版本：spring-cloud-openfeign-core-2.1.1.RELEASE.jar，spring-webmvc-5.1.14.RELEASE.jar，jetty-server-9.4.41.v20210516.jar，tomcat-embed-core-9.0.48.jar

问题背景

生产服务请求下游服务时偶发抛出下面的异常，下游服务已经很久没有人发布并且没有修改任何配置，而且是偶发，这个问题很奇怪，服务使用的Spring cloud openfeign，由于不熟悉Spring cloud与openfeign，先梳理学习Spring cloud openfeign bean的初始化与定义

feign.codec.DecodeException: Could not extract response: no suitable HttpMessageConverter found for response type [class com.....master.models.APIResponse] and content type [application/xhtml+xml;charset=UTF-8]
    at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:180)
    at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:140)
    at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:78)
    at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:103)
    at com.sun.proxy.$Proxy401.sendEmail(Unknown Source)
    at ......
    Caused by: org.springframework.web.client.RestClientException: Could not extract response: no suitable HttpMessageConverter found for response type [class com.......APIResponse] and content type [application/xhtml+xml;charset=UTF-8]
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:121)
    at org.springframework.cloud.openfeign.support.SpringDecoder.decode(SpringDecoder.java:59)
    at org.springframework.cloud.openfeign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:62)
    at feign.optionals.OptionalDecoder.decode(OptionalDecoder.java:36)
    at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:176)
    ... 106 more

Spring Cloud openfeign

初始化过程

应用启动SpringBootApplication，引入自动配置sdk，自动配置：org.springframework.cloud.openfeign.FeignAutoConfiguration

FeignAutoConfiguration

1. 构建feignContext：org.springframework.cloud.openfeign.FeignContext，FeignContext是Spring应用上下文与feignClient配置org.springframework.cloud.openfeign.FeignClientSpecification的组合
2. 按需（@ConditionalOnClass(name = “feign.hystrix.HystrixFeign”)）构建feignTargeter：HystrixTargeter，默认（@ConditionalOnMissingClass(“feign.hystrix.HystrixFeign”)）：DefaultTargeter
3. 按需构建HttpClientFeignConfiguration，条件见：下方代码
4. 按需构建OkHttpFeignConfiguration，条件见：下方代码

	@ConditionalOnClass(ApacheHttpClient.class)
	@ConditionalOnMissingClass("com.netflix.loadbalancer.ILoadBalancer")
	@ConditionalOnMissingBean(CloseableHttpClient.class)
	@ConditionalOnProperty(value = "feign.httpclient.enabled", matchIfMissing = true)
	protected static class HttpClientFeignConfiguration {
        ...
    }
	@Configuration
	@ConditionalOnClass(OkHttpClient.class)
	@ConditionalOnMissingClass("com.netflix.loadbalancer.ILoadBalancer")
	@ConditionalOnMissingBean(okhttp3.OkHttpClient.class)
	@ConditionalOnProperty("feign.okhttp.enabled")
	protected static class OkHttpFeignConfiguration {
        ...
    }

查看注解：@EnableFeignClients(basePackages = {“…”}),客户端注入过程切入点，该注解会通过Spring Import注解导入bean定义，注解指定导入bean定义类：org.springframework.cloud.openfeign.FeignClientsRegistrar

注册BeanDefinitions流程

1. registerDefaultConfiguration
2. registerFeignClients

registerDefaultConfiguration

1. 获取EnableFeignClients注解默认属性配置
2. 如果默认配置包含（defaultConfiguration），则将默认配置注册为bean（FeignClientSpecification）。beanName：default…notification.Application

registerFeignClients

1. 使用ClassPathScanningCandidateComponentProvider扫描器按照EnableFeignClients配置的basePackages扫描当前资源resourceLoader，扫描过滤器AnnotationTypeFilter筛选出FeignClient注解的AnnotatedBeanDefinition
2. 根据FeignClient注解的属性（configuration）定义构建FeignClientSpecification的BeanDefinition，注册客户端配置方法代码如下

	private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,
			Object configuration) {
		BeanDefinitionBuilder builder = BeanDefinitionBuilder
				.genericBeanDefinition(FeignClientSpecification.class);
		builder.addConstructorArgValue(name);
		builder.addConstructorArgValue(configuration);
		registry.registerBeanDefinition(
				name + "." + FeignClientSpecification.class.getSimpleName(),
				builder.getBeanDefinition());
	}

registerFeignClient

1. 构建客户端bean：FeignClientFactoryBean
2. 将bean封装为：BeanDefinitionHolder注册至工厂
   1. bean名称为className
   2. feign client bean别名使用注解属性（FeignClient）：qualifier，兜底使用：contextId+FeignClient，contextId读取注解属性优先级：contextId>name，name注解属性优先级：serviceId>name>value

