Auto-configured JsonMapper backs off when any type of ObjectMapper is defined

[Saljack]

I want to create a bean CsvMapper (or any other *Mapper), but if I create it then the autoconfigured default JsonMapper isn't created because there is @ConditionalOnMissingBean(ObjectMapper.class) and the CsvMapper extends ObjectMapper. This is problematic, and it can be unexpected behaviour. The only solution for this is to define a custom ObjectMapper with the same configuration as is in the autoconfiguration (copy paste code from the autoconfiguration).

If you change the condition to @ConditionalOnMissingBean(JsonMapper.class), then it is resolved and users can use multiple different *Mappers. The problem is that ObjectMapper is commonly used but if there is a new major version of Spring Boot 4 and if it uses the new version of Jackson 3, then it can be a good time to resolve it correctly.

See https://github.com/spring-projects/spring-boot/blob/main/module/spring-boot-jackson/src/main/java/org/springframework/boot/jackson/autoconfigure/JacksonAutoConfiguration.java#L114C28-L114C40

[Deepakgarg02]

Currently, the default JSON mapper is created with:

@Bean
@Primary
@ConditionalOnMissingBean(ObjectMapper.class)
JsonMapper jacksonJsonMapper(JsonMapper.Builder builder) {
    return builder.build();
}

Because CsvMapper, XmlMapper, and other *Mappers all extend ObjectMapper, defining one of them prevents the default JsonMapper from being created. This leads to unexpected behavior when users simply want to add an additional mapper (e.g., CSV + JSON), but JSON autoconfiguration gets disabled.

Update the condition to:

@ConditionalOnMissingBean(JsonMapper.class)

This way: The default JsonMapper is only skipped if the user has already defined a JsonMapper.
Other ObjectMapper subtypes (CsvMapper, XmlMapper, etc.) can coexist without blocking the JSON configuration.
Backward compatibility is preserved since JsonMapper still extends ObjectMapper.

Why: Avoids surprising behavior for users who add a CsvMapper/XmlMapper.
Removes the need for copy-pasting autoconfig code just to keep the default JSON mapper.
Clearer separation of concerns: only the JSON mapper replaces JSON config.

[wilkinsona]

Thanks for trying to help, @Deepakgarg02, but your comment is a (more verbose) rewording of the existing analysis of the problem.