1. First, we 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. Then, we create our asynchronous session bean:

@Stateless
public class UserBean {
    
    @Asynchronous
    public Future<User> getUser(){
        long id = new Date().getTime();
        User user = new User(id, "User " + id);
        return new AsyncResult(user);
    }
    
    @Asynchronous
    public void doSomeSlowStuff(User user){
        try {
            TimeUnit.SECONDS.sleep(5);
        } catch (InterruptedException ex) {
            System.err.println(ex.getMessage());
        }
    }
}

3. And finally, we create the endpoint that will call the bean:

@Stateless
@Path("asyncService")
public class AsyncService {
    
    @Inject
    private UserBean userBean;
    
    @GET
    public void asyncService(@Suspended AsyncResponse response){
        try {
            Future<User> result = userBean.getUser();
            
            while(!result.isDone()){
                try {
                    TimeUnit.SECONDS.sleep(1);
                } catch (InterruptedException ex) {
                    System.err.println(ex.getMessage());
                }
            }
            
            response.resume(Response.ok(result.get()).build());
        } catch (InterruptedException | ExecutionException ex) {
            System.err.println(ex.getMessage());
        }
    }
}