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 follows:
* 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:
...
<form-beans>
...
<-- Registration form bean -->
<form-bean name="registrationForm"
type="org.apache.struts.webapp.example.RegistrationForm"/>
...
</form-beans>
...
<action-mappings>
...
<-- Edit user registration -->
<action path="/editRegistration"
type="org.apache.struts.webapp.example.EditRegistrationAction"
name="registrationForm"
scope="request"
validate="false"/>
...
<-- Save user registration -->
<action path="/saveRegistration"
type="org.apache.struts.webapp.example.SaveRegistrationAction"
name="registrationForm"
input="registration"
scope="request"/>
...
</action-mappings>
Note the following features of this approach:
* Both the /editRegistration and /saveRegistration
actions use the same form bean.
* 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:
<action path="/logoff"
type="org.apache.struts.webapp.example.LogoffAction">
<forward name="success" path="/index.jsp"/>
</action>
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.
Can you give me a simple example of using
the requiredif Validator rule?
First off, there's an even newer Validator
rule called
validwhen, which is almost certainly what you want to
use,
since it is much easier and more powerful.
It will be available in the first release after 1.1 ships.
The example shown below could be coded with validwhen
as:
<form name="medicalStatusForm">
<field
property="pregnancyTest" depends="validwhen">
<arg0 key="medicalStatusForm.pregnancyTest.label"/>
<var>
<var-name>test</var-name>
<var-value>((((sex == 'm') OR (sex == 'M'))
AND (*this* == null)) OR (*this* != null))</test>
</var>
</field>
Let's assume you have a medical information form
with three fields,
sex, pregnancyTest, and testResult.
If sex is 'f' or 'F',
pregnancyTest is required. If pregnancyTest
is not blank,
testResult is required. The entry in your
validation.xml
file would look like this:
<form name="medicalStatusForm">
<field
property="pregnancyTest" depends="requiredif">
<arg0 key="medicalStatusForm.pregnancyTest.label"/>
<var>
<var-name>field[0]</var-name>
<var-value>sex</var-value>
</var>
<var>
<var-name>fieldTest[0]</var-name>
<var-value>EQUAL</var-value>
</var>
<var>
<var-name>fieldValue[0]</var-name>
<var-value>F</var-value>
</var>
<var>
<var-name>field[1]</var-name>
<var-value>sex</var-value>
</var>
<var>
<var-name>fieldTest[1]</var-name>
<var-value>EQUAL</var-value>
</var>
<var>
<var-name>fieldValue[1]</var-name>
<var-value>f</var-value>
</var>
<var>
<var-name>fieldJoin</var-name>
<var-value>OR</var-value>
</var>
</field>
<field
property="testResult" depends="requiredif">
<arg0 key="medicalStatusForm.testResult.label"/>
<var>
<var-name>field[0]</var-name>
<var-value>pregnancyTest</var-value>
</var>
<var>
<var-name>fieldTest[0]</var-name>
<var-value>NOTNULL</var-value>
</var>
</field>
</form>
When is the best time to validate input?
This is an excellent question. Let's step back a second and think about a
typical mid to large size application. If we start from the back end and work
toward the view we have:
1) Database: Most modern databases are going to validate for required fields,
duplicate records, security constraints, etc.
2) Business Logic: Here you are going to check for valid data relationships and
things that make sense for the particular problem you are triing
to solve.
... This is where struts comes into the picture, by now the system should be
pretty well bulletproof. What we are going to do is make validation friendlier
and informative. Rember it is OK to have duplicate
validations...
3) ActionErrors validate(ActionMapping
map, HttpServletRequest req)
is where you can do your validation and feed back to the view, information
required to correct any errors. validate is run after the form has been reset
and after the ActionForm properties have been set
from corresponding view based input. Also remember you can turn validation off
with validate="false" in the action mapping in the struts-config.xml.
This is done by returning an ActionErrors collection
with messages from your ApplicationResources.properties
file.
