[Regression] 404 without ACCEPT request header results in a 406 in Spring Framework 5.1.6

[mixcloud-downloader]

Affects: 5.1.6

Since the Upgrade from Spring-Boot-2.1.3.RELEASE to Spring-Boot-2.1.4.RELEASE, any error (e.g. a 404 or an 500) an expected 404 without an ACCEPT request header result in a 406 response and this message in the logs:

DefaultHandlerExceptionResolver : Resolved [org.springframework.web.HttpMediaTypeNotAcceptableException: Could not find acceptable representation]

This regression happens only when setting the default content type to TEXT_HTML:

@Configuration
class DefaultContentTypeConfiguration implements WebMvcConfigurer {

	@Override
	public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
		configurer.defaultContentType(TEXT_HTML);
	}
}

See spring-attic/spring-framework-issues#191 for an example.

On a site note: This is the second bug in a very short time (the other was #22851) which goes unnoticed with Spring's classic MockMVC testing approach. It makes me thing that using MockMVC is not a too good idea any more.

[rstoyanchev]

This appears to be related to the changes for #22598. An empty ProducesRequestCondition returns a constant EMPTY_CONDITION instead of this which causes it to lose the configured ContentNegotiationManager, which in Spring Boot falls back on "text/html" by default.