Cannot replace fasterxml jackson with gson

[gtiwari333]

Describe the bug
I'm trying to replace fasterxml-jackson dependency with gson from my simple app with just two dependencies: spring-boot-starter-web and spring-cloud-function-web dependencies.

    implementation('org.springframework.boot:spring-boot-starter-web') {
        exclude group: 'org.springframework.boot', module: 'spring-boot-starter-json'
    }
    implementation 'com.google.code.gson:gson'
    implementation("org.springframework.cloud:spring-cloud-function-web") {
        exclude group: 'com.fasterxml.jackson.datatype'
        exclude group: 'com.fasterxml.jackson.core'
        exclude group: 'com.fasterxml.jackson.module'
    }

I also have the following config to use gson mapper as preferred mapper for both http converter and cloud function.

spring.http.converters.preferred-json-mapper=gson
spring.cloud.function.preferred-json-mapper=gson

Scenario 1

When i run my empty project, i get following error (full stacktrace is on the repository)

java.lang.IllegalStateException: Error processing condition on org.springframework.cloud.function.observability.ObservationAutoConfiguration
	at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:60) ~[spring-boot-autoconfigure-3.5.0.jar:3.5.0]
	at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:99) ~[spring-context-6.2.7.jar:6.2.7]
	at gt.cloudfunctionjson.CloudFunctionJsonApplication.main(CloudFunctionJsonApplication.java:14) ~[main/:na]
Caused by: java.lang.IllegalStateException: Failed to introspect Class [org.springframework.cloud.function.context.config.ContextFunctionCatalogAutoConfiguration$JsonMapperConfiguration] from ClassLoader [jdk.internal.loader.ClassLoaders$AppClassLoader@4e0e2f2a]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:483) ~[spring-core-
	... 17 common frames omitted
Caused by: java.lang.NoClassDefFoundError: com/fasterxml/jackson/databind/Module

	... 33 common frames omitted
Caused by: java.lang.ClassNotFoundException: com.fasterxml.jackson.databind.Module
	... 37 common frames omitted

Scenario 2

When I provide my own JsonMapper bean to bypass config initialization from org.springframework.cloud.function.context.config.ContextFunctionCatalogAutoConfiguration.JsonMapperConfiguration#jsonMapper, i get a similar error from different place.

    @Bean
    JsonMapper gsonMapper(Gson gson) {
        return new GsonMapper(gson);
    }

	at org.springframework.beans.factory.support.ConstructorResolver.instantiate(ConstructorResolver.java:657) ~[spring-beans-6.2.7.jar:6.2.7]
	at gt.cloudfunctionjson.CloudFunctionJsonApplication.main(CloudFunctionJsonApplication.java:14) ~[main/:na]
Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [org.springframework.cloud.function.json.JsonMapper]: Factory method 'gsonMapper' threw exception with message: com/fasterxml/jackson/core/JsonProcessingException
	at org.springframework.beans.factory.support.SimpleInstantiationStrategy.lambda$instantiate$0(SimpleInstantiationStrategy.java:199) ~[spring-beans-6.2.7.jar:6.2.7]
	... 21 common frames omitted
Caused by: java.lang.NoClassDefFoundError: com/fasterxml/jackson/core/JsonProcessingException
	at gt.cloudfunctionjson.CloudFunctionJsonApplication.gsonMapper(CloudFunctionJsonApplication.java:20) ~[main/:na]
org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:393) ~[spring-context-6.2.7.jar:6.2.7]
	at gt.cloudfunctionjson.CloudFunctionJsonApplication$$SpringCGLIB$$0.gsonMapper(<generated>) ~[main/:na]
	... 24 common frames omitted
Caused by: java.lang.ClassNotFoundException: com.fasterxml.jackson.core.JsonProcessingException

	... 33 common frames omitted

The problem is with JsonMapper class. It requires fasterxml jackson dependency to be present in classpath to create GsonMapper bean. Also the isJsonString, isJsonStringRepresentsCollection and isJsonStringRepresentsMap are not generic enough when either fasterxml or gson is present.

package org.springframework.cloud.function.json;

public abstract class JsonMapper {

	private static final ObjectMapper mapper = new ObjectMapper().enable(DeserializationFeature.FAIL_ON_TRAILING_TOKENS);

versions

Spring Boot: 3.5.0
Spring Cloud: 2025.0.0
JDK: 21

Sample

https://github.com/gtiwari333/spring-cloud-function-without-jackson-xml