1. Let's first create a User POJO:

public class User implements Serializable{

    private Long id;
    private String name;
    
    public User(long id, String name){
        this.id = id;
        this.name = name;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    
}

2. And then, let's create a REST endpoint with event and observer features:

@Stateless
@Path("asyncService")
public class AsyncService {
    
    @Inject
    private Event<User> event;
    
    private AsyncResponse response;
    
    @GET
    public void asyncService(@Suspended AsyncResponse response){
        long id = new Date().getTime();
        this.response = response;
        event.fireAsync(new User(id, "User " + id));
    }
    

    public void onFireEvent(@ObservesAsync User user){
        response.resume(Response.ok(user).build());
    }
}