[BUG][KOTLIN] Fails to compile after updating to 7.25.0 with useJackson3=false useSpringBoot4=true

[pkubowicz]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

7.25.0 breaks backward compatibility, by force-setting useJackson3=true for Spring 4 and jvm-spring-restclient

It's incorrect to enforce Jackson 3 with those settings: Spring 4.0 and 4.1 work with Jackson 2, it's just deprecated. RestClient still works with Jackson 2.

openapi-generator version

7.25.0, it's a regression compared to 7.24.0

OpenAPI declaration file content or url

Generation Details

See 'steps to reproduce'

Steps to reproduce

I only generate sources and use my own build.gradle.kts, throwing away generator's one.

dependencies {
    api(platform("org.springframework.boot:spring-boot-dependencies:4.0.8"))

    api("org.springframework:spring-web")

    // api because Serializer.jacksonObjectMapper is public
    api("com.fasterxml.jackson.core:jackson-databind")
    api("org.openapitools:jackson-databind-nullable:0.2.11")
    implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
}

openApiGenerate {
    configOptions.set(
        mapOf(
            "omitGradleWrapper" to "true",
            "library" to "jvm-spring-restclient",
            "useSpringBoot3" to "true",
            "useSpringBoot4" to "true",
            "useJackson3" to "false",
            "serializationLibrary" to "jackson",
        )
    )
}

What complied with 7.24.0, fails to compile in 7.25.0 because Serializer.kt is now:

import tools.jackson.databind.DeserializationFeature
import tools.jackson.databind.json.JsonMapper
import tools.jackson.databind.cfg.DateTimeFeature
import com.fasterxml.jackson.annotation.JsonInclude
import tools.jackson.module.kotlin.jsonMapper
import tools.jackson.module.kotlin.kotlinModule

object Serializer {
    @JvmStatic
    val jacksonObjectMapper: JsonMapper = jsonMapper {
        addModule(kotlinModule())
        changeDefaultPropertyInclusion { it.withValueInclusion(JsonInclude.Include.NON_ABSENT) }
        disable(DateTimeFeature.WRITE_DATES_AS_TIMESTAMPS)
        disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
    }
}

Because I explicitly set useJackson3=false, the generated code should import com.fasterxml.jackson

Related issues/PRs

#24709

Suggest a fix

Remove the addition of setUseJackson3(true) as pointed out in https://github.com/OpenAPITools/openapi-generator/pull/24709/changes#r3917417022

[Picazsoo]

I agree with your analysis. The fix should be to set useJackson3=true as the default when useSpringBoot4=true, while still respecting any explicit user configuration. This way:

1. Projects using Spring Boot 4 that want Jackson 3 get it by default
2. Users who prefer Jackson 2 can explicitly set useJackson3=false to opt-out
3. Any explicit user setting is honored, whether true or false

The current behavior of forcing useJackson3=true and ignoring the user's explicit useJackson3=false setting breaks backward compatibility. The generated code should match what the user requests, not override it.