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problem in understanding jstl set tag

 
sangeeta kapoor
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HF page 446

"In the <c:set> tag, the target attribute in the tag seems like it should work just like "id" in the <jsp:setProperty>, but it doesn't work that way"

But there is no "id" in <jsp:setProperty" . Is it talking about <jsp:useBean>


"With the target attribute, you do not type in a String literal that represents the name under which the attribute was bound to page,scope etc"

What does this mean?

I tried the following

Person.java


JSP

This does work fine. But i am using the "id" in <jsp:useBean> And if it really is <jsp:useBean> above instead of <jsp:setProperty> , how does this work.
Also What are the other ways in which a bean can be created and given as a value to "target"


Page 447 6th Bullet point

If you put in a String literal that represents the "id" name of the bean or Map, it won't work. In other words , "target" is not for the attribute name of the bean or Map-its for the actual attribute object.

What is the difference between attribute object and attribute name

if I say

<% com.Person p = new com.Person();
request.setAttribute("person", p);
%>

then in this case is "person" a attribute name or object? I think it's a name which represents an object p.
Then how can we differentiate between the two.
Moreover on the next page to which the request is submitted I only have access to "person", I can't get "p" directly.

Plz help me understand this.
 
Nitish Bahadur
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You are right.

The id in <jsp:setProperty> is the <jsp:useBean> 'id'. The <c:set> target attribute does not allow a string literal "person". If it did allow the string literal, then how would the <c:set> know that the author of the JSP page really wanted a 'person object' and not string literal "person" string.

Attributes have names and values. The name is a key, similar to Map keys, and the value is a java.lang.Object. "person", in your example, is the Attribute name, and the object referenced by "p" is the Attribute value.
 
Francois Roland
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About your question about the difference between attribute object and attribute name.

What you object (or instance, call it the way you like) is is really just no more than a chunk of data stored on the heap. This is what you create when you type new com.Person().

In your code, you decided to store this object in a local variable you called p. One line later, you put the content of the p variable into your request attributes collection (yes, you can think of this as a java.util.Map). So there is really no reason you can get a direct acces to you p variable by any other mean than a scriptlet (remember that all scriptlets of the page are put into the same and unique method when you JSP is translated).

In fact, you could have written something like this:
 
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