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Java EE - Dependency Injection -

Java EE - Dependency Injection -. Pierre-Johan CHARTRE pierre-johan.chartre@logica.com. Coupling vs dependency.

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Java EE - Dependency Injection -

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  1. Java EE - Dependency Injection - Pierre-Johan CHARTRE pierre-johan.chartre@logica.com

  2. Coupling vs dependency • “In computer science, coupling or dependency is the degree to which each program module relies on each one of the other modules.” http://en.wikipedia.org/wiki/Coupling_%28computer_programming%29 • Hight coupling (concrete): your dependency is an instance of a concrete class. • private ArrayList<String> myList; • Low coupling (abstract/interface): your dependency is an instance of an abstract class. • private List<String> myList; • Limitations: • Component replacing, refactoring • Test writing, mock • Component overriding

  3. Example: A hight coupling app MyScreen <<Servlet>> MyService <<Object>> MyDAO <<Object>> MyObject <<Object>> uses uses uses uses OracleDriver <<jar>>

  4. How to remove coupling ? • A solution based on Design Patterns • Creation design patterns • Factory • Singleton • Structural design patterns • Proxy • Architectural design pattern • Inversion of Control & Dependency Injection

  5. How to remove coupling ? • Using the Factory Design Pattern ?

  6. How to remove coupling ? • Service Provider Interface (SPI) • the Java 3 approach • Define implementations in a text file META-INF/services/com.isima.spi.IDAO com.isima.MyTestDAO com.isima.MyDAOOracle • Get implementations ServiceLoader loader = ServiceLoader.load(com.isima.spi.IDAO.class); Iterator iterator = loader.iterator(); while (iterator.hasNext()) { IDAO dao = (IDAO) iterator.next(); dao.find(); } • Limitation: static injection IDAO <<Object>> MyTestDAO <<Object>> MyDAOOracle <<Object>>

  7. Dependency Injection • « OK, we removed coupling, but it will be better if it’s dynamic ! » • It’s « Inversion of Control » (IoC)

  8. Dependency Injection • « OK, but we could introduce more features ! » • Singleton for only one instance • Multiple implementation for the same interface • Default implementation • AOP • Mock for unit tests • Annotation support • …

  9. Example: A low coupling app MyScreen <<Servlet>> IService <<Object>> IDAO <<Object>> MyObject <<Object>> uses uses uses MyTestService <<Object>> MyService <<Object>> uses MyTestDAO <<Object>> MyDAOOracle <<Object>> OracleDriver <<jar>> • 3 Configurations • 1 for screen testing • 1 for service testing • 1 for DAO testing

  10. J2EE is a standard ! • JSR 330: Dependency Injection for Java @Inject @InMyServlet private IService service; • A J2EE is one JSR, multiple implementations • WELD • OpenWebBeans • Google Guice • Spring • …

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