we include this on the builder.

Am I missing something?

I’m looking towards the annotation processor, but we can figure that out later. Let’s move forward with your suggestions.