1. SOEN 343 Software Design Section H Fall 2006 Dr Greg Butler

2. Outline Week 9 lectures: Evaluate Good Design
Page Controller and Front Controller
Note Command Pattern
Web EA Do-It-All TS Re-design
GRASP Principles
Pure Fabrication
Indirection

3. Recall the Greeting EA
A single servlet which offers the greeting “Hello”.
We refactored it, offering two alternative designs.

4. Review Question On a blank sheet of paper …
Provide three class diagrams corresponding to the three design solutions of the Greeting EA.

5. Evaluating a Design
Which is a better design?
What is a good design?

6. What is a good design? Satisfies user needs, requirements.
Reveals intent.
Is a simple as possible.
Minimizes cost of … likely changes.
And …
High cohesion.
Low coupling.

7. Greeting EA Evolution … What if …
Support a personalized greeting.
Change “look” of output (e.g. bold the name).
Get full name from db.

8. Comparing Design Solutions
Property
Sol’n 1
Sol’n 2
Sol’n 3
Correct 
Intent clear
Simplest
Acc. Change
Cohesion
Coupling

9. GRASP: Controller Who handles a system event?
E.g. “List Movies”
Main choices: assign to a design entity representing
Overall system, or subsystem (façade controller).
A Use Case scenario (often named, e.g. ListMovieHandler).

10. GRASP: Controller Who handles a system event?
E.g. “List Movies”
Main choices: assign to a design entity representing
Overall system, or subsystem (façade controller).
A Use Case scenario (often named, e.g. ListMovieHandler).

11. Presentation Domain Data Source Controller Patterns Page Controller
Template View
Presentation
Front Controller
Transform View
Domain Model
Transaction Script
Domain
Data Mapper
Active Record
Table Module
Data Mapper
Row Data Gateway
Table Data Gateway
Data Source
Table Data Gateway

12. Page Controller (done)

13. Front Controller
Fowler: 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.

14. Front Controller GoF design pattern: Command
Use a Command object to encapsulate the steps that are executed by a command.
Command::doIt() executes command
Allows Command::unDoIt() etc
GoF design pattern: Decorator
Decorator::m() adds additional behaviour, but delegates to real Object::m() for real behaviour
Eg, logging command execution before/after executing command

15. Page vs Front Controller – Differences in URL
Page Controllers for A1 …
Front Controller for A1
Base URL:
…/frontController ?command=ViewTaskList
…/frontController ?command=AddTaskGrade&title=abc&…

16. Front Controller

17. Front Controller

18. Front Controller (Fowler) Command (GoF) Patterns
FrontCommand
+ init ( )
+ processRequest ( )
RemoveStudentCommand
ViewStudInfoCommand
+ processRequest ( )
+ processRequest ( )

19. Front Controller: Sequence Diagram

20. Front Controller: ProcessRequest

21. Front Controller (cont’d)

22. Front Controller: Advantages?

23. Web EA Redesign Exercise
Refactoring a DoItAll Servlet, let me count the ways …

24. Do-it-all Servlet or Script
Presentation
Do-it-all Servlet or Script
Domain
Data Source

25. Do-it-all Servlet or Script, Class
public class DoItAll extends javax.servlet.http.HttpServlet {
protected void doGet(
HttpServletRequest request,
HttpServletResponse response)
throws ... // … }

26. Redesigning This Application
Redesign DoItAll so as to increase the class cohesion.
Create new classes. Which EA patterns would you use?
Distribute the doGet() method code (given on the next slide) into the new classes. Identify which lines would go into which classes.

27. Do-it-all Transaction Script, doGet()
String lastName = request.getParameter(“…");
Connection dbc = DbRegistry.getDbConnection();
String findSql = "SELECT * from … where LastName = ?”;
PreparedStatement dbQuery = dbc.prepareStatement(findSql);
dbQuery.setString(1, lastName);
ResultSet rs = dbQuery.executeQuery();
rs.next();
String firstName = rs.getString("FirstName");
lastName = lastName.toUpperCase();
PrintWriter out = response.getWriter();
response.setContentType("text/html");
out.println("<h1>Hello "+firstName+" "+lastName+"</h1>");

28. Do-it-all Servlet Redesign: Separating Concerns
The next few slides show ways in which we can separate concerns; i.e. distribute the functionality of the do-it-all script into separate classes.
Combinations other than the ones shown are possible.

29. Redesigning Do-it-all (2 classes) [#1]
Presentation
Do-it-all
redesign
Domain
Data Source
Note:

30. Redesigning Do-it-all (3 classes) [#3]
Presentation
Do-it-all
redesign
Domain
Data Source

31. Redesigning Do-it-all (4 classes) [#4]
Presentation
Do-it-all
redesign
Domain
Data Source

32. Redesigning Do-it-all : Adding … (5 classes) [#5]
Presentation
Do-it-all
redesign
Domain
Data Source

33. Redesigning Do-it-all : Adding … [#5b]
Presentation
Do-it-all
redesign
Domain
Data Source

34. Redesigning Do-it-all (5 classes) [#5c]
Presentation
Do-it-all
redesign
Domain
Data Source
Is there anything wrong with this design? (Hint: consider the dependencies and the layering style.)

35. Redesigning Do-it-all (6 classes) [#6]
Presentation
Do-it-all
redesign
Domain
Data Source

36. Pure Fabrication
Problem: Existing objects, ie domain objects, are not appropriate to have the responsibility
Solution suggested by Information Expert not appropriate
Might violate high cohesion, low coupling
Solution: Fabricate (ie create, make up) a new object to hold the responsibility

37. GRASP: Pure Fabrication
Assign a highly cohesive set of responsibilities to an artificial or convenience class that does not represent a problem domain concept—something made up, to support high cohesion, low coupling, and reuse.
Can you think of an example from EA?

38. GRASP: Pure Fabrication
Design of objects
Representational decomposition
Behavorial decomposition
It’s OK for OO software to have objects representing behaviour, ie use case, process, function, strategy, TS
But do not overdo it
All GoF design patterns are Pure Fabrications

39. Indirection
Problem: How to assign responsibility to avoid direct coupling? How to de-couple objects?
Solution: Assign responsibility to an intermediatory object to mediate between the two components
Example: Adapter pattern
Intermediatory to external tax calculators

40. Fig

41. Indirection
“Most problems in computer science can be solved by another level of indirection”