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Java EE - Dependency Injection Pierre-Johan CHARTRE [email protected] 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 Example: A hight coupling app MyScreen MyService <<Servlet>> MyDAO <<Object>> uses MyObject <<Object>> uses <<Object>> uses uses OracleDriver <<jar>> 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 How to remove coupling ? • Using the Factory Design Pattern ? How to remove coupling ? • Service Provider Interface (SPI) – the Java 3 approach – Define implementations in a text file IDAO <<Object>> 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 MyTestDAO MyDAOOracle <<Object>> <<Object>> Dependency Injection • « OK, we removed coupling, but it will be better if it’s dynamic ! » – It’s « Inversion of Control » (IoC) 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 … Example: A low coupling app MyScreen IService <<Servlet>> IDAO <<Object>> MyObject <<Object>> uses uses MyTestService MyService <<Object>> <<Object>> <<Object>> uses uses MyTestDAO MyDAOOracle <<Object>> 3 Configurations •1 for screen testing •1 for service testing •1 for DAO testing <<Object>> OracleDriver <<jar>> 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 …
