Ques 46. 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:
...
...
<-- Registration form bean -->
type="org.apache.struts.webapp.example.RegistrationForm"/>
...
...
...
<-- Edit user registration -->
type="org.apache.struts.webapp.example.EditRegistrationAction"
name="registrationForm"
scope="request"
validate="false"/>
...
<-- Save user registration -->
type="org.apache.struts.webapp.example.SaveRegistrationAction"
name="registrationForm"
input="registration"
scope="request"/>
...
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 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.
Ques 47. 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.
Ques 48. 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:
Ques 49. 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.
Ques 50. 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">
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.
