I'm in the process of upgrading our project from 2.4.2 to 2.5.6. While in the progress of testing this, we started seeing a lot or errors saying "This ApplicationConversionService cannot be modified".
It looks like this is caused by us using ApplicationConversionService.getSharedInstance().addConverter() and a change that was introduced in https://github.com/spring-projects/spring-boot/issues/26088
The top bullet in the RC1 mentions this as a new feature. https://github.com/spring-projects/spring-boot/releases?page=3
Looks like a breaking change to me? I think it would be nice if it was properly documented as such, and with at least a pointer to the documentation of how to add converters instead.
Comment From: wilkinsona
Good catch, thank you. This should have been mentioned in the release notes.