Solved

HttpServlet and ServletRequestAttributeListener explantion

Posted on 2015-02-01
3
42 Views
Last Modified: 2015-02-11
This is from the book I am reading:

Given this code from an otherwise valid HttpServlet that has also been
registered as a ServletRequestAttributeListener:

public void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException, ServletException {
         req.setAttribute(“a”, “b”);
         req.setAttribute(“a”, “c”);
          req.removeAttribute(“a”);
}
        public void attributeAdded(ServletRequestAttributeEvent ev) {
        System.out.print(“ A:” + ev.getName() + “->” + ev.getValue());
}
       public void attributeRemoved(ServletRequestAttributeEvent ev) {
       System.out.print(“ M:” + ev.getName() + “->” + ev.getValue());
}
       public void attributeReplaced(ServletRequestAttributeEvent ev) {
       System.out.print(“ P:” + ev.getName() + “->” + ev.getValue());
}

Open in new window


What logging output is generated?

And answer is:
C. A:a->b P:a->b M:a->c

And explanation from book is:
Tricky! The getValue method returns the OLD value of the attribute if the attribute was replaced.

My question is how this could be?
Particularly this part of sequence is not clear to me: P:a->b
Why would it be once again P:a->b instead of P:a->c ?
0
Comment
Question by:KPax
  • 2
3 Comments
 
LVL 27

Accepted Solution

by:
dpearson earned 500 total points
ID: 40583362
Just to clarify this output, can we call them "Added", "Replaced" and "Removed" so we're looking at this:

Added:a->b
Replaced:a->b
Removed:a->c

Now the question is why does Replaced return "b" for the value instead of "c"?

The simple answer is because that's what the docs say it should do:
http://docs.oracle.com/javaee/7/api/javax/servlet/ServletRequestAttributeListener.html

void attributeReplaced(ServletRequestAttributeEvent srae)
Receives notification that an attribute has been replaced on the ServletRequest.
Parameters:
srae - the ServletRequestAttributeEvent containing the ServletRequest and the name and (old) value of the attribute that was replaced


Now maybe the more interesting question is why are they doing this?

Well usually APIs like this are designed to pass you the old value, because you always have the option to ask for the current value in the callback.  So if they pass you the old value - you have more information available to you than if they just passed in the current value.  (There's no way to ask "what value did this attribute use to have?" after it's gone).

So with this API design you could write a listener than took some action whenever the "a:b" was removed - either by an explicit remove call or by replacing it with another value.  If they only passed in the new value, you couldn't write that listener (without storing the values that were added yourself).

Hope that helps make it clearer why it's this way.

Doug
0
 

Assisted Solution

by:KPax
KPax earned 0 total points
ID: 40593317
I found this explanation as well

The getName() method returns the String name of the attribute that triggered the event. The getValue() method returns the object value of the attribute that triggered the event. Watch out! It returns the old value, not the new one. In other words, it returns the value the attribute had BEFORE the change that triggered the event!
0
 

Author Closing Comment

by:KPax
ID: 40602721
I found this solution in a book  "Servlets and JSP, 2 ed"
0

Featured Post

Master Your Team's Linux and Cloud Stack!

The average business loses $13.5M per year to ineffective training (per 1,000 employees). Keep ahead of the competition and combine in-person quality with online cost and flexibility by training with Linux Academy.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

An old method to applying the Singleton pattern in your Java code is to check if a static instance, defined in the same class that needs to be instantiated once and only once, is null and then create a new instance; otherwise, the pre-existing insta…
Go is an acronym of golang, is a programming language developed Google in 2007. Go is a new language that is mostly in the C family, with significant input from Pascal/Modula/Oberon family. Hence Go arisen as low-level language with fast compilation…
Viewers learn about the “for” loop and how it works in Java. By comparing it to the while loop learned before, viewers can make the transition easily. You will learn about the formatting of the for loop as we write a program that prints even numbers…
Viewers will learn about the regular for loop in Java and how to use it. Definition: Break the for loop down into 3 parts: Syntax when using for loops: Example using a for loop:

840 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question