Auto-configure Jackson mapper beans with a bean-based HandlerInstantiator

[grassehh]

Hello,

In Spring Boot 3, the ObjectMapper configuration used to automatically configure a SpringHandlerInstantiator (through Jackson2ObjectMapperBuilder).

Now that Jackson2ObjectMapperBuilder is deprecated in favor of using Jackson builders, the new JacksonAutoConfiguration class does not configure JacksonHandlerInstantiator by default inside JsonMapper.

This results in my custom serializer to fail to instantiate now because it injects a bean/
It would be nice to recover this default behavior from Spring Boot 3 so that Spring Boot automatically configures JacksonHandlerInstantiator in the JsonMapper.

What do you think ?

[wilkinsona]

Thanks for raising this.

It's a little bit awkward as JacksonHandlerInstantiator is in spring-web and, now that Jackson2ObjectMapperBuilder isn't involved, there's no guarantee that spring-web will be on the classpath when using Boot's Jackson auto-configuration.

We could conditionally auto-configure the use of JacksonHandlerInstantiator but that would result in different behavior with and without spring-web on the classpath which could take people by surprise.

Alternatively, we could have our own copy of JacksonHandlerInstantiator in spring-boot-jackson so that we can always configure it. I think it could be package-private to avoid confusion with spring-web's version.

I'd like to discuss options with the team. In the meantime, assuming that you have spring-web on the classpath, you could set things up using a customizer:

@Bean
JsonMapperBuilderCustomizer instantiatorCustomizer(AutowireCapableBeanFactory beanFactory) {
	return (builder) -> builder.handlerInstantiator(new JacksonHandlerInstantiator(beanFactory));
}

[wilkinsona]

We discussed this today and we're going to take the package-private route.