Mockito : Java mocking framewor

Introduction

This comprehensive Mockito guide will walk you through the most popular Java mocking framework for unit testing. Learn how to create mocks, stub method calls, verify interactions, handle exceptions, and write clean, maintainable tests. Whether you're testing service layers, repositories, or complex dependencies, Mockito provides powerful tools for isolating units under test and creating reliable test doubles. Master mocking, spying, argument matchers, and advanced verification techniques to build robust test suites.

A. Getting Started

Mockito Introduction: Why Mocking Matters — Understanding test isolation, dependencies, and the need for mocks.
Setting Up Mockito — Maven/Gradle dependencies and creating your first mock.
Creating Mocks with @Mock and mock() — Different ways to create mocks and when to use each.

B. Stubbing Methods

when().thenReturn() Basics — Stubbing method calls to return specific values.
Stubbing Exceptions — Making mocks throw exceptions with thenThrow().
Consecutive Calls and thenAnswer() — Handling multiple calls and dynamic stubbing.
Stubbing Void Methods — Using doNothing(), doThrow(), and doAnswer().

C. Verification

verify() - Basic Verification — Verifying method calls and interactions.
Verification Modes — times(), atLeast(), atMost(), never(), and only().
Argument Capturing — Using ArgumentCaptor to inspect method arguments.
Verifying Call Order — Using InOrder to verify sequence of calls.

D. Argument Matchers

Built-in Matchers — any(), eq(), isNull(), isNotNull(), contains(), etc.
Custom Argument Matchers — Creating ArgumentMatcher for complex matching logic.
Matcher Rules and Best Practices — Understanding matcher limitations and proper usage.

E. Spies and Partial Mocking

Understanding Spies — When to use @Spy vs @Mock and real object wrapping.
Stubbing Spy Methods — Overriding specific methods while keeping real behavior.
Spy vs Mock: When to Use Each — Choosing between spies and mocks for your tests.

F. Spring Boot Integration

@MockBean and @SpyBean — Mocking Spring beans in integration tests.
@ExtendWith(MockitoExtension.class) — JUnit 5 integration with Mockito.
Testing Service Layers — Mocking repositories and dependencies in Spring services.
Testing Controllers — Mocking services in @WebMvcTest and @MockMvc tests.

G. Advanced Topics

Mocking Static Methods — Using mockStatic() for static method mocking.
Mocking Constructors — Mocking object construction with mockConstruction().
Mocking Final Classes — Configuring Mockito to mock final classes and methods.
Deep Stubs — Using RETURNS_DEEP_STUBS for nested object mocking.
Custom Answers — Implementing Answer interface for complex stubbing logic.

H. Best Practices & Patterns

AAA Pattern (Arrange-Act-Assert) — Structuring tests with Mockito.
Avoiding Over-Mocking — When not to mock and testing real behavior.
Writing Maintainable Tests — Best practices for readable and maintainable test code.
Common Mistakes and Pitfalls — Avoiding common Mockito errors and anti-patterns.
Testing Exception Scenarios — Comprehensive exception testing strategies.

Prerequisites

Java 8+ knowledge
Understanding of unit testing concepts
Basic familiarity with JUnit 4 or JUnit 5
Experience with dependency injection (optional but helpful)
Optional: Spring Boot familiarity for integration sections

Conclusion

By the end of this series, you'll have mastered Mockito and be able to write effective unit tests with proper mocking, stubbing, and verification. You'll understand when to use mocks vs spies, how to handle complex scenarios, and integrate Mockito seamlessly into Spring Boot applications. This knowledge will help you write isolated, maintainable tests that verify your code's behavior without depending on external systems or complex setups.