Kotlin Spring Boot Patterns

Current Versions (verify before scaffolding new project)

Component	Version	Notes
Spring Boot	4.0.6	First GA: 4.0.0 on 2025-11-20. Built on Spring Framework 7.0.7+, Java 17 baseline (Java 25 supported), Kotlin 2.2 minimum.
Kotlin	2.3.21	Required for Spring Boot 4.
Spring Framework	7.0.7+	Pulled transitively.

If asked for an older Spring Boot 3.x scaffold (e.g. for legacy compat), use 3.5.x as the floor — never 3.4 or earlier.

Jackson 3 is the default. Package moved from com.fasterxml.jackson → tools.jackson. Use jackson-module-kotlin from the new coords; the old jackson 2.x still works but adds dual classpath.
HTTP clients namespace. Properties moved under spring.http.clients.* (was spring.http.client.*).
Kotlin compiler flag. Add -Xannotation-default-target=param-property to freeCompilerArgs so @field: style annotations on data class params behave correctly with Spring's reflection.
AOT / native is first-class — keep reflection use minimal; avoid kotlin-reflect outside framework needs.
Removed in 4.0: legacy WebMvcConfigurer defaults that no-op'd, @EnableConfigurationProperties is no longer needed when using @ConfigurationPropertiesScan.