Dear Martin,
Thanks for your reply. Although you raise some good points, we still
have some trouble understanding the logic behind Struts' behaviour.
The Struts documentation encourages the use of the Form Bean across
actions, thus coupling -- whether tightly or loosely -- the actions
together. Or, you could look at it the other way around and say that the
Form Bean "provides a coupling mechanism".
Although we agree that it is the natural behaviour of Struts to
populate the form bean from the request parameters, we still have trouble
understanding why it is natural that this would be done with EVERY
forwarded-to action (beyond the fact that the code currently does this). In
fact, it seems very unnatural that it would be done over and over --
especially when one may wish to populate the form bean AFTER Struts
populates it from the request parameters.
I don't think that having one Action set a Form Bean attribute and
having another Action read that attribute is considered "tightly coupling".
Is it unusual to use the Form Bean as a method of defining something that
contains properties for all of the fields that will be used across pages
(and thus across requests AND Actions) like in a Wizard interface?
Although it may seem like our example code was tightly coupled, it
really doesn't matter what happens with the forwards in between actions. You
are right in assuming that we could be forwarding to another servlet before
forwarding back to another action. However, if we did this, we still would
not want the form bean re-populated from the request when entering any
subsequent action.
When forwarding to a JSP, however, this is a completely different
matter. Because returning from a JSP will yield an entirely new request; and
thus the whole problem disappears.
Take this scenario as a loosely-coupled example of another place
where this problem could arise:
The user forwards to an action from a JSP. The Action populates the
form bean. The Action forwards to a servlet, which does its own thing. Then
that servlet forwards to another servlet, which does yet another thing. Then
a third non-Action servlet is forwarded to, which actually removes the Form
Bean from the session (for some unknown reason). When that servlet forwards
to another Action, the Action will re-create the removed Form Bean and
re-populate it from the request.
This is very loosely-coupled. In fact, the two actions don't care
about one another at all. The problem is that a very unnatural thing
happens; the Form Bean is recreated AND repopulated! Both of these
behaviours are contrary to what the code is trying to do.
It is true that actions do not know where they are going to or where
they are coming from; they don't need to. But Struts should know if it has
processed the request parameters or not, so that it does not do the same
work over and over, and so that it does not override the Form Bean
attributes set by servlets/Actions.
Perhaps Struts should have a dirty marker that is placed in the
request after Struts processes the request parameters, so that it does not
re-process them? It could set a request Attribute and the ActionServlet
could read this boolean value to see if the Form Bean has been populated
from the request parameters yet or not. Because some people already have
applications which rely on the current behaviour, it might also be a good
idea to have this dirty marker be activated/disactivated from the Struts
config file?
Perhaps we are doing things completely incorrectly, but if we are,
then what is the right way? Your suggestion of having our Actions agreeing
on retrieving the Form Bean from some other location would actually make our
Actions very tightly coupled, whereas right now they are not. This also
bypasses Struts completely. What we are doing in the mean time is sending
our own values in the HTTPRequest from the JSP without using Struts form
elements, and setting the attributes of the Form Bean ourselves in the
Action class -- thus bypassing Struts which would do it upon EVERY
Action.perform() call. This bypasses Struts too, and we would like to
actually use the Struts form elements in our JSPs; but if we do... we run
into the problem of Struts re-setting our Form Bean attributes on every
Action.perform() call. Is there anything we can do?
Sincerely,
Daryl and Terry.
-----Original Message-----
From: Martin Cooper [mailto:[EMAIL PROTECTED]]
Sent: Sunday, April 21, 2002 1:45 AM
To: Struts Developers List
Subject: Re: ActionServlet re-populates form upon Actoin => Action call
What you are seeing is a natural consequence of the Servlet spec, and in
particular the behaviour of the RequestDispatcher. Here are some points to
consider:
* When you forward a request, all of the request parameters are preserved
and presented to whatever processes the forwarded request. Therefore, it is
natural that those original values be used to populate any form bean
associated with a forwarded-to action, should the target of the forward
happen to be a Struts action.
* A forward need not target a Struts action. It may go to a JSP page,
another servlet in the web app, or something else. You may happen to know
that, for your application, the target is another Struts action, but Struts
cannot assume that.
* A forwarded-to action does not know that it was forwarded to from another
action. It could have been forwarded to from a JSP page, a non-Struts
servlet, or something else. Therefore, Struts cannot assume that the form
bean has already been populated by some prior action.
It seems that you have some very specific assumptions built into your app.
In particular, your actions are very tightly coupled. For example, your
actions "know" that they are forwarding to another action, and further, they
"know" the type and instance of the form bean that the other action will
use. This type of coupling is one of the things that Struts really tries
hard to avoid.
If you really need this kind of tight coupling between your actions, then
you can have your actions "agree" on some other location from which they can
obtain their form bean, rather than using the one that Struts will pass to
your execute() method.