代码如下

	private void registerFeignClient(BeanDefinitionRegistry registry,
			AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
		String className = annotationMetadata.getClassName();
		BeanDefinitionBuilder definition = BeanDefinitionBuilder
				.genericBeanDefinition(FeignClientFactoryBean.class);
		validate(attributes);
		definition.addPropertyValue("url", getUrl(attributes));
		definition.addPropertyValue("path", getPath(attributes));
		String name = getName(attributes);
		definition.addPropertyValue("name", name);
		String contextId = getContextId(attributes);
		definition.addPropertyValue("contextId", contextId);
		definition.addPropertyValue("type", className);
		definition.addPropertyValue("decode404", attributes.get("decode404"));
		definition.addPropertyValue("fallback", attributes.get("fallback"));
		definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
		definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
		String alias = contextId + "FeignClient";
		AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
		boolean primary = (Boolean) attributes.get("primary"); // has a default, won't be
																// null
		beanDefinition.setPrimary(primary);
		String qualifier = getQualifier(attributes);
		if (StringUtils.hasText(qualifier)) {
			alias = qualifier;
		}
		BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
				new String[] { alias });
		BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
	}

FeignClientFactoryBean

1. 获取FeignClient bean：org.springframework.cloud.openfeign.FeignClientFactoryBean#getObject
2. 应用上下文中获取FeignContext
3. 创建Feign.Builder：org.springframework.cloud.openfeign.FeignClientFactoryBean#feign
   1. 从FeignContext上下文获取构造者feign.Feign.Builder
   2. 为构建者设置encoder，从FeignContext上下文获取feign.codec.Encoder：org.springframework.cloud.openfeign.support.PageableSpringEncoder->SpringEncoder->SpringFormEncoder->feign.codec.Encoder.Default
   3. 为构建者设置decoder，从FeignContext上下文获取feign.codec.Decoder：feign.optionals.OptionalDecoder->org.springframework.cloud.openfeign.support.ResponseEntityDecoder->SpringDecoder
   4. 为构建者设置contract，从FeignContext上下文获取feign.Contract：使用feign.hystrix.HystrixDelegatingContract#HystrixDelegatingContract代理包装自定义实现OpenFeignSpringMvcContract（继承自feign.OpenFeignBaseContract）
   5. 配置feign（configureFeign）：如果存在FeignClientProperties配置，则按照配置设置：loggerLevel/connectTimeout/readTimeout/Retryer/ErrorDecoder/RequestInterceptor/decode404/encoder/decoder/contract。注意配置优先级，如果spring上下文中存在配置bean，则：如果当前配置是默认配置，则当前配置<default配置<contextId对应的配置。否则：default配置<contextId对应的配置<当前配置。如果spring上下文中不存在配置bean，则仅按照当前配置设置feign
4. 如果不存在url配置：org.springframework.cloud.openfeign.FeignClientFactoryBean#loadBalance
5. 否则
   1. 获取Client：org.springframework.cloud.openfeign.FeignClientFactoryBean#getOptional。如果client不为空并且是LoadBalancerFeignClient类型则获取其代理的实际Client
   2. 获取Targeter：org.springframework.cloud.openfeign.FeignClientFactoryBean#get
   3. 构建目标bean：org.springframework.cloud.openfeign.Targeter#target（feign.Target.HardCodedTarget）

当前案例默认无url，即步骤3：org.springframework.cloud.openfeign.FeignClientFactoryBean#loadBalance

