Do I have to have a separate ActionForm
bean for every HTML form?
This is an interesting question. As a newbie, it is a good practice to
create a new ActionForm for each action sequence. You can use
DynaActionForms to help reduce the effort required, or use the code
generation facilities of your IDE.
Some issues to keep in mind regarding reuse of form beans are as
* Validation - You might need to use different validation rules
depending upon the action that is currently being executed.
* Persistence - Be careful that a form populated in one action is not
unexpectedly reused in a different action. Multiple entries in struts-config.xml
for the same ActionForm subclass can help (especially if you store your
form beans in session scope). Alternatively, storing form beans in
request scope can avoid unexpected interactions (as well as reduce the
memory footprint of your application, because no server-side objects
will need to be saved in between requests.
* Checkboxes - If you do as recommended and reset your boolean
properties (for fields presented as checkboxes), and the page you are
currently displaying does not have a checkbox for every boolean property
on the form bean, the undisplayed boolean properties will always appear
to have a false value.
* Workflow - The most common need for form bean reuse is workflow. Out
of the box, Struts has limited support for workflow, but a common
pattern is to use a single form bean with all of the properties for all
of the pages of a workflow. You will need a good understanding of the
environment (ActionForms, Actions, etc.) prior to being able to put
together a smooth workflow environment using a single form bean.
As you get more comfortable, there are a few shortcuts you can take in
order to reuse your ActionForm beans. Most of these shortcuts depend on
how you have chosen to implement your Action / ActionForm combinations.
How can I prepopulate a form?
The simplest way to prepopulate a form is to have an Action whose sole
purpose is to populate an ActionForm and forward to the servlet or JSP
to render that form back to the client. A separate Action would then be
use to process the submitted form fields, by declaring an instance of
the same form bean name.
The struts-example example application that is shipped with Struts
illustrates this design pattern nicely. Note the following definitions
from the struts-config.xml file:
<-- Registration form bean -->
<-- Edit user registration -->
<-- Save user registration -->
Note the following features of this approach:
* Both the /editRegistration and /saveRegistration actions use the same
* When the /editRegistration action is entered, Struts will have
pre-created an empty form bean instance, and passed it to the execute()
method. The setup action is free to preconfigure the values that will be
displayed when the form is rendered, simply by setting the corresponding
form bean properties.
* When the setup action completes configuring the properties of the form
bean, it should return an ActionForm that points at the page which will
display this form. If you are using the Struts JSP tag library, the
action attribute on your <html:form> tag will be set to /saveRegistration
in order for the form to be submitted to the processing action.
* Note that the setup action (/editRegistration) turns off validation on
the form that is being set up. You will normally want to include this
attribute in the configuration of your setup actions, because you are
not planning to actually process the results -- you simply want to take
advantage of the fact that Struts will precreate a form bean instance of
the correct class for you.
* The processing action (/saveRegistration), on the other hand, leaves
out the validate attribute, which defaults to true. This tells Struts to
perform the validations associated with this form bean before invoking
the processing action at all. If any validation errors have occurred,
Struts will forward back to your input page (technically, it forwards
back to an ActionForward named "registration" in this case, because the
example webapp uses the inputForward attribute in the element -- see the
documentation describing struts-config.xml for more information) instead
of calling your processing action.
Can I have an Action without a form?
Yes. If your Action does not need any data and it does
not need to make any data available to the view or
controller component that it forwards to, it doesn't need
a form. A good example of an Action with no ActionForm is
the LogoffAction in the struts example application:
<forward name="success" path="/index.jsp"/>
This action needs no data other than the user's session, which
it can get from the Request, and it doesn't need to prepare any
view elements for display, so it does not need a form.
However, you cannot use the <html:form> tag without
an ActionForm. Even if you want to use the <html:form>
tag with a simple Action that does not require input,
the tag will expect you to use some type of ActionForm,
even if it is an empty subclass without any properties.