Front Controller

JSP

A Java Server Page (JSP) is an HTML file that may contain JSP components.

JSP components include:

scriptlets <%, <%!, <%=, <%-- ... %>

directives <%@ ... %>

actions <jsp:action> ... </jsp:action>

Model-View-Controller

Model-View-Controller architecture divides the web app into three components:

The model provides application data and logic. It is independent of the views and controller. In our examples the model usually takes the form of a Java bean.

The controller is responsible for receiving and executing user input commands. In the Service-to-Worker pattern the controller accesses the model directly. In the Dispatcher-View pattern the controller simply selects a view.

A view is responsible for sending HTML to the client. The view may fetch dynamic content from the model.

FrontController.jsp

<html>
<head> <title> Front Controller </title> </head>
<body>

A front controller forwards client requests to a selected view.
<br />

</body>
</html>

Build and Deploy

Request myapp/FrontController.jsp from localhost:8080

request.html

<html>
<head> <title> A simple form </title> </head>
<body>

<form action = "/myapp/FrontController.jsp" method = "post">
Send a command to the server (type return to send):
<input type = "text" name = "cmmd" value = "???" size = "30"/>
</form>

</body>
</html>

View.jsp

<html>
<head> <title> A Simple View </title> </head>
<body>

A view sends static and dynamic HTML to the client. <br />
Here's what I know about you: <br />

<ul>
<li> command = <%= request.getParameter("cmmd") %> </li>
<li> locale = <%= request.getLocale().getDisplayCountry() %> </li>
<li> computer = <%= request.getRemoteHost() %> </li>
</ul>

</body>
</html>

FrontController.jsp (enhanced)

<html>
<head> <title> Front Controller </title> </head>
<body>

A front controller forwards client requests to a selected view.
<br/>

<%!
   String selectView(String cmmd) {
      String view = "View.jsp";
      // convert cmmd into view
      return view;
   }
%>

<%
   String command = request.getParameter("cmmd");
   String view = selectView(command);
%>

<jsp:forward page = "<%= view %>" />

</body>
</html>

Build

Request request.html

HelperBean.java

public class HelperBean implements java.io.Serializable {
   private int count = 0;
   public String fetchContent() {
      return " count = " + count++;
   }
}

View.java

<html>
<head><title> A Simple View </title></head>
<body>

<jsp:useBean id = "helper" scope = "request"
class = "HelperBean" />

A view sends static and dynamic HTML to the client. <br />

Dynamic content is usually provided by a helper bean:
<%= helper.fetchContent() %>
<br />

</body>
</html>

Build

Request Page

Change Bean Scope

<jsp:useBean id = "helper" scope = "session"
class = "HelperBean" />

Re-request Page

Include Content from Other Files

<html>
<head><title> A Simple View </title></head>
<body>

A view may include HTML generated from other files: <br />
<jsp:include page = "request.html" flush = "true" />

</body>
</html>

Build and Request

Front Controller (Working)

<html>
<head> <title> Front Controller </title> </head>
<body>

A front controller forwards client requests to a selected view.
<br/>

<%
// Dispatcher FC assumes view = cmmd
String view = request.getParameter("cmmd");
%>

<jsp:forward page = "<%= view %>" />

</body>
</html>

request.html (Enhanced)

<html>
<head> <title> A simple form </title> </head>
<body>

<form action = "/myapp/FrontController.jsp" method = "post">
   <input type = "hidden" name = "cmmd" value = "/View.jsp">
   Name: <input type = "text"
                 name = "name"
                 value = "???"
                 size = "30"/> <br />
   Phone: <input type = "text"
                 name = "phone"
                 value = "???"
                 size = "30"/><br />
   <input type = "radio"
          name = "party"
          value="Democrat" checked/> Democrat <br />
   <input type = "radio"
          name = "party"
          value="Republican"/> Republican <br />
   <input type = "submit" />
</form>

</body>
</html>

View.jsp

<html>
<head><title> A Simple View </title></head>
<body>

Good evening <%= request.getParameter("name") %>.

<% if (request.getParameter("party").equals("Democrat")) %>
You must be very sad.
<% else %>
You must be very glad.

</body>
</html>