How it works...

Well, there's a lot happening here! First, we should have a look at the @Produces annotation. This is a Jakarta CDI annotation that says to the server, Hey! This method knows how to construct a User object.

Since we didn't create a default constructor for the User class, the getUser method from our factory will be injected into our context as one.

The second annotation is our custom @Profile annotation, which has our ProfileType enumeration as a parameter. It is the qualifier for our UserProfile objects.

Now, let's have a look at these declarations:

@Profile(ProfileType.ADMIN)
public class ImplAdmin implements UserProfile{
   ...
}

@Profile(ProfileType.OPERATOR)
@Default
public class ImplOperator implements UserProfile{
   ...
}

This code will teach Jakarta CDI how to inject a UserProfile object:

• If the object is annotated as @Profile(ProfileType.ADMIN), use ImplAdmin.
• If the object is annotated as @Profile(ProfileType.OPERATOR), use ImplOperator.
• If the object is not annotated, use ImplOperator, as it has the @Default annotation.

We can see these in action in our endpoint declaration:

 @Inject
 @Profile(ProfileType.ADMIN)
 private UserProfile userProfileAdmin;
    
 @Inject
 @Profile(ProfileType.OPERATOR)
 private UserProfile userProfileOperator;
    
 @Inject
 private UserProfile userProfileDefault;

So, Jakarta CDI is helping us use the context to inject the right implementation of the UserProfile interface.

By taking a look at the endpoint methods, we will see the following:

 @GET
 @Path("getUser")
 public Response getUser(@Context HttpServletRequest request, 
         @Context HttpServletResponse response) 
         throws ServletException, IOException{
     
        request.setAttribute("result", user);
        request.getRequestDispatcher("/result.jsp")
        .forward(request, response);
        return Response.ok().build();
 }

Note that we included HttpServletRequest and HttpServletResponse as parameters for our method, but annotated them as @Context. So, even though this is not a servlet context (when we have easy access to request and response references), we can ask Jakarta CDI to give us a proper reference to them.

Finally, we have our user event engine, as follows:

 @Inject
 private Event<User> userEvent;

    ...

 private ProfileType fireUserEvents(ProfileType type){
     userEvent.fire(user);
     userEvent.fireAsync(user);
     return type;
 }
    
 public void sendUserNotification(@Observes User user){
     System.out.println("sendUserNotification: " + user);
 }
  
 public void sendUserNotificationAsync(@ObservesAsync User user){
     System.out.println("sendUserNotificationAsync: " + user);
 }

So, we are using the @Observes and @ObserversAsync annotations to say to Jakarta CDI, Hey CDI! Watch over the User object... when somebody fires an event over it, I want you to do something.

And for something, Jakarta CDI understands this as calling the sendUserNotification and sendUserNotificationAsync methods. Try it for yourself!

Obviously, @Observers will be executed synchronously, while @ObservesAsync will be executed asynchronously.