Optional request body should be supported by feign

In Spring MVC it is possible to annotate body parameter as optional using @RequestBody(required = false) parameter annotation, e.g.:

@FeignClient("foo-service")
public interface FooService {
    @RequestMapping(value = "/foo", method = RequestMethod.POST)
    void doFoo(@RequestBody(required = false) FilterDto filter);
}

However, Spring-Feign contract does not consider the optionality flag which leads to an IllegalStateException on proxy bootstrapping in consumer when calling something like:

fooService.doFoo(null);

java.lang.IllegalArgumentException: Body parameter 0 was null
    at feign.Util.checkArgument(Util.java:102)
    at feign.ReflectiveFeign$BuildEncodedTemplateFromArgs.resolve(ReflectiveFeign.java:323)
    at feign.ReflectiveFeign$BuildTemplateByResolvingArgs.create(ReflectiveFeign.java:213)
    at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:72)
    at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:103)

It is expected that null can be passed as parameter since it is optional.

Using:

• spring-cloud-starter-feign:1.1.0.RELEASE
• feign-core:8.16.2

Start by tracing the Spring-Feign contract handling of @RequestBody(required = false), then inspect the failure path in feign/ReflectiveFeign.java shown in the stack trace. Reproduce the FooService example with a null body and verify that optional request bodies no longer fail during template resolution.