1. 获取Client：org.springframework.cloud.openfeign.FeignClientFactoryBean#getOptional。常用实现：ApacheHttpClient，OkHttpClient，feign.Client.Default。AnnotationConfigApplicationContext上下文（每一个应用都有一个独立的上下文，拥有同一个parent Spring应用上下文）仅管理FeignClient注解的bean。从中获取对应的bean（org.springframework.cloud.context.named.NamedContextFactory#getInstance(java.lang.String, java.lang.Class)）。当前案例使用自定义封装ReWriteHeaderFeignClient客户端，继承自：org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient
2. 获取Targeter：org.springframework.cloud.openfeign.HystrixTargeter
3. 构建目标bean：org.springframework.cloud.openfeign.Targeter#target（feign.Target.HardCodedTarget）

Client

配置入口：org.springframework.cloud.openfeign.ribbon.FeignRibbonClientAutoConfiguration-》org.springframework.cloud.openfeign.ribbon.HttpClientFeignLoadBalancedConfiguration#feignClient
默认实现：org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient#LoadBalancerFeignClient

1. lbClientFactory：配置入口org.springframework.cloud.openfeign.ribbon.FeignRibbonClientAutoConfiguration#cachingLBClientFactory
2. clientFactory：配置入口org.springframework.cloud.netflix.ribbon.RibbonAutoConfiguration#springClientFactory

构建目标bean

构建Feign：feign.hystrix.HystrixFeign.Builder#build(feign.hystrix.FallbackFactory<?>)

1. 实现InvocationHandler：feign.hystrix.HystrixInvocationHandler
2. 包装contract：feign.hystrix.HystrixDelegatingContract#HystrixDelegatingContract
3. 调用父类build方法构建Feign：feign.Feign.Builder#build

public Feign build() {
  SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
      new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
          logLevel, decode404, closeAfterDecode, propagationPolicy);
  ParseHandlersByName handlersByName =
      new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,
          errorDecoder, synchronousMethodHandlerFactory);
  return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
}

SynchronousMethodHandler.Factory工厂相关属性

1. client:常用实现：ApacheHttpClient，OkHttpClient，feign.Client.Default。当前案例使用自定义封装ReWriteHeaderFeignClient客户端
2. retryer:feign.Retryer.Default#Default()
3. requestInterceptors
   1. 自定义：OpenFeignRequestInterceptor
   2. 自定义：GrayPatternMatcherRequestInterceptor
   3. 自定义：GzipFeignAcceptGzipEncodingInterceptor继承FeignAcceptGzipEncodingInterceptor，如果配置feign.compression.response.enabled=true，则启用该拦截器，追加header Accept-Encoding -> gzip
   4. 自定义：FeignEnvFlagInterceptor
4. decode404：false
5. closeAfterDecode：true
6. propagationPolicy：feign.Feign.Builder#propagationPolicy（NONE）

ParseHandlersByName相关属性

1. options：feign.Request.Options#Options()，connectTimeoutMillis默认10s，readTimeoutMillis默认60s
2. encoder，decoder，contract：同Feign.Builder
3. queryMapEncoder：feign.QueryMapEncoder.Default
4. errorDecoder：自定义实现new ErrorDecoder()
5. contract：HystrixDelegatingContract

ReflectiveFeign相关属性

1. invocationHandlerFactory：feign.InvocationHandlerFactory.Default。当前案例为内部实现类，工厂创建的handler为：HystrixInvocationHandler
2. queryMapEncoder：同上
3. handlersByName：ParseHandlersByName

创建目标实例
feign.ReflectiveFeign#newInstance

1. 获取nameToHandler映射：feign.ReflectiveFeign.ParseHandlersByName#apply
   1. 解析与校验元数据：feign.Contract#parseAndValidatateMetadata，当前案例：自定义实现重写父类方法feign.OpenFeignBaseContract#parseAndValidateMetadata
   2. processAnnotationOnClass
   3. processAnnotationOnMethod
   4. processAnnotationsOnParameter
   5. checkState
   6. checkMapString：HeaderMap
   7. checkMapKeys：QueryMap
   8. 回调handler：feign.FeignClientMethodMetadataParseHandler#parsed
2. 根据default与nameToHandler获取methodToHandler映射
3. 创建InvocationHandler：feign.InvocationHandlerFactory.Default#create
4. 创建目标实例代理：java.lang.reflect.Proxy#newProxyInstance(java.lang.ClassLoader, java.lang.Class<?>[], java.lang.reflect.InvocationHandler)
5. 如果是default方法：feign.Util#isDefault，绑定feign.DefaultMethodHandler至动态代理
6. 返回代理对象

isDefault默认方法定义

	// Default methods are public non-abstract, non-synthetic, and non-static instance methods
    // declared in an interface.
    // method.isDefault() is not sufficient for our usage as it does not check
    // for synthetic methods. As a result, it picks up overridden methods as well as actual default
    // methods.

