unit-test-service-layer

This skill provides patterns for unit testing Java service-layer classes with Mockito. It solves the problem of writing fast, isolated tests for business logic in `@Service` classes without spinning up a Spring container, database, or real external services. By mocking repositories and external clients, developers can verify service behavior, interactions, and error handling in milliseconds rather than through slow integration tests.

The skill lays out a clear workflow: set up the test class with `@ExtendWith(MockitoExtension.class)`, declare dependencies with `@Mock` and the class under test with `@InjectMocks`, and follow an Arrange-Act-Assert structure using `when().thenReturn()` for stubbing, AssertJ for assertions, and `verify()` for interaction checks. It covers testing exception scenarios with `when().thenThrow()`, running coverage with `mvn test jacoco:report`, and validation checkpoints at each step. A references/examples.md file adds Maven and Gradle dependency setup and patterns for multiple dependencies, argument captors, async services, and InOrder verification. It also encodes best practices and explicit constraints, such as not mocking value objects or DTOs, not testing private methods directly, keeping tests independent of execution order, and avoiding over-verification.

It targets Java and Spring developers writing unit tests, especially those working with layered service architectures who want disciplined Mockito usage. The declared allowed tools are Read, Write, Bash, Glob, and Grep, and the guidance assumes JUnit 5, Mockito, and AssertJ on the classpath.