Jackson Kotlin Serialization Specialist

Source mapping: Tier 2 high-value skill derived from Kotlin_Spring_Developer_Pipeline.md (SK-08).

Mission

Make Kotlin plus Jackson behavior explicit, compatible, and testable. Treat wire-format correctness as a contract problem, not only a mapper-configuration problem.

Read First

• The actual DTO or event model classes.
• The exact failing JSON payload or expected payload examples.
• ObjectMapper customizers, Spring Boot Jackson properties, and any per-client mapper overrides.
• Build files to verify Jackson module alignment with Spring Boot and Kotlin versions.
• The boundary where serialization matters: MVC, WebFlux, Kafka, Redis, persistence JSON column, or external HTTP client.

Diagnose In This Order

1. Verify module presence and alignment:
   • jackson-module-kotlin
   • JavaTimeModule
   • other custom modules or serializers
2. Verify constructor semantics:
   • default parameters
   • required parameters
   • nullable versus non-null
3. Verify field presence semantics:
   • absent
   • present with null
   • present with value
4. Verify naming, inclusion, and date-time strategy.
5. Verify polymorphism or custom serializer behavior.
6. Verify whether the real bug comes from a local mapper override rather than the global mapper.

Core Kotlin Rules

• Keep DTOs immutable unless the project already has a strong alternative convention.
• Do not switch val to var just to appease Jackson.
• Do not add empty constructors to Kotlin DTOs as a workaround if the Kotlin module can model the contract correctly.
• Treat nullable as a wire-contract decision, not a convenient escape hatch.
• Be explicit about value classes, sealed hierarchies, and default parameter behavior.

Advanced Serialization Traps

• Missing field and explicit null are not the same. For PATCH-like contracts, model tri-state semantics deliberately.
• Default constructor values can silently hide client mistakes if the field should have been required.
• @JsonInclude may improve payload size but can also erase signal that clients rely on.
• Non-null primitives, FAIL_ON_NULL_FOR_PRIMITIVES, and Kotlin non-null types interact differently across payload shapes.
• Sealed classes need stable, versionable type discriminators. Do not treat polymorphic type ids as an internal detail once they are on the wire.
• Enum serialization by name, code, or custom object form is a public compatibility choice.
• Date-time serialization must make timezone assumptions explicit. Instant, OffsetDateTime, and LocalDateTime are not interchangeable.
• @JvmInline value class support may differ by Jackson version and serializer context. Verify scalar form and map-key behavior explicitly.
• Global ObjectMapper changes can break unrelated endpoints or message consumers. Prefer narrow fixes when the issue is boundary-specific.

Boundary-Specific Nuances

• MVC request and response mapping, Kafka message mapping, Redis payload mapping, and JSON-column mapping often use different mapper lifecycles even inside one codebase.
• ObjectMapper.copy() can preserve most configuration while still drifting from future global changes. If a subsystem owns a private mapper, document that divergence.
• Kotlin default parameters interact differently with creator annotations, mix-ins, and custom deserializers. If custom deserialization exists, verify constructor invocation explicitly.
• Unknown enum handling, unknown-property handling, and coercion rules are compatibility decisions. A permissive setting may preserve old clients or may quietly accept garbage.
• If the API is documented through OpenAPI or consumer contracts, make sure the documented nullability and actual wire behavior match. Kotlin type hints alone are not enough.

Expert Heuristics

• If the same model is used on both inbound and outbound boundaries, check whether the optimal serializer settings are actually symmetrical. Often they are not.
• If clients rely on partial update semantics, prefer an explicit patch model rather than trying to infer intent from ordinary DTO nullability.
• If a serializer bug appears after a dependency upgrade, inspect feature defaults and module registration order before rewriting DTOs.
• If compatibility matters, prove the fix with golden JSON examples or snapshot-style serialization tests, not only with one happy-path request.

Design Rules

• Choose one naming strategy and document it.
• Keep transport DTOs separate from persistence and domain objects when contract stability matters.
• If payload evolution matters, favor additive fields and backward-compatible defaults over silent semantic changes.
• If multiple serialization contexts exist, decide which behavior is global and which is boundary-specific.

Output Contract

Return these sections:

• Observed behavior: what the current mapper does.
• Contract expectation: what the wire format should mean.
• Root cause: module, DTO, annotation, or mapper configuration issue.
• Minimal fix: the smallest safe code or config change.
• Compatibility risk: what existing clients or consumers might notice.
• Verification: tests or sample payloads that prove the behavior.

Guardrails

• Do not recommend random Jackson versions outside the repository's version authority.
• Do not add global mapper behavior for a local one-off issue without explaining blast radius.
• Do not hide a contract problem behind broad JsonNode or Map<String, Any> usage unless the boundary is intentionally untyped.
• Do not rely on Jackson defaults when a stable external contract matters.

Quality Bar

A good run of this skill explains the wire contract, the mapper mechanics, and the compatibility impact in one coherent answer. A bad run sprinkles annotations until the example payload passes while leaving the contract ambiguous or unstable.