Dependency Injection
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Dependency Injection DESIGN PATTERNS USING SPRING AND GUICE DHANJI R. PRASANNA MANNING Greenwich (74° w. long.) www.allitebooks.com For online information and ordering of this and other Manning books, please visit www.manning.com. The publisher offers discounts on this book when ordered in quantity. For more information, please contact Special Sales Department Manning Publications Co. Sound View Court 3B fax: (609) 877-8256 Greenwich, CT 06830 email: [email protected] ©2009 by Manning Publications Co. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by means electronic, mechanical, photocopying, or otherwise, without prior written permission of the publisher. Many of the designations used by manufacturers and sellers to distinguish their products are claimed as trademarks. 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Copyeditor: Linda Recktenwald Sound View Court 3B Typesetter: Gordan Salinovic Greenwich, CT 06830 Cover designer: Leslie Haimes ISBN 978-1-933988-55-9 Printed in the United States of America 1 2 3 4 5 6 7 8 9 10 – MAL – 14 13 12 11 10 09 www.allitebooks.com Dependency injection: what’s all the hype? 1 1 1.1 Every solution needs a problem 2 Seeing objects as services 2 1.2 Pre-DI solutions 4 Construction by hand 5 ■ The Factory pattern 7 ■ The Service Locator pattern 12 1.3 Embracing dependency injection 13 The Hollywood Principle 13 ■ Inversion of Control vs. dependency injection 15 1.4 Dependency injection in the real world 17 Java 17 ■ DI in other languages and libraries 19 1.5 Summary 19 Time for injection 21 2 2.1 Bootstrapping the injector 22 2.2 Constructing objects with dependency injection 23 www.allitebooks.com 2.3 Metadata and injector configuration 26 XML injection in Spring 27 ■ From XML to in-code configuration 30 Injection in PicoContainer 31 ■ Revisiting Spring and autowiring 34 2.4 Identifying dependencies for injection 36 Identifying by string keys 37 ■ Limitations of string keys 42 Identifying by type 44 ■ Limitations of identifying by type 46 Combinatorial keys: a comprehensive solution 47 2.5 Separating infrastructure and application logic 51 2.6 Summary 52 Investigating DI 54 3 3.1 Injection idioms 55 Constructor injection 55 ■ Setter injection 56 ■ Interface injection 60 Method decoration (or AOP injection) 62 3.2 Choosing an injection idiom 65 Constructor vs. setter injection 66 ■ The constructor pyramid problem 69 The circular reference problem 71 ■ The in-construction problem 75 Constructor injection and object validity 78 3.3 Not all at once: partial injection 81 The reinjection problem 81 ■ Reinjection with the Provider pattern 82 The contextual injection problem 84 ■ Contextual injection with the Assisted Injection pattern 86 ■ Flexible partial injection with the Builder pattern 88 3.4 Injecting objects in sealed code 92 Injecting with externalized metadata 93 ■ Using the Adapter pattern 95 3.5 Summary 96 Building modular applications 99 4 4.1 Understanding the role of an object 100 4.2 Separation of concerns (my pants are too tight!) 101 Perils of tight coupling 102 ■ Refactoring impacts of tight coupling 105 ■ Programming to contract 108 ■ Loose coupling with dependency injection 111 4.3 Testing components 112 Out-of-container (unit) testing 113 ■ I really need my dependencies! 114 More on mocking dependencies 115 ■ Integration testing 116 4.4 Different deployment profiles 118 Rebinding dependencies 118 ■ Mutability with the Adapter pattern 119 4.5 Summary 121 www.allitebooks.com Scope: a fresh breath of state 123 5 5.1 What is scope? 124 5.2 The no scope (or default scope) 125 5.3 The singleton scope 128 Singletons in practice 131 ■ The singleton anti-pattern 135 5.4 Domain-specific scopes: the web 139 HTTP request scope 141 ■ HTTP session scope 149 5.5 Summary 154 More use cases in scoping 156 6 6.1 Defining a custom scope 157 A quick primer on transactions 157 ■ Creating a custom transaction scope 158 ■ A custom scope in Guice 160 ■ A custom scope in Spring 164 6.2 Pitfalls and corner cases in scoping 166 Singletons must be thread-safe 167 ■ Perils of scope-widening injection 169 6.3 Leveraging the power of scopes 180 Cache scope 181 ■ Grid scope 181 ■ Transparent grid computing with DI 183 6.4 Summary 184 From birth to death: object lifecycle 186 7 7.1 Significant events in the life of objects 187 Object creation 187 ■ Object destruction (or finalization) 189 7.2 One size doesn’t fit all (domain-specific lifecycle) 191 Contrasting lifecycle scenarios: servlets vs. database connections 191 ■ The Destructor anti-pattern 196 ■ Using Java’s Closeable interface 197 7.3 A real-world lifecycle scenario: stateful EJBs 198 7.4 Lifecycle and lazy instantiation 201 7.5 Customizing lifecycle with postprocessing 202 7.6 Customizing lifecycle with multicasting 205 7.7 Summary 207 Managing an object’s behavior 210 8 8.1 Intercepting methods and AOP 211 A tracing interceptor with Guice 212 ■ A tracing interceptor with Spring 214 How proxying works 216 ■ Too much advice can be dangerous! 