Feign client support for optional request param

Question

Does Feign client support optional request param?

For example, I have an endpoint, but I am not finding a way to actually make the param1 optional using feign client.

@GetMapping(path = "endpoint1")
ResponseEntity request(@RequestParam(name = "param1", required = false, defaultValue = "key") String param1){}

Answer 1

I managed to use Optional request params with Feign by creating a customised FeignFormatterRegistrar. Here is the code:

package feignformatters;

import org.springframework.cloud.openfeign.FeignFormatterRegistrar;
import org.springframework.format.FormatterRegistry;
import org.springframework.stereotype.Component;

import java.util.Optional;

@Component
public class OptionalFeignFormatterRegistrar implements FeignFormatterRegistrar {
    
    @Override
    public void registerFormatters(FormatterRegistry registry) {
        registry.addConverter(
            Optional.class, 
            String.class, 
            optional -> {
                if (optional.isPresent())
                    return optional.get().toString();
                else
                    return "";
            });
    }
}

The following client started working fine with the previous component loaded in the project:

@FeignClient("book-service")
public interface BookServiceClient {

    @GetMapping("/books")
    public List<Book> getBooks(
        @RequestParam("page") Optional<Integer> pageNum,
        @RequestParam("size") Optional<Integer> pageSize,
        @RequestParam("reader") Optional<Long> readerId);
}