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The DVD Manager Application
This application is used to manage a set of DVDs:
- List all of them.
- Show the details for a specific DVD.
- Update data for a specific DVD.
- Create a new DVD.
- Delete a specific DVD.
This figure shows the flow between the application's two pages:
The "List page" looks like this:
The "Detail page":
In a real, serious application, the DVD data would be stored on some back-end, probably a database. To simplify this example, the DVD data is stored in a static area in a DVDManager class. This class has methods like:
-
getDvds()
-
findDvd(int key)
-
updateDvd(int key,...)
-
deleteDvd(int key)
A DVD class is used to store data for a DVD and each DVD has a unique key, implemented through an int value. An actor is kept in an Actor class, and each DVD instance keeps an array of Actor instances. Check out the DVDManager class
here.
The advice given above will now be applied to the List and Detail pages.
The "List Page"
This is a page without a form, but it's still useful to create a "Page action
class," which is responsible for:
- Getting the list with all DVDs as a collection of beans. This is the preferred format for any Struts
.jsp page.
- Place the collection in the request.
So there are no pre-conditions for this page action. But if the list of DVDs is large, one could easily imagine that some kind of "filter" or "mask" would be necessary to limit the number of DVDs that are listed. See the source code for the List action
here.
Since the List page has no form, there is really no need for a relay action. You'll pass the key for the selected DVD as a parameter on a hyperlink to the next action.
The "Detail Page"
This page is, of course, much more interesting. Following Rule #3, pass the key of the selected DVD in the request. Naturally, the receiver is the Page action for the Detail page. To place the key in the request, you need an action that takes the key from a parameter and places it in the request:
The code for the "Flow to Detail action" is
here,
and the code for the "Detail page action" is
here.
Following Rule #1, the page action looks up the DVD details and then stores the data in the Detail form bean. Finally, it retrieves names and keys for all actors to be used by the actors drop down.
As you can see, the ability to "chain" Struts actions is used in several situations.
Rule #5: Don't be afraid to chain Struts actions.
Chaining results in small modules with loose, configurable bindings. This is generally a desirable application property.
In most common situations, the overhead involved in chaining is minimal. However, you should note, that for each action that defines a form (using the name-parameter
in struts-config) Struts populates the form from the request and also validates
the data if validation="true".
The "update," "create," "delete," and "cancel" actions—all available through
buttons on the Detail page—are simple to implement. The relay action directs
each action to a separate class. Here is—as an example—the "update" class:
public class UpdateDVDAction extends MyAction {
public ActionForward myExecute(
ActionMapping mapping,
ActionForm form,
HttpServletRequest request,
HttpServletResponse response) throws Exception {
// Get the data from the Form bean
DynaValidatorForm df = (DynaValidatorForm) form;
int key = (Integer) df.get("key"); /* 1 */
String title = (String) df.get("title");
int length = (Integer) df.get("length");
int[] actors = (int[]) df.get("actors"); /* 2 */
// Update the DVD
DVDManager manager = new DVDManager();
manager.updateDVD(key, title, length, actors);
// Use the key when the DVD is displayed again
request.setAttribute("key", key); /* 3 */
// Forward control to the next page
return doForward(mapping, "OK");
}
}
Notes:
- The key for the DVD is stored in a hidden form field.
- The actors have keys just like the DVDs.
- Since you must flow back to the same page, you must keep the contract: store a key in the request for the Detail page action.
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