unit-test-service-layer

SKILL.md

Isolated unit testing patterns for Spring service layer using Mockito and JUnit 5.

Covers mocking injected dependencies, verifying service interactions, and testing business logic without database or external API calls

Includes patterns for exception handling, complex workflows, argument capturing, and verification of call order and frequency

Supports testing async/reactive services with CompletableFuture and provides best practices for constructor injection and spy-based partial mocking

Demonstrates common anti-patterns to avoid and troubleshooting guidance for typical Mockito and JUnit 5 issues

Unit Testing Service Layer with Mockito

Overview

Provides patterns for unit testing @Service classes using Mockito. Mocks repository calls, verifies method invocations, tests exception scenarios, and stubs external API responses. Enables fast, isolated tests without Spring container or database.

When to Use

Testing business logic in @Service classes

Mocking repository and external client dependencies

Verifying service interactions with mocked collaborators

Testing error handling and edge cases in services

Writing fast, isolated unit tests (no database, no API calls)

Instructions

Follow this workflow to test service layer with Mockito, including validation checkpoints:

1. Setup Test Class