Question

I’ve been messing around with servlets and JSPs and I am a bit confused at something:

1. I’ve made a servlet (controller) which would dispatch the request to a JSP

2. I’ve set some attributes to the request object using setAttribute() method in my
   servlet.

3. I can access the request object’s parameters and attributes within the JSP without any
   problem.

4. Now I’ve stored the request object as an attribute in the session object using
   session.setAttribute(“test”, request).

5. I’ve written a second JSP (navigating to it from the first JSP would be through
   Javascript when I click a particular button- by using the window.location function and
   giving the address of the second JSP as the value)

6. In the second JSP, when I retrieve the request object from the session object, I get a
   null value from all the attributes of the retrieved request object.
7. I can access the parameters of the retrieved request object BUT only if I had retrieved
   the parameters atleast once in my first JSP using request.getParameter() method
   otherwise they return null in my second JSP.

I am really new to this stuff and am confused about this behaviour. Why were my request object’s attributes being ‘erased’ while the parameters remain intact (as long I had accessed the parameters in my first JSP; which is even more bewildering to me as it does not make sense IMO)

Any explanation would be appreciated! Thanking you in advance.

Answer 1

This is just an educated guess, but I think the problem is that request objects in your container of choice might be lazy about its parameters: when you ask it for a parameter it reaches out to some external context and pulls the required data, at the same time caching it.

Nevertheless, the reason of the strange behavior is not really important. The problem should be solved by not saving requests in session. A request object is only your handle to the current request, not a data store by itself. It might be using any mechanism underneath, for all we know the attributes may be stored in threadlocals. There is absolutely no contract that would make a request act as an archive of any sort. For example: what would it mean if I asked such a stored request for the security principal? Would I mean “the current principal of the session”? Would I mean “the principal as of the moment when the request was created”?

EDIT:

Out of pure curiosity I just took a peek at Tomcat’s implementation (I have no idea which container you are using) and found that it supports my claims: not only most of the data is gathered lazily, but the request object is recycled! So if you try store it in a session and then use, you might find that you are using someone’s else request.