动态代理InvocationHandler

  static final class Default implements InvocationHandlerFactory {

    @Override
    public InvocationHandler create(Target target, Map<Method, MethodHandler> dispatch) {
      return new ReflectiveFeign.FeignInvocationHandler(target, dispatch);
    }
  }

请求过程

关键对象属性

FeignInvocationHandler

1. target：HardCodedTarget
   1. type：目标对象class，例如：MyTestAPI
   2. name：my-feign-client
   3. url：http://my-feign-client
2. dispatch：key代理对象方法，例如：MyTestAPI.helloWorld()，value：feign.SynchronousMethodHandler

SynchronousMethodHandler

1. 与SynchronousMethodHandler.Factory相同部分不再重复，搜索上方关键字：SynchronousMethodHandler.Factory
2. target：HardCodedTarget
3. decoder，options，errorDecoder：同ParseHandlersByName.decoder
4. metadata：feign.MethodMetadata
5. buildTemplateFromArgs：feign.ReflectiveFeign.BuildEncodedTemplateFromArgs#BuildEncodedTemplateFromArgs/BuildTemplateByResolvingArgs/BuildFormEncodedTemplateFromArgs
6. client: 同工厂（SynchronousMethodHandler.Factory），当前案例为自定义实现：ReWriteHeaderFeignClient

FeignLoadBalancer

1. lb:ZoneAwareLoadBalancer->com.netflix.loadbalancer.DynamicServerListLoadBalancer，定期更新server列表updateListOfServers
2. config：com.netflix.client.config.DefaultClientConfigImpl

ZoneAwareLoadBalancer

1. serverListImpl：org.springframework.cloud.netflix.ribbon.eureka.DomainExtractingServerList

DynamicServerListLoadBalancer

1. list：com.netflix.niws.loadbalancer.DiscoveryEnabledNIWSServerList

DiscoveryEnabledNIWSServerList

1. eurekaClientProvider:com.netflix.niws.loadbalancer.LegacyEurekaClientProvider，get获取返回EurekaClient：com.netflix.discovery.DiscoveryManager#getDiscoveryClient/getEurekaClient
2. obtainServersViaDiscovery：刷新服务实例列表方法，按照服务名称（vipAddresses，例如：MyEurekaService1,MyEurekaService2）获取实例列表：com.netflix.discovery.EurekaClient#getInstancesByVipAddress(java.lang.String, boolean, java.lang.String)->com.netflix.discovery.DiscoveryClient#getInstancesByVipAddress(java.lang.String, boolean, java.lang.String)

