Question

Struts bean:define tag

Asked by: colr__

If I have the following struts tag lib call in my JSP:

<bean:define id="repMap" name="repMap"/>

Where does the bean 'repMap' actually come from? It appears that it is set as an attribute to the request object in the Action class, but is this the only place it is mapped? Does it have a mapping in xml somewhere too? I cant find a reference to that name anywhere else, but I just need confirmation on that.

The struts documentation ive consulted says it is a bean instance, but I know that the object is of type HashMap. What makes it a bean?

This is really confusing me - im trying to pick up struts for a new job, so would love some help!

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Asked On
2007-03-05 at 04:47:19ID22427988
Tags

struts

,

bean

,

define

,

tag

Topics

Jakarta Struts

,

Java Server Pages (JSP)

,

J2EE

Participating Experts
2
Points
500
Comments
6

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Answers

 

by: mrcoffee365Posted on 2007-03-05 at 10:15:46ID: 18656177

bean:define is generally used to define a local variable.  I'm not sure what it does for you if you don't give it a value.

This page has a nice explanation of the different bean declarations you can have in your page:
http://javaboutique.internet.com/tutorials/StrutsTags/bean_tags.html

 

by: owenli27Posted on 2007-03-05 at 11:03:50ID: 18656510

<bean:define id="repMap" name="repMap"/>
- Since there is no 'scope' attribute, so default rules applied by PageContext.findAttribute() to search for the named 'repMap' attribute in page, request, session and application scope in order.  In this case, you have 'repMap' attribute existed in request scope.

- Thers is also no 'toScope' attribute in <bean:define> tag,  the id will be created in 'page' scope as default.

So, This line actually copy an bean instance named 'repMap' from request attribute to this JSP page.  The new bean instance is also called 'repMap' in this JSP page, but it is a different object from one in request attribute.  

 

by: mrcoffee365Posted on 2007-03-05 at 11:57:27ID: 18656872

owenli27 -- great explanation.

It's also discussed in the link I gave above (partway down the page):

"With the bean tags you can create a lot of different beans (which then can be accessed with the logic tags--see the next section). Here are some useful examples of the bean tags:

A bean with a String constant       <bean:define id="string"
  value="Struts in Javaboutique"/>

Copy an existing bean         <bean:define id="copy" name="dvd"/>

Copy an existing bean and create it in a new scope       <bean:define id="copy" name="dvd"
  scope="request" toScope="session"/>

Copy a single property from a bean        <bean:define id="title"
  name="copy" property="title"/>

 

by: colr__Posted on 2007-03-06 at 01:28:42ID: 18660337

Excellent guys, that expains it.

One question owenli27 - do you know *why* its a copy and not the original? Is this a safety feature so you cant change the values of the request attributes?

 

by: owenli27Posted on 2007-03-06 at 06:16:12ID: 18661603

To make a local copy give developer flexibility. It also depends on your proggram business logic.  for examples, there are 2 cases as below:

1. Your current JSP page may forward request to another JSP along with your Request object and your next JSP page may needs the original bean instance, not the modified local  bean instance.

2. If your next JSP page needs the modified bean instance in your current JSP, so you can set Request attribute to replace original bean by using the modified bean.


 

 

by: owenli27Posted on 2007-03-06 at 06:32:03ID: 18661717

Sorry.  Please ignore my last comments and I misread your question.   About *why* its a copy and not a object reference which refer to original bean instance in Request attribute.  I think that <bean:define> is explained to copy bean instance, not getting bean instance reference.
Normally, when we call session.getAttribute("anyName") or request.getAttribute("anyName")  will give us a object reference.  Any changes will automaticlly reflect to object stored in session/request as attribute (Except String object because it is immutable)

20120131-EE-VQP-002

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