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Implementing MVC from scratch

For simple and moderately complex applications,  implementing MVC from scratch with RequestDispatcher is straightforward and flexible.

The most important point about MVC is the idea of separating the business logic and data access layers from the presentation layer.  Here is a quick summary of the required steps:
1. Define beans to represent the data.
Beans are just Java objects that follow a few simple conventions. Your first step is define beans to represent the results that will be presented to the user
2. Use a servlet to handle requests.
In most cases, the servlet reads request parameters(shorthand using BeanUtils.populateBean)
3. Populate the beans. The servlet invokes business logic(application-specific code) or data-access code to obtain the results. The results are placed in the beans that were defined in step 1.

ValueObject value = new ValueObject(...);
HttpSession session = request.getSession();
session.setAttribute("key", value);

Next, the servlet would forward to a JSP page that uses the following
to retrieve the data:
<jsp:useBean id="key" type="somePackage.ValueObject"
scope="session" />

4. Store the bean in the request, session , or servlet context. The servlet calls setAttribute on the request , session , or servlet context objects to store a reference to the beans that represent the results of the request.

5. Forward the request to a JSP page. The servlet determines which JSP page is appropriate to the situation and uses the forward method of RequestDispatcher to transfer control to that page.

String operation = request.getParameter("operation");
if (operation == null) {
operation = "unknown";
String address;
if (operation.equals("order")) {
address = "/WEB-INF/Order.jsp";
} else if (operation.equals("cancel")) {
address = "/WEB-INF/Cancel.jsp";
} else {
address = "/WEB-INF/UnknownOperation.jsp";
//RequestDispatcher: Defines an object that receives requests from the clicent and sends them to any //resource(such as a servlet, HTML file , or JSP file) on the server. The servlet container creates the //RequestDispatcher object, which is used as a wrapper around a server resource located at a particular //path or given by a particular name.
RequestDispatcher dispatcher =
dispatcher.forward(request, response);
// in JSp, you can use <jsp:forward page="<%= destination %>" />

6. Extract the data from the beans. the JSP page accesses beans with jsp:useBean and a scope matching the location of Step 4. The page then uses jsp:getProperty to output the bean properties. The JSP page does not create or modify the bean; it merely extracts and displays data that the servlet created.

<jsp:useBean id="key" type="somePackage.SomeBeanClass"
scope="session" />


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