SOEN 6011Software Engineering Processes
Section SS Fall 2007Dr Greg Butler
http://www.cs.concordia.ca/~gregb/home/soen6011-f2007.html
Week 6• Servlets and JSP• Larman Controller Principle• Fowler Front Controller
– GoF Command pattern– Java reflection to instantiate Command
Servlets, The General Idea• Applet as a client side Java application.• Servlet as a server side technology for
implementing part of the functionality of an application.
• HTTP (URL) requests cause a servlet to be:– Instantiated (if it did not exist).– Run.– The servlet builds a response which is then sent back
to the client (usually in the form of HTML).
Servlet: HelloWebpublic class HelloWebServlet extends HttpServlet
{protected void doGet(HttpServletRequest request,HttpServletResponse response)throws ServletException, IOException {ServletOutputStream out = response.getOutputStream();out.println("Hello Web!");}
}
Java Server Page
• Servlet:– Embed HTML inside code.
• JSP– Embed code inside HTML.
HelloWeb.jsp
<%@ page contentType="text/html; charset=iso-8859-1" language="java" ... %>
<html><body>Hello Web!</body></html>
Hello.jsp
<%@ page contentType="text/html; charset=iso-8859-1" language="java" ... %>
<html><body>Hello <%= request.getParameter("name") %></body></html>
Java Server Page
• Implemented as a (special kind of) servlet.
Controller Illustration: Web-based EA
Server :Tut06ControllerServlet
HTTP Get
doGet(req,resp)
g:Greeting
req:
setAttribute("Greeting",g)
forward(".../Greeting.jsp")
Greeting.jsp
getAttribute(...)
g
...
getGreeting()
PATTERN: Controller
• What object in the domain (or application coordination layer) receives requests for work from the UI layer?
???
Presentation
ApplicationLogic
Video Store
Record Rental
Video ID
...
...
...
... ...
Clerk
rent(videoID)
Now what happens?
PATTERN: Controller (Larman 17.13)
Problem: What object in the domain (or application coordination layer) receives requests for work from the UI layer?
System operations (see SSD): major input events to the system
The controller is the first object beyond the UI layer that is responsible for receiving or handling a system operations message.
Note that UI objects delegate system operation request to a controller.
PATTERN: Controller (Larman 17.13)
Solution: Assign the responsibility to a class representing one of the following choices:
• A façade controller, which represents1. the overall system2. A root object3. A device that the software is running within, or4. A major subsystem
• A use case or session controller which represents a use case scenario in which the system operation occurs
Fig. 17.21
Which class of object should be responsible for receiving this system event message?
It is sometimes called the controller or coordinator. It does not normally do the work, but delegates it to other objects.
The controller is a kind of "facade" onto the domain layer from the interface layer.
actionPerformed( actionEvent )
: ???
: Cashier
:SaleJFrame
presses button
enterItem(itemID, qty)
UI Layer
Domain Layer
system operation message
Dom
ain
Data
Sou
rce
Transaction Script Domain Model
Data Mapper
Row Data Gateway
Front Controller
Template View
Transform View
Page Controller
Pres
enta
tion
Active Record
Fowler’s EA Patterns
Table Module
Table Data Gateway
Front ControllerFowler: A controller that handles all requests
for a web site.
The Front Controller consolidates all request handling by channeling requests through a single handler object. This object can carry out common behaviour which can be modified at runtime with decorators. This handler then dispatches to command objects for behaviour particular to a request.
Front ControllerGoF design pattern: Command
Use a Command object to encapsulate the steps that are executed by a command.
Command::doIt() executes commandAllows Command::unDoIt() etc
GoF design pattern: DecoratorDecorator::m() adds additional behaviour, but delegates
to real Object::m() for real behaviourEg, logging command execution before/after executing
command
Page vs Front Controller – Differences in URL Page Controllers for A1 • http:/stu.cs/343A1/ViewTaskList• http:/stu.cs/343A1/AddTask• http:/stu.cs/343A1/RemoveAllTasks• …
Front Controller for A1• Base URL: http:/stu.cs/343A1/frontController• …/frontController
?command=ViewTaskList• …/frontController
?command=AddTaskGrade&title=abc&…
Front Controller
FrontServlet
# processRequest ( )- getCommand ( )- getCommandClass ( )
FrontCommand
+ init ( )+ processRequest ( )
AddStudent
+ processRequest ( )
Servlet
+ init ( )+ destroy ( )# processRequest ( )# doGet ( )# doPost ( )+ getServletInfo ( )+ forward ( )+ forwardAbsolute ( )+ getUrlBase ( )
# controller
0..1
Error
+ processRequest ( )
Front Controller
Front Controller: Sequence Diagram
Front Controller: ProcessRequest
Front Controller (cont’d)