Here you have access to the request so you can see what kinds of action is
being requested to fine tune your validations. The <html:error>
tag allows you to dump all errors on your page or a particular error associated
with a particular property. The input attribute of the struts-config.xml action
allows you to send validation errors to a particular jsp
/ html / tile page.
4) You can have the system perform low level validations and client side
feedback using a ValidatorForm or its derivatives.
This will generate javascript and give instant
feedback to the user for simple data entry errors. You code your validations in
the validator-rules.xml file. A working knowledge of regular expressions is
necessary to use this feature effectively.
How can I avoid validating a form
before data is
entered?
The simplest way is to have two actions. The first one has the job
of setting the form data, i.e. a blank registration screen. The second action
in our writes the registration data to the database. Struts would take care of
invoking the validation and returning the user to the correct screen if
validation was not complete.
The EditRegistration action in the struts example
application illustrates this:
< action path="/editRegistration">
type="org.apache.struts.webapp.example.EditRegistrationAction"
attribute="registrationForm"
scope="request"
validate="false">
<forward name="success path="/registration.jsp"/>
</action>
When the /editRegistration action is invoked, a registrationForm is created and added to the request, but
its validate method is not called. The default value of the validate attribute
is true, so if you do not want an action to trigger form validation, you need
to remember to add this attribute and set it to false.
How can I create a wizard workflow?
The basic idea is a series of actions with next, back, cancel and finish
actions with a common bean. Using a LookupDispatchAction
is reccomended as it fits the design pattern well and
can be internationalized easily. Since the bean is shared, each choice made
will add data to the wizards base of information. A sample of struts-config.xml
follows:
< form-beans>
<form-bean name="MyWizard"
type="forms.MyWizard" />
</form-beans>
<!-- the first screen of the wizard (next action only available) -->
<!-- no validation, since the finish action is not available -->
<actions>
<action path="/mywizard1"
type="actions.MyWizard"
name="MyWizard"
validate="false"
input="/WEB-INF/jsp/mywizard1.jsp">
<forward name="next"
path="/WEB-INF/jsp/mywizard2.jsp" />
<forward name="cancel"
path="/WEB-INF/jsp/mywizardcancel.jsp"
/>
</action>
<!-- the second screen of the wizard (back, next and finish) -->
<!-- since finish action is available, bean should validated, note
validation should not necessarily validate if back action requested, you
might delay validation or do conditional validation -->
<action path="/mywizard2"
type="actions.MyWizard"
name="MyWizard"
validate="true"
input="/WEB-INF/jsp/mywizard2.jsp">
<forward name="back"
path="/WEB-INF/jsp/mywizard1.jsp" />
<forward name="next"
path="/WEB-INF/jsp/mywizard3.jsp" />
<forward name="finish"
path="/WEB-INF/jsp/mywizarddone.jsp" />
<forward name="cancel"
path="/WEB-INF/jsp/mywizardcancel.jsp"
/>
</action>
<!-- the last screen of the wizard (back, finish and cancel only) -->
<action path="/mywizard3"
type="actions.MyWizard"
name="MyWizard"
validate="true"
input="/WEB-INF/jsp/mywizard3.jsp">
<forward name="back"
path="/WEB-INF/jsp/mywizard2.jsp" />
<forward name="finish"
path="/WEB-INF/jsp/mywizarddone.jsp" />
<forward name="cancel"
path="/WEB-INF/jsp/mywizardcancel.jsp"
/>
</action>
The pieces of the wizard are as follows:
forms.MyWizard.java - the form bean holding the
information required
actions.MyWizard.java - the actions of the wizard,
note the use of LookupDispatchAction allows for one
action class with several methods. All the real work will be done in the
'finish' method.
mywizard[x].jsp - the data
collection jsp's
mywizarddone.jsp - the 'success' page
mywizardcancel.jsp - the 'cancel' page
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