Test Suite Builder

Source mapping: Tier 1 critical skill derived from Kotlin_Spring_Developer_Pipeline.md (SK-14).

Mission

Choose the lightest test that proves the behavior. Generate tests that catch regressions instead of reproducing the implementation line by line.

Start With A Test Strategy

Before writing code, classify what needs to be proven:

• pure business logic
• HTTP contract and validation
• repository query behavior
• serialization behavior
• security rules
• cross-component integration
• database or message-broker integration

Three-Layer Strategy

• Use unit tests for domain logic, calculations, decision tables, and deterministic branching.
• Use slice tests for framework boundaries:
  • @WebMvcTest or WebTestClient for HTTP
  • @DataJpaTest for repositories
  • focused JSON or security slices when available in the project
• Use @SpringBootTest and Testcontainers only when a realistic application graph or infrastructure boundary must be proven.

Kotlin Testing Rules

• Prefer JUnit 5.
• Prefer MockK over Mockito unless the repository already standardizes on something else.
• Use readable backtick test names when that matches the project style.
• Use runTest and the coroutine test toolkit for coroutine-heavy code.
• Build reusable fixtures, builders, or object mothers instead of duplicating inline object construction.

What To Cover

• Success path.
• Validation failures.
• Business rule failures.
• Edge cases around nullability, optional fields, empty collections, and duplicates.
• At least one regression-oriented test for the bug or change that motivated the work.

What Not To Do

• Do not default to @SpringBootTest when a slice test or unit test is enough.
• Do not write tests that only verify mock interactions and prove nothing observable.
• Do not couple assertions to private implementation details when public behavior is enough.
• Do not use real time, random values, or shared mutable state without control.

Output Contract

Return these sections:

• Test plan: which layers to use and why.
• Generated tests: the concrete test classes or patch plan.
• Test data support: builders, fixtures, or factories to add.
• Coverage gaps: important cases still not covered.
• Verification: commands to run and which tests should fail before the fix.

Framework-Specific Checks

• Match MockMvc vs WebTestClient to MVC vs WebFlux.
• Match Testcontainers setup to the actual database or broker in the repository.
• If serialization or validation is the bug, include a test that proves the wire contract, not only the service logic.
• If transaction behavior matters, add a test that proves rollback or uniqueness behavior in the real persistence layer.

Advanced Testing Nuances

• A transactional test that always rolls back can hide commit-time failures. Use explicit flush or real commit boundaries when unique constraints, triggers, or transaction listeners matter.
• H2 is not a safe stand-in for Postgres or MySQL when dialect, JSON, locking, index use, or transaction semantics matter. Prefer the real engine with Testcontainers for those cases.
• @MockBean is powerful but expensive. Overusing it turns integration tests into slow unit tests with hidden wiring.
• Security tests should usually prove both 401 and 403, not just the happy authorized path.
• Async and event-driven flows need deterministic waiting strategies such as controlled schedulers, latches, or Awaitility. Do not scatter sleeps.
• Concurrency and deadlock behavior cannot be proven in a single-threaded rollback test. Use multiple transactions and explicit synchronization when reviewing those cases.
• Reused containers improve speed but require strict state isolation. Do not trade determinism away for a faster green build.
• Use deterministic time and ID providers when business logic depends on clocks or UUIDs.

Expert Heuristics

• If a bug was caused by framework wiring, add at least one test above unit level.
• If a bug was caused by domain branching, do not drag the whole Spring context into the fix.
• When in doubt, choose the smallest test that would have failed before the change.
• Treat test code as production code for readability and maintenance. Fixtures and helpers should reduce noise, not hide behavior.

Guardrails

• Keep tests deterministic.
• Keep setup explicit and local to the scenario.
• Prefer one clear reason for failure per test.
• Do not silently introduce slow infrastructure-heavy tests into fast unit test suites.

Quality Bar

A good run of this skill produces a test suite that is fast where possible and realistic where necessary. A bad run floods the project with context-heavy tests, brittle mocks, and no clear explanation of why each test level exists.