Dmitri Namiot wrote:
> looks like very logical and clear model, but see:
> you submit your forms to servelt. (I also like
> one servlet per application, just more easy to
> maintain). So your logic (application logic) is
> supported by servlet. What in that case the added
> value from JSP-beans to web-application ?
> Yes, we have some business rules implementation,
> localized within some java classes. But we can use these
> classes from servlet. I am feeling myself some trouble
> to split beans/servlets during own "jsp-projects" for the
> clients. Looks like in the most cases it is still "servlet-
> projects" because any attempt to add some dynamic/behavior
> to your JSP pages directly means java-coding within the page
> what is not a good.
>
Let's review the MVC roles of the various components for a moment:
* Servlet == Controller. It decides which business logic
method to call, and then calls it.
* Beans == Model. The business logic does something,
and usually creates results that need to be displayed or
saved for later use.
* JSP Page == View. It displays the results.
To make this concrete, let's assume a very simple form that let's you ask for a
particular
customer to be displayed:
<form method="post" action="/myservlet/displayCustomer">
Account Number:
<input type="text" name="account">
<input type="submit">
</form>
The servlet that receives this would look at the "/displayCustomer" part of the URI
(this is
not the only way to do it, but it's easy) and calls a business method to perform the
lookup.
The business logic extracts the account number, and uses it to perform a JDBC lookup
of a row
in the CUSTOMERS table.
Now, the question becomes, "how do I transfer the column values for this customer to
the JSP
page for display?" A really simple way is to create a CustomerBean that represents
this
customer, with properties for each column you care about. Say, ...
CustomerBean cb = new CustomerBean();
cb.setName(resultset.getString("name"));
cb.setAddress(resultset.getAddress("address"));
// and so on
To transfer the bean to the JSP page, you need to pass it somehow. There's three
choices --
with the request, in the session, or in the servlet context. Because we only need this
customer for long enough to display it, the request is the right way. Let's store
this bean,
and then forward control to the correct JSP page for displaying the result.
request.setAttribute("customerBean", cb);
RequestDispatcher rd =
getServletContext().getRequestDispatcher("/displayCustomer.jsp");
rd.forward(request, response);
In the "/displayCustomer.jsp" JSP page, you can now refer to the bean that was passed
just
like any other bean:
<jsp:useBean id="customerBean" scope="request"
type="CustomerBean" />
Customer Name: <%= customerBean.getName() %>
Customer Address: <%= customerBean.getAddress() %>
No Java logic in the JSP page is required except the scriptlet fragments to get the
property
values.
So, we used a bean to represent (or "model") data that was in the database. We
created the
bean in the servlet, and passed it to the JSP page. After the page is done, the bean
is
thrown away (because we passed it as a request attribute). You'd use one of the other
scopes
for things that needed to last longer than the current request.
Why go to all the extra work? Well, consider the impact of a couple possible changes
that
might happen to your application later:
* You might go back and change the look and feel of the
customer display page. You can do this with no impact
on the servlet or the business logic.
* You might change the database from which you read
customer information -- perhaps you now have to join
two different tables to create a customer bean. You can
do this by modifying only the business logic -- the JSP
page doesn't care at all how the customer bean got
created; all it needs to know is that the business logic
left it there.
Craig McClanahan
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