Practical Unit Testing with JUnit and Mockito

Introduction

The book «Practical Unit Testing with JUnit and Mockito» by Tomek Kaczanowski is one of the best practical guides for writing high-quality unit tests in Java. Published in 2013, it remains relevant today because the fundamental principles of testing and working with the JUnit and Mockito frameworks remain unchanged. The book is aimed at developers who already write code in Java but want to systematize their knowledge of testing and learn how to create reliable, maintainable tests.

What This Book Is About

The book systematically guides the reader from basic concepts of unit testing to advanced techniques using mocks and stubs. The author does not simply list the capabilities of JUnit and Mockito, but explains why tests should be written in a certain way. The main focus is on:

• Creating isolated tests that do not depend on external resources (databases, file systems, network).
• Using Mockito to create stubs and verify interaction between objects.
• Proper organization of test code: naming, structure, FIRST principles (Fast, Isolated, Repeatable, Self-validating, Timely).
• Refactoring existing code to improve testability.
• Integrating tests into the build process (Maven, Gradle) and CI/CD.

The book contains many examples in Java that can be immediately applied in real projects.

Who This Book Is For

The book will be useful for:

• Java developers of any level who want to improve the quality of their code through testing.
• Beginner automation testers who want to understand the inner workings of unit tests.
• Team leads and architects implementing a testing culture in their team.
• Students learning Java and wanting to master professional practices.

Basic knowledge of Java and a general understanding of testing are sufficient for reading.

What the Reader Will Learn

After reading the book, the reader will be able to:

• Write clean, understandable, and maintainable unit tests using JUnit 4 (and the basics of JUnit 5).
• Effectively use Mockito to create mocks, spies, and stubs.
• Apply testing patterns: Arrange-Act-Assert, Given-When-Then.
• Avoid common test anti-patterns (fragile tests, testing implementation instead of behavior).
• Refactor code to improve its testability (dependency injection, interface extraction).
• Configure automatic test execution in Maven/Gradle.
• Understand the difference between unit tests, integration tests, and system-level tests.

Strengths

The main advantages of the book:

• Practical focus. Each chapter contains exercises and examples that can be completed independently.
• Clear structure. The material is divided into logical blocks: from simple to complex.
• In-depth explanation of Mockito. The author not only shows the syntax but explains when and why to use mocks, stubs, and spies.
• Emphasis on test quality. The book teaches not only how to write tests but also how to make them reliable, fast, and informative.
• Modern relevance. Despite the year of publication, 90% of the material is applicable to current versions of JUnit and Mockito.

Who It Is Recommended For

The book is a must-read for anyone who writes in Java and wants to stop being afraid of testing. It will be an excellent addition to classic books on TDD (e.g., by Kent Beck) and will help form good habits. If you are just starting your journey in test automation — start with this book. If you are an experienced developer but your tests still break with any change