Testing Spring Boot Applications

Most Spring Boot applications do not fail because the code cannot compile. They fail at the boundaries—business rules, database transactions, security controls, API contracts, configuration, and external infrastructure.

Testing Spring Boot Applications is a practical guide to building fast, reliable, and maintainable automated test suites for modern Spring Boot projects. Instead of treating testing as a collection of annotations, this book shows you how to select the right test scope, detect meaningful defects, and create confidence without filling your project with slow, fragile, or excessively mocked tests.

Using one connected, production-style Spring Store application, you will progress from focused unit tests to realistic PostgreSQL-backed integration and full HTTP testing.

Inside, you will learn how to:

• Write clear unit tests with JUnit Jupiter and AssertJ
• Isolate repositories and external collaborators with Mockito
• Test services, business rules, exceptions, and prevented side effects
• Verify REST controllers, validation, status codes, JSON, pagination, and error responses with MockMvc
• Test JPA repositories, queries, projections, relationships, constraints, and transaction behavior
• Validate Flyway migrations against PostgreSQL-compatible infrastructure
• Load and configure integration tests with @SpringBootTest
• Control profiles, properties, clocks, fixtures, and database state
• Test authentication, roles, permissions, method security, and resource ownership
• Verify JWT signatures, claims, expiration, invalid issuers, incorrect audiences, tampering, and revocation
• Run disposable PostgreSQL databases with Testcontainers and Spring Boot service connections
• Test external API boundaries without calling live production services
• Organize unit and integration tests with Maven Surefire and Failsafe
• Automate verification with GitHub Actions and use JaCoCo coverage responsibly
• Diagnose flaky tests, reduce execution time, and maintain dependable continuous integration

The book also explains what each testing technique can and cannot prove, helping you avoid common mistakes such as loading Spring for every test, trusting mocks as proof of database compatibility, disabling security filters, relying on automatic rollback, or treating coverage percentages as a quality score.

Designed for Java developers who already understand basic Spring Boot development, this fourth book in the Spring Boot Development Series uses Java 21, Spring Boot 4.x, PostgreSQL, Maven, and current testing practices.

By the end, you will be able to design a balanced testing architecture that provides fast development feedback, protects critical application boundaries, supports safer refactoring, and prepares your Spring Boot applications for reliable releases.