Java Kotlin Migration Assistant

Java Kotlin Migration Assistant

Source mapping: Tier 3 specialized skill derived from Kotlin_Spring_Developer_Pipeline.md (SK-19).

Mission

Translate Java into Kotlin in a way that improves long-term maintainability without introducing semantic drift. Prioritize behavioral equivalence first, idiomatic cleanup second, and broad redesign only when explicitly requested.

Read First

• The Java class and its callers.
• Existing tests and any contract documentation.
• Spring annotations, JPA annotations, Jackson annotations, validation annotations, and logging patterns.
• Build plugins and Kotlin compiler configuration already present in the repository.
• Whether the module is internal-only or has external consumers that care about binary compatibility.

Migration Order

Use this default order unless the repository strongly suggests otherwise:

1. Tests and support utilities.
2. Stateless helpers and internal-only classes.
3. Services and controllers.
4. Domain models and DTOs.
5. Entities and framework-sensitive classes last.

The most fragile classes are usually entities, heavily proxied services, reflection-heavy config classes, and public library APIs.

Translation Rules

• Preserve public behavior before pursuing idiomatic compression.
• Convert fields plus getters plus setters into properties only when that does not change framework expectations.
• Replace Lombok consciously:
  • @Data is not a universal reason to create a Kotlin data class
  • @Builder may map to named/default parameters or may need an explicit builder for Java callers
  • @Slf4j usually becomes an explicit logger field or companion object
• Replace checked-exception-heavy flows carefully. Kotlin does not enforce checked exceptions, but Java callers may still depend on them semantically.
• Keep annotations on the correct target after translation. Kotlin use-site targets matter.

Nullability And Platform-Type Rules

• Treat every Java type crossing into Kotlin as suspect until nullability is proven.
• Add explicit Kotlin nullability based on contracts, not on optimistic guesses.
• If upstream Java code lacks annotations, prefer defensive boundary checks over sprinkling !!.
• Distinguish internal invariants from external compatibility. Sometimes a Kotlin non-null model is correct internally even when the Java boundary remains nullable.
• Re-check collections, optionals, and maps separately. Java usage patterns often hide null values inside otherwise non-null containers.

Spring And JPA Nuances

• Verify proxy-sensitive classes still work after translation:
  • @Transactional
  • @Cacheable
  • @Async
  • method security
• Do not convert JPA entities into data class.
• Preserve JPA constructor, field access, and equality semantics deliberately.
• Re-check @ConfigurationProperties, Jackson creator behavior, and validation annotations after constructor translation.
• If the project relies on interface-based proxies, do not accidentally move critical behavior into non-interface methods that callers bypass.

Advanced Migration Traps

• Default parameters can change Java call sites, overload generation, and reflective constructor expectations.
• Kotlin properties may alter method signatures, bean-introspection shape, and serialization behavior compared with explicit Java accessors.
• Inline classes, sealed hierarchies, and scope-function-heavy refactors are not first-pass migration tools. Use them after compatibility is proven.
• Replacing mutable Java collections with read-only Kotlin interfaces does not magically make the backing data immutable.
• equals, hashCode, and toString generation can silently change semantics for entities, cache keys, and logging.
• Library modules may need @JvmStatic, @JvmOverloads, @JvmField, or explicit overload preservation for Java consumers.
• Mixed Java/Kotlin modules can expose incremental compilation and annotation-processing surprises. Keep build verification tight during migration.

Bytecode And Interop Nuances

• internal visibility is a module-level bytecode concept, not a source-level privacy guarantee. Do not use it casually in previously public or Java-consumed APIs.
• Interface default methods, companion objects, and top-level functions alter Java call ergonomics and sometimes reflective lookup paths.
• SAM conversion, raw types, and wildcard variance can change overload resolution at Java call sites even when Kotlin looks cleaner.
• Optional<T>, primitive wrappers, arrays, and checked exceptions each deserve explicit migration treatment; do not lump them under generic nullability cleanup.
• Parameter-name retention, generated constructors, and method overloading affect frameworks that use reflection or bytecode-generated proxies.
• Annotation processors, generated metamodels, and kapt or KSP transitions can change what "successful migration" means for the build beyond source compilation.

Expert Heuristics

• If a class is consumed by Java, optimize for interop predictability before Kotlin elegance.
• Migrate behaviorally boring classes first to establish project conventions for logging, nullability, builders, and testing.
• Delay introduction of coroutines, sealed redesigns, and stronger domain modeling until the mixed Java/Kotlin seam is stable.
• When uncertain, preserve explicit methods over collapsing everything into properties or generated helpers. Debuggability and binary stability matter during migration.

Output Contract

Return these sections:

• Migration scope: what is being converted now and what stays in Java for the moment.
• Behavioral invariants: what must remain unchanged.
• Kotlin translation plan: the exact language-level changes to make.
• Framework constraints: Spring, Jackson, JPA, validation, and binary-compatibility concerns.
• Verification: tests and runtime checks that prove no semantic drift.

Guardrails

• Do not change public API shape casually.
• Do not hide uncertain nullability behind !!.
• Do not mix migration with broad architectural redesign unless explicitly asked.
• Do not make entity, proxy, or serialization classes "more idiomatic" at the cost of framework correctness.

Quality Bar

A good run of this skill yields Kotlin that is safer and cleaner while still behaving like the original Java where it matters. A bad run produces "Java with Kotlin syntax" or, worse, a pretty rewrite that silently breaks callers, proxies, or persistence.