WebMvcConfigurer is overridden by WebMvcAutoConfiguration

[dpash]

I noticed that as a result of #11105, my custom WebMvcConfigurer.configureContentNegotiation(ContentNegotiationConfigurer configurer) changes were being overwritten by the defaults from WebMvcProperties. Regardless of the @Order value given, WebMvcAutoConfiguration.configureContentNegotiation(ContentNegotiationConfigurer configurer) always seems to happen after the user configurer.

This means the only way to configure the content negotiation is through the property files.

In 1.5, I was calling configurer.favorParameter(true), but this gets overwritten by the default property, false, in 2.0.

@bclozel mentioned in #11105 that this appears to be a general ordering problem with WebMvcConfigurer, hence raising this issue.

[bclozel]

In WebMvcAutoConfiguration, Spring Boot provides a WebMvcAutoConfigurationAdapter (unordered). If developers provide their own WebMvcConfigurer, both are unordered, so Ordered.LOWEST_PRECEDENCE.

The developer's one is processed first (contributed by the application configuration) while the auto-configuration one is processed after (contributed during the auto-configuration phase), overwriting all choices made by the first one.

We should order the auto-configuration configurer at 0, so that:

• by default, developer's configuration will run after and enforce their opinion
• we give enough room for developers to run things before if they want to