Java Application Architecture Modularity Patterns with Examples Using OSGi

"I'm dancing! By god I'm dancing on the walls. I'm dancing on the ceiling. I'm ecstatic. I'm overjoyed. I'm really, really pleased." From the Foreword by Robert C. Martin (a.k.a. Uncle Bob) This isn't the first book on Java application architecture. No doubt it won't be the last. But rest assured, this book is different. The way we develop Java applications is about to change, and this book explores the new way of Java application architecture. Modularity in software development is a concept that dates back to a seminal paper written by David Parnas in 1971, and modularity as a more general concept dates back several civilizations. How is it that modularity is such a big deal in Java application architecture going forward? The answer is that modularity is coming to the Java platform, and it will change the way we develop Java applications. This book will help you Design modular software that is extensible, reusable, maintainable, and adaptable Design modular software today, in anticipation of platform support for modularity Break large software systems out into a flexible composite of collaborating modules Understand where to place your architectural focus Migrate large-scale monolithic applications to applications with a modular architecture Clearly articulate the advantages of modular software to your team Over the past several years, module frameworks have been gaining traction on the Java platform, and upcoming versions of Java will include a new module system that will allow you to leverage the power of modularity to build more resilient, malleable, extensible, and flexible software systems. But you'll only be able to realize the benefits if you understand how to design more modular software systems. Java Application Architecturelays the foundation you'll need to incorporate modular design thinking into your development initiatives. Before it walks you through 18 patterns that will help you realize the benefits of modular software architecture, it lays a solid foundation that shows you why modularity is a critical weapon in your arsenal of design tools. Finally, it wraps up with several examples that demonstrate the patterns in action and the benefits. Throughout, you'll find ample examples that illustrate the concepts. By beginning to design modular applications today, you are positioning yourself for the platform and architecture of tomorrow. That's why Uncle Bob is dancing.

Kirk Knoernschild is a software developer who has filled most roles on the software development team. Kirk is the author of Java Design: Objects, UML, and Process (Addison-Wesley, 2002), and he contributed to No Fluff Just Stuff 2006 Anthology (Pragmatic Bookshelf, 2006). Kirk is an open source contributor, has written numerous articles, and is a frequent conference speaker. He has trained and mentored thousands of software professionals on topics including Java/J2EE, modeling, software architecture and design, component-based development, service-oriented architecture, and software process. You can visit his website at techdistrict.kirkk.com.

Foreword by Robert C. Martin xix

Foreword by Peter Kriens xxi

Acknowledgments xxv

About the Author xxvii

 

Introduction 1

Object-Oriented Design 2

Logical versus Physical Design 3

Modularity 4

Who This Book Is For 6

How This Book Is Organized 7

Pattern Form 10

Pattern Catalog 12

The Code 13

An Opening Thought on the Modularity Patterns 14

Reference 14

 

Part I: The Case for Modularity 15

 

Chapter 1: Module Defined 17

1.1 Defining a Module 17

1.2 Succinct Definition of a Software Module 20

1.3 Conclusion 20

 

Chapter 2: The Two Facets of Modularity 21

2.1 The Runtime Model 21

2.2 The Development Model 22

2.3 Modularity Today 25

2.4 Conclusion 27

 

Chapter 3: Architecture and Modularity 29

3.1 Defining Architecture 29

3.2 A Software Architecture Story 30

3.3 The Goal of Architecture 33

3.4 Modularity: The Missing Ingredient 36

3.5 Answering Our Questions 43

3.6 Conclusion 44

3.7 References 44

 

Chapter 4: Taming the Beast Named Complexity 45

4.1 Enterprise Complexity 46

4.2 Technical Debt 47

4.3 Design Rot 48

4.4 Cyclic Dependencies–The Death Knell 50

4.5 Joints, Modules, and SOLID 56

4.6 Managing Complexity 57

4.7 Benefits of Modularity 59

4.8 Conclusion 60

4.9 References 60

 

Chapter 5: Realizing Reuse 61

5.1 The Use/Reuse Paradox 62

5.2 The Reuse Disclaimer 63

5.3 Reuse or Use 64

5.4 Modular Tension 65

5.5 Modular Design 66

5.6 Conclusion 67

5.7 Reference 68

 

Chapter 6: Modularity and SOA 69

6.1 All the Way Down, Revisited 69

6.2 Granularity–Architecture’s Nemesis 72

6.3 An Alternate View 79

6.4 Conclusion 80

 

Chapter 7: Reference Implementation 83

7.1 Why No OSGi? 83

7.2 Background on This Exercise: Building the System 84

7.3 Version 1 85

7.4 First Refactoring 87

7.5 Second Refactoring 90

7.6 Third Refactoring 93

7.7 Fourth Refactoring 95

7.8 Fifth Refactoring 98

7.9 Sixth Refactoring 99

7.10 Seventh Refactoring 102

7.11 The Postmortem 103

7.12 Conclusion 110

7.13 Reference 110

 

Part II: The Patterns 111

 

Chapter 8: Base Patterns 115

Manage Relationships 116

Module Reuse 125

Cohesive Modules 139

 

Chapter 9: Dependency Patterns 145

Acyclic Relationships 146

Levelize Modules 157

Physical Layers 162

Container Independence 170

Independent Deployment 178

Reference 185

 

Chapter 10: Usability Patterns 187

Published Interface 188

External Configuration 200

Default Implementation 206

Module Facade 212

 

Chapter 11: Extensibility Patterns 221

Abstract Modules 222

Implementation Factory 229

Separate Abstractions 237

Reference 244

 

Chapter 12: Utility Patterns 245

Colocate Exceptions 246

Levelize Build 253

Test Module 263

 

Part III: Poma and OSGi 271

 

Chapter 13: Introducing OSGi 273

13.1 Some History 273

13.2 Benefits of OSGi 274

13.3 Digesting OSGi 276

13.4 OSGi Bundle 277

13.5 OSGi Runtime Management 279

13.6 The Two Facets of Modularity, Revisited 279

13.7 OSGi and the Patterns 279

 

Chapter 14: The Loan Sample and OSGi 283

14.1 Getting Started 283

14.2 The Manifests 285

14.3 μServices 286

14.4 Installation and Execution 292

14.5 Conclusion 293

 

Chapter 15: OSGi and Scala 295

15.1 Getting Started 295

15.2 The Scala Code 296

15.3 Scala Bean Configuration 299

15.4 Scala μService Configuration 299

15.5 Building the Scala Module 300

15.6 Installation and Execution 300

15.7 Conclusion 301

 

Chapter 16: OSGi and Groovy 303

16.1 Getting Started 303

16.2 The Groovy Code 304

16.3 Groovy Bean Configuration 306

16.4 Groovy Service Configuration 307

16.5 Building the Groovy Module 307

16.6 Installation and Execution 308

16.7 Conclusion 309

 

Chapter 17: Future of OSGi 311

17.1 OSGi as an Enabler 312

17.2 The Disruption 312

17.3 The Power of Ecosystems 314

17.4 The Ecosystem 316

17.5 Conclusion 317

 

Appendix: SOLID Principles of Class Design 319

Single Responsibility Principle (SRP) 320

Open Closed Principle (OCP) 320

Liskov Substitution Principle (LSP) 323

Dependency Inversion Principle (DIP) 325

Interface Segregation Principle 327

Composite Reuse Principle (CRP) 329

References 335

 

Index 337