[Fwd: [MVC-Programmers@basebeans.com] [Struts Tips] #2 - Use DispatchAction to organize related operations]

[Vic Cekvenich <vi...@basebeans.com>]

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hth, Vic

-------- Original Message --------
Subject: [MVC-Programmers@basebeans.com] [Struts Tips] #2 - Use 
DispatchAction to organize related operations
Date: Mon, 25 Mar 2002 09:11:07 -0500
From: Ted Husted <hu...@apache.org>
Reply-To: mvc-programmers@basebeans.com

Newsgroups: MVC-Programmers
References: <3C...@apache.org>

Any software application is defined by the things it can do for you. In
a Struts Web application, the things an application does is usually
defined by its action-mapping elements. An action-mapping is designed to
be the target of an HTML form, and is often used with hyperlinks as
well.

Each action-mapping can specify a Struts Action class as its handler. In
larger applications, developers can find themselves managing dozens or
even hundreds of Action classes.

In practice, many of these Action classes handle related operations,
often evidenced by their name. A package might include separate
RegCreate, RegSave, and RegDelete Actions, which just perform different
operations on the same RegBean object. Since all of these operations are
usually handled by the same JSP page, it would be handy to also have
them handled by the same Struts Action.

A very simple way to do this is to have the submit button modify a field
in the form which indicates which operation to perform.

<SCRIPT>
function set(target) {
  document.forms[0].dispatch.value=target;
}
</SCRIPT>

<html:hidden property="dispatch" value="error"/>
<html:submit onclick="set('save');">SAVE</html:submit>
<html:submit onclick="set('create');">SAVE AS NEW</html:submitl>
<html:submit onclick="set('delete);">DELETE</html:submit>

Then, in the Action you can setup different methods to handle the
different operations, and branch to one or the other depending on which
value is passed in the dispatch field.

String dispatch = myForm.getDispatch();

if ("create".equals(dispatch)) { ...

if ("save".equals(dispatch)) { ...

The Struts Dispatch Action is designed to do exactly the same thing, but
without messy branching logic. The base perform method will check a
dispatch field for you, and invoke the indicated method. The only catch
is that the dispatch methods must use the same signature as perform.
This is a very modest requirement, since in practice you usually end up
doing that anyway.

To convert an Action that was switching on a dispatch field to a
DispatchAction, you simply need to create methods like this

     public ActionForward create(ActionMapping mapping,
                  ActionForm form,
                  HttpServletRequest request,
                  HttpServletResponse response)
     throws IOException, ServletException { ...

     public ActionForward save(ActionMapping mapping,
                  ActionForm form,
                  HttpServletRequest request,
                  HttpServletResponse response)
     throws IOException, ServletException { ...

Cool. But do you have to use a property named dispatch? No, you don't.
The other step is to specify the name of of the "dispatch" property as
the "parameter" property of the action-mapping. So a mapping for our
example might look like this:

             <action
                 path="/reg/dispatch"
                 type="app.reg.RegDispatch"
                 name="regForm"
                 scope="request"
                 validate="true"
                 parameter="dispatch"/> // Which parameter to use

If you wanted to use the property "o" instead, as in o=create, you would
change the mapping to

             <action
                 path="/reg/dispatch"
                 type="app.reg.RegDispatch"
                 name="regForm"
                 scope="request"
                 validate="true"
                 parameter="o"/> // Look for o=dispatchMethod

Again, very cool. But why use a JavaScript button in the first place?
Why not use several buttons named "dispatch" and use the values to
specify the operation.

You can, but the value of the button is also its label. This means if
the page designers want to label the button something different, they
have to coordinate with the Action programmer. Worse, localization
becomes virtualy impossible.

If you prefer not to use JavaScript buttons, you can use the
DispatchLookup Action instead. This works much like the DispatchAction,
but requires more setup. We'll explore the DispatchLookup Action in Tip
#3.

HTH, Ted.


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