请求过程

1. 动态代理调用：feign.ReflectiveFeign.FeignInvocationHandler#invoke
2. 调用代理方法：feign.SynchronousMethodHandler#invoke
3. 创建RequestTemplate：feign.ReflectiveFeign.BuildTemplateByResolvingArgs#create
   1. 解析参数构建EncodedTemplate对象：feign.ReflectiveFeign.BuildEncodedTemplateFromArgs#resolve
   2. encode request body：org.springframework.cloud.openfeign.support.PageableSpringEncoder#encode->org.springframework.cloud.openfeign.support.SpringEncoder#encode
   3. org.springframework.http.converter.json.MappingJackson2HttpMessageConverter->org.springframework.http.converter.AbstractHttpMessageConverter#write将request body写入org.springframework.cloud.openfeign.support.SpringEncoder.FeignOutputMessage
   4. 如果请求没有Content-Type header默认值设置为：application/json;charset=UTF-8
   5. 写入encoded body，并追加Content-Length header
4. 克隆Retryer：feign.Retryer.Default#clone
5. 发起请求并解析响应结果：feign.SynchronousMethodHandler#executeAndDecode
6. 构建请求：feign.SynchronousMethodHandler#targetRequest
   1. 回调拦截器处理请求：feign.RequestInterceptor
   2. 创建request请求：feign.Target.HardCodedTarget#apply-》feign.RequestTemplate#request
7. 客户端执行请求：feign.SynchronousMethodHandler#client#execute(request, options)：org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient#execute
8. 按照客户端名称创建负载均衡客户端：org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient#lbClient-》org.springframework.cloud.openfeign.ribbon.CachingSpringLoadBalancerFactory#create，从Spring client Factory中获取ILoadBalancer+IClientConfig，构建FeignLoadBalancer或RetryableFeignLoadBalancer
9. 负载均衡，构建LB命令：com.netflix.client.AbstractLoadBalancerAwareClient#buildLoadBalancerCommand，如果LoadBalancerCommand.server为null则选择server：com.netflix.loadbalancer.reactive.LoadBalancerCommand#selectServer-》com.netflix.loadbalancer.LoadBalancerContext#getServerFromLoadBalancer-》com.netflix.loadbalancer.ZoneAwareLoadBalancer#chooseServer-》自定义实现LB规则…platform.ribbon.wrapper.ZoneAvoidanceAndGrayAndLoadBasedRule#choose（请求eureka刷新服务器列表com.netflix.discovery.DiscoveryClient#refreshRegistry，按照LB规则选择服务器）
10. 提交命令执行请求。自定义实现com.netflix.loadbalancer.reactive.ServerOperation：重构Server请求URI：com.netflix.client.AbstractLoadBalancerAwareClient#executeWithLoadBalancer(S, com.netflix.client.config.IClientConfig)-》com.netflix.loadbalancer.LoadBalancerContext#reconstructURIWithServer
11. 完成请求返回响应结果feign.Response
12. decode response
13. org.springframework.web.client.HttpMessageConverterExtractor.extractData
14. 如果转换器类型为GenericHttpMessageConverter，根据responseType+contentType获取转换器GenericHttpMessageConverter
15. 否则根据responseClass+contentType获取转换器
16. HttpMessageConverter将response转换为目标方法返回值类型

问题分析

客户端

ReWriteHeaderFeignClient自定义封装调用链路

1. feign.SynchronousMethodHandler#invoke->executeAndDecode->自定义实现ReWriteHeaderFeignClient#execute ->org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient#execute->org.springframework.cloud.openfeign.ribbon.LoadBalancerFeignClient#lbClient->自定义实现EncodeHeaderCachingSpringLoadBalancerFactory#create->FeignLoadBalancer(如果存在重试工厂loadBalancedRetryFactory则使用RetryableFeignLoadBalancer）->自定义实现EncodeHeaderCachingSpringLoadBalancerFactory.EncodeFeignLoadBalancer#executeWithLoadBalancer->LoadBalancerCommand.submit->异步调用org.springframework.cloud.openfeign.ribbon.FeignLoadBalancer#execute-》org.springframework.cloud.openfeign.ribbon.FeignLoadBalancer.RibbonRequest.client()->自定义实现ResetTimeoutFeignClient#execute-》自定义实现JettyHttpClient#execute-〉org.eclipse.jetty.client.HttpRequest#send(org.eclipse.jetty.client.api.Response.CompleteListener)-》请求完成回调org.eclipse.jetty.client.util.FutureResponseListener#onComplete-》将org.eclipse.jetty.client.HttpResponse封装为org.eclipse.jetty.client.HttpContentResponse#HttpContentResponse->自定义实现JettyHttpClient#toFeignResponse（代码见下方）-》org.springframework.cloud.openfeign.ribbon.FeignLoadBalancer#execute将Response封装为RibbonResponse->feign.SynchronousMethodHandler#decode

return Response.builder().status(status).reason(reason).headers(headers).request(feignRequest).body(body).build();

