1. Java EE - Dependency Injection -
Pierre-Johan CHARTRE

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.”
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
private IService service;
A J2EE is one JSR, multiple implementations
WELD
OpenWebBeans
Google Guice
Spring …