Java EE 6 introduced dependency injection mechanism, which makes it extremely easy to instantiate bean objects and pass the reference to other java services.
Injecting EJB into web services, servlets or EJBs can be easily done by using @javax.ejb.EJB annotation.
However, it doesn't work in the same way for JAX-RS services. Server would report NPE if @EJB annotation is used to inject EJB instance. The reason is that JAX-RS services are not managed beans.
An easy way to fix it is to make the JAX-RS service a stateless session bean by annotating it with @javax.ejb.Stateless.
A better alternative is to make JAX-RS service a managed bean. Detailed steps are as follows:
1. Add an empty beans.xml to WEB-INF directory. The purpose of this is to indicate that the web application is JCDI enabled.
2. Add a no-argument constructor to the JAX-RS service class. This makes the service class a JCDI bean. If some properties need to be set with some values (from http headers, for example.), it can be done inside the @javax.annotation.PostConstruct method.
3. Annotate the JAX-RS service class with @javax.enterprise.context.RequestScoped. This defines the service has http request scope.
4. Inject EJB instance with @javax.inject.Inject annotation.