How is setPropertyActionListener really working?

Question

I'm studying JSF and I believe I'm missing something as I don't get setPropertyActionListener. In the Java EE 5 Tutorial:
     <c:forEach items="#{bookDBAO.books}" var="book" varStatus="stat">
          <c:set var="book" scope="request" value="${book}"/>
          <h:commandButton id="add" action="#{catalog.add}"
                 value="#{bundle.CartAdd}">   
                 <f:setPropertyActionListener
                        target="#{requestScope.book}"
                        value="#{book}"/>
          </h:commandButton>
          <c:remove var="book" scope="request"/>
     </c:forEach>and
If you are familiar with JSTL, you know that the var attribute of the forEach tag is always in page scope. However, the book data must be in request scope so that the page that displays after the user has clicked the button has access to the book data. Therefore, the setPropertyActionListener tag is used to set the current book object into request scope when the user activates the button.
But doesn't this mean setPropertyActionListener is copying book into itself, it is in the request scope already? And how is the book in the request scope when the button is activated in the first place, this is the postback and I suppose the JSTL code does not get executed.

Answer 1

The JSTL is executed and the view is created when the page is loaded the first time. The SetPropertyActionListner is executed on postback.
Then why to I need to put the book in the request scope if the <c:set> is not executed in the same request as the SetPropertyActionListener?
This is the SetPropertyActionListener documentation:
The ActionListener instance created and installed by this tag has the following behavior and contract:
- Only create and register the ActionListener instance the first time the component for this tag is created.
- The "target" and "value" tag attributes are ValueExpression instances and are stored unevaluated as instance variables of the listener.
- When the listener executes, call getValue() on the "value" ValueExpression. Pass the result to a call to setValue() on the "target" ValueExpression.
Where is the #{book} value when the action is executed on postback? If it's in the request scope then who is putting it there and why do I need the action
if it's already there? It seems that the value expression is evaluated only when the action is invoked but how does it find the right book instance for the corresponding iteration?