Is it possible to @Inject a @RequestScoped bean into a @Stateless EJB?
Is it possible to inject a request-scoped CDI bean into a Stateless session bean?
I had asked a related question and thought the specific CDI @RequestScoped into @Stateless question merited its own post.
Passing state between EJB methods / @RequestScoped and @Stateless
I also asked a similar question about JMS @MessageDriven beans - basically want to know the same about @Stateless.
Answer
You can absolutely do what you mention and use @RequestScoped beans in an @Stateless session bean and an @MessageDriven bean. This is a core part of the CDI spec and TCK and guaranteed portable.
Note on MDBs
Do be aware that there is a test for a @Stateless bean that uses a @RequestScoped bean, but there is no test that guarantees a @MessageDriven bean can reference @RequestScoped beans. This was just an oversight and is already fixed for the Java EE 7 TCK. So just be aware that if it doesn't work for the MDB case, it may not be your fault :)
The workaround would be to simply have your MDB delegate to a SessionBean of any kind as @Stateless, @Stateful, and @Singleton all have @RequestScoped tests.
Making the EJB, itself, scoped
While @Stateless, @Singleton and @MessageDriven can have scoped references injected via @Inject, they cannot be @RequestScoped or any other scope. Only the @Stateful model is flexible enough to support scopes. In other words, you can annotate the @Stateful bean class itself as @RequestScoped, @SessionScoped, etc..
In simple terms @Stateless, @Singleton have fixed "scopes" already. @Singleton is essentially @ApplicationScoped and @Stateless would perhaps be some made-up scope like @InvocationScoped, if that existed. The lifecycle of an @MessageDriven bean is entirely up to the Connector that drives it and is therefore also not allowed to have user-defined scope.