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Pages With and Without Forms
The purpose of Web applications is to show pages in a browser. Pages can be divided into two groups: either they contain one or more "forms" or they are form-less. A form contains "controls"—like text input fields, drop downs, checkboxes, and buttons. The form-less pages are, by far, the most simple. As a programmer, you only need to put your data in one or more beans, or collection of beans, and then use the bean:write tag to show this data. More interesting are the form pages, and the rest of the article will give advice on techniques and standards for coding these pages. For simplicity, assume that you only have one form per page. It's not difficult to extend the techniques to handle more than one form.
Every form must have an accompanying "form bean." Think of this bean as a buffer between the action classes and the browser. To get the data entered in the form, the action class retrieves it from the form bean. To place data in the form's controls, Struts gives you the option of using any bean you like, so this gives us room for:
Rule #1: Data that must be shown in a form control should be made available through the form bean.
This makes the scenario "symmetric"—data is entered and retrieved from the
same source. A common situation in applications is a page flow from one page to
the same page. Using Rule #1, the data is not lost from the form during
the flow.
Here's an example that shows how to place and retrieve data from the form. To show the text "JAVA" in a text field called "language", you code something like this in the action class:
// Using real form beans
DetailForm df = (DetailForm)form;
df.setLanguage("JAVA");
- - -
// Using dynamic form beans:
DynaValidatorForm df = (DynaValidatorForm) form;
df.set("language", "JAVA");
In the .jsp page, write:
<html:text property="language"/>
Since you take the language value from the form bean, you don't have to use the "name" parameter of the html:text tag.
Keep the form bean in request scope if possible. This saves memory space and helps to avoid name clashes in the request namespace. So the purpose of Rule #1 is simplicity and consistency.
Now, take a look at the action class that invokes the .jsp page. It should be considered a special action. It is responsible for:
- Checking "preconditions": Are you able to show the data you want on the page?
- Copying data to the form bean.
- Storing other data (in the request or the session objects) to be used by the .
jsp page. This could be read-only data or the data that appears in drop downs.
This kind of action class will be called a "Page Action."
Rule #2: Make a "Page Action" for every page.
Use time to make it clear what the preconditions are and note them in the source code (and other documentation). A page action that shows "dynamic data," for example a specific customer's data, will need the customer key or id passed to it in some way.
Rule #3: Use the request object to pass data to a page action.
Again, this advice is to keep things simple and to save memory space. Every Java class working in the servlet environment can place data on the request object, so it's a general technique. Often you may want to pass data through parameters, for example in the URL, but you should remember that there is no easy way to create new parameters on the request.
All this advice gives you solid control over how pages are built, but what about submission of forms? This is also an area that can benefit from implementing standards. With a standard Struts setup, you submit the form to the class mentioned in the html:form's
action parameter. If the page only offers one type of functionality, then it's simple to let the receiving action process the request. Often, however, a page offers more than one function. An example is a page that can update, delete, or create some entity. Struts has some helper classes (DispatchAction, etc.) that
can direct various functions to specific Java methods. I prefer to create a "relay-action" that simply sends the request to specific, separate action classes. The
commonly used technique to recognize the various functions is to place the name of the function in a hidden form field. This can be done with simple JavaScript. If you prefer a JavaScript-less solution, you may use form buttons with different names and then test for the presence of these names in the relay action.
Here's what is needed in the .jsp page:
. . .
<script>
function go(action) {
document.forms[0].action.value=action;
}
</script>
. . .
<html:form ...
. . .
<html:submit onclick="go('update')" value="Update"/>
<input type=hidden name=action>
</html:form>
. . .
This example shows how a submit button will call the JavaScript function "go," which inserts the action name in the hidden field called "action." A relay action class becomes very simple:
public final class DetailDVDRelayAction extends MyAction {
public ActionForward myExecute(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response) throws Exception {
// Forward control to the given action
return doForward(mapping, request.getParameter("action"));
}
}
The corresponding struts-config file must list all the actions the relay action can receive, for example:
<action path="/detailDVDRelay"
type="keld.playground.DetailDVDRelayAction"
name="detailForm"
validate="false"
scope="request">
<forward name="create" path="/createDVD.do"/>
<forward name="update" path="/updateDVD.do"/>
<forward name="delete" path="/deleteDVD.do"/>
<forward name="cancel" path="/cancelDVD.do"/>
</action>
Rule #4: Use a relay action class for each form.
Now it's time to apply these rules to a real application.
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