Modular Spring Boot Applications: Design Maintainable Modular Monoliths with Clear Boundaries, Domain Events, Testing, and Scalable Spring Boot Architecture (Spring Boot Development Series)

Build Spring Boot applications that stay maintainable as they grow.

As Spring Boot projects become larger, clean controllers and services are no longer enough. Features begin to overlap, repositories are shared across unrelated areas, dependencies become difficult to control, and seemingly small changes can affect large parts of the application.

Modular Spring Boot Applications shows you how to solve these problems by transforming a growing Spring Boot application into a well-structured modular monolith with clear business boundaries, controlled dependencies, reliable module communication, and maintainable architecture.

Using the production-style Spring Store application developed throughout the Spring Boot Development Series, you will learn how to organize your application around real business capabilities such as Customer, Catalog, Inventory, Order, and Identity.

Inside this practical guide, you will learn how to:

• Design modules around business capabilities instead of technical layers

• Improve cohesion and reduce unnecessary coupling

• Organize Spring Boot applications using package-by-feature

• Build and verify modules with Spring Modulith

• Create clear public module APIs while protecting internal implementation

• Define named interfaces and allowed dependencies

• Detect circular and illegal module dependencies

• Choose between direct module calls and application events

• Handle transactional events and reliable event publication

• Keep repositories and data ownership inside the correct modules

• Manage cross-module transactions and consistency

• Design idempotent event-processing workflows

• Preserve authentication, authorization, and resource ownership boundaries

• Test modules with @ApplicationModuleTest

• Test event-driven workflows using Spring Modulith Scenario

• Generate architecture documentation and module diagrams

• Observe module interactions in production

• Refactor existing Spring Boot applications incrementally

• Decide when a module should remain inside the monolith—or become an independent service

The book uses Java 21, Spring Boot 4.x, Spring Modulith, PostgreSQL, Spring Data JPA, Hibernate, Flyway, Spring Security, JUnit, Testcontainers, Actuator, and Micrometer while avoiding unnecessary distributed-system complexity.

Rather than treating microservices as the automatic destination for every growing application, this book shows how a well-designed modular monolith can provide strong ownership, cleaner boundaries, easier testing, simpler transactions, and operational simplicity within a single deployable application.

Modular Spring Boot Applications is ideal for Java and Spring Boot developers who already understand the fundamentals and want to design larger applications that remain understandable, testable, and easier to evolve.

This is Book 6 and the final volume of the Spring Boot Development Series, completing the Spring Store journey from REST API development through persistence, security, testing, deployment, and professional modular architecture.