Doubt regarding Listeners

[Srinivasan thoyyeti]

Hi pramod,

Specifiaction

setAttribute
public void setAttribute(java.lang.String name,
java.lang.Object value)Binds an object to this session, using the name specified. If an object of the same name is already bound to the session, the object is replaced.
After this method executes, and

1. if the new object implements HttpSessionBindingListener, the container calls HttpSessionBindingListener.valueBound. The container then notifies any HttpSessionAttributeListeners in the web application.

2. If an object was already bound to this session of this name that implements HttpSessionBindingListener, its HttpSessionBindingListener.valueUnbound method is called.

3. If the value passed in is null, this has the same effect as calling removeAttribute().

Parameters:
name - the name to which the object is bound; cannot be null
value - the object to be bound
Throws:
IllegalStateException - if this method is called on an invalidated session

I am again re-emphasizing that step2 1,2,3 are mutually exclusive if conditions. That means they have no-depedencies.

I DON"T THINK SPECIFICATION DICTATING THE ORDER THERE.
ORDER WILL BE SPECIFIED IF THERE ARE DEPENDENCIES.

For example, If you take point (1). There it is combination of two atomic steps.
1.
1.1 if the new object implements HttpSessionBindingListener, the container
calls HttpSessionBindingListener.valueBound.
1.2 The container then notifies any HttpSessionAttributeListeners in the
web application.
***** HERE 1.2 can't execute before 1.1 This is order

Hence Websphere is a robust application server.

Proof for step 1 automicity (on websphere)

Check with this listener

package edu.servlet.http;
 
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
 
public class X implements HttpSessionBindingListener, HttpSessionAttributeListener {
private String id;
public X(String str) { id= str;}
public X() { }
public String getId(){
return id;
}
public void setId(String id){
this.id = id;
}

public void valueBound(HttpSessionBindingEvent event) {
System.out.println("B");
}
 
public void valueUnbound(HttpSessionBindingEvent event) {
System.out.println("UB");
}
public String toString(){
return id;
}
public void attributeAdded(HttpSessionBindingEvent hsbe) {
System.out.println("Attribute added: (name = "+hsbe.getName()+","+hsbe.getValue()+")");

}
public void attributeRemoved(HttpSessionBindingEvent hsbe) {
System.out.println("Attribute removed: (name = "+hsbe.getName()+","+hsbe.getValue()+")");

}
public void attributeReplaced(HttpSessionBindingEvent hsbe) {
System.out.println("Attribute replaced: (name = "+hsbe.getName()+","+hsbe.getValue()+")");

}
} 

Now call:

package edu.servlet.http;
 
import java.io.IOException;
 
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
public class ServletX extends HttpServlet{
public void doGet(HttpServletRequest req,HttpServletResponse res) throws IOException,ServletException{

req.getSession().setAttribute("key",new X("one")); // B
req.getSession().setAttribute("key",new X("two"));
req.getSession().setAttribute("key",null);
req.getSession().removeAttribute("key");
}
}

Result is :
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O B
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O Attribute added: (name = key,one)
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O UB
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O B
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O Attribute replaced: (name = key,one)
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O UB
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O Attribute removed: (name = key,two)
[8/10/07 19:51:17:453 IST] 00000020 SystemOut O Attribute removed: (name = key,null)

Thats all guys,
This is my opinion. Any contradictions invited.
because discussion can only makes us clear with this.

[Raj Menon]

Hi,

I thank each and every one who responded so well with their explanations.
I couldn't able to check this forum yesterday to post my opinion.

Any way tomorrow i'm taking this exam.
Hope i won't get this question in the exam.
If i get,i will go by the suggestion given by Srinivasan -
If both are present - I'll select both
If only either of the two exits - No Choice (Have to select that one)
I'll share my experiences once i'm done with the exam.

And finally going by all the practical tests and their explanations by every one regarding this Listener Bound/Unbound problem,
Shall we have to conclude that Different Containers provide different implementation to this Listener Bound/Unbound concept because of lack of clarity in the Spec?

[ August 10, 2007: Message edited by: Rajesh Kodali ]
[ August 10, 2007: Message edited by: Rajesh Kodali ]

[Srinivasan thoyyeti]

Hi Rajesh,

CONCLUSION:
Specification need not be specific about the order IF there are no dependencies. and this type of questions will not appear in real exam if they apply some thought and common sense.

In setAttribute specification After setAttribute call, Depending on value set need to call one if they satisfy the condition specified.

A.if (cond) { valueBound(), [attributeAdded, attributeReplaced] }
B.if (cond) { valueUnBound(). }
C.if (cond) { attributeRemoved() }

The implementing function should include checks for A,B,C. Thats all.
They can check order like this: (A,B,C), (B,A,C).


Thanks every body who participated in this Thread and Rajesh you inited this therad and made my eyes wide open.

We had a very good discussion.
[ August 10, 2007: Message edited by: Srinivasan thoyyeti ]

[Cristiano Sganzerla]

Nice thread, this cant be on the exam, its container dependent !!!