FeignHttpMessageConverters concurrency issue

Describe the bug
When mutiple concurrent threads call the org.springframework.cloud.openfeign.support.FeignHttpMessageConverters#getConverters method on a non-initialized class, some non-initialized converters could be returned. This leads to similar exceptions when processing requests/responses. After the class is initialized, no exceptions are thrown.

feign.codec.EncodeException: Could not write request: no suitable HttpMessageConverter found for request type [com.MyClass]
	at org.springframework.cloud.openfeign.support.SpringEncoder.encodeWithMessageConverter(SpringEncoder.java:168)
	at org.springframework.cloud.openfeign.support.SpringEncoder.encode(SpringEncoder.java:110)
	at org.springframework.cloud.openfeign.support.PageableSpringEncoder.encode(PageableSpringEncoder.java:105)
	at feign.RequestTemplateFactoryResolver$BuildEncodedTemplateFromArgs.resolve(RequestTemplateFactoryResolver.java:274)
	at feign.RequestTemplateFactoryResolver$BuildTemplateByResolvingArgs.create(RequestTemplateFactoryResolver.java:107)
	at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:48)
	at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:104)
	at org.springframework.cloud.openfeign.FeignCachingInvocationHandlerFactory$1.proceed(FeignCachingInvocationHandlerFactory.java:66)
	at org.springframework.cache.interceptor.CacheInterceptor.lambda$invoke$0(CacheInterceptor.java:54)
	at org.springframework.cache.interceptor.CacheAspectSupport.invokeOperation(CacheAspectSupport.java:424)
	at org.springframework.cache.interceptor.CacheAspectSupport.execute(CacheAspectSupport.java:410)
	at org.springframework.cache.interceptor.CacheInterceptor.invoke(CacheInterceptor.java:64)
	at org.springframework.cloud.openfeign.FeignCachingInvocationHandlerFactory.lambda$create$0(FeignCachingInvocationHandlerFactory.java:53)
	at jdk.proxy2/jdk.proxy2.$Proxy166.list(Unknown Source)
	... 189 filtered

java.lang.IllegalArgumentException: 'messageConverters' must not contain null elements
	at org.springframework.util.Assert.noNullElements(Assert.java:436)
	at org.springframework.web.client.HttpMessageConverterExtractor.<init>(HttpMessageConverterExtractor.java:78)
	at org.springframework.web.client.HttpMessageConverterExtractor.<init>(HttpMessageConverterExtractor.java:71)
	at org.springframework.cloud.openfeign.support.SpringDecoder.decode(SpringDecoder.java:56)
	at org.springframework.cloud.openfeign.support.ResponseEntityDecoder.decode(ResponseEntityDecoder.java:62)
	at feign.optionals.OptionalDecoder.decode(OptionalDecoder.java:38)
	at feign.InvocationContext.decode(InvocationContext.java:121)
	... 201 more
Wrapped by: <#64f634dc> feign.codec.DecodeException: 'messageConverters' must not contain null elements
	at feign.InvocationContext.decode(InvocationContext.java:125)
	at feign.InvocationContext.proceed(InvocationContext.java:94)
	at feign.ResponseHandler.handleResponse(ResponseHandler.java:69)
	at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:109)
	at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:53)
	at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:104)
	at org.springframework.cloud.openfeign.FeignCachingInvocationHandlerFactory$1.proceed(FeignCachingInvocationHandlerFactory.java:66)
	at org.springframework.cache.interceptor.CacheInterceptor.lambda$invoke$0(CacheInterceptor.java:54)
	at org.springframework.cache.interceptor.CacheAspectSupport.invokeOperation(CacheAspectSupport.java:424)
	at org.springframework.cache.interceptor.CacheAspectSupport.execute(CacheAspectSupport.java:410)
	at org.springframework.cache.interceptor.CacheInterceptor.invoke(CacheInterceptor.java:64)
	at org.springframework.cloud.openfeign.FeignCachingInvocationHandlerFactory.lambda$create$0(FeignCachingInvocationHandlerFactory.java:53)
	at jdk.proxy2/jdk.proxy2.$Proxy166.list(Unknown Source)
	... 172 filtered

Workaround

Current workaround is declaring own converters bean and call initialization explicitly at application startup:

    @Bean
    @ConditionalOnMissingBean
    public FeignHttpMessageConverters feignHttpMessageConverters(
            ObjectProvider<HttpMessageConverter<?>> messageConverters,
            ObjectProvider<HttpMessageConverterCustomizer> customizers) {
        var feignHttpMessageConverters = new FeignHttpMessageConverters(messageConverters, customizers);
        // init converters
        feignHttpMessageConverters.getConverters();
        return feignHttpMessageConverters;
    }

Start by reading FeignHttpMessageConverters#getConverters and the constructor usage shown in the workaround. Reproduce concurrent calls before initialization and inspect whether incomplete or null converters can be returned. Done means concurrent request and response processing no longer produces the reported converter exceptions, with coverage for initialization under concurrency.