Get Your Hands Dirty on Clean Architecture (2nd edition)
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Get Your Hands Dirty on Clean Architecture (2nd edition)

A Hands-on Guide to Creating Clean Web Applications with Code Examples in Java

About the Book

Looking for inspiration on how to build a web application with a clean, maintainable architecture can be frustrating. Many web sources and books do a great job of explaining the concepts of a clean architecture, but fall short in giving hands-on advice on how to implement them.

This book fills this void by converting the concepts of a Hexagonal Architecture into actual code. It concentrates on one of the most common forms of applications we're building today - a web application with an underlying database.

The book starts with a discussion about the conventional layered architecture style and which advantages the domain-centric architecture styles of Robert C. Martin's Clean Architecture and Alistair Cockburn's Hexagonal Architecture bring to the table. It then quickly dives into hands-on chapters that show a way of how to manifest a Hexagonal Architecture in actual code.

After discussing each of the layers in a hexagonal architecture style, the book goes into detail about different mapping strategies between those layers, how to assemble the architecture elements to an application, how to enforce the architecture boundaries, which shortcuts produce which technical debt, and when we might willingly take on this technical debt anyways.

If you prefer a print version of the book, you can get it on Amazon.

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    • Software Architecture
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About the Author

Tom Hombergs
Tom Hombergs

Tom is a software engineer by profession and by passion with more than a decade of experience in many different software projects for many different clients across various industries. In software projects, he takes on the roles of software developer, architect and coach, with a focus on the Java ecosystem.

He found that writing is the best way to learn, so he likes to dive deep into topics he encounters in his software projects to create texts that give structure to the chaotic world of software development. He regularly writes about software development on his blog at and is an occasional speaker at conferences.

Reader Testimonials

Gernot Starke
Gernot Starke

Co-founder of and founding member of iSAQB (International Software Architecture Qualification Board).

Tom Hombergs has done a terrific job in explaining clean architecture - from concepts to code. Really wish more technical books would be as clear as that one!

Marten Deinum
Marten Deinum

Spring Framework contributor and author of "Spring 5 Recipes" and "Spring Boot 2 Recipes"

Love your book. One of the most practical books on hexagonal architecture I have seen/read so far.

Sebastian Kempken
Sebastian Kempken

Software architect at Adcubum

A book taken right out of the machine room of software development. Tom talks straight from his experience and guides you through the day-to-day trade-offs necessary to deliver clean architecture.

Table of Contents

  • Foreword
  • Preface
    • What is the goal of this book?
    • Who should read this book?
    • The example application
    • Print version
    • Feedback
    • Notes about the second edition
  • 1. Maintainability
    • What does maintainability even mean?
    • Maintainability enables functionality
    • Maintainability generates developer joy
    • Maintainability supports decision-making
    • Maintaining maintainability
  • 2. What’s Wrong with Layers?
    • They promote database-driven design
    • They’re prone to shortcuts
    • They grow hard to test
    • They hide the use cases
    • They make parallel work difficult
    • How does this help me build maintainable software?
  • 3. Inverting Dependencies
    • The Single Responsibility Principle
    • A tale about side effects
    • The Dependency Inversion Principle
    • Clean Architecture
    • Hexagonal Architecture
    • How does this help me build maintainable software?
  • 4. Organizing Code
    • Organizing by layer
    • Organizing by feature
    • An architecturally expressive package structure
    • The role of dependency injection
    • How does this help me build maintainable software?
  • 5. Implementing a Use Case
    • Implementing the domain model
    • A use case in a nutshell
    • Validating input
    • The power of constructors
    • Different input models for different use cases
    • Validating business rules
    • Rich versus anemic domain model
    • Different output models for different use cases
    • What about read-only use cases?
    • How does this help me build maintainable software?
  • 6. Implementing a Web Adapter
    • Dependency Inversion
    • Responsibilities of a web adapter
    • Slicing controllers
    • How does this help me build maintainable software?
  • 7. Implementing a Persistence Adapter
    • Dependency inversion
    • Responsibilities of a persistence adapter
    • Slicing port interfaces
    • Slicing persistence adapters
    • An example with Spring Data JPA
    • What about database transactions?
    • How does this help me build maintainable software?
  • 8. Testing Architecture Elements
    • The test pyramid
    • Testing a domain entity with unit tests
    • Testing a use case with unit tests
    • Testing a web adapter with integration tests
    • Testing a persistence adapter with integration tests
    • Testing main paths with system tests
    • How much testing is enough?
    • How does this help me build maintainable software?
  • 9. Mapping between Boundaries
    • The “No Mapping” strategy
    • The “Two-Way” mapping strategy
    • The “Full” mapping strategy
    • The “One-Way” mapping strategy
    • When to use which mapping strategy?
    • How does this help me build maintainable software?
  • 10. Assembling the Application
    • Why even care about assembly?
    • Assembling via plain code
    • Assembling via Spring’s classpath scanning
    • Assembling via Spring’s Java Config
    • How does this help me build maintainable software?
  • 11. Taking Shortcuts Consciously
    • Why shortcuts are like broken windows
    • The responsibility of starting clean
    • Sharing models between use cases
    • Using domain entities as the input or output model
    • Skipping incoming ports
    • Skipping services
    • How does this help me build maintainable software?
  • 12. Enforcing Architecture Boundaries
    • Boundaries and dependencies
    • Visibility modifiers
    • Post-compile fitness function
    • Build artifacts
    • How does this help me build maintainable software?
  • 13. Managing Multiple Bounded Contexts
    • One hexagon per bounded context?
    • Decoupled bounded contexts
    • Appropriately coupled bounded contexts
    • How does this help me build maintainable software?
  • 14. A Component-Based Approach to Software Architecture
    • Modularity through components
    • Case study: Building a “Check Engine” component
    • Enforcing component boundaries
    • How does this help me build maintainable software?
  • 15. Deciding on an Architecture Style
    • Start simple
    • Evolve the domain
    • Trust your experience
    • It depends
  • Changelog
    • First edition
    • Second edition
  • Notes

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