OpenFeign doesn't work with kotlin suspend method

[XhstormR]

I have an API interface where the methods are kotlin suspend methods:

@FeignClient(path = "/api/masker", contextId = "MaskerApi", name = Const.SERVICE, configuration = [FeignConfig::class])
interface MaskerApi {

    @PostMapping("/mask")
    suspend fun mask(
        @Valid @RequestBody maskRequest: MaskRequest,
    ): RestResponse<List<CharSequence>>

    @PostMapping("/matches")
    suspend fun matches(
        @Valid @RequestBody maskRequest: MaskRequest,
    ): RestResponse<List<Boolean>>
}

But when I run the program, the program throws an exception:

Caused by: java.lang.IllegalStateException: Method has too many Body parameters: public abstract java.lang.Object io.github.xhstormr.masker.web.api.masker.MaskerApi.matches(io.github.xhstormr.masker.model.masker.MaskRequest,kotlin.coroutines.Continuation)
Warnings:
- 
	at feign.Util.checkState(Util.java:121) ~[feign-core-11.7.jar:na]
	at feign.Contract$BaseContract.parseAndValidateMetadata(Contract.java:142) ~[feign-core-11.7.jar:na]
	at org.springframework.cloud.openfeign.support.SpringMvcContract.parseAndValidateMetadata(SpringMvcContract.java:193) ~[spring-cloud-openfeign-core-3.1.0.jar:3.1.0]
	at feign.Contract$BaseContract.parseAndValidateMetadata(Contract.java:65) ~[feign-core-11.7.jar:na]
	at feign.ReflectiveFeign$ParseHandlersByName.apply(ReflectiveFeign.java:151) ~[feign-core-11.7.jar:na]
	at feign.ReflectiveFeign.newInstance(ReflectiveFeign.java:49) ~[feign-core-11.7.jar:na]
	at feign.Feign$Builder.target(Feign.java:268) ~[feign-core-11.7.jar:na]
	at org.springframework.cloud.openfeign.DefaultTargeter.target(DefaultTargeter.java:30) ~[spring-cloud-openfeign-core-3.1.0.jar:3.1.0]
	at org.springframework.cloud.openfeign.FeignClientFactoryBean.loadBalance(FeignClientFactoryBean.java:373) ~[spring-cloud-openfeign-core-3.1.0.jar:3.1.0]
	at org.springframework.cloud.openfeign.FeignClientFactoryBean.getTarget(FeignClientFactoryBean.java:421) ~[spring-cloud-openfeign-core-3.1.0.jar:3.1.0]
	at org.springframework.cloud.openfeign.FeignClientFactoryBean.getObject(FeignClientFactoryBean.java:396) ~[spring-cloud-openfeign-core-3.1.0.jar:3.1.0]
	at org.springframework.cloud.openfeign.FeignClientsRegistrar.lambda$registerFeignClient$0(FeignClientsRegistrar.java:235) ~[spring-cloud-openfeign-core-3.1.0.jar:3.1.0]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.obtainFromSupplier(AbstractAutowireCapableBeanFactory.java:1249) ~[spring-beans-5.3.14.jar:5.3.14]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1191) ~[spring-beans-5.3.14.jar:5.3.14]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582) ~[spring-beans-5.3.14.jar:5.3.14]
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:542) ~[spring-beans-5.3.14.jar:5.3.14]
	... 28 common frames omitted

This kotlin.coroutines.Continuation parameter is generated by the kotlin compiler at compile time, so I can't modify this parameter, how can I tell FeignClient to ignore this Continuation parameter?

[XhstormR]

When I make a successful call to make an http request, but it fails when converting the http call result json into an object. When I remove the suspend modifier in the method, try again and it will be successful.

