Dependency Injection in EJB3.0

To facilitate test driven development(TDD), the EJB 3.0 specification allows you to use annotations to inject dependencies through annotations on fields or setter methods. 

Instead of complicated XML ejb-refs or resource refs, you can use the @EJB and @Resource annotations to set the value of a field or to call a setter method within your session bean with anything registered within JNDI. 

You can use the @EJB annotation to inject EJB references and @Resource to access datasources. 

The example shows two ways to get access to the Calculator EJB. 

One is:

@EJB
private Calculator calculator;

or

private Calculator set;

@EJB(beanName="org.jboss.tutorial.injection.bean.CalculatorBean")
public void setCalculator(Calculator c)
{
   set = c;
}

When the ShoppingCartBean instance is created, the EJB container will set the calculator field using the jndiName of that particular referenced EJB. 

Another example should be the message driven context inside an MDB.

@Resource
private MessageDrivenContext mdc; 

The @javax.annotation.Resource annotation allows you to inject resources. 

@Resource(mappedName="DefaultDS")
private javax.sql.DataSource ds;

In JBoss, whenever the mappedName() attribute is specified (

@Resource, @EJB), jboss will use that as the GLOBAL jndi name to look it up.

The @Resource annotation is used to inject these singletons as well.

   @Resource
 javax.ejb.SessionContext ctx;
   @Resource
 javax.ejb.TimerService timer;
   @Resource
 javax.ejb.UserTransaction ut;

@EJB and @Resource also create an entry within the 
JNDI ENC of the bean.  So, the above @EJB injection

will create an entryfor the reference calculator beanver is 

currently being refactored in this area, 

so an EJB3 specific namespace is being used.