Attempt to submit a form using Map<String, String> results in EncodeException

[gkresic]

One of our Feign clients looked like:

import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;

@FeignClient(name = "authClient", url = "${auth.url}", configuration = AuthClient.Configuration.class)
public interface AuthClient {

    @PostMapping(value = "/oauth2/token", consumes = MediaType.APPLICATION_FORM_URLENCODED_VALUE)
    AuthResponse getAccessToken(@RequestBody Map<String, ?> request);

}

This works. However, changing request type from Map<String, ?> to Map<String, String> causes Feign to fail with:

feign.codec.EncodeException Unable to make field transient java.util.HashMap$Node[] java.util.HashMap.table accessible: module java.base does not "opens java.util" to unnamed module @531be3c5

During my investigation, I learned about Feign's close relationship with Map<String, ?> via feign.codec.Encoder#MAP_STRING_WILDCARD, but even if this is not a bug, but a design choice, it just seems like it should be handled more gracefully.

io.github.openfeign:feign-core:12.5
io.github.openfeign:feign-okhttp:13.2.1
io.github.openfeign.form:feign-form:3.8.0
io.github.openfeign.form:feign-form-spring:3.8.0
io.github.openfeign:feign-slf4j:12.5
org.springframework.cloud:spring-cloud-starter-openfeign:4.0.6
OpenJDK 21

[velo]

Thanks for the report. This project is community-maintained, and the maintainer will not implement fixes or features directly. If you want this to move forward, please open a pull request with automated tests that cover the change.