java.lang.IllegalStateException: Failed to execute ApplicationRunner
   at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:761) ~[spring-boot-2.6.2.jar:2.6.2]
   at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:748) ~[spring-boot-2.6.2.jar:2.6.2]
   at org.springframework.boot.SpringApplication.run(SpringApplication.java:309) ~[spring-boot-2.6.2.jar:2.6.2]
   at org.springframework.boot.SpringApplication.run(SpringApplication.java:1301) ~[spring-boot-2.6.2.jar:2.6.2]
   at org.springframework.boot.SpringApplication.run(SpringApplication.java:1290) ~[spring-boot-2.6.2.jar:2.6.2]
   at io.github.xhstormr.masker.ApplicationKt.main(Application.kt:54) ~[main/:na]
Caused by: java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class io.github.xhstormr.masker.model.response.RestResponse (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; io.github.xhstormr.masker.model.response.RestResponse is in unnamed module of loader 'app')
   at io.github.xhstormr.masker.web.api.masker.MaskerApi.maskSync(MaskerApi.kt:32) ~[main/:na]
   at java.base/java.lang.invoke.MethodHandle.invokeWithArguments(MethodHandle.java:710) ~[na:na]
   at feign.DefaultMethodHandler.invoke(DefaultMethodHandler.java:141) ~[feign-core-11.7.jar:na]
   at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:100) ~[feign-core-11.7.jar:na]
   at com.sun.proxy.$Proxy116.maskSync(Unknown Source) ~[na:na]
   at io.github.xhstormr.masker.Application.init$lambda-0(Application.kt:45) ~[main/:na]
   at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:758) ~[spring-boot-2.6.2.jar:2.6.2]
   ... 5 common frames omitted

[OlgaMaciaszek]

Hello, @XhstormR. We have never introduced kotlin support in Spring Cloud OpenFeign, so there might be many Kotlin constructs that will not work. Putting the issue into ice-box as currently we are not working on Kotlin support.

[aSemy]

It looks like OpenFeign recently merged in support for Kotlin suspend functions OpenFeign/feign#1706. When that PR is released and spring-cloud-openfeign can update, will suspend functions 'just work'?

[OlgaMaciaszek]

We'll review it. If it requires only minimal changes, we'll possibly implement support here also. However, if it's more involved, then Kotlin support is currently not a priority. However, we would review a PR if it's submitted.

[XhstormR]

OpenFeign 12.0 was released with support for Kotlin coroutines. By looking at the test case CoroutineFeignTest, we simply need to build the Client using CoroutineFeign to get invocation support for the Kotlin coroutine.

https://github.com/OpenFeign/feign/blob/master/kotlin/src/test/kotlin/feign/kotlin/CoroutineFeignTest.kt#L148

[Alex-Schiff]

Since the Feign class and the CoroutineFeign class have nothing in common, it is not trivial to add this support to spring-cloud-openfeign. I think it is a better fit for https://github.com/spring-projects-experimental/spring-cloud-openfeign-async if that ever gets written.

[dgensert]

Actually Coroutine Feign is great solution and works pretty well. The issue is that the Builder expects a non-AsyncClient and that I cannot use CoroutineFeign.CoroutineBuilder as Feign.Builder. If it was possible to allow @FeignClient to leverage a CoroutineFeign builder directly would work nicely. The AsyncHandler should get invoked, but the regular ResponseHandler get's invoked, while the parameter Expansion Continuation fails. There is a draft project achieving this to inspect, but the functionality is lackluster, maybe this could be achieved relatively easily in this project: https://github.com/sunny-chung/spring-starter-feign-coroutine

[OlgaMaciaszek]

Hello @dgensert, Spring Cloud OpenFeign has been marked as feature-complete as we suggest moving over to Spring Interface Clients. We still fix CVEs and bugs, but are not working on any new features in this project.

[dgensert]

Hi @OlgaMaciaszek , I am not sure which you are pointing me towards, since searching for the term is kind of ambiguous.
I am guessing you are refering to this https://docs.spring.io/spring-framework/reference/integration/rest-clients.html#rest-http-interface. It is great and all but strategically I am not sure if this is the right way.

The development and roadmap of feign itself is going in the right direction, supporting async, planning webclient and other resilience function. We are leveraging Kotlin Coroutines with Spring on JVM (having a multiplatform setup in mind) and are really happy to abstract API integration to feign. Switching to a lower level abstraction, like linked above would mean losing a lot of benefits. Lastly, this would join quite well with openapi-generators to generate the correct integrations.

I am not sure if my request is so far off a suitable maintenance for the project.

[OlgaMaciaszek]

Hello @dgensert This decision has been thoroughly discussed and taken long time ago. There's no way to provide integration for WebClient or reactive streams in SC OpenFeign without major changes in Feign and at the point of that decision the issue to add such support has been unresolved for a few years. Also, as Framework authors, it makes sense for us to support the interface client model directly within the Spring Framework, as it both fits the model and the scope of concerns we usually cover and is easier for us to maintain than a third party integration. It also allowed us to add aligned support for RSocket and there's work in place to provide a similar solution for GRPC.

In any case if it's a small change, you can provide a PR for us to review, but at this point I cannot promise we will decide to go ahead with the change after review. The outlined scoped of a maintenance project for us is only fixing CVEs and blocking bugs. We sometimes review and accept small PRs from the community as well.