查看所有自定义实现的类没有重写Content-Type header的类，故而排除

服务端

处理流程

1. 接收派发请求：org.springframework.web.servlet.DispatcherServlet#doDispatch
2. 根据请求查找处理句柄：org.springframework.web.servlet.DispatcherServlet#getHandler
3. 查找Handler适配器（即目标方法适配器）：org.springframework.web.servlet.DispatcherServlet#getHandlerAdapter
4. 预处理句柄回调：org.springframework.web.servlet.HandlerExecutionChain#applyPreHandle-》org.springframework.web.servlet.HandlerInterceptor#preHandle
5. 实际执行处理句柄：org.springframework.web.servlet.HandlerAdapter#handle
6. 回处理完成后置调拦截器：org.springframework.web.servlet.HandlerInterceptor#postHandle
7. 处理派发请求处理结果：org.springframework.web.servlet.DispatcherServlet#processDispatchResult
   1. 如果存在异常则处理：org.springframework.web.servlet.DispatcherServlet#processHandlerException
   2. 如果存在ModelAndView，则渲染mv：org.springframework.web.servlet.DispatcherServlet#render
   3. 如果是并发异步处理则返回
   4. 否则回调拦截器：org.springframework.web.servlet.HandlerInterceptor#afterCompletion
8. 如果是并发异步处理则回调拦截器：org.springframework.web.servlet.AsyncHandlerInterceptor#afterConcurrentHandlingStarted
9. 如果是multipart请求清理multipart：org.springframework.web.servlet.DispatcherServlet#cleanupMultipart
10. 发布事件：org.springframework.web.servlet.FrameworkServlet#publishRequestHandledEvent
11. javax.servlet.FilterChain

实际执行处理句柄

org.springframework.web.servlet.DispatcherServlet#doDispatch->org.springframework.web.servlet.HandlerAdapter#handle

1. org.springframework.web.servlet.HandlerAdapter#handle，当前案例实现类：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter-》org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter#handle
2. org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#handleInternal
3. org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#invokeHandlerMethod
   1. org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#getDataBinderFactory
   2. org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#getModelFactory
   3. org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#createInvocableHandlerMethod
   4. 设置argumentResolvers（见图1）
   5. 设置returnValueHandlers（见图2）
   6. 设置parameterNameDiscoverer：org.springframework.core.DefaultParameterNameDiscoverer
   7. 设置ignoreDefaultModelOnRedirect，默认true
   8. 设置asyncRequestTimeout，默认null
   9. 设置TaskExecutor：new SimpleAsyncTaskExecutor(“MvcAsync”)
   10. 注册callableInterceptors，默认空数组：CallableProcessingInterceptor
   11. 注册deferredResultInterceptors，默认空数组：DeferredResultProcessingInterceptor
   12. 执行目标方法：org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod#invokeAndHandle
   13. 反射调用目标方法，得到返回值
   14. 如果返回值为空或者responseStatusReason不为空设置setRequestHandled后返回
   15. 选择支持返回值类型的处理句柄：org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite#selectHandler，当前案例：org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor
   16. 回调返回值拦截器：org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite#handleReturnValue，当前案例：org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor#handleReturnValue
   17. 创建输入输出消息对象：org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver#createInputMessage，org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#createOutputMessage
   18. 写入数据：org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#writeWithMessageConverters(T, org.springframework.core.MethodParameter, org.springframework.http.server.ServletServerHttpRequest, org.springframework.http.server.ServletServerHttpResponse)
   19. 如果是isResourceType，写入header：Accept-Ranges，存在异常则写入header：Content-Range，当前案例：否
   20. 如果outputMessage存在Content-Type则使用，当前案例：不存在
   21. 否则获取request请求的Accept类型org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#getAcceptableMediaTypes
   22. 获取请求producible类型（对应注解produces属性：@PostMapping(value = “/hello”, produces = “application/json;charset=utf8”)）rg.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#getProducibleMediaTypes(javax.servlet.http.HttpServletRequest, java.lang.Class<?>, java.lang.reflect.Type)：当前案例请求中org.springframework.web.servlet.HandlerMapping#PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE参数不存在，走兜底：org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver#allSupportedMediaTypes
   23. 根据请求方acceptableTypes获取可兼容的producibleTypes作为可使用的Content-Type MediaType列表：mediaTypesToUse，当前案例：Content-Type列表见：注释1
   24. 排序可兼容的mediaTypes：org.springframework.http.MediaType#sortBySpecificityAndQuality，排序结果见：注释2
   25. 选择第一个具体的MediaType：org.springframework.util.MimeType#isConcrete
   26. 获取匹配的转换器：org.springframework.http.converter.HttpMessageConverter，当前案例：org.springframework.http.converter.json.MappingJackson2HttpMessageConverter
   27. 写入body前置通知回调：org.springframework.web.servlet.mvc.method.annotation.ResponseBodyAdvice#beforeBodyWrite
   28. 如果存在则写入Response header：Content-Disposition
   29. 写入body数据：MappingJackson2HttpMessageConverter-》org.springframework.http.converter.AbstractGenericHttpMessageConverter#write
   30. 添加默认header：org.springframework.http.converter.AbstractHttpMessageConverter#addDefaultHeaders，Content-Type，Content-Length（当前案例均为null不写入：org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter#getContentLength）
   31. 向outputMessage写入body数据：org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter#writeInternal
   32. 构建ModelAndView：org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter#getModelAndView
4. 请求完成后置处理：org.springframework.web.context.request.AbstractRequestAttributes#requestCompleted
5. 如果Response不包含header（Cache-Control），并且org.springframework.web.method.annotation.SessionAttributesHandler#hasSessionAttributes，处理cache：org.springframework.web.servlet.support.WebContentGenerator#applyCacheSeconds(javax.servlet.http.HttpServletResponse, int)
6. 否则准备Response：org.springframework.web.servlet.support.WebContentGenerator#prepareResponse
   1. cacheControl不为空，则设置header（Cache-Control），或header（Pragma），或header（Expires）
   2. 否则设置header（Cache-Control），或header（Pragma），或header（Expires）为指定值，例如：no-cache，no-store，1L
   3. 如果varyByRequestHeaders不为空，设置header（Vary）