219 www.allitebooks.com 8.2 Enterprise use cases for interception 221 Transactional methods with warp-persist 222 ■ Securing methods with Spring Security 224 8.3 Pitfalls and assumptions about interception and proxying 228 Sameness tests are unreliable 228 ■ Static methods cannot be intercepted 230 ■ Neither can private methods 231 ■ And certainly not final methods! 233 ■ Fields are off limits 234 ■ Unit tests and interception 236 8.4 Summary 238 Best practices in code design 240 9 9.1 Objects and visibility 241 Safe publication 244 ■ Safe wiring 245 9.2 Objects and design 247 On data and services 247 ■ On better encapsulation 252 9.3 Objects and concurrency 257 More on mutability 258 ■ Synchronization vs. concurrency 261 9.4 Summary 264 Integrating with third-party frameworks 266 10 10.1 Fragmentation of DI solutions 267 10.2 Lessons for framework designers 270 Rigid configuration anti-patterns 271 ■ Black box anti-patterns 276 10.3 Programmatic configuration to the rescue 280 Case study: JSR-303 280 10.4 Summary 286 Dependency injection in action! 289 11 11.1 Crosstalk: a Twitter clone! 290 Crosstalk’s requirements 290 11.2 Setting up the application 290 11.3 Configuring Google Sitebricks 294 11.4 Crosstalk’s modularity and service coupling 295 11.5 The presentation layer 296 The HomePage template 298 ■ The Tweet domain object 301 Users and sessions 302 ■ Logging in and out 304 11.6 The persistence layer 308 Configuring the persistence layer 310 11.7 The security layer 311 11.8 Tying up to the web lifecycle 312 11.9 Finally: up and running! 313 11.10 Summary 314 appendix A The Butterfly Container 315 appendix B SmartyPants for Adobe Flex 320 index 323 Dependency injection: what’s all the hype? This chapter covers: ■ Seeing an object as a service ■ Learning about building and assembling services ■ Taking a tour of pre-existing solutions ■ Investigating the Hollywood Principle ■ Surveying available frameworks “We all agree that your theory is crazy, but is it crazy enough?” —Niels Bohr So you’re an expert on dependency injection (DI); you know it and use it every day. It’s like your morning commute—you sleepwalk through it, making all the right left turns (and the occasional wrong right turns before quickly correcting) until you’re comfortably sitting behind your desk at work. Or you’ve heard of DI and Inversion of Control (IoC) and read the occasional article, in which case this is your first com- mute to work on a new job and you’re waiting at the station, with a strong suspicion you are about to get on the wrong train and an even stronger suspicion you’re on the wrong platform. 1 2 CHAPTER 1 Dependency injection: what’s all the hype? Or you’re somewhere in between; you’re feeling your way through the idea, not yet fully convinced about DI, planning out that morning commute and looking for the best route to work, MapQuesting it. Or you have your own home-brew setup that works just fine, thank you very much. You’ve no need of a DI technology: You bike to work, get a lot of exercise on the way, and are carbon efficient. Stop! Take a good, long breath. Dependency injection is the art of making work come home to you. 1.1 Every solution needs a problem Most software today is written to automate some real-world process, whether it be writ- ing a letter, purchasing the new album from your favorite band, or placing an order to sell some stock. In object-oriented programming (OOP), these are objects and their interactions are methods. Objects represent their real-world counterparts. An Airplane represents a 747 and a Car represents a Toyota; a PurchaseOrder represents you buying this book; and so on. Of particular interest is the interaction between objects: An airplane flies, while a car can be driven and a book can be opened and read. This is where the value of the automation is realized and where it is valuable in simplifying our lives.