jdiscuss question

[Jingh Yi]

Associate the events with appropriate listener interface:

When an attribute is replaced in the session:

a. HttpSesionActivationListener
b. HttpSessionBindingListener
c. HttpSessionListener
d. None of these




Answer is d. Why not b?

[kapil munjal]

because there is httpsessionattributelistener.

httpsessionbindinglistener only tells if a new attribute is bound or some attribute is unbound from the session.

[Jingh Yi]

But HttpSessionBindingListener does listen to attribute replacement. Like in the following HttpServlet:

  
public class MusicServlet extends HttpServlet {
public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException,
                    java.io.IOException {
Dog dog1 = new Dog();
Dog dog2 = new Dog();
dog1.setBreed("domation");
dog2.setBreed("poodle");
HttpSession session = req.getSession();
session.setAttribute("dog", dog1);
session.setAttribute("dog", dog2);
...

The HttpSessionBindingListener class:

 
public class Dog implements HttpSessionBindingListener, Serializable
{
private String breed;
 
public Dog() {
 
}
/*public Dog(String breed) {
this.breed = breed;
}*/
 
public String getBreed() {
return breed;
}
 
public void setBreed(String value) {
breed = value;
}
 
public void valueBound(HttpSessionBindingEvent event) {
String name = event.getName();
Object value = event.getValue();
System.out.println("Value Bound: " + name + ": " + value);
}
 
public void valueUnbound(HttpSessionBindingEvent event) {
String name = event.getName();
Object value = event.getValue();
System.out.println("Value Unbound: " + name + ": " + value);
} 

When the dog1 is replaced by dog2, HttpSessionBindingListener does print out lines.

Why is it not correct answer?

Thanks,
Jenny

[Regis Santos]

Jenny,

The listener HttpSessionBindingListener is for the object to know when it has been added (bound) or will be removed (unbound) in a session.

If you want to know when anything has been added, replaced or removed as an attribute in a session, you have to use HttpSessionAttributeListener. Pay attention: HttpSessionAttributeListener must be declared in the DD.

In you example, the Dog implements HttpSessionBindingListener and it is notified when it is added or removed in a session.

Regards,

[Sergey Tyulkin]

What mock exam do you use. I saw similar question in EnthuWare mock exam, and people on their forum says that HttpSessionBindingListener is a correct answer for such questions.
We can only hope not to encounter such questions in the real exam