【Bug 排除】OpenFeign：InvalidDefinitionException: Cannot construct instance of `reactor.core.publisher.Mono`

问题描述

OepnFeign 客户端使用 WebFlux 时遇到错误

• FeignClient：

    @GetMapping("/{deviceId}/test")
    Mono<R> test(@PathVariable(value = "deviceId") String deviceId);

调用后报错：

feign.codec.DecodeException: Type definition error: [simple type, class reactor.core.publisher.Mono]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `reactor.core.publisher.Mono` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (PushbackInputStream); line: 1, column: 1]
	at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:182) ~[feign-core-10.4.0.jar:na]
	Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Assembly trace from producer [reactor.core.publisher.MonoFlatMap] :
	reactor.core.publisher.Mono.flatMap
	org.springframework.web.reactive.result.method.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:137)
Error has been observed at the following site(s):

解决办法：

查看 Spring 官网文档，发现目前（2020-12-17）OpenFeign 暂不支持 WebFlux ：

https://docs.spring.io/spring-cloud-openfeign/docs/2.2.5.RELEASE/reference/html/#reactive-support

官方建议使用：https://github.com/Playtika/feign-reactive 解决