--
Martin Cooper
----- Original Message -----
From: "Terrence Xavier" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Thursday, April 18, 2002 11:21 AM
Subject: ActionServlet re-populates form upon Actoin => Action call
> Dear Struts Developers,
>
> We have noticed some unexpected behavior with Struts. Basically, if
> the HTTPRequest specifies attributes to modify in a struts form, they get
> modified on EVERY call to the ActionServlet. An example of this is as
> follows. Say that we have a JSP that sets a form attribute, and then we
> submit to an Action class. From that Action, we modify the same form
> attributes, and then forward to another Action. The problem we are seeing
is
> that the form attributes get re-set upon entry into the second Action,
thus
> overwriting our changed values in the form from the first Action.
> The normal expected behaviour is that we can change the form
> attributes in an Action, and then forward to another action and read those
> changed values. However, due to how the ActionServlet processes
> HTTPRequests, those values will be replaced with the values specified in
the
> HTTPRequest. Since the parameters in the HTTPRequest are read-only, we
> cannot override this functionality.
> We have done extensive testing of this "bug" over the past few days.
> We have written a simple project to demonstrate the issue, and have tested
> it with Struts 0.5, 1.0.2, and 1.1b1. Each of those versions of Struts
> functions the same with regard to this issue.
>
> Here is what is happening in more detail:
>
> The ActionServlet processes the HTTPRequest and populates the form
> bean from the parameters in the HTTPRequest. This occurs before
> instantiating the Action class to "perform". Thus, if we forward from
Action
> to Action, when the same request is passed by the servlet engine, the form
> bean gets populated twice -- before and after the first Action. So form
bean
> values set within the Action that are also present in the HTTPRequest get
> overridden with the values from the HTTPRequest.
> Our simple project (attached to this e-mail) demonstrates this.
> Here's the flow of that project:
>
>
> JSP => Action1 => Action2 => etc...
> | | | | |
> V V V V V
> set(x)=2 (x=2) set(x)=4 (x=2) get(x)=2
>
>
> In our project, whenever we enter and leave an action, we print out
> a string indicating that we are entering or leaving the action. We also
have
> print statements inside every getter and setter in the form bean that will
> tell us whenever anything is retrieved or set in the form. This is the
> output we get from our application (with extra a couple comments to help
> explain what is going on):
>
> *********** startActionOne **************START
> formAction : setFormAction() originalValue
> *********** startActionOne **************END
> +++++++++++ One.jsp +++++++++
> formAction : setFormAction() changedbyOneJSP <-- Struts sets the
form
> bean attribute from JSP.
> *********** ActionOne **************START
> formAction : getFormAction() changedbyOneJSP
> formAction : setFormAction() changedbyActionOne
> *********** ActionOne **************END
> formAction : setFormAction() changedbyOneJSP <-- Unexpected
behavior
> occurs here
> *********** StartActionTwo **************START
> formAction : setFormAction() changedbystartActionTwo
> *********** StartActionTwo **************END
> +++++++++++ Two.jsp +++++++++ getFormAction() = changedbyOneJSP
>
>
> It is easy to see why this behaviour would be unexpected. After
> explicitly setting the "formAction" attribute of the form in ActionOne, it
> is re-set by Struts before ActionTwo is "performed".
>
> Given that this is how Struts functions, our questions are these:
>
> 1) We have work-arounds by using session request attributes, but we
> would like to set form values in actions. How do we do this within the
> Struts framework and have them retain their values?
>
> 2) Since Struts seems to process the request twice, setting the form
> attributes over and over again for each action that is forwarded to, does
it
> make sense to change Struts so that it does not do the same work on the
same
> HTTPRequest object more than once?
>
> 3) Is this well-known issue?
>
> We would appreciate any input you can offer.
>
> Sincerely,
>
> Daryl Beattie and Terrence Xavier
>
> P.S. The example code is configured to work with Struts 1.1b1 right now.
We
> used the same project to test the other versions, but have since changed
it
> to use 1.1b1. We did not include the jars in our zip file, so if you want
to
> try it you will have to copy the Struts jars (and its dependent jars) in
the
> WEB-INF/lib folder.
>
>
>
>
----------------------------------------------------------------------------
----
> --
> To unsubscribe, e-mail:
<mailto:[EMAIL PROTECTED]>
> For additional commands, e-mail:
<mailto:[EMAIL PROTECTED]>
--
To unsubscribe, e-mail: <mailto:[EMAIL PROTECTED]>
For additional commands, e-mail: <mailto:[EMAIL PROTECTED]>
--
To unsubscribe, e-mail: <mailto:[EMAIL PROTECTED]>
For additional commands, e-mail: <mailto:[EMAIL PROTECTED]>
