WebConversionService breaks embedded value resolving

[snicoll]

See this issue and in particular this comment

There is a sample attached with a Spring Boot project that demonstrates the issue.

[philwebb]

I don't understand why the BackgroundPreinitializer breaks things. We just create then throw away a DefaultFormattingConversionService. Does DateTimeFormatAnnotationFormatterFactory.getFormatter get stored in a static somewhere?

[snicoll]

Yep, I don't either. I've debugged the application in Spring Boot and something should call EmbeddedValueResolutionSupport#setEmbeddedValueResolver but it's not clear as why it's not called. I don't think the background thing has any effect.

Perhaps @rstoyanchev knows a bit more about that part before I start digging again?

[rstoyanchev]

I'm not sure either.

[mhalbritter]

Like @snicoll and @philwebb suspected, this has nothing to do with background preinitialization.

In org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.EnableWebMvcConfiguration#mvcConversionService, we create a WebConversionService. It extends DefaultFormattingConversionService and calls super(false), meaning the embeddedValueResolver is null at that point. It then calls org.springframework.format.support.DefaultFormattingConversionService#addDefaultFormatters, which calls org.springframework.format.datetime.DateFormatterRegistrar#registerFormatters, which calls org.springframework.format.support.FormattingConversionService#addFormatterForFieldAnnotation. In there, we have this code:

if (this.embeddedValueResolver != null && annotationFormatterFactory instanceof EmbeddedValueResolverAware embeddedValueResolverAware) {
  embeddedValueResolverAware.setEmbeddedValueResolver(this.embeddedValueResolver);
}

this.embeddedValueResolver is null at that point, so annotationFormatterFactory.setEmbeddedValueResolver(...) is never called, which explains why the breakpoint there never hits.

I have some changes in https://github.com/mhalbritter/spring-boot/tree/mh/8923, but I'd like to get a second pair of eyes on that change. It does fix the problem in the reproducer project.