Repopulating Select Lists after Validation Error

[Asleson, Ryan]

 
Hello,
 
We're transitioning from a Struts 1.x and Tiles development environment
to Struts 2 and Sitemesh, and I need some help getting around some of
the differences.
 
Here's what I'm planning so far.  I want to have an abstract BaseAction
that extends ActionSupport.  Since we are no longer using Tiles, we
can't count on using Tiles Controller classes to prepare a page for
display -- for example, retrieving the contents of drop down lists from
the database.  In my BaseAction, I have overridden the execute() method
to call a "lifecycle" method called prepareAction() that is meant to be
an extension point for subclasses to retrieve any data the is needed to
display the page.
 
For example, let's say there's a drop-down select box on a page, and its
contents come from a database.  A concrete implementation of BaseAction
should override prepareAction() to retrieve the list items for the drop
down list.  
 
Now, here's where things get sticky:  Assume the page above is
displayed.  The user enters some form data and posts the page.  However,
there are some validation errors.  Struts 2 will automatically redisplay
the page.... but will the drop-down list be populated with list items,
or will it be empty?
 
In the Struts 1.x/Tiles world, where we have a Tiles Controller backing
the JSP, the list items would be repopulated by the Controller.  But
with Tiles no longer in play, what will repopulate the list?  I'm pretty
sure that the prepareAction() method that was used when the page is
first displayed will not be called when there are validation errors,
resulting in an empty drop-down list.
 
So, here are my questions:
 
1.  Is my idea to have a BaseAction that provides an extension point for
subclasses, where things like database access should occur, a bad idea?
 
2.  What is the recommended way to repopulate drop-down selects after a
validation error, if the list items need to be retrieved from a
database?
 
Thank you!!
 
-Ryan
 
 

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[Laurie Harper]

Asleson, Ryan wrote:

>  
> Hello,
>  
> We're transitioning from a Struts 1.x and Tiles development environment
> to Struts 2 and Sitemesh, and I need some help getting around some of
> the differences.
>  
> Here's what I'm planning so far.  I want to have an abstract BaseAction
> that extends ActionSupport.  Since we are no longer using Tiles, we
> can't count on using Tiles Controller classes to prepare a page for
> display -- for example, retrieving the contents of drop down lists from
> the database.  In my BaseAction, I have overridden the execute() method
> to call a "lifecycle" method called prepareAction() that is meant to be
> an extension point for subclasses to retrieve any data the is needed to
> display the page.

Why add a custom lifecycle method? Why not just let sub-classes
implement Preparable?

> For example, let's say there's a drop-down select box on a page, and its
> contents come from a database.  A concrete implementation of BaseAction
> should override prepareAction() to retrieve the list items for the drop
> down list.  

By implementing Preparable that logic would go in either prepare() or
prepareInput() (where 'input' is the name of the method on the action
that is called to display the form).

> Now, here's where things get sticky:  Assume the page above is
> displayed.  The user enters some form data and posts the page.  However,
> there are some validation errors.  Struts 2 will automatically redisplay
> the page.... but will the drop-down list be populated with list items,
> or will it be empty?

If there are validation errors, Struts will forward control to the
action result mapped as 'input'. You can either arrange for the
drop-down list data to be loaded before validation fires (e.g. using
Preparable.prepare()) or, if you don't want to perform that load during
a successful form submit, you can do it as part of the 'input' result
processing.

> In the Struts 1.x/Tiles world, where we have a Tiles Controller backing
> the JSP, the list items would be repopulated by the Controller.  But
> with Tiles no longer in play, what will repopulate the list?  I'm pretty
> sure that the prepareAction() method that was used when the page is
> first displayed will not be called when there are validation errors,
> resulting in an empty drop-down list.

If a validation error occurs, Struts will not call the action method for
handling the form submit so no, you can't rely on a method being called
from there in this case.

> So, here are my questions:
>  
> 1.  Is my idea to have a BaseAction that provides an extension point for
> subclasses, where things like database access should occur, a bad idea?

Not necessarily a bad idea, but not necessarily optimal. For example,
since your actions would all be dependent on super.execute() being
invoked, it may make it messy to target other methods on the action
(e.g. calling action.input() to setup and display a form and
action.execute() to process it on submit).

> 2.  What is the recommended way to repopulate drop-down selects after a
> validation error, if the list items need to be retrieved from a
> database?

Take a look at  the Preparable interface and the 'prepare' interceptor
for one approach. Another may be to use the 'action-chain' result type
for your action's 'input' result mapping, and keep the data retrieval in
the chained action.

I recommend taking a look at some of the example applications (e.g.
Mailo Reader and Showcase) to see how this has been handled elsewhere.
There's More Than One Way To Do It (TM) :-)

L.



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