EJB3 instantiation

Question

I got some propably trivial question.

If I got defined EJB3+ interface, lets say it's remote like that:

@Remote
public class FooServiceRemote {
   void foo();
}

and one implementation

@Stateless
public class FooService implements FooServiceRemote {
   void foo() { ... }
}

How does the application server by default resolves what implementation to use (and call via proxy) if he knows only @EJB annotation for dependency injection like on interface:

public class SomeClass {
   @EJB
   private FooServiceRemote fooService;
}

Is it done by reflection (shortening name of interface)? Or he scans possible implementations of such interface, choosing one. Or.. ? And what if I want to create more implementations of one interface, is it possible and how to specify what implementation should be instantiated (maybe it is possible via some argument of annotation).

Thanks:-)

Answer 1

In the rare case that you need to have two beans implementing the same interface (not a good practice), you can name them and choose which one you want by name.

@Stateless(name="FooService1")
public class FooService1 implements FooService { }

@Stateless(name="FooService2")
public class FooService2 implements FooService { }

public class SomeClass {
   @EJB(beanName="FooService1") 
   private FooService fooService;
}

Other possible approaches are looking it up using JNDI or the mappedName property. See the javadoc for EJB annotation here: http://download.oracle.com/javaee/6/api/javax/ejb/EJB.html

Answer 2

Just a fix,

@Remote
interface FooServiceRemote {
   void foo();
}

@Stateless
public class FooService implements FooServiceRemote {
   void foo() { ... }
}

With this, application server knows which classes implements the specified interface.

If you have two classes, you must specifify which class do you need.