Spring Gateway集成OpenFeign启动失败 缺少HttpMessageConverters Bean

问题现象 #

使用Spring Boot 2.7.0搭建Spring Cloud API网关，原有逻辑运行正常，将RestTemplate替换为OpenFeign后启动失败。

环境依赖配置 #

build.gradle核心配置如下：

plugins {
    id 'org.springframework.boot' version '2.7.0'
    id 'io.spring.dependency-management' version '1.0.11.RELEASE'
    id 'java'
    id 'org.liquibase.gradle' version '2.1.0'
    id 'groovy'
}

ext {
    set('springCloudVersion', "2021.0.3")
}
    
// 已注释Servlet栈web依赖，使用WebFlux栈
// implementation 'org.springframework.boot:spring-boot-starter-web'        
implementation 'org.springframework.boot:spring-boot-starter-webflux'
implementation "org.springframework.cloud:spring-cloud-dependencies:${springCloudVersion}"        
implementation 'org.springframework.cloud:spring-cloud-starter-circuitbreaker-resilience4j'
implementation 'org.springframework.cloud:spring-cloud-starter-netflix-eureka-client'
implementation 'org.springframework.cloud:spring-cloud-starter-openfeign'
implementation 'org.springframework.cloud:spring-cloud-starter-gateway'
implementation 'org.springframework.cloud:spring-cloud-starter-config'
implementation 'org.springframework.cloud:spring-cloud-starter-bootstrap'

启动类添加了对应开启注解：

@SpringBootApplication
@EnableDiscoveryClient
@EnableFeignClients
public class ApiGatewayApplication {
    // 启动类核心逻辑
}

编写的Feign客户端代码如下：

@FeignClient(name="identity-service")
public interface IdentityServiceClient {

    @GetMapping("/api/all")
    public List<ApiKey> getAllApiKeys();

}

报错信息 #

启动时抛出核心错误如下：

o.s.c.openfeign.FeignClientFactoryBean   : For 'identity-service' URL not provided. Will try picking an instance via load-balancing.
DiscoveryClientOptionalArgsConfiguration : Eureka HTTP Client uses RestTemplate
....
onfigReactiveWebServerApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'apiGatewayApplication': Invocation of init method failed; nested exception is feign.codec.DecodeException: No qualifying bean of type 'org.springframework.boot.autoconfigure.http.HttpMessageConverters' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}
com.netflix.discovery.DiscoveryClient    : Shutting down DiscoveryClient ...
com.netflix.discovery.DiscoveryClient    : Unregistering ...
com.netflix.discovery.DiscoveryClient    : DiscoveryClient_API-GATEWAY/192.168.1.72:api-gateway:8081 - deregister  status: 404
com.netflix.discovery.DiscoveryClient    : Completed shut down of DiscoveryClient
 


***************************
APPLICATION FAILED TO START
***************************

Description:

A component required a bean of type 'org.springframework.boot.autoconfigure.http.HttpMessageConverters' that could not be found.


Action:

Consider defining a bean of type 'org.springframework.boot.autoconfigure.http.HttpMessageConverters' in your configuration.

初步排查特征 #

• 公开示例文档均未提及需要手动配置HttpMessageConverters，该组件按自动装配逻辑应随spring-web、spring-boot-starter-webflux依赖加载。
• 不通过@Autowired注入Feign客户端、业务服务时，应用可正常启动；一旦使用@Autowired注入上述组件，就会复现报错。

根因分析 #

1. Spring Cloud Gateway基于WebFlux响应式栈构建，默认不会触发Servlet栈相关的自动配置，而HttpMessageConverters的自动配置类是绑定Spring MVC（Servlet栈）场景的，纯WebFlux环境下默认不会创建该Bean。
2. OpenFeign默认的编码器、解码器实现强依赖HttpMessageConverters完成请求/响应的序列化反序列化。Feign客户端默认是懒初始化的：如果代码中没有注入Feign客户端，就不会触发Feign实例的初始化流程，不会校验依赖的Bean；一旦通过@Autowired注入Feign客户端，就会触发Feign实例初始化，加载默认解码器时找不到HttpMessageConverters Bean，直接抛出启动错误。
3. 注意：不要为了获取这个Bean引入spring-boot-starter-web依赖，Servlet栈的web依赖会和WebFlux栈的Gateway产生冲突，导致网关以Servlet容器模式启动，网关路由、过滤器等核心功能会失效。

解决方案 #

在配置类中手动注册HttpMessageConverters Bean即可，不需要引入额外的Servlet栈依赖：

import org.springframework.boot.autoconfigure.http.HttpMessageConverters;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.http.converter.HttpMessageConverter;

@Configuration
public class FeignConfig {
    @Bean
    public HttpMessageConverters httpMessageConverters(ObjectProvider<HttpMessageConverter<?>> converters) {
        return new HttpMessageConverters(converters.orderedStream().toList());
    }
}

添加该配置后重启应用，Feign客户端即可正常注入、调用。如果需要完全适配响应式场景，也可以替换使用Spring Cloud官方提供的响应式Feign组件，适配WebFlux栈的编解码逻辑，不需要额外注册上述Bean。

内容的提问来源于stack exchange，提问作者sonoerin