Mutation of Jackson2ObjectMapperBuilder in one injection point may affect subsequent usages

[wilkinsona]

Jackson2ObjectMapperBuilder is stateful and we have a few places where it's injected and mutated:

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/http/JacksonHttpMessageConvertersConfiguration.java

Lines 63 to 68 in 3c28622

	@Bean
	@ConditionalOnMissingBean
	public MappingJackson2XmlHttpMessageConverter mappingJackson2XmlHttpMessageConverter(
	Jackson2ObjectMapperBuilder builder) {
	return new MappingJackson2XmlHttpMessageConverter(builder.createXmlMapper(true).build());
	}

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jackson/JacksonAutoConfiguration.java

Lines 102 to 107 in 3c28622

	@Bean
	@Primary
	@ConditionalOnMissingBean
	ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
	return builder.createXmlMapper(false).build();
	}

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/rsocket/RSocketStrategiesAutoConfiguration.java

Lines 76 to 85 in 3c28622

	@Bean
	@Order(0)
	@ConditionalOnBean(Jackson2ObjectMapperBuilder.class)
	public RSocketStrategiesCustomizer jacksonCborRSocketStrategyCustomizer(Jackson2ObjectMapperBuilder builder) {
	return (strategy) -> {
	ObjectMapper objectMapper = builder.createXmlMapper(false).factory(new CBORFactory()).build();
	strategy.decoder(new Jackson2CborDecoder(objectMapper, SUPPORTED_TYPES));
	strategy.encoder(new Jackson2CborEncoder(objectMapper, SUPPORTED_TYPES));
	};
	}

As things stand, depending on auto-configuration ordering, whether or not you have Jackson's XML format on the classpath, and when precisely the Jackson2ObjectMapperBuilder is used, you may or may not get an XML mapper. Perhaps that's OK and we expect consumers of the builder to always call createXmlMapper(true|false) to meet their needs but I'm not sure that the current behaviour is intentional and I think it could be a bit brittle and confusing. #17425 is an example of that.

[philwebb]

We're going to try making this a prototype bean.