问题原因

响应体如果没有指定Content-Type，那么就会从兼容请求体Accept的Content-Type中选择一个进行响应，也就是说请求方使用了非json格式的Accept引起，为了验证问题，我们查看线上pinpoint（开源的链路监控平台）监控，出现问题的请求的Accept header如下，与报错原因也就对应上了

accept=text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7

解决方法

1. 请求方使用适配的Accept类型，例如：application/json;charset=utf8或*/*
2. 接口PostMapping/RequestMapping等注解增加配置
   a. 下游接口可以增加produces配置响应内容为json格式，即content-type header：org.springframework.web.bind.annotation.PostMapping#produces
   b. 上游接口可以增加consumes配置请求内容为json格式，即accept header：org.springframework.web.bind.annotation.PostMapping#consumes
3. 实现接口org.springframework.http.converter.HttpMessageConverter#write写入OutMessage Content-Type，例如：open-feign在编译时写入header：org.springframework.cloud.openfeign.support.SpringEncoder#encode-》write，服务端是在将return type写入outputMessage时处理（Writes the given return type to the given output message）：org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#writeWithMessageConverters(T, org.springframework.core.MethodParameter, org.springframework.http.server.ServletServerHttpRequest, org.springframework.http.server.ServletServerHttpResponse)
4. 为OutputMessage设置适配的Content-Type header org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor#createOutputMessage
   1. 增加自定义ServletHandler，例如：xxl-job中的方法，代码见附件3
   2. 实现预处理handle拦截器处理：org.springframework.web.servlet.HandlerInterceptor#preHandle

注意：方法2-4要根据实际业务场景是否可以使用，客户端期望xhtml，但是实际响应json，是否有潜在的风险？
Q&A

1. 接口PostMapping/RequestMapping等注解增加headers配置是否可以？org.springframework.web.bind.annotation.PostMapping#headers
   1. 不可以，因为该header是添加再Request请求中，需要添加在Response中的header才可以

注释

注释1

0 = {MediaType@22077} "application/octet-stream"
1 = {MediaType@22078} "text/plain"
2 = {MediaType@22079} "application/xml"
3 = {MediaType@22080} "text/xml"
4 = {MediaType@22081} "application/x-www-form-urlencoded"
5 = {MediaType@22082} "application/cbor"
6 = {MediaType@22083} "application/*+xml"
7 = {MediaType@22084} "multipart/form-data"
8 = {MediaType@22085} "application/json"
9 = {MediaType@22086} "application/*+json"
10 = {MediaType@22057} "*/*"

注释2

0 = {MediaType@22085} "application/json"
1 = {MediaType@22085} "application/json"
2 = {MediaType@22082} "application/cbor"
3 = {MediaType@22293} "application/xml"
4 = {MediaType@22296} "application/xml"
5 = {MediaType@22086} "application/*+json"
6 = {MediaType@22292} "application/*+json"
7 = {MediaType@22294} "text/xml"
8 = {MediaType@22295} "application/*+xml"
9 = {MediaType@22297} "text/xml"
10 = {MediaType@22298} "application/*+xml"

附件

图